Electrical Notes and Articles

Total Losses in Power Distribution and Transmission Lines-Part 2 JULY 2, 2013

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3 Votes

(2) Non-Technical (Commercial Losses): Non-technical losses are at 16.6%, and related to meter reading, defective meter and error in meter reading, billing of customer energy consumption, lack of administration, financial constraints, and estimating unmetered supply of energy as well as energy thefts.

Main Reasons for Non-Technical Losses: (1) Power Theft : Theft of power is energy delivered to customers that is not measured by the energy meter for the customer. Customer tempers the meter by mechanical jerks, placement of powerful magnets or disturbing the disc rotation with foreign matters, stopping the meters by remote control.

(2) Metering Inaccuracies: Losses due to metering inaccuracies are defined as the difference between the amount of energy actually delivered through the meters and the amount registered by the meters. All energy meters have some level of error which requires that standards be established. Measurement Canada, formerly Industry Canada, is responsible for regulating energy meter accuracy. Statutory requirements5 are for meters to be within an accuracy range of +2.5% and – 3.5%. Old technology meters normally started life with negligible errors, but as their mechanisms aged they slowed down resulting in under-recording. Modern electronic meters do not under-record with age in this way. Consequently, with the introduction of electronic meters, there should have been a progressive reduction in meter errors. Increasing the rate of replacement of mechanical meters should accelerate this process

(3) Un metered Losses for very small Load: Unmetered losses are situations where the energy usage is estimated instead of measured with an energy meter. This happens when the loads are very small and energy meter installation is economically impractical. Examples of this are street lights and cable television amplifiers.

(4) Un metered supply: Unmetered supply to agricultural pumps is one of the major reasons for commercial losses. In most states, the agricultural tariff is based on the unit horsepower (H.P.) of the motors. Such power loads get sanctioned at the low load declarations. Once the connections are released, the consumers increasing their connected loads, without obtaining necessary sanction, for increased loading, from the utility. Further estimation of the energy consumed in unmetered supply has a great bearing on the estimation of T&D losses on account of inherent errors in estimation. Most of the utilities deliberately overestimate the unmetered agricultural consumption to get higher subsidy from the State Govt. and also project. reduction in losses. In other words higher the estimates of the unmetered consumption, lesser the T&D loss figure and vice versa. Moreover the correct estimation of unmetered consumption by the agricultural sector greatly depends upon the cropping

pattern, ground water level, seasonal variation, hours of operation etc.

(5) Error in Meter Reading: Proper Calibrated Meter should be used to measure Electrical Energy. Defective Energy Meter should be replaced immediately. The reason for defective meter are Burning of meters, Burn out Terminal Box of Meter due to heavy load, improper C.T.ratio and reducing the recording, Improper testing and calibration of meters.

(6) Billing Problems: Faulty and untimely serving Bill should be main part of non-Technical Losses. Normal Complain regarding Billing are Not Receipt of Bill, Late Receipt of Bill, Receiving wrong Bill , Wrong Meter Reading, Wrong Tariff, wrong Calculations.

How to reduce Technical Losses: (1) Converting LV Line to HV Line: Many Distribution pockets of Low Voltage (430V) in Town are surrounded by higher voltage feeders. At this lower voltage, more conductor current flows for the same power delivered, resulting in higher I2R losses. Converting old LV (430V) feeders to higher voltage the Investment Cost is high and often not economically justifiable but If parts of the LV (430V) Primary feeders are in relatively good condition, installing multiple step-down power transformers at the periphery of the 430 volt area will reduce copper losses by injecting load current at more points (i.e., reducing overall conductor current and the distance traveled by the current to serve the load).

(2) Large Commercial / Industrial Consumer get direct Line from Feeder: Design the distribution network system in such a way that if it is Possible than large consumer gets direct Power Line from feeder.

(3) Adopting High Voltage Distribution Service (HVDS) for Agricultural Customer: In High Voltage direct service (HVDS) ,11KV line direct given to cluster of 2 to 3 Agricultural Customer for Agricultural Pump set and employed small distribution Transformer (15KVA) for given these 2 to 3 customer through smallest ( almost negligible) LT distribution Lines. In HVDS there is less distribution losses due to minimum length of Distribution Line, High quality of Power Supply with no Voltage drop, Less Burn out of motor due to less voltage fluctuation and Good quality of Power, to avoid overloading of Transformer.

(4) Adopting Arial Bundle Conductor (ABC): Where LT Line are not totally avoidable use Arial Bundle Conductor to minimize faults in Lines, to avoid direct theft from Line (Tampering of Line).

(5) Reduce Number of Transformer: Reduce the number of transformation steps. Transformers are responsible for almost half of network losses. High efficiency distribution transformers can make a large impact on reduction of Distribution Losses

(6) Utilize Feeder on its Average Capacity:

By overloading of Distribution Feeder Distribution Losses will be increase. The higher the load on a power line, the higher its variable losses. It has been suggested that the optimal average utilization rate of distribution network cables should be as low as 30% if the cost of losses is taken into account.

(7) Replacements of Old Conductor/Cables: By using the higher the cross-section area of Conductor / cables the losses will be lower but the same time cost will be high so by forecasting the future Load an optimum balance between investment cost and network losses should be maintained.

(8) Feeder Renovation / Improvement Program: Re conductoring of Transmission and Distribution Line according to Load. Identification of the weakest areas in the distribution system and strengthening /improving them. Reducing the length of LT lines by relocation of distribution sub stations or installations of additional new distribution transformers. Installation of lower capacity distribution transformers at each consumer premises instead of cluster formation and substitution of distribution transformers with those having lower no load losses such as amorphous core transformers. Installation of shunt capacitors for improvement of power factor. Installation of single-phase transformers to feed domestic and nondomestic load in rural areas. Providing of small 25kVA distribution transformers with a distribution box attached to its body, having provision for installation of meters, MCCB and capacitor. Lying of direct insulated service line to each agriculture consumer from distribution transformers Due to Feeder Renovation Program T&D loss may be reduced from 60-70 % to 15-20 %.

(9) Industrial / Urban Focus Program: Separations of Rural Feeders from Industrial Feeders. Instantly release of New Industrial or HT connections. Identify and Replacement of slow and sluggish meters by Electronics type meters. In Industrial and agricultural Consumer adopt One Consumer, one Transformer scheme with meter should be Introduced. Change of old Service Line by armored cable. Due to Feeder Renovation Program T&D loss may be reduced from 60-70 % to 15-20 %.

(10)

Strictly Follow Preventive Maintenance Program:

Required to adopt Preventive Maintenance Program of Line to reduce Losses due to Faulty / Leakage Line Parts. Required to tights of Joints, Wire to reduce leakage current.

How to reduce Non-Technical Losses: (1) Making mapping / Data of Distribution Line: Mapping of complete primary and secondary distribution system with all parameters such as conductor size, line lengths etc. Compilation of data regarding existing loads, operating conditions, forecast of expected loads etc. Preparation of long-term plans for phased strengthening and improvement of the distribution systems along with transmission system.

(2) Implementation of energy audits schemes:

It should be obligatory for all big industries and utilities to carry out Energy Audits of their system. Further time bound action for initiating studies for realistic assessment of the total T&D Losses into technical and nontechnical losses has also to be drawn by utilities for identifying high loss areas to initiate remedial measures to reduce the same. The realistic assessment of T&D Loss of a utility greatly depends on the chosen sample size which in turn has a bearing on the level of confidence desired and the tolerance limit of variation in results. In view of this it is very essential to fix a limit of the sample size for realistic quick estimates of losses.

(3) Mitigating power theft by Power theft checking Drives: Theft of electric power is a major problem faced by all electric utilities. It is necessary to make strict rule by State Government regarding Power theft. Indian Electricity Act has been amended to make theft of energy and its abatement as a cognizable offense with deterrent punishment of up to 3 years imprisonment. The impact of theft is not limited to loss of revenue, it also affects power quality resulting in low voltage and voltage dips. Required to install proper seal management at Meter terminal Box, at CT/PT terminal to prevent power theft. Identify Power theft area and required to expedite power theft checking drives. Installation of medium voltage distribution (MVD) networks in theft-prone areas, with direct connection of each consumer to the low voltage terminal of the supply transformer. All existing un metered services should be immediately stopped.

(4)

Replacement of Faulty/Sluggish Energy Meter:

It is necessary to replacement of Faulty or sluggish Meter by Distribution Agency to reduce un metered Electrical energy. Required to test Meter periodically for testing of accuracy of meter. Replacement of old erroneous electromechanical meters with accurate Electro static Meter (Micro presser base) for accurate measurement of energy consumption. Use of Meter boxes and seals them properly to ensure that the meters are properly sealed and cannot be tampered.

(5) Bill Collection facility: Increase Bill’s Payment Cells, Increasing drop Box facility in all Area for Payment Collection. E-Payment facility gives more relief to Customer for bill Payment and Supply agency will get Payment regularly and speedily from Customer. Effectively disconnect the connection of defaulter Customer who does not pay the Bill rather than give them chance to pay the bill.

(6) Reduce Debit areas of Sub Division: Recovery of old debts in selected cases through legal, communication and judicial actions. Ensuring police action when required to disconnect connection of defaulter Consumer.

(7) Watchdog effect on users. Users must aware that the distribution Agency can monitor consumption at its convenience. This allows the company fast detection of any abnormal consumption due to tampering or by-passing of a meter and enables the company to take corrective action. The result is consumer discipline. This has been shown to be extremely effective with all categories of large and medium consumers having a history of stealing electricity. They stop stealing once they become aware that the utility has the means to detect and record it. These measures can significantly increase the revenues of utilities with high non-technical losses.

(8) Loss Reduction Programmed:

The increased hours of supply to Agriculture and Rural domestic consumers have resulted in higher loss levels.

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Total Losses in Power Distribution & Transmission Lines-Part 1 JULY 1, 2013

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Introduction: Power generated in power stations pass through large & complex networks like transformers, overhead lines, cables & other equipments and reaches at the end users. It is fact that the Unit of electric energy generated by Power Station does not match with the units distributed to the consumers. Some percentage of the units is lost in the Distribution network. This difference in the generated & distributed units is known as Transmission and Distribution loss. Transmission and Distribution loss are the amounts that are not paid for by users. T&D Losses= (Energy Input to feeder(Kwh)-Billed Energy to Consumer(Kwh)) / Energy Input kwh x100 Distribution Sector considered as the weakest link in the entire power sector. Transmission Losses is approximate 17% while Distribution Losses is approximate 50%. There are two types of Transmission and Distribution Losses 1. Technical Losses 2. Non Technical Losses (Commercial Losses)

(1) Technical Losses: The technical losses are due to energy dissipated in the conductors, equipment used for transmission Line, Transformer, sub- transmission Line and distribution Line and magnetic losses in transformers. Technical losses are normally 22.5%, and directly depend on the network characteristics and the mode of operation. The major amount of losses in a power system is in primary and secondary distribution lines. While transmission and sub-transmission lines account for only about 30% of the total losses. Therefore the primary and secondary distribution systems must be properly planned to ensure within limits. The unexpected load increase was reflected in the increase of technical losses above the normal level Losses are inherent to the distribution of electricity and cannot be eliminated. There are two Type of Technical Losses.

(a) Permanent / Fixed Technical losses: Fixed losses do not vary according to current. These losses take the form of heat and noise and occur as long as a transformer is energized. Between 1/4 and 1/3 of technical losses on distribution networks are fixed losses. Fixed losses on a network can be influenced in the ways set out below.

Corona Losses. Leakage Current Losses. Dielectric Losses. Open-circuit Losses. Losses caused by continuous load of measuring elements Losses caused by continuous load of control elements.

(b) Variable Technical losses Variable losses vary with the amount of electricity distributed and are, more precisely, proportional to the square of the current. Consequently, a 1% increase in current leads to an increase in losses of more than 1%. Between 2/3 and 3/4 of technical (or physical) losses on distribution networks are variable Losses. By increasing the cross sectional area of lines and cables for a given load, losses will fall. This leads to a direct trade-off between cost of losses and cost of capital expenditure. It has been suggested that optimal average utilization rate on a distribution network that considers the cost of losses in its design could be as low as 30 per cent. joule losses in lines in each voltage level impedance losses Losses caused by contact resistance.

Main Reasons for Technical Losses: (1) Lengthy Distribution lines: In practically 11 KV and 415 volts lines, in rural areas are extended over long distances to feed loads scattered over large areas. Thus the primary and secondary distributions lines in rural areas are largely radial laid usually extend over long distances. This results in high line resistance and therefore high I2R losses in the line. Haphazard growths of sub-transmission and distribution system in to new areas. Large scale rural electrification through long 11kV and LT lines.

(2) Inadequate Size of Conductors of Distribution lines: The size of the conductors should be selected on the basis of KVA x KM capacity of standard conductor for a required voltage regulation but rural loads are usually scattered and generally fed by radial feeders. The conductor size of these feeders should be adequate.

(3) Installation of Distribution transformers away from load centers: Distribution Transformers are not located at Load center on the Secondary Distribution System. In most of case Distribution Transformers are not located centrally with respect to consumers. Consequently, the farthest consumers obtain an extremity low voltage even though a good voltage levels maintained at the transformers secondary. This again leads to higher line losses. (The reason for the line losses increasing as a result of decreased voltage at the consumers end Therefore in order to reduce the voltage drop in the line to the farthest consumers, the distribution transformer should be located at the load center to keep voltage drop within permissible limits.

(4) Low Power Factor of Primary and secondary distribution system: In most LT distribution circuits normally the Power Factor ranges from 0.65 to 0.75. A low Power Factor contributes towards high distribution losses. For a given load, if the Power Factor is low, the current drawn in high And the losses proportional to square of the current will be more. Thus, line losses owing to the poor PF can be reduced by improving the Power Factor. This can be done by application of shunt capacitors.

Shunt capacitors can be connected either in secondary side (11 KV side) of the 33/11 KV power transformers or at various point of Distribution Line. The optimum rating of capacitor banks for a distribution system is 2/3rd of the average KVAR requirement of that distribution system. The vantage point is at 2/3rd the length of the main distributor from the transformer. A more appropriate manner of improving this PF of the distribution system and thereby reduce the line losses is to connect capacitors across the terminals of the consumers having inductive loads. By connecting the capacitors across individual loads, the line loss is reduced from 4 to 9% depending upon the extent of PF improvement.

(5) Bad Workmanship: Bad Workmanship contributes significantly role towards increasing distribution losses. Joints are a source of power loss. Therefore the number of joints should be kept to a minimum. Proper jointing techniques should be used to ensure firm connections. Connections to the transformer bushing-stem, drop out fuse, isolator, and LT switch etc. should be periodically inspected and proper pressure maintained to avoid sparking and heating of contacts. Replacement of deteriorated wires and services should also be made timely to avoid any cause of leaking and loss of power.

(6) Feeder Phase Current and Load Balancing: One of the easiest loss savings of the distribution system is balancing current along three-phase circuits. Feeder phase balancing also tends to balance voltage drop among phases giving three-phase customers less voltage unbalance. Amperage magnitude at the substation doesn’t guarantee load is balanced throughout the feeder length. Feeder phase unbalance may vary during the day and with different seasons. Feeders are usually considered “balanced” when phase current magnitudes are within 10.Similarly, balancing load among distribution feeders will also lower losses assuming similar conductor resistance. This may require installing additional switches between feeders to allow for appropriate load transfer. Bifurcation of feeders according to Voltage regulation and Load.

(7) Load Factor Effect on Losses: Power consumption of Customer varies throughout the day and over seasons. Residential customers generally draw their highest power demand in the evening hours. Same commercial customer load generally peak in the early afternoon. Because current level (hence, load) is the primary driver in distribution power losses, keeping power consumption more level throughout the day will lower peak power loss and overall energy losses. Load variation is Called load factor and It varies from 0 to 1. Load Factor=Average load in a specified time period / peak load during that time period. For example, for 30 days month (720 hours) peak Load of the feeder is 10 MW. If the feeder supplied a total energy of 5,000 MWH, the load factor for that month is (5,000 MWh)/ (10MW x 720) =0.69. Lower power and energy losses are reduced by raising the load factor, which, evens out feeder demand variation throughout the feeder. The load factor has been increase by offering customers “time-of-use” rates. Companies use pricing power to influence consumers to shift electric-intensive activities during off-peak times (such as, electric water and space heating, air conditioning, irrigating, and pool filter pumping). With financial incentives, some electric customers are also allowing utilities to interrupt large electric loads remotely through radio frequency or power line carrier during periods of peak use. Utilities can try to design in higher load factors

by running the same feeders through residential and commercial areas

(8) Transformer Sizing and Selection: Distribution transformers use copper conductor windings to induce a magnetic field into a grain-oriented silicon steel core. Therefore, transformers have both load losses and no-load core losses. Transformer copper losses vary with load based on the resistive power loss equation (P loss = I2R). For some utilities, economic transformer loading means loading distribution transformers to capacity-or slightly above capacity for a short time-in an effort to minimize capital costs and still maintain long transformer life. However, since peak generation is usually the most expensive, total cost of ownership (TCO) studies should take into account the cost of peak transformer losses. Increasing distribution transformer capacity during peak by one size will often result in lower total peak power dissipation-more so if it is over Loaded. Transformer no-load excitation loss(iron loss) occurs from a changing magnetic field in the transformer core whenever it is energized. Core loss varies slightly with voltage but is essentially considered constant. Fixed iron loss depends on transformer core design and steel lamination molecular structure. Improved manufacturing of steel cores and introducing amorphous metals (such as metallic glass) have reduced core losses.

(9) Balancing 3 phase loads Balancing 3-phase loads periodically throughout a network can reduce losses significantly. It can be done relatively easily on overhead networks and consequently offers considerable scope for Cost effective loss reduction, given suitable incentives.

(10) Switching off transformers One method of reducing fixed losses is to switch off transformers in periods of low demand. If two transformers of a certain size are required at a substation during peak periods, only one might be required during times of low demand so that the other transformer might be switched off in order to reduce fixed losses. This will produce some offsetting increase in variable losses and might affect security and quality of supply as well as the operational condition of the transformer itself. However, these trade-offs will not be explored and optimized unless the cost of losses are taken into account.

(11)

Other Reasons for Technical Losses:

Unequal load distribution among three phases in L.T system causing high neutral currents. leaking and loss of power Over loading of lines. Abnormal operating conditions at which power and distribution transformers are operated Low voltages at consumer terminals causing higher drawl of currents by inductive loads. Poor quality of equipment used in agricultural pumping in rural areas, cooler air-conditioners and industrial loads in urban areas.

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Calculate TC Size & Voltage Drop due to starting of Large Motor JUNE 5, 2013

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10 Votes

Calculate TC Size & Voltage Drop due to starting of Large Motor Calculate Voltage drop in Transformer ,1000KVA,11/0.480KV,impedance 5.75%, due to starting of 300KW,460V,0.8 Power Factor, Motor code D(kva/hp).Motor Start 2 times per Hour and The allowable Voltage drop at Transformer Secondary terminal is 10%.

Motor current / Torque: Motor Full Load Current= (Kwx1000)/(1.732x Volt (L-L)x P.F) Motor Full Load Current=300×1000/1.732x460x0.8= 471 Amp. Motor Locked Rotor Current =Multiplier x Motor Full Load Current

Locked Rotor Current (Kva/Hp) Motor Code

Min

Max

A

3.15

B

3.16

3.55

C

3.56

4

D

4.1

4.5

E

4.6

5

F

5.1

5.6

G

5.7

6.3

H

6.4

7.1

J

7.2

8

K

8.1

9

L

9.1

10

M

10.1

11.2

N

11.3

12.5

P

12.6

14

R

14.1

16

S

16.1

18

T

18.1

20

U

20.1

22.4

V

22.5

Min Motor Locked Rotor Current (L1)=4.10×471=1930 Amp Max Motor Locked Rotor Current(L2) =4.50×471=2118 Amp Motor inrush Kva at Starting (Irsm)=Volt x locked Rotor Current x Full Load Currentx1.732 / 1000 Motor inrush Kva at Starting (Irsm)=460 x 2118x471x1.732 / 1000=1688 Kva

Transformer: Transformer Full Load Current= Kva/(1.732xVolt) Transformer Full Load Current=1000/(1.732×480)=1203 Amp. Short Circuit Current at TC Secondary (Isc) =Transformer Full Load Current / Impedance. Short Circuit Current at TC Secondary= 1203/5.75= 20919 Amp Maximum Kva of TC at rated Short Circuit Current (Q1) = (Volt x Iscx1.732)/1000. Maximum Kva of TC at rated Short Circuit Current (Q1)=480x20919x1.732/1000= 17391 Kva. Voltage Drop at Transformer secondary due to Motor Inrush (Vd)= (Irsm) / Q1 Voltage Drop at Transformer secondary due to Motor Inrush (Vd) =1688/17391 =10% Voltage Drop at Transformer Secondary is 10% which is within permissible Limit. Motor Full Load Current 15% kVA transformer.

Method

Advantages

Disadvantages

Individual capacitors

Most technically efficient, most flexible

Higher installation & maintenance cost

Fixed bank

Most economical, fewer installations

Less flexible, requires switches and/or circuit breakers

Automatic bank

Best for variable loads, prevents over voltages, low installation cost

Higher equipment cost

Combination

Most practical for larger numbers of motors

Least flexible

Type of Capacitor as per Construction: (1) Standard duty Capacitor: Construction: Rectangular & Cylindrical(Resin filled / Resin coated-Dry) Application: Steady inductive load. Non linear up to 10%. For Agriculture duty.

(2) Heavy-duty: Construction: Rectangular & Cylindrical (Resin filled / Resin coated-Dry/oil/gas) Application: Suitable for fluctuating load. Non linear up to 20%. Suitable for APFC Panel. Harmonic filtering (3) LT Capacitor: Application: Suitable for fluctuating load. Non linear up to 20%. Suitable for APFC Panel & Harmonic filter application.

Selecting Size of Capacitor Bank: The size of the inductive load is large enough to select the minimum size of capacitors that is practical. For HT capacitors the minimum ratings that are practical are as follows:

System Voltage

Minimum rating of capacitor bank

3.3 KV , 6.6KV

75 Kvar

11 KV

200 Kvar

22 KV

400 Kvar

33 KV

600 Kvar

Unit sizes lower than above is not practical and economical to manufacture. When capacitors are connected directly across motors it must be ensured that the rated current of the capacitor bank should not exceed 90% of the no-load current of the motor to avoid self-excitation of the motor and also over compensation. Precaution must be taken to ensure the live parts of the equipment to be compensated should not be handled for 10 minutes (in case of HT equipment) after disconnection of supply. Crane motors or like, where the motors can be rotated by mechanical load and motors with electrical braking systems, should never be compensated by capacitors directly across motor terminals. For direct compensation across transformers the capacitor rating should not exceed 90 % of the no-load KVA of the

motor.

Selection of Capacitor as per Non Liner Load: For power Factor correction it is need to first decide which Type of capacitor is used. Selection of Capacitor is depending upon many factor i.e. operating life, Number of Operation, Peak Inrush current withstand capacity. For selection of Capacitor we have to calculate Total Non Liner Load like UPS, Rectifier, Arc/Induction Furnace, AC/DC Drives, Computer, CFL Blubs, and CNC Machines. Calculation of Non liner Load, Example: Transformer Rating 1MVA,Non Liner Load 100KVA % of non Liner Load= (Non Liner Load/Transformer Capacity) x100 = (100/1000) x100=10%. According to Non Linear Load Select Capacitor as per Following Table.

% Non Liner Load

Type of Capacitor

20 Pico Farads

Yes

No

No

No

Speed of Response Timing

Mechanical clock

function

works, dashpot

Time of Accuracy

Requirement of Draw Out

Range of settings Isolation Voltage

Output Capacitance Deterioration due to

Operation Relay

No

Partially

Programmable

Programmable

No

No

Possible

Yes

Not Possible

Possible

Possible

Possible

Flags, targets

LEDs

LEDs, LCD

LEDs, LCD

Self monitoring

No

Yes

Yes

Yes

Parameter

Plug setting, dial

Thumb wheel,dual

Keypad for numeric values,through

Keypad for numeric values,through

setting

setting

in line switches

computer

computer

Not possible

Not possible

possible

possible

Programming SCADA Compatibility Operational value indication Visual indication

Fault Disturbance Recording

Relay’s Nomenclature as per ANSI: No

Type of Relay

2

Time delay relay

3

3 Checking or Interlocking relay

21

21 Distance relay

25

Check synchronizing relay

27

Under voltage relay

30

Annunciation relay

32

Directional power (Reverse power) relay

37

Low forward power relay

40

Field failure (loss of excitation)

46

Negative phase sequence relay

49

Machine or Transformer Thermal relay

50

Instantaneous Over current relay

51

A.C. IDMT Over current relay

52

Circuit breaker

52a

Circuit breaker Auxiliary switch “Normally open” (‘a ‘contact)

52b

Circuit breaker Auxiliary switch “Normally closed” (‘b’contact)

55

Power Factor relay

56

Field Application relay

59

Overvoltage relay

64

Earth fault relay

67

Directional relay

68

Locking relay

74

Alarm relay

76

D.C Over current relay

78

Phase angle measuring or out of step relay

79

AC Auto reclose relay

80

Monitoring loss of DC supply

81

Frequency relay

81 U

Under frequency relay

81 O

Over frequency relay

83

Automatic selective control or transfer relay

85

Carrier or pilot wire receive relay

86

Tripping Relay

87

Differential relay

87G

Generator differential relay

87G

Generator differential relay

87G

T Overall differential relay

87U

UAT differential relay

87NT

Restricted earth fault relay

95

Trip circuit supervision relay

99

Over flux relay

186A

Auto reclose lockout relay

186B

Auto reclose lockout relay

Relays for Transmission & Distribution Lines protection: No

1

2

Line

Protection

400 KV Transmission Line

220 KV Transmission Line

3

132 KV Transmission Line

4

33 KV Lines

5

11KV Line

Main-I: Non switched or Numerical Distance Scheme Main-II: Non switched or Numerical Distance Scheme Main-I : Non switched distance scheme (Fed from Bus PTs) Main-II: Switched distance scheme (Fed from line CVTs) With a changeover facility from bus PT to line CVT and vice-versa Main Protection: Switched distance scheme (fed from bus PT). Backup Protection: 3 Nos. directional IDMT O/L Relays and 1 No. Directional IDMT E/L relay. Non-directional IDMT 3 Over Current and 1 Earth Fault relays Non-directional IDMT 2 Over Current and 1 Earth Fault relays

Reference: Handbook of Switchgear –Bhel Digital/Numerical Relays -T.S.M. Rao

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Transformer IDMT Over Current & Earth Fault Relay Setting NOVEMBER 13, 2012

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5 Votes

Transformer IDMT Over Current & Earth Fault Relay Setting:

Calculate LT & HT Side IDMT Over Current Relay Setting (50/51) Calculate LT & HT Side IDMT Over Current Relay Pick up setting. Calculate LT & HT Side IDMT Over Current Relay PSM. Calculate LT & HT Side IDMT Over Current Relay Time setting. Calculate Actual Operating time according to Various IES Curve. Calculate LT & HT Side Low current Setting(I>). Calculate LT & HT Side High Current Setting(I>>) Calculate LT & HT Side Actual Operating Time of Relay(t>) Calculate LT & HT Side Actual Operating Time of Relay(t>>)

Calculate LT & HT Side IDMT Earth Fault Relay Setting (50N/51N) Calculate LT & HT Side IDMT Earth Fault Relay Pick up setting. Calculate LT & HT Side IDMT Earth Fault Relay PSM Calculate LT & HT Side IDMT Earth Fault Relay Time setting Calculate Actual Operating time according to Various IES Curve. Calculate LT & HT Side Low current Setting(Ie>) Calculate LT & HT Side High Current Setting(Ie>>) Calculate LT & HT Side Actual Operating Time of Relay(te>) Calculate LT & HT Side Actual Operating Time of Relay(te>>)

Calculate Differential Protection Relay setting: Calculate Percentage Differential Current at Normal tapping Calculate Percentage Differential Current at Highest tapping Calculate Percentage Differential Current at Lowest tapping

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IDMT Over Current Relay Grading Calculation (50/51) NOVEMBER 5, 2012

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4 Votes

IDMT Over Current Relay Grading Calculation (50/51):

Calculate IDMT Over current Relay Grading up to 5 Nos of Radial Bus bar System. Calculate Relay Pick up setting of IDMT Over current Relay from Downstream to Source End. Calculate PSM of Various IDMT Relay from Downstream to Source End. Calculate Time setting of Various IDMT Relay from Downstream to Source End. Calculate Total Grading Time of Various IDMT Relay from Downstream to Source End. Calculate Actual Relay Operating time according to Various IES Curve.

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IDMT Relay Setting & Curves NOVEMBER 1, 2012

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IDMT Relay Setting & Curves

Calculation of Actual Plug (Relay Pick up) setting of Various IDMT Relay. Calculate PSM of Various IDMT Relay.

Calculate Time setting of Various IDMT Relay. Calculate Total Grading Time of Various IDMT Relay. Calculate Actual Operating time according to Various IES Curve. Draw Various IDMT Relay characteristic Curve.

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Calculation of IDMT Over current & Earth Fault Relay Setting: OCTOBER 13, 2012

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6 Votes

IDMT Over current & Earth Fault Relay Setting:

Calculate IDMT Over Current Relay Setting (50/51) Calculate IDMT Over Current Relay Pick up setting. Calculate IDMT Over Current Relay PSM. Calculate IDMT Over Current Relay Time setting. Calculate Actual Operating time according to Various IES Curve. Calculate Low current Setting(I>). Calculate High Current Setting(I>>) Calculate Actual Operating Time of Relay(t>) Calculate Actual Operating Time of Relay(t>>) Calculate IDMT Earth Fault Relay Setting (50N/51N) Calculate IDMT Earth Fault Relay Pick up setting. Calculate IDMT Earth Fault Relay PSM Calculate IDMT Earth Fault Relay Time setting Calculate Actual Operating time according to Various IES Curve. Calculate Low current Setting(Ie>) Calculate High Current Setting(Ie>>) Calculate Actual Operating Time of Relay(te>) Calculate Actual Operating Time of Relay(te>>)

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Abstract of over current Protection of Transformer (NEC 450.3) SEPTEMBER 18, 2012

8 COMMENTS

13 Votes

Introduction: The over current protection required for transformers is consider for Protection of Transformer only.Such over current protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer. When voltage is switched on to energize a transformer, the transformer core normally saturates. This results in a large inrush current which is greatest during the first half cycle (approximately0.01 second) and becomes progressively less severe over the next several cycles (approximately 1 second) until the transformer reaches its normal magnetizing current. To accommodate this inrush current, fuses are often selected which have time-current withstand values of at least 12 times transformer primary rated current for 0.1 second and 25 times for 0.01 second. Some small dry-type transformers may have substantially greater inrush currents. To avoid using over sized conductors, over current devices should be selected at about 110 to 125 percent of the transformer full-load current rating. And when using such smaller over current protection, devices should be of the time-delay type (on the primary side) to compensate for inrush currents which reach 8 to 10 times the full-load primary current of the transformer for about 0.1 s when energized initially. Protection of secondary conductors has to be provided completely separately from any primary-side protection. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the transformer installation. Over current protection for a transformer on the primary side is typically a circuit breaker. In some instances where there is not a high voltage panel, there is a fused disconnect instead. It is important to note that the over current device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer

Over current Protection of Transformers > 600 V (NEC 450.3-A) 1) Unsupervised Location of Transformer (Impedance 600V): Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next higher Standard size. or Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next higher Standard size. Over Current Protection at Secondary Side (Secondary Voltage 600V):

Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next higher Standard size. or Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current. Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5% Full Load Current At Primary side=750000/(1.732X11000)=39A Rating of Primary Fuse = 3X39A= 118A, So Standard Size of Fuse =125A. OR Rating of Primary Circuit Breaker =6X39A=236A, So Standard Size of Circuit Breaker =250A. Full Load Current at Secondary side=750000/ (1.732X415) =1043A. Rating of Secondary of Fuse / Circuit Breaker = 1.25X1043A=1304A, So Standard Size of Fuse =1600A.

2) Unsupervised Location of Transformer (Impedance 6% to 10 %)

Over Current Protection at Primary Side (Primary Voltage >600V): Rating of Pri. Fuse at Point A= 300% of Primary Full Load Current or Next higher Standard size. Rating of Pri. Circuit Breaker at Point A= 400% of Primary Full Load Current or Next higher Standard size. Over Current Protection at Secondary Side (Secondary Voltage 600V): Rating of Sec. Fuse at Point B= 225% of Sec. Full Load Current or Next higher Standard size. Rating of Sec. Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size. Example: 10MVA, 66KV/11KV 3Phase Transformer, Impedance of Transformer is 8% Full Load Current At Primary side=10000000/(1.732X66000)=87A Rating of Pri. Fuse = 3X87A= 262A, So Next Standard Size of Fuse =300A. OR Rating of Pri. Circuit Breaker =4X87A=348A, So Next Standard Size of Circuit Breaker =400A. Full Load Current at Secondary side=10000000/ (1.732X11000) =525A. Rating of Sec. Fuse = 2.25X525A=1181A, So Next Standard Size of Fuse =1200A. OR Rating of Sec. Circuit Breaker =2.5X525A=1312A, So Next Standard Size of Circuit Breaker =1600A.

3) Supervised Location (in Primary side only) of Transformer:

Over Current Protection at Primary Side (Primary Voltage >600V): Rating of Pri. Fuse at Point A= 250% of Primary Full Load Current or Next higher Standard size.

Rating of Pri. Circuit Breaker at Point A= 300% of Primary Full Load Current or Next higher Standard size.

4) Supervised Location of Transformer (Impedance Up to 6%):

Over Current Protection at Primary Side (Primary Voltage >600V): Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next Lower Standard size. Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next Lower Standard size. Over Current Protection at Secondary Side (Secondary Voltage 600V): Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next Lower Standard size. Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current or Next Lower Standard size. Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5% Full Load Current At Primary side=750000/(1.732X11000)=39A Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size of Fuse =110A. OR Rating of Primary Circuit Breaker =6X39A=236A, So Next Lower Standard Size of Circuit Breaker =225A. Full Load Current at Secondary side=750000/ (1.732X415) =1043A. Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, So Standard Size of Fuse =2500A.

5) Supervised Location of Transformer (Impedance 6% to 10%):

Over Current Protection at Primary Side (Primary Voltage >600V): Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next Lower Standard size. Rating of Pri. Circuit Breaker at Point A= 400% of Pri. Full Load Current or Next Lower Standard size. Over Current Protection at Secondary Side (Secondary Voltage 600V): Rating of Sec. Fuse at Point B= 225% of Sec. Full Load Current or Next Lower Standard size. Rating of Sec. Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next Lower Standard size. Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 8% Full Load Current At Primary side=750000/(1.732X11000)=39A Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size of Fuse =110A. OR Rating of Primary Circuit Breaker =4X39A=157A, So Next Lower Standard Size of Circuit Breaker =150A.

Full Load Current at Secondary side=750000/ (1.732X415) =1043A. Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, So Standard Size of Fuse =2500A.

Difference in C.B between Supervised & Unsupervised Location Here we see two notable conditions while we select Fuse / Circuit Breaker in Supervised Location and Unsupervised Location. First notable Condition is Primary Over current Protection. In Unsupervised Location Fuse in Primary side is 300% of Primary Current or Next Higher Standard size and in Supervised Location is 300% of Primary Current or Next Lower Standard size. Here Primary Over current Protection is same in both conditions (300%) But Selecting Size of Fuse/Circuit Breaker is Different. Lets us Check with the Example for 750KVA, 11KV/415V 3Phase Transformer. Full Load Current At Primary side=750000/(1.732X11000)=39A In Unsupervised Location: Rating of Primary Fuse = 3X39A= 118A, So Next Higher Standard Size =125A In Supervised Location: Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size =110A Second notable Condition is Secondary Over current Protection increased from 125% to 250% for unsupervised to Supervised Location.

Summary of over current Protection for more than 600V: Maximum Rating of Over current Protection for Transformers more than 600 Volts LocationLimitations

Any location

Supervisedlocations only

TransformerRated Impedance

Primary Protection (More than 600 Volts)

Secondary Protection More than 600 Volts

Less than 600 Volts

Circuit Breaker

Fuse Rating

Circuit Breaker

Fuse Rating

C.B or Fuse

Less than 6%

600% (NH)

300% (NH)

300 %( NH)

250% (NH)

125% (NH)

6% To 10%

400% (NH)

300% (NH)

250% (NH)

225% (NH)

125% (NH)

Any

300% (NH)

250% (NH)

Not required

Not required

Not required

Less than 6%

600%

300%

300%

250%

250%

6% To 10%

400%

300%

250%

225%

250%

NH: Next Higher Standard Size.

Over current Protection of Transformers< 600 V (NEC 450.3-B) 1) Only Primary side Protection of Transformer:

Over Current Protection at Primary Side (Less than 2A):

Rating of Pri. Fuse / C.B at Point A= 300% of Pri. Full Load Current or Next Lower Standard size. Example: 1KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=1000/(1.732X480)=1A Rating of Primary Fuse = 3X1A= 3A, So Next Lower Standard Size of Fuse =3A. Over Current Protection at Primary Side (2A to 9A): Rating of Sec. Fuse / C.B at Point A= 167% of Pri. Full Load Current or Next Lower Standard size. Example: 3KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=3000/(1.732X480)=4A Rating of Primary Fuse = 1.67X4A= 6A, So Next Lower Standard Size of Fuse =6A. Over Current Protection at Primary Side (More than 9A): Rating of Pri. Fuse / C.B at Point A= 125% of Pri. Full Load Current or Next Higher Standard size. Example: 15KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=15000/(1.732X480)=18A Rating of Primary Fuse = 1.25X18A= 23A, So Next Higher Standard Size of Fuse =25A.

2) Primary and Secondary side Protection of Transformer:

Over Current Protection at Primary Side (Less than 2A): Rating of Pri. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Next Lower Standard size. Over Current Protection at Primary Side (2A to 9A): Rating of Sec. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Next Lower Standard size. Over Current Protection at Primary Side (More than 9A): Rating of Pri. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Lower Higher Standard size. Example: 25KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=125000/(1.732X480)=30A Rating of Primary Fuse = 2.50X30A= 75A, So Next Lower Standard Size of Fuse =70A. Over Current Protection at Secondary Side (Less than 9A): Rating of Pri. Fuse / C.B at Point B= 167% of Sec. Full Load Current or Lower Standard size. Example: 3KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=3000/(1.732X230)=8A Rating of Primary Fuse = 1.67X8A= 13A, So Next Lower Standard Size of Fuse =9A. Over Current Protection at Secondary Side (More than 9A): Rating of Pri. Fuse / C.B at Point A= 125% of Pri. Full Load Current or Higher Standard size. Example: 15KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=15000/(1.732X230)=38A Rating of Primary Fuse = 1.25X38A= 48A, So Next Higher Standard Size of Fuse =50A.

Summary of over current Protection for Less than 600V: Maximum Rating of Over current Protection for Transformers Less than 600 Volts ProtectionMethod

Primary Protection

Secondary Protection

More than 9A

2A to 9A

Less than 2A

More than 9A

Less than 9A

Primary only protection

125% (NH)

167%

300%

Not required

Not required

Primary and secondary protection

250%

250%

250%

125% (NH)

167%

NH: Next Higher Standard Size.

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Difference between Bonding, Grounding and Earthing SEPTEMBER 7, 2012

18 COMMENTS

9 Votes

Introduction: One of the most misunderstood and confused concept is difference between Bonding, Grounding and Earthing. Bonding is more clear word compare to Grounding and Earthing but there is a micro difference between Grounding and Earhing. Earthing and Grounding are actually different terms for expressing the same concept. Ground or earth in a mains electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment. Earthing is more commonly used in Britain, European and most of the commonwealth countries standards (IEC, IS), while Grounding is the word used in North American standards (NEC, IEEE, ANSI, UL). We understand that Earthing and Grounding are necessary and have an idea how to do it but we don’t have crystal clear concept for that. We need to understand that there are really two separate things we are doing for same purpose that we call Grounding or Earthing. The Earthing is to reference our electrical source to earth (usually via connection to some kind of rod driven into the earth or some other metal that has direct contact with the earth). The grounded circuits of machines need to have an effective return path from the machines to the power source in order to function properly (Here by Neutral Circuit). In addition, non-current-carrying metallic components in a System, such as equipment cabinets, enclosures, and structural steel, need to be electrically interconnected and earthed properly so voltage potential cannot exist between them. However, troubles can arise when terms like “bonding,” “grounding,” and “earthing” are interchanged or confused in certain situations. In TN Type Power Distribution System, in US NEC (and possibly other) usage: Equipment is earthed to pass fault Current and to trip the protective device without electrifying the device enclosure. Neutral is the current return path for phase. These Earthing conductor and Neutral conductor are connected together and earthed at the distribution panel and also at the street, but the intent is that no current flow on earthed ground, except during momentary fault conditions. Here we may say that Earthing and grounding are nearly same by practice. But In the TT Type Power Distribution System (In India) Neutral is only earthed (here it is actually called Grounding) at distribution source (at distribution transformer) and Four wires (Neutral and Three Phase) are distributed to consumer. While at consumer side all electrical equipments body are connected and earthed at consumer premises (here it is called Earthing). Consumer has no any permission to mix Neutral with earth at his premises here earthing and grounding is the different by practice. But in both above case Earthing and Grounding are used for the same Purpose. Let’s try to understand this terminology one by one.

Bonding: Bonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and a pipe, or these may be two Equipments. Bonding has to be done by connecting of all the metal parts that are not supposed to be carrying current during normal operations to bringing them to the same electrical potential. Bonding ensures that these two things which are bonded will be at the same electrical potential. That means we would not get electricity building up in one equipment or between two different equipment. No current flow can take place between two bonded bodies because they have the same potential. Bonding, itself, does not protect anything. However, if one of those boxes is earthed there can be no electrical energy build-up. If the grounded box is bonded to the other box, the other box is also at zero electrical potential. It protects equipment & Person by reducing current flow between pieces of equipment at different potentials. The primary reason for bonding is personnel safety, so someone touching two pieces of equipment at the same time does not receive a shock by

becoming the path of equalization if they happen to be at different potentials. The Second reason has to do with what happens if Phase conductor may be touched an external metal part. The bonding helps to create a low impedance path back to the source. This will force a large current to flow, which in turn will cause the breaker to trip. In other words, bonding is there to allow a breaker to trip and thereby to terminate a fault. Bonding to electrical earth is used extensively to ensure that all conductors (person, surface and product) are at the same electrical potential. When all conductors are at the same potential no discharge can occur.

Earthing: Earthing means connecting the dead part (it means the part which does not carries current under normal condition) to the earth for example electrical equipment’s frames, enclosures, supports etc. The purpose of earthing is to minimize risk of receiving an electric shock if touching metal parts when a fault is present. Generally green wire is used for this as a nomenclature. Under fault conditions the non-current carrying metal parts of an electrical installation such as frames, enclosures, supports, fencing etc. may attain high potential with respect to ground so that any person or stray animal touching these or approaching these will be subjected to potential difference which may result in the flow of a current through the body of the person or the animal of such a value as may prove fatal. To avoid this non-current carrying metal parts of the electrical system are connected to the general mass of earth by means of an earthing system comprising of earth conductors to conduct the fault currents safely to the ground. Earthing has been accomplished through bonding of a metallic system to earth. It is normally achieved by inserting ground rods or other electrodes deep inside earth. Earthing is to ensure safety or Protection of electrical equipment and Human by discharging the electrical energy to the earth.

Grounding: Grounding means connecting the live part (it means the part which carries current under normal condition) to the earth for example neutral of power transformer. Grounding is done for the protections of power system equipment and to provide an effective return path from the machine to the power source. For example grounding of neutral point of a star connected transformer. Grounding refers the current carrying part of the system such as neutral (of the transformer or generator). Because of lightening, line surges or unintentional contact with other high voltage lines, dangerously high voltages can develop in the electrical distribution system wires. Grounding provides a safe, alternate path around the electrical system of your house thus minimizing damage from such occurrences. Generally Black wire is used for this as a nomenclature. All electrical/electronic circuits (AC & DC) need a reference potential (zero volts) which is called ground in order to make possible the current flow from generator to load. Ground is May or May not be earthed. In Electrical Power distribution it is either earthed at distribution Point or at Consumer end but it is not earthed in Automobile( for instance all vehicles’ electrical circuits have ground connected to the chassis and metallic body that are insulated from earth through tires). There may exist a neutral to ground voltage due to voltage drop in the wiring, thus neutral does not necessarily have to be at ground potential. In a properly balanced system, the phase currents balance each other, so that the total neutral current is also zero. For individual systems, this is not completely possible, but we strive to come close in aggregate. This balancing allows maximum efficiency of the distribution transformer’s secondary winding

Micro Difference between earthing & Grounding: There is no major difference between earthing and Grounding, both means “Connecting an electrical circuit or device to the Earth”. This serves various purposes like to drain away unwanted currents, to provide a reference voltage for circuits needing one, to lead lightning away from delicate equipment. Even though there is a micro difference between grounding & earthing.

(1) Difference in Terminology: In USA term Grounding is used but in UK term Earthing is used.

(2) Balancing the Load Vs Safety: Ground is a source for unwanted currents and also as a return path for main current some times. While earthing is done not for return path but only for protection of delicate equipments. It is an alternate low resistance path for current. When we take out the neutral for a three phase unbalanced connection and send it to ground, it is called grounding. Grounding is done to balance unbalanced load. While earthing is used between the equipment and earth pit so as to avoid electrical shock and equipment damage.

(3) Equipment Protection Vs Human Safety: Earthing is to protect the circuit elements whenever high voltage is passed by thunders or by any other sources while Grounding is the common point in the circuit to maintain the voltage levels. Earth is used for the safety of the human body in fault conditions while Grounding (As neutral earth) is used for the protection of equipments. Earthing is a preventive measure while Grounding is just a return path The ground conductor provides a return path for fault current when a phase conductor accidentally comes in contact with a grounded object. This is a safety feature of the wiring system and we would never expect to see grounding conductor current flow during normal operation. Do not Ground the Neutral Second time When It is grounded either at Distribution Transformer or at Main service Panel of Consumer end. Grounding act as neutral. But neutral cannot act as ground.

(4) System Zero Potential Vs Circuit Zero Potential: Earthing and Grounding both is refer to zero potential but the system connected to zero potential is differ than Equipment connected to zero potential .If a neutral point of a generator or transformer is connected to zero potential then it is known as grounding. At the same time if the body of the transformer or generator is connected to zero potential then it is known as earthing. The term “Earthing means that the circuit is physically connected to the ground and it is Zero Volt Potential to the Ground (Earth) but in case of “Grounding” the circuit is not physically connected to ground, but its potential is zero(where the currents are algebraically zero) with respect to other point, which is also known as “Virtual Grounding.” Earth having zero potential whereas neutral may have some potential. That means neutral does not always have zero potential with respect to ground. In earthing we have Zero Volt potential references to the earth while in grounding we have local Zero Volt potential reference to circuit. When we connect two different Power circuits in power distribution system, we want to have the same Zero Volt reference so we connect them and grounds together. This common reference might be different from the earth potential.

Illegal Practice of interchange Purpose of Grounding & earthing wire Neutral wire in grid connections is mandatory for safety. Imagine a person from 4th floor in a building uses Earth wire (which is earthed in the basement at Basement) as neutral to power his lights. Another Person from 2nd floor has a normal setup and uses neutral for the same purpose. Neutral wire is also earthed at the ground level (as per USA practice Neutral is Grounded (earthed) at Building and as per Indian Practice it is Grounded (earthed) at Distribution Transformer). However, ground wire (Neutral wire) has a much lower electrical resistance than Earth Wire (Earthing) which results in a difference of electrical potential (i.e. voltage) between them. This voltage is quite a hazard for anyone touching a Earth wire (Metal Body of Equipment) as it may have several tens of volts. The second issue is legality. Using ground wire instead of neutral makes you an energy thief as the meter uses only the Phase and neutral for recording your energy consumption. Many Consumers make energy theft by using Earthing wire as a Neutral wire in an Energy meter.

Conclusion: Ground is a source for unwanted currents and also as a return path for main current. While earthing is done not for return path but only for protection of delicate equipments. It is an alternate low resistance path for current. Earth is used for the safety of the human body in fault conditions while Grounding (As neutral earth) is used for the protection of equipments.

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Safety Clearance for Transformer SEPTEMBER 2, 2012

3 COMMENTS

8 Votes

Safety Clearance for Transformer: Clearance from Outdoor Liquid Insulated Transformers to Buildings (NEC):

Liquid

Liquid Volume (m3)

Fire Resistant Wall

NonCombustible Wall

Combustible Wall

Vertical Distance

Less Flammable

NA

0.9 Meter

0.9 Meter

0.9 Meter

0.9 Meter

38 m3

4.6 Meter

4.6 Meter

15.2 Meter

15.2 Meter

19 m3

7.6 Meter

15.2 Meter

30.5 Meter

30.5 Meter

Mineral Oil

Clearance between Two Outdoor Liquid Insulated Transformers (NEC): Liquid

Liquid Volume (m3)

Distance

Less Flammable

NA

0.9 Meter

38 m3

7.6 Meter

19 m3

15.2 Meter

Mineral Oil

Dry Type Transformer in Indoor Installation (NES 420.21): Voltage

Distance (min)

Up to 112.5 KVA

300 mm (12 in.) from combustible material unless separated from the combustible material by a heat-insulated barrier.

Above 112.5 KVA

Installed in a transformer room of fire-resistant construction.

Above 112.5 KVA with Class 155 Insulation

separated from a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

Dry Type Transformer in Outdoor Installation (NES 420.22): Voltage

Distance (min)

Above 112.5 KVA with Class 155 Insulation

separated from a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

Non Flammable Liquid-Insulated Transformer in Indoor Installation (NES 420.21): Voltage

Distance (min)

Over 35KV

Installed indoors Vault (Having liquid confinement area and a pressure-relief vent for absorbing any gases generated by arcing inside the tank, the pressure-relief vent shall be connected to a chimney or flue that will carry such gases to an environmentally safe area

Above 112.5 KVA

Installed in a transformer room of fire-resistant construction.

Above 112.5 KVA (Class 155 Insulation)

separated from a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

Oil Insulated Transformer in Indoor Installation (NES 420.25): Voltage

Distance (min)

Up to 112.5 KVA

Installed indoors Vault (With construction of reinforced concrete that is not less than 100 mm (4 in.) thick.

Up to 10 KVA & Up to 600V

Vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting

Up to 75 KVA & Up to 600V

Vault shall not be required if where the surroundingStructure is classified as fire-resistant construction.

Furnace transformers (Up to 75 kVA)

Installed without a vault in a building or room of fire resistant construction

Transformer Clearance from Building (IEEE Stand): Transformer

Distance from Building (min)

Up to 75 KVA

3.0 Meter

75 KVA to 333 KVA

6.0 Meter

More than 333 KVA

9.0 Meter

Transformer Clearance Specifications (Stand: Georgia Power Company): Description of Clearance

Distance (min)

Clearance in front of the transformer

3.0 Meter

Between Two pad mounted transformers (including Cooling fin)

2.1 Meter

Between Transformer and Trees, shrubs, vegetation( for unrestricted natural cooling )

3.0 Meter

The edge of the concrete transformer pad to nearest the building

4.2 Meter

The edge of the concrete transformer pad to nearest building wall, windows, or other openings

3.0 Meter

Clearance from the transformer to edge of (or Canopy)

3.0 Meter

building (3 or less stories) Clearance in front of the transformer doors and on the left side of the transformer, looking at it from the front. (For operation of protective and switching devices on the unit.)

3.0 Meter

Gas service meter relief vents.

0.9 Meter

Fire sprinkler values, standpipes and fire hydrants

1.8 Meter

The water’s edge of a swimming pool or any body of water.

4.5 Meter

Facilities used to dispense hazardous liquids or gases

6.0 Meter

Facilities used to store hazardous liquids or gases

3.0 Meter

Clear vehicle passageway at all times, immediately adjacent of Transformer

3.6 Meter

Fire safety clearances can be reduced by building a suitable masonry fire barrier wall (2.7 Meter wide and 4.5 Meter Tall) 0.9 Meter from the back or side of the Pad Mounted Transformer to the side of the combustible wall Front of the transformer must face away from the building.

Clearance of Transformer-Cable-Overhead Line (Stand: Georgia Power Company): Horizontal Distance (mm) Description of Clearance

to pad-mounted transformers

to buried HV cable

to overhead HV Line

Fuel tanks

7.5 Meter

1.5 Meter

7.5 Meter

Granaries

6.0 Meter

0.6 Meter

15 Meter

Homes

6.0 Meter

0.6 Meter

15 Meter

Barns, sheds, garages

6.0 Meter

0.6 Meter

15 Meter

Water wells

1.5 Meter

1.5 Meter

15 Meter

0.6 Meter

Height of Antenna + 3.0 Meter

Antennas

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Safety Clearance for Electrical Panel AUGUST 26, 2012

5 COMMENTS

10 Votes

Safety Clearance for Electrical Panel: Working Space around Indoor Panel/Circuit Board (NES 312.2): Voltage

Exposed live parts to Not live parts( or grounded parts )

Up to 150 V

0.914 Meter (3 Ft)

150 V to 600 V

0.914 Meter (3 Ft)

Exposed live parts to Grounded parts (concrete, brick, and walls).

Exposed live parts on both sides

0.914 Meter (3 Ft)

0.914 Meter (3 Ft)

1.07 Meter (3’6”)

1.22 Meter (4 Ft)

Clearance around an Indoor electrical panel (NES 110.26): Description of Clearance Left to Right the minimum clearance

Distance (min) 0.9 Meter (3 Ft)

Distance between Panel and wall

1.0 Meter

Distance between Panel and Ceiling

0.9 Meter

Clear Height in front of Panel>480V

2.0 Meter

Clear Height in front of Panel 4, they are to be installed in a horizontal

Minimum Approach Distance of Crane or Moving Part from Live Conductor: Without Safety Observation Voltage

For ordinary Person

Un insulated portions

Insulated portions

Upto1KV

2 Meter

1.0 Meter

3.0 Meter

11 KV

2 Meter

1.4 Meter

3.0 Meter

22 KV

2.4 Meter

2 Meter

3.0 Meter

33 KV

2.4 Meter

2 Meter

3.0 Meter

66 KV

2.8 Meter

2 Meter

3.0 Meter

132 KV

3.0 Meter

3.0 Meter

3.0 Meter

220 KV

4.8 Meter

4.8 Meter

6.0 Meter

330 KV

6.0 Meter

6.0 Meter

6.0 Meter

500 KV

8.0 Meter

8.0 Meter

8.0 Meter

Minimum Fixed Clearances for Electrical Apparatus (Isolation Points): Voltage

Fixed Clearance

Up to 11Kv

0.320 Meter

11KV to 33KV

0.320 Meter

33KV to 66KV

0.630 Meter

66KV to 132KV

1.1 Meter

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Electrical Safety Clearance (Part-3) AUGUST 10, 2012

6 COMMENTS

4 Votes

Standard: Northern Ireland Electricity (NIE), 6/025 ENA

Clearances of Electrical Line to Ground and Roads: Description of Clearance

0.4 KV

11 KV

33KV

110KV

220 KV

400KV

Line conductor to any point not over road

5.2 Meter

6.1 Meter

6.4 Meter

6.4 Meter

7.0 Meter

7.0 Meter

Line conductor to road surface

5.8 Meter

6.1 Meter

6.4 Meter

6.4 Meter

7.4 Meter

8.1 Meter

Line conductor to road surface of high load routes

6.9 Meter

6.9 Meter

6.9 Meter

7.2 Meter

8.5 Meter

9.2 Meter

Bare live metalwork ( transformer terminals, jumper connections, etc)

4.6 Meter

4.6 Meter

4.6 Meter

-

-

-

Clearances of Electrical Line to Other Objects: Description of Clearance

33 kV). Cable

Min. Distance

Power cable of voltage exceeding 33 kV shall be laid

Min 1.2 Meter depth

Underground telecommunication cable shall be with underground power cable of voltage exceeding 33 kV.

Min 0.6 Meter Separate from Power Cable

Safe approach limits for people: 11KV

33KV

66KV

132KV

275KV

Voltage

214V to 415 KV

Person using manually

1.3 Meter

2.0 Meter

3.0 Meter

4.0 Meter

5.0 Meter

6.0 Meter

operated tool Person using power operated tool Share this:

3.0 Meter

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3.0 Meter

3.0 Meter

4.0 Meter

Email

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LinkedIn

5.0 Meter

6.0 Meter

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Electrical Safety Clearance (Part-1) AUGUST 8, 2012

4 COMMENTS

3 Votes

Standard: Qatar General Electricity & Water Corporation Minimum Safety Clearance of Pipelines from Electrical Tower: Voltage Description 33KV/66KV/132KV

220KV/400KV

25 Meter

50 Meter

Not within 25 Meter

Not within 50 Meter

With All Underground services crossing the way leave (from the nearest tower foundation)

35 Meter (Min)

50 Meter (Min)

The nearest side of the road reservation to the nearest tower foundation

25 Meter

35 Meter,50 Meter

25 Meter (Min)

35 Meter(Min) ,50 Meter(Min)

25 Meter

35 Meter,50 Meter

Min 1.5 Meter

Min 1.5 Meter

The right of way (ROW) (widths on either side of the center line of overhead transmission lines ) Other than the roadways and boundary fences of security establishments any temporary or permanent structures / buildings, parapet walls,

The pipelines (water, oil/gas etc.) crossing (respectively away from the nearest base of tower leg). Pipe-lines shall not be laid parallel to the overhead line within the limits of a way leave (ROW). Cables crossing the transmission lines way leave (ROW) respectively away from the nearest base of tower leg. Foundations and civil structures (Temporary or permanent) will not be permitted in the close proximity to the cable circuit. A minimum horizontal

distance from such structures to the nearest edge of the cable trench shall be observed Clearance among Electrical Line-Telephone- Water-Sewerage -Gas Line Service

Vertical Clearance(Min)

Water Line (to cross below EHV cable level)

0.5 Meter

Sewerage Mains (to cross below EHV cable level)

1.0 Meter

Drainage Mains (to cross below EHV cable level)

0.5 Meter

Gas pipes

0.6 Meter

Telephone lines

0.5 Meter

LV / 11kV cables

0.5 Meter

Safety Clearance for Excavation of Land : Type of Excavation

Distance

Use of heavy mechanical excavators (other than hand operated pneumatic jack hammers) or driving sheet piles

Not less than 3 Meter from the edge of cable, cover, cable joint

Heavy machinery engaged in the civil construction or road works

operating load/thrust/ weight will not be applied directly on the cable installation

Trench excavations parallel to the cable installations

Minimum separation of 1 Meter to the nearest edge of cable tile

Laying of metal pipes over a long distance parallel to cable

Not permitted unless the Step and Touch Potentials at any point of the pipe line do not exceed 65 Volts.

(A) Inside Towns Distance between Tower’s foundation to Pipeline in parallel and intersections. Voltage

Min Distance

380/220V

0.5 Meter

20KV

2 Meter

63KV

7 Meter

132KV

10 Meter

230KV and Above

20 Meter

Distance between underground power cables to wall of gas pipelines in parallel routes. Voltage

Min Horizontal Distance

Min Vertical Distance

380/220V

1 Meter

0.5 Meter

20KV

2 Meter

1 Meter

63KV

3 Meter

1.5 Meter

(B) Outside of Towns: Distance between Tower’s foundation to Pipeline in parallel and intersections. KV

Min Distance in Parallel Route (Up to 5 Km)

Min Distance in Parallel Route (Above 5 Km)

20KV

20 Meter

30 Meter

63KV

30 Meter

40 Meter

132KV

40 Meter

50 Meter

230KV

50 Meter

60 Meter

400KV

60 Meter

60 Meter

Distance between overhead lines to gas pipelines at intersections. KV

Min Distance

20KV

8 Meter

63KV

9 Meter

132KV

10 Meter

230KV

11 Meter

400KV

12 Meter

Distance between Tower’s foundations to gas pipelines at intersections. KV

Min Distance

20KV

20 Meter

63KV and Higher

30 Meter

Right Of Way (R.O.W) From Roads: Highway High Way : (38 meter from one side of Central Line of

Distance 76 Meter

Highway) First Class State Road=(22.5 meter from one side of Central Line of Highway)

45 Meter

Second Class State Road=(17.5 meter from one side of Central Line of Highway)

35 Meter

Third Class State Road =(12.5 meter from one side of Central Line of Highway)

25 Meter

Forth Class State Road=(7.5 meter from one side of Central Line of Highway)

15 Meter

General Electrical Safety Clearance: KV

Description

Distance

Up to 11 KV

At points where the lines cross roads or railways

Min 6 Meter Height

Up to 11 KV

parallel to roads the

Min 5.5 Meter Height

Up to 11 KV

lines cross totally desert regions where no traffic is possible

Min 5.5 Meter Height

20 KV to 66 KV

All Location

Min 6 Meter Height

Up to 11 KV

Conductor Joint

No joint shall be closer than 3 meters to a point of support

33 KV & 66 KV

Conductor Joint

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Impact of Floating Neutral in Power Distribution JULY 28, 2012

24 COMMENTS

17 Votes

Introduction: If The Neutral Conductor opens, Break or Loose at either its source side (Distribution Transformer, Generator or at Load side (Distribution Panel of Consumer), the distribution system’s neutral conductor will “float” or lose its reference ground Point. The floating neutral condition can cause voltages to float to a maximum of its Phase volts RMS relative to ground, subjecting to its unbalancing load Condition. Floating Neutral conditions in the power network have different impact depending on the type of Supply, Type of installation and Load balancing in the Distribution. Broken Neutral or Loose Neutral would damage to the connected Load or Create hazardous Touch Voltage at equipment body. Here We are trying to understand the Floating Neutral Condition in T-T distribution System.

What is Floating Neutral? If the Star Point of Unbalanced Load is not joined to the Star Point of its Power Source (Distribution Transformer or Generator) then Phase voltage do not remain same across each phase but its vary according to the Unbalanced of the load. As the Potential of such an isolated Star Point or Neutral Point is always changing and not fixed so it’s called Floating Neutral.

Normal Power Condition & Floating Neutral Condition Normal Power Condition: On 3-phase systems there is a tendency for the star-point and Phases to want to ‘balance out’ based on the ratio of leakage on each Phase to Earth. The star-point will remain close to 0V depending on the distribution of the load and subsequent leakage (higher load on a phase usually means higher leakage). Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).

3 Phase 3 Wire System: Three phases has properties that make it very desirable in electric power systems. Firstly the phase currents tend to cancel one another (summing to zero in the case of a linear balanced load). This makes it possible to eliminate the neutral conductor on some lines. Secondly power transfer into a linear balanced load is constant. 3 Phase 4 Wire System for Mix Load: Most domestic loads are single phase. Generally three phase power either does not enter domestic houses or it is split out at the main distribution board. Kirchhoff’s Current Law states that the signed sum of the currents entering a node is zero. If the neutral point is the node, then, in a balanced system, one phase matches the other two phases, resulting in no current through neutral. Any imbalance of Load will result in a current flow on neutral, so that the sum of zero is maintained. For instance, in a balanced system, current entering the neutral node from one Phase side is considered positive, and the current entering (actually leaving) the neutral node from the other side is considered negative. This gets more complicated in three phase power, because now we have to consider phase angle, but the concept is exactly the same. If we are connected in Star connection with a neutral, then the neutral conductor will have zero current on it only if the three phases have the same current on each. If we do vector analysis on this, adding up sin(x), sin(x+120), and sin(x+240), we get zero. The same thing happens when we are delta connected, without a neutral, but then the imbalance occurs out in the distribution system, beyond the service transformers, because the distribution system is generally a Star Connected. The neutral should never be connected to a ground except at the point at the service where the neutral is initially grounded (At Distribution Transformer). This can set up the ground as a path for current to travel back to the service. Any break in the ground path would then expose a voltage potential. Grounding the neutral in a 3 phase system helps stabilize phase voltages. A non-grounded neutral is sometimes referred to as a “floating neutral” and has a few limited applications.

Floating Neutral Condition:

Power flows in and out of customers’ premises from the distribution network, entering via the Phase and leaving via the neutral. If there is a break in the neutral return path electricity may then travel by a different path. Power flow entering in one Phase returns through remaining two phases. Neutral Point is not at ground Level but it Float up to Line Voltage. This situation can be very dangerous and customers may suffer serious electric shocks if they touch something where electricity is present.

Broken neutrals can be difficult to detect and in some instances may not be easily identified. Sometimes broken neutrals can be indicated by flickering lights or tingling taps. If you have flickering lights or tingly taps in your home, you may be at risk of serious injury or even death.

Voltage Measurement between Neutral to Ground: A rule-of-thumb used by many in the industry is that Neutral to ground voltage of 2V or less at the receptacle is okay, while a few volts or more indicates overloading; 5V is seen as the upper limit. Low Reading: If Neutral to ground voltage is low at the receptacle than system is healthy, If It is high, then you still have to determine if the problem is mainly at the branch circuit level, or mainly at the panel level. Neutral to ground voltage exists because of the IR drop of the current traveling through the neutral back to the Neutral to ground bond. If the system is correctly wired, there should be no Neutral to Ground bond except at the source transformer (at what the NEC calls the source of the Separately Derived System, or SDS, which is usually a transformer). Under this situation, the ground conductor should have virtually no current and therefore no IR drop on it. In effect, the ground wire is available as a long test lead back to the Neutral to ground bond. High Reading: A high reading could indicate a shared branch neutral, i.e., a neutral shared between more than one branch circuits. This shared neutral simply increases the opportunities for overloading as well as for one circuit to affect another. Zero Reading: A certain amount of Neutral to ground voltage is normal in a loaded circuit. If the reading is stable at close to 0V. There is a suspect an illegal Neutral to ground bond in the receptacle (often due to lose strands of the neutral touching some ground point) or at the subpanel. Any Neutral to ground bonds other than those at the transformer source (and/or main panel) should be removed to prevent return currents flowing through the ground conductors.

Various Factors which cause Neutral Floating: There are several factors which are identifying as the cause of neutral floating. The impact of Floating Neutral is depend on the position where Neutral is broken 1)

At The Three Phase Distribution Transformer: Neutral failure at transformer is mostly failure of Neutral bushing. The use of Line Tap on transformer bushing is identified as the main cause of Neutral conductor failure at transformer bushing. The Nut on Line Tap gets loose with time due to vibration and temperature difference resulting in hot connection. The conductor start melting and resulting broke off Neutral. Poor workmanship of Installation and technical staff also one of the reasons of Neutral Failure.

A broken Neutral on Three phases Transformer will cause the voltage float up to line voltage depending upon the load balancing of the system. This type of Neutral Floating may damage the customer equipment connected to the Supply. Under normal condition current flow from Phase to Load to Load to back to the source (Distribution Transformer). When Neutral is broken current from Red Phase will go back to Blue or Yellow phase resulting Line to Line voltage between Loads. Some customer will experience over voltage while some will experience Low voltage. 2)

Broken Overhead Neutral conductor in LV Line: The impact of broken overhead Neutral conductor at LV overhead distribution will be similar to the broken at transformer. Supply voltage floating up to Line voltage instead of phase Voltage. This type of fault condition may damage customer equipment connected to the supply.

3)

Broken of Service Neutral Conductor: A broken Neutral of service conductor will only result of loss of supply at the customer point. No any damages to customer equipments.

4)

High Earthing Resistance of Neutral at Distribution Transformer: Good Earthing Resistance of Earth Pit of Neutral provide low resistance path for neutral current to drain in earth. High Earthing Resistance may provide high resistance Path for grounding of Neutral at Distribution Transformer. Limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting.

5)

Over Loading & Load Unbalancing: Distribution Network Overloading combined with poor load distribution is one of the most reason of Neutral failure. Neutral should be properly designed so that minimum current will be flow in to neutral conductor. Theoretically the current flow in the Neutral is supposed to be zero because of cancellation due to 120 degree phase displacement of phase current. IN= IR