Facts

FOR INTERNAL CIRCULATION ONLY

user’s manual of Construction (part two)

Sub-Stations Volume-8 Series Compensation and FACTS

Construction Management

Power Grid Corporation of India Limited (A Government of India Enterprise) DOCUMENT CODE NO. : CC/CM/SS/SC/2003

MARCH, 2003

CONTENTS Chapter-1 Introduction Page No. 1.1

Why Series Compensation

1.2

The Principal of Operation

1.3

Advantages of Series Compensation Chapter-2 Equipment and Functioning

2.1

Circuit Diagram

2.2

Main Equipment for Series Compensation

2.3

Function of Various Components

2.4

Functioning of the System Chapter-3 Civil Works

3.1

Soil Investigation

3.2

Levelling Chapter-4 Capacitors

4.1

Receipt of Capacitors

4.2

Storage of Capacitors

4.3

Installation

4.4

Selecting Accessories

4.5

Pre-Commissioning Checks

4.6

Recommended Commissioning Procedure

4.7

Normal Maintenance of HT Capacitor Bank

4.8

Preventive Maintenance Chapter-5 Spark Gaps

5.1

Theory & Operation

5.2

Setting of the Spark Gap

5.3

Installation

5.4

Maintenance Chapter-6

6.1

Conclusions

Chapter-1 INTRODUCTION

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CHAPTER ONE ___________________________________________________________________________

INTRODUCTION Back to contents page

1.1

WHY SERIES COMPENSATION Ever increasing requirement of laying transmission lines are causing ever increasing problems of right of way. Besides more and more Transmission lines require more and more funds.

The solution of above problems is

becoming increasingly difficult as well as time consuming.

Under the

circumstances an effective method has been developed to increase the power Transmission capacity of the Transmission Line (or new lines) by installation of static capacitors in series with the Transmission Line. This addition of capacitors in series with the Transmission Line is called series compensation. The series compensation can be fixed i.e. the capacitance in the line remains fixed and cannot be altered. It is also possible to provide alteration of series capacitance by means of thyristors. Such a system becomes Flexible AC Transmission System (FACTS). It is generally possible to derive 50-80% higher power from Transmission Line by utilizing series compensation. Introduction of Series Capacitance is more useful for long lines having more inductive reactance.

1.2

The Principal of Operation Back to contents page

Series compensation uses passive capacitor banks as the means of reducing the effect of inductance. Addition of capacitance in series with the Transmission line modifies the reactance (Inductive reactance-Capacitive reactance) of the line.

The Transmission line as such offers resistance and inductive reactance. Both the resistance and. inductive reactance have adverse effects on the operation of Transmission lines. The resistance causes power losses as well as the drop in the voltage. The inductive reactance causes drop in voltage as well as creates the problems of decreased power factor and increases instability of the Transmission Line. By adding series capacitance in transmission line the inductive reactance gets partially neutralized, thus decreasing the adverse effects of inductive reactance and enhancing the Power Transmission Capability of the Transmission Line. The limit of maximum Power transmitted by a Transmission Line is determined by the following formula:-.

PL = VsVr/X1

Where PL=Limit of maximum Power Transmission capability

Vs = Sending end voltage

Vr = Receiving end voltage

X1 = Inductive reactance in the line.

It is obvious that if the inductive reactance of line is effectively reduced then the Power transmission capability increases. In fact the maximum power which can be transmitted by introducing series compensation is given by the following formula :-

PL = VsVr/X1-XC

Where XC = Reactance of Series Capacitance added in the line.

The above may be rewritten as

PL = VsVr/X1 (1-Xc/X1)

From the denominator, we can define the percentage compensation as follows:

Percentage Compensation = Xc/X1) x 100

1.3

Advantages of Series Compensation Back to contents page

1.3.1

The series compensation reduces the overall inductive reactance of the system. As explained earlier the stability of a power system depends upon the inductive reactance of each line. The decrease in inductive reactance results in higher stability limit of the line. In other words the capability to transfer power on the same line increases depending upon the amount of series compensation of capacitors.

1.3.2

Due to the fact that the load current passes through the series capacitors, the capacitive volt/ampere output is dependent upon the current passing through

the line. As the current increases the capacitive volt ampere increases and vice versa. In this way the series capacitor compensation offers self regulation which is very useful from the point of view of functioning of power system. This also results in improved voltage regulation, which means drop in voltage due to flow of load current is small.

1.3.3

Since series capacitance offers capacitive volt ampere to the system, the requirement of shunt compensation by capacitors at the distribution network comes down.

1.3.4

The series compensation provides a means of regulating power flowing through two parallel lines by introducing series capacitance in one of the lines, the power flow in each line can be regulated as per the requirements. This provides the flexibility of power flow through the system and gives optimum power transfer through of the Transmission Lines in parallel.

1.3.5

The series capacitors has the effect of increasing power factor of the system or in other words they reduce the current flowing through the system for the same power. Therefore, more power can be transmitted for this reason also.

Chapter-2 EQUIPMENT AND FUNCTIONING

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CHAPTER TWO ___________________________________________________________________________ EQUIPMENT AND FUNCTIONING Back to contents page 2.1

Circuit Diagram

The circuit diagram of the system installed at Ballabgarh substation is enclosed herewith.

2.2

Main equipment for Series Compensation

Back to contents page

Following is the list of the main equipments installed at Ballabhgarh substation :Sl. No. 1.

Item Description 400 KV SF6 CIRCUIT

Rating

Qty.

400 KV, 3150A

1

35, 1, 1200a, 3x50.64 MVAR

192

10.4, 1600a, 3x26, 62 mvar

96

150 K/Vp TO 210 K/Vp (40 KA

2

BREAKERS (BPCB) 2.

FIXED CAPACITORS, 27%

3.

VARIABLE CAPACITORS, 8%

4.

SPARK GAP

FOR 1 Sec.) 5.

MOV FOR FIXED CAPACITOR

MCOV=53 KV, PROTECTIVE

26

LEVEL=120 KVp, ENERGY RATING=35.0 M 6.

MOV FOR VARIABLE

MCOV=23 KV, PROTECTIVE

CAPACITOR

LEVEL=52 KVp, ENERGY RATING=15.0 MJ

23

7.

CT FOR PLATFORM LEAKAGE

1200-800/5A-5A, 5P20-20VA

1

PROTECTION 8.

CT FOR SPARK GAP

1200-800/5A-5A, 5P20-20VA

1

9.

CT FOR LINE CURRENT

1200-800/5A-5A, 5P10-10VA

1

10.

CT FOR UNBALANCE 3

200-100/5A-5A, 5P10-10 VA

1

400-200/5A-5A, 5P10-10VA

1

1200-800/5A-5A, 5P20-20VA

3

1200-800/5A-5A, 5P20-20VA

3

PROTECTION FOR FIXED CAPACITOR BANK 11.

CT FOR UNBALANCE PROTECTION FOR VARIABLE CAPACITOR BANK

12.

CT FOR UNBALANCE PROTECTION FOR VARIABLE CAPACITOR BANK

13.

CT FOR MOV CIRCUIT FOR VARIABLE CAPACITOR BANK

14.

CIRCUIT BREAKER CT

1200-800/5A-5A, 5P20-20VA

1

15.

CT FOR PLATFORM POWER

3.6 KV, 1600A, 40 KA FOR 1

1

SUPPLY

SEC.

DAMPING CIRCUIT

47h, 1200a, 10.5, 6.5 mj, 66

16.

1

kV class, 40 KA FOR 1 Sec. 17.

OPTICAL SIGNAL-COLUMN

-

1

BOTTOM 18.

BY PASS ISOLATOR

400 KV, 2000A

1

19.

CAPACITOR ISOLATOR

400 KV, 2000A

2

2.3

Function of Various Components

Back to contents page

2.3.1

400 KV SF-6 Circuit Breaker

This Circuit Breaker is used as a by-pass Circuit Breaker i.e. when the overvoltage occurs across the bank of capacitors due to fault currents, then this circuit Breaker closes so that the current passes through the Circuit Breaker and overvoltage across the capacitor-bank drops down, thus protecting capacitors. Generally this breaker opens when fault has been cleared by other circuit breakers (Fault in this line or outside depending upon fault location).

2.3.2

Fixed Capacitors

Fixed capacitors are inserted in series with the line and are fixed (in the sense not variable). These set of capacitors remain connected in the line except during abnormal conditions. During normal conditions these act as series capacitors in the line.

2.3.3

Variable Capacitors

These capacitors are also serving the same function as fixed capacitors except that the capacitance/inductance inserted in the line can be varied automatically cy silicon controlled rectifiers. This variation done based upon system conditions and variations in system conditions, to obtain steady state and transient control, respectively.

2.3.4

Spark Gap

These spark gaps are inserted in parallel with capacitor banks such that in case of over voltage across capacitors, the spark gaps breakdown and become conducting, thus decreasing the overvoltages across capacitors to protect them.

2.3.5

MOV for fixed capacitors

The capacitors are very sensitive to over-voltages. Even short duration overvoltages may damage the capacitors. MOV (Metal-Oxide Variastors) provide the fastest and smoothest removal and re-insertion of resistance to by pass and reinstate the capacitors in the series circuit, as and when required, to control overvoltages.

The spark gaps and by-pass Circuit Breakers provide back up protection to the capacitors, i.e. if MOV fail then spark gap and circuit breakers come into play.

MOV for Variable Capacitors

The function of this MOV is the same as that of MOV for fixed capacitors. However the voltage ratings of MOV to be adjusted depending upon the voltages likely to appear across variable capacitors in normal and abnormal conditions.

2.3.6

CT for Platform Leakage Protection

The metallic platform (on which capacitors, spark gaps, MOV etc are mounted) is insulated from earth. The leakage current from platform to earth is a measure of the health of this insulation. The abnormal increase of leakage current is a danger signal and requires immediate checking/replacement of insulation. (insulators)

2.3.7

CT for Spark Gap

This is used to transform the current passing through the spark gap, to measurable values.

2.3.8

CT for Line current

This CT transforms the normal and abnormal currents passing through the series capacitor system, to measurable values.

2.3.9

CT for unbalance Protection for fixed Capacitor Bank

The capacitor Bank consists of sets of series and parallel connected capacitors. With reference to the single line diagram, the CT is connected such that no current flows through CT during normal conditions. In case of any fault in any one or more capacitors, the current flowing through CT shall indicate fault in capacitor and in that case this capacitor bank needs to be isolated, checked and repaired if necessary.

2.3.10

CT for unbalance for variable capacitor bank

The function of this CT is the same as for fixed capacitor Bank.

2.3.11

CT for MOV circuit for fixed Capacitor Bank

This CT transforms the current passing through the fixed capacitor MOV circuit, particularly during fault condition.

2.3.12 CT for MOV circuit for variable Capacitor Bank

This CT transforms the current passing through the variable capacitor MOV circuit particularly during fault condition.

2.3.13

Circuit Breaker CT

This CT transforms the current flowing through by-pass circuit breaker when the breaker is closed.

2.3.14

CT for Platform Power Supply

This CT transforms the entire current flowing to the platform and equipment thereon.

2.3.15

Damping Circuit

For safety of capacitors and system, it is very important that the discharge current amplitude from the capacitors should be limited and oscillation should be damped. These discharge current/oscillations are generated (generally due to capacitors discharging) when spark gap operates or by-pass circuit breaker closes) thus providing the route for discharge current. The damping circuit

needs careful design depending upon the rating of capacitors and other circuit parameters.

2.3.16

Optical Signal Column Bottom

In order to transmit various signals from CT’s, etc. an optical signal column is used. This is an insulating column, the top of which is at platform voltage and bottom at earth voltage. Inside the column, the signals are transmitted by optical fibre cable device.

2.3.17

By-Pass Isolator

This Isolator by-passes the entire capacitor bank and associated equipment, platform etc. This is useful for operating the line without series capacitors while the line maintenance work is being carried out on series capacitors and associated equipment.

2.3.18

Capacitor Isolators

These two isolators are used to isolate the capacitors and associated equipment from the line voltage such that maintenance work can be done.

2.4

Functioning of the system

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2.4.1

Normal Operation

During normal operation, series capacitors are in the line circuit, the capacitor isolators are in closed position. The line current passes through the series capacitors and all advantages of series capacitance are realized.

In case there is a fault on the compensated line section, heavy currents shall flow through the line and also through the capacitors. This will develop high voltage across capacitors, which is harmful to the capacitors. Hence, due to high voltage, MOV and spark gap shall by pass the capacitors. MOV (metal oxide variastor, generally ZnO) is the fastest to respond to high voltage and regain high resistance in case voltage across it comes down. Also the by-pass circuit breaker shall close to by-pass the current. The fault is normally cleared by external (line) breakers within 100 ms. In autoreclose systems a dead time of about 1000 ms is normally allowed (after clearance of fault) for deionization of the fault. During this dead interval, the spark gap also gets deionized and regains insulation. MOV also restores its high resistance properties immediately after fault is cleared. The by-pass circuit breaker also open out during this period. Hence auto reclosing of line circuit breaker restores the normal condition if the fault does not persist.]

In case the fault persists, the process as above shall get repeated, but line breakers will not close after one reclosure. Hence the series capacitor system shall be ready to be switched in. When the fault is eliminated & line breaker is closed, the system shall operate under normal conditions, with series capacitance in circuit.

2.4.2

Advantages of Thyristor Controlled Series Compensation (TCSC)

Thyristers provide a means of very fast response to changing system conditions. The high speed switching capability of thyristors provides a method for controlling line power flow by changing the value of series compensation in the line. Whenever there are disturbances in the line like switching off lines, switching on/off heavy loads or generators etc. the thyristors provide fast and automatic control method by varying the series capacitance in the line. In addition to the variation, the steady state power flow can also be regulated by TCSC. Its control can be utilized to modulate the line reactance and provide damping to system swings.

Thus the TCSC provides fast control in steady state as well as the transient stability problems.

The TCSC also provides a method for reducing a potential sub-synchronous resonance problem at thermal generators electrically close to transmission lines, by series compensation. Sometimes, such located Series Compensation (fixed) has been limited to range of 20% to 40% due to inability to mitigate sub-synchronous resonance with fixed series capacitors. But by using a small percentage of thyristor control, the total compensation can be increased significantly.

Chapter-3 CIVIL WORKS

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CHAPTER THREE ___________________________________________________________________________ CIVIL WORKS Back to contents page

3.1

Soil Investigation:

3.1.1

Soil Investigation is carried out at site and result of soil investigation are forwarded to Corporate Centre for design of various foundations.

3.1.2

Detailed soil investigation is carried out at site to arrive at sufficiently accurate, general as well as specific information about the soil profile and necessary soil parameters of the site in order that the foundations of various structures can be designed and constructed safely & rationally.

3.1.3

Engineering department at Corporate Centre prepares the foundation drawings and approved drawings are sent to site for casting.

3.1.4

Engineering Department also releases various other erection drawings for different works like cable trench design drawings, cover slab design drawings, overall layout of equipments in switchyard, equipments erection key drawings etc. for erection works at site. During this period, the site leveling work is carried out at site in order to smoothen the undulations.

3 .2

Levelling

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3.2.1

The FACTS is installed in the switchyard area of existing substation or in a new substation or in the extension of existing substation.

3.2.2

The concerned land may be barren/cultivated land depending upon the situation. The acquired land may contain tree, bushes, drains etc. that require clearning/clearing before starting the leveling works in the yard.

3.2.3

Switchyard area is important and preferably it should be brought to a single level. However, in only unavoidable circumstances or where it is uneconomical to go for leveling the soil than keeping a multi layered/in steps levels, the different levels may be kept.

3.2.4

Now spot levels will have to be taken in the yard area before making an assessment for the leveling i.e. for assessing the requirement of soil for low level areas and cutting of soil from high level area to bring the whole yard area to a normal formation level.

3.2.5

In the ideal case of leveling there is no requirement of soil for low level areas and cutting of soil from high level area to bring the whole yard area to a normal formation level.

3.2.6

In the ideal case of leveling there is no requirement for borrowed earth and quantity of earth excavated from the high level and fill it in the low lying areas is equal. This the most economical method of leveling. Care is to be taken such that the earth is not excavated below the formation level.

3.2.7

For compaction earth is filled in the low lying areas in layers of 20cm. Thickness then watered and compacted by rollers/dozers.

3.2.8

The method and equipment used to compact the fill material to a density that will give the allowable soil bearing pressure required for the foundations, roads, etc. In each layer of fill material. Each layer of earth embankment when compacted should be as close to optimum moisture as practicable. Embankment material which does not contain sufficient moisture to obtain proper compaction should be wetted. If the material contains an excess of moisture, then it should be allowed to dry before rolling by hand rollers/dozers. No compaction is carried out in rainy weather.

3.2.9

The levels in the entire area (after finishing the leveling work) should be taken and checked up with the desired formation level. Final dressing up and finishing should be done in case if levels are not found satisfactory. The care should however be taken during compaction. Measurement for leveling work (i.e. excavation & filling) is a cumbersome process and it should be done strictly as per the specifications. All the level records must be noted in field leveling book duly signed by the concerned personnel of contractor of site. The drawings of level before starting the leveling and then final levels should be maintained. The measurements should be recorded very carefully as per the technical specification. Care should be taken that with the movement of trucks, dozers etc. any other structure in the vicinity is not affected or uprooted.

Chapter-4 CAPACITORS

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CHAPTER FOUR ___________________________________________________________________________

CAPACITORS Back to contents page 4.1

Receipt of Capacitors

4.1.1

Ensure that proper material handling equipments are available at site for unloading of capacitor units.

Do not use the bushings for lifting or slinging or shifting capacitors.

4.1.2

Check for :

Proper condition of packing case

No. of packages

Contents of packages

4.1.3

Check for damages specially for :

a)

Any breakages of bushings or insulators

b)

Leakages of oil from bushings top/base

c)

Leakages from capacitor container

d)

Denting/damage to terminal cover bank frames, busbars and other accessories.

4.1.4

Ensure that all components, parts have been received strictly as per contract drawings.

Users are entitled to receive, as a minimum, the following details :

4.1.5

-

Schematic Diagram

-

Brief Constructional details

-

Important Components and their functions

-

Vulnerable areas

Action in the event of damages Lodge insurance claim, inform manufacturer and mention on LR/RR.

In case of Broken bushings, major leakage, report to manufacturer and send the unit back for repairs at factory. Do not try to repair the unit at site.

For minor leakages apply fast curing Araldite or M-Seal compound.

4.2

STORAGE OF CAPACITORS

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If the capacitors have to be stored for a while, please ensure the following :

4.2.1

Preferably store the equipment indoor or under cover with bushings of units in vertical direction.

4.2.2

Do not store capacitor units on top of one another.

4.2.3

Do not store capacitor units near a heat source.

4.2.4

While in storage, periodically check for oil leakages and also for open or accessible copper parts, which are likely to be stolen.

4.3

INSTALLATION

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All HT installations, in general, require prior approval by the Electrical Inspector for compliance with the provisions of Indian Electricity Act.

Care has to be taken with regard to the clearances of all live parts. Capacitors are generally mounted in open (outdoor).

4.3.1

Install capacitors in cool dry place, which should also be free from excessive dust and chemical fumes. Capacitors should be installed away from heat generating bodies.

4.3.2

Clean capacitors, specially porcelain bushings, with cloth for dust and oil stains.

4.3.3

Tighten all electrical and mechanical connections. Avoid over tightening of capacitor bushing terminals. Also take care while connecting busbars so that no stress comes on capacitor bushing.

4.3.4

Ensure that the ambient temperature at capacitor location does not exceed beyond the limits specified in Indian Standards for Power Capacitors.

Upper limit of

Maximum ambient temperature in 0C

temperature

Mean over

Mean over

Mean over

category in Deg. C

1 hour

24 hour

1 year

40

40

30

20

45

45

40

30

50

50

45

35

4.3.5

Ensure free circulation of air around the capacitors.

4.3.6

Banking structure, capacitors, fuses and busbar work should be done as per drawings. Adequate clearances should be maintained.

Rated Voltage

Clearance in mm

in KV

Indoor

Outdoor

Phase to Phase

Phase to earth

Phase to phase

Phase to earth

Upto 6.6

89

64

178

140

11

127

77

229

178

22

242

140

330

280

33

356

223

432

381

When capacitors are mounted on an open structure of angles and channels, their live parts are exposed and accessible. Such banks are required to be

elevated to a minimum height of eight m from ground level by providing elevating structures. (for 400 KV Systems).

4.3.7

When the capacitor has external fuse protection it should be ensured that the fuse is connected to the individual capacitor unit. Do not connect a fuse to more than one capacitor unit.

4.3.8

Care must be taken to provide sufficient magnetic clearance for reactor (if used in the system) as prescribed by standards/manufacturer.

4.4

SELECTING ACCESSORIES

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For trouble free satisfactory performance of capacitor choosing proper peripheral equipment is essential. Here are some recommendations :

4.4.1

Series Reactors Strictly follow the guidelines prescribed by individual manufacturers. Ensure proper selection and use of the equipment.

4.4.2

Lightening Arrestors Distribution type lightning arrestors are designed for light duty as against station type for heavy duty lightning arresters. Use the latter since every time the lightning arresters shorts the system to ground, there is a heavy capacitor current discharge through the same. Each set of lightning arrestor must have its individual and separate earthing plates and an earthing system. This can then

be tied into the general earthing system. Lightning arresters must be used when the HT mains are subjected to heavy impulses such as those due to frequent lightning strikes; those due to switching surges; due to frequent operations of very high HP motors etc. or those due to cyclic impulses caused by the firing of high power thyristors and rectifiers on HT mains.

4.4.3

Fuses Although individual or bulk protective fuses are sometimes chosen with a rating of 2 times capacitor current, this will not provide adequate protection to the capacitor. It is, therefore, recommended to select the fuse rating by matching the 1st characteristic curve of the fuse with capacitor overcurrent withstand characteristic. Fuses may be of two types :

a)

Expulsion type with replaceable fuse element.

b)

HRC sealed ceramic type.

HRC fuses are better in performance than expulsion fuses with respect to fault clearance capacity and tolerance in fusing current but are more expensive.

In either case, spares must be maintained for replacement and bare copper wires must never be used.

4.4.4

Isolators Any isolator, which is used on a capacitor bank, must be operated only with the Breaker switched off. Interlocks provided for this purpose must be checked for effectiveness.

4.4.5

Circuit Breakers Select a circuit breaker from an established manufacturer and preferably with a proven or certified capacity to operate a capacitor installation. Both Vacuum Circuit Breakers and SF6 Circuit Breakers are suitable for capacitor switching. Circuit breakers should be capable of withstanding the current and frequency.

Selected circuit breakers or a contactor must have sufficient breaking capacity in MVA to meet the installation Short Current MVA at this point.

When capacitor banks are operated in parallel there are heavy inrush currents into the bank which is switched into the circuit. This factor must be duly considered at the initial stage while selecting a circuit breaker.

4.4.6

Protection The Capacitors should be provided with following protection;

Over Voltage Protection

Over current protection

Earth Fault Protection

Voltage/Current unbalance Protection

Discharge Time Delay Protection

The relay settings on circuit breaker panel should be made corresponding to the maximum over voltage and unbalance conditions mentioned below :-

Over Voltage

:

1.1 times the rated voltage

Over current

:

1.3 times the rated current

Neutral

Displacement :

10% max. between phases.

(Voltage Displacement) Current Unbalance Protection

:

To prevent more than 50% over voltage on this healthy elements of an internally fused capacitor unit. 10% over voltage on other units.

Time

delay

between :

300 Secs. (min.)

Switching OFF & ON

4.4.7

Cables Cables wherever possible to use, for capacitor installations must be derated by a factor of 2. The cable lugs must be properly fitted and cable ends and cable joints must be securely sealed. Should a cable develop a fault, all capacitors will discharge into this, causing a serious blow up at the fault and blowing of external and/or internal fuses in capacitors.

Take care especially with XLPE cables to select appropriate cable lugs.

4.5

PRECOMMISSIONING CHECK

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Individual testing of Capacitor Units is advisable before making the connection. Following tests should be carried out on all Capacitor Units. Manufacturers recommendations are more important.

4.5.1

Meggar Test Charge the capacitor between terminals with 500 V or 1000 V DC Megger. The Megger shows first short circuit & then slowly, charge builds up. This indicates that the the Capacitor unit is electrically O.K. If megger indication is zero, it means the Capacitor Unit is internally short circuited and if indication is infinity, particularly in the beginning, the unit has an internal open circuit.

Internal discharge resistor may not allow the Megger value to go up.

After test discharge the Capacitor by shorting the terminals through a resistor. A healthy Capacitor Unit will discharge with a spark and noise.

4.5.2

Insulation Resistance Test Megger value between terminals shorted together and container may be checked with a 500 volts DC megger to ensure that there is no earth fault. The megger value should not be less than 50 mega ohms. As per IS : 2834.

This test is not applicable to the Capacitor Units having single bushing and other terminal connected to the body of the Capacitor Unit.

4.5.3

Rating Capacitor rating can be checked by one of the following two methods :-

4.5.4

Current Method The Capacitor current may be checked by applying low voltage to its terminals. The measured value can be extrapolated to its rated voltage and the output at rated voltage may be calculated using following formula : KVAR = (i) Voltage x Current x 3 x 10-3

(for 3 Units) (ii) Voltage x Current x 10-3

(for 1 Units)

4.5.5

Capacitance Method If portable, battery operated capacitance meter is available at site, the Capacitance of individual unit/Bank may be checked directly and output may be calculated using following formula :

KVAR = (i) 2 x f x C x V2 x 10-9

(for 1 Capacitor Unit)

(ii) 4 x f x C x V2 x 10-9

(for 3  Capacitor Unit)

Where

KVAR = KVAR of Capacitors

F = Rated frequency in Hertz

C = Capacitance between terminals in MFD (Microfarad)

V=Rated Voltage (terminal – terminal) of Capacitors in volts.

In case the Capacitors are supplied alongwith associated equipments like Series Reactor, Residual Voltage transformer (RVT), Neutral Current transformer (NCT), CT, PT etc. they should be checked for continuity of windings, insulation resistance, oil level etc. as per the standard practice.

4.5.6

System Parameters It is important to check the line voltage, frequency and initial power factor of the system. It should be strictly observed that the line voltage and frequency matches the rated voltage and frequency of the equipments for which they are designed.

4.6

RECOMMENDED COMMISSIONING PROCEDURE

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4.6.1

The recommendation of the manufacturer are very important and should be complied with. In general the following guidelines are given :

If the Capacitors are supplied with associated equipments, the following procedure should be followed for commissioning :

Keep all the Capacitor Units disconnected from the busbars. Switch ON the installation only with Series Reactor/RVT/Isolators/LA etc. in the circuit. This is to check for any malfunctioning of the associated equipments.

Any malfunctioning on the part of these equipments can lead to failure of capacitors and hence these are important precautionary checks. Once the observations are satisfactory, capacitors also can be connected and the bank energized.

4.6.2

Check the line voltages. If these are more than the rated voltage for capacitor system, do not switch on the capacitors. As the load builds up, the line voltages will fall. Switch on the capacitors only then.

4.6.3

Make sure that there is enough load on the H.T. mains. Thus, if a 1 MVAR bank is being energized, there should be a steady load of atleast 2 MVA or MW. Ascertain this by timing the energy meter over a 15 minutes interval.

It is risky to switch on a capacitor bank with a load which fluctuates largely from instant to instant, as in the case of an arc furnace load in its initial melting stage.

4.6.4

Remove one unit deliberately from the balanced half of the bank. Adjust the neutral unbalanced current/voltage relay at the calculated value where possible or at ½ to 1/3 position. Switch on the bank and continue adjusting the relay till it just trips the breaker. Put the setting at slightly below this value.

4.6.5

Put the disconnected unit back into the half bank, the two halves are now balanced. Switch on the capacitor

4.6.6

If a simple service oscilloscope is available, set this to 50 Hz and connect it across the ammeter terminals of the ammeter on the capacitor system. Adjust properly and study the current wave shape. If it shows excess of 3rd, 5th or 7th harmonics, investigate and try to eliminate these. Otherwise, the capacitors will be overloaded and become unduly hot. If this is not possible on the spot, trace the wave shape as accurately as possible and send this to the manufacturer with full details of the load and the system. If the wave shape shows sparking or pulses of high magnitude, switch-off and investigate possible sources. Sharp pulses or spikes of any nature destroy a capacitor in no time. These can be and should be eliminated.

4.6.7

Once the bank is energized and switched-off, wait for full five minutes. Then earth the bus system across which you expect to work and keep this earthing while work is being carried on. A standard earthing pole approved by the electrical inspector, with an open trailing copper flexible, which is earthed at one end and tied to the metal probe of the earthing rod at the other end, may be used.

Earthing within less than five minutes of switching-off can blow the fuse of the capacitors and damage them.

If a capacitor system is momentarily earthed and the earthing rod is removed, it can get recharged to a dangerous voltage and can cause accidents. It is best to keep this earthing rod permanently on the bus or live part where work is

being carried out. Also remember to remove this when the work is over and the capacitors are to be reenergized.

4.7

NORMAL MAINTENANCE OF HT CAPACITOR BANK

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4.7.1

Keep a weekly record of temperature rise for the first six months. If any unit develops more than 20 Deg. C temperature rise, investigate and eliminate the trouble. Failing this, replace the unit.

If no spare units are available disconnect one more healthy unit from each of the remaining phases and balance the half.

IMPORTANT However, the check is not applicable for Filter Banks as this method may change L:C ratio.

4.7.2

Check the condition of HRC fuses.

4.7.3

Check for leakage/seepage of oil on any equipment. These cannot be tolerated. Repair immediately by applying fast curing M-Seal or Araldite.

4.7.4

There is a short or a trip on any phase :

Identify and replace the damaged unit. If no spare is available, remove equal units from other phases and balance.

Before recommissioning, check the capacitance values and if possible tan delta on all units in this phase as well as in other phases.

4.8

PREVENTIVE MAINTENANCE

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Capacitors should be maintained periodically once a month.

Capacitors including bushings should be cleaned of dust etc. All connections should be checked for tightness.

Any rusty portion should be cleaned and then given a coat of paint.

Other auxiliary equipments specially breakers should be maintained as per manufacturer’s recommended practice.

Oil of series reactor and RVT should be checked for its break don strength periodically.

Normally capacitors once installed and operated as recommended here should give long and trouble free service. However, in the event of encountering any trouble, users may refer to the table below which gives some of the common troubles encountered with its causes and remedies to take appropriate action.

Trouble

Cause

Remedial action

1.

Leakage of oil from

a)

capacitor bushing or

Minor transit

i)

Check for hollowness.

ii)

For minor leakage M-welded

damage

welded joints.

joints does not stop replace the Unit and refer to manufacturer for possible repairs. b) Due to tension

i)

coming on bushing

Re-do, the connections to avoid tension coming on bushings, repair capacitor as stated above.

c)

Due to over-

c)

heating of unit 2.

Over heating of

a)

Poor ventilation

See Serial heating No. 2 for action.

a)

Units

Provide free circulation of cooling air.

b) Excessive ambient

b)

Arrange forced ventilation.

c)

c)

See Sr. No. 3 for action.

d)

Refer to manufacturer.

a)

Reduce voltage by changing

Drawing excessive current.

d) None of the above Causes. 3.

Capacitor drawing

a)

High voltage

high current

transformer taps or switch off the bank. b) Harmonic current

b)

flowing

If current beyond permissible limit switch off capacitor bank and take remedial measures to reduce harmonics flowing in capacitors.

4.

Capacitors drawing less current

a)

Low voltage

a)

Current directly proportional to voltage – No repair needed.

b) Failure of

b)

If one or more units have failed

capacitor unit with

and it has not been detected by

blown fuses.

neutral unbalance relay check whether unit has failed or fuse has spuriously blown. If unit fails replace the unit.

c)

Partial failure of

c)

unit

In case of internal fused capacitors sometimes partial failure in various units occur which cannot be detected by neutral unbalance relay. Check the whole bank for capacitance and regroup them. Remove units having capacitance less than 95% of rated value. Also check if neutral unbalance limit is set correctly.

5.

Expulsion fuse

a)

blowing too

Expulsion fuse

a)

rating low

Check expulsion rating. Manufacturer may be consulted

frequently but unit

giving complete bank

consulted Healthy.

particulars. b) Expulsion fuse

b)

Meant for one

Correct the mistake. Connect one fuse with one unit.

connected to more than unit. 6.

Expulsion fuse not

a)

Blowing but unit

Expulsion fuse

a)

rating may be high

Check expulsion Fuse rating. Manufacturer may be consulted

failed.

giving complete bank and system particulars.

7.Abnormal bulging/bursting.

a)

Gas formation due to internal arcing causing unit to Bulge or burst.

a)

Replace the unit refer to the manufacturer.

8.

Abnormal sound and

a)

External earth fault a)

Clean the bushings and check

black marks bushing

between terminal

installation for any short.

and near by.

and container b) High voltage

b)

surges due to

Consult the manufacturer’s application engineer of breaker.

lightning or due to Restriking. 9.

Terminal over heated a)

Loose contact

a)

or melted

Tighten the connection. Replace melted terminal unit for repair.

b) Corrosion

b)

Check for bi-metallic contacts. Avoid using washer & nut etc. of different metals.

10. Capacitor bank

a)

Co-ordination of

a)

Check and consult

tripping unbalance

expulsion fuse

manufacturer’s application on

Protection But

blowing with

engineer.

expulsion fuse not

neutral protection

blown.

not proper

Chapter-5 SPARK GAPS

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CHAPTER FIVE ___________________________________________________________________________

SPARK GAPS Back to contents page 5.1 Theory & Operation

The spark gap is the gap between two conductors generally of spherical shape. This gap generally consists of air which breaks down when the voltage across the conductors reaches beyond certain value. This value depends upon the dimensions and shape of spark gap and spacing between electrodes. Spark gap is connected in parallel with the equipment to be protected (against over voltage) such that when the voltage across equipment exceeds a certain preset value then the spark gap becomes conducting and the voltage across the equipment falls down. However the spark gap resumes its insulating properties when the voltage across it becomes very low or if the current through it is stopped by breaking its circuit. In FACTS, the spark gap operation is as follows :

Spark gap in flexible AC Transmission systems is utilized in 2 ways. One is to use it as a simple spark gap which conducts when the voltage across it exceeds a certain voltage, thus reducing the voltage across parallel connected equipment. This method of operation is called conventional operation. In this method also first trigatron gap ignites, which in turn ignites main gaps. There

is a trigatron spark gap which acts like a precision gap and the main gap ignites when the voltage over trigatron spark gap reaches the ignition voltage that has been adjusted and trigatron gap ignites. The procedure is explained in subsequent paras. Please also refer to fig. 5.1 The second method consists of ignition (conduction) of spark-gap by means of a triggering device called trigatron. This method is called forced triggered operation. This operation can be utilized to trigger the saprk gap even when the voltage across spark gap is normal voltage but some condition in the system requires conduction of the spark gap. The ignition of trigatron spark gap is initiated by means of an electronic circuit or, depending upon the system conditions.

The forced triggered ignition system is used in the metal-oxide variastor (MOV) protection scheme in case the MOV’s have failures. They can be electrically disconnected from the system and the spark gap can be used as a conventional spark gap. Then the series capacitor bank can be operated as a conventional series capacitor bank.

The spark gap consists of two spark gap enclosures which are placed one on top of the other. Inside the enclosures there are the main electrodes Trigatrons (T1 and T2), current limiting resistors and two voltage divider capacitors (C1 and C2) are located under the lower enclosure. Two voltage divider capacitors (C3 and C4) are located between the enclosures.

The voltage divider capacitors C1, C2, C3, C4 divide the voltage so that the gap between the main electrodes is affected by half of the total bank voltage.

The main gaps are adjusted so that in no climate conditions (at normal operating voltages) they ignite before the trigatron spark gap is triggered.

5.1.1

Theory of operation of forced triggening

The forced triggered protection is the primary function in FACTS.

The ignition procedure in forced triggening protection obeys process described below.

Trigatron is triggered by a high voltage pulse. There is an ignition cable lead from the coil into the spark electrode of Trigatron spark gap. The ignition

spark arces between the outer surface of the electrode ball of Trigatron and the spark plug, which is fitted inside the electrode ball.

This tiny ignition arc decreases the voltage withstand capability in Trigatron spark gap by causing ionization, hence Trigatron fires.

5.1.2

Theory of forced triggered ignition

When the control electronics gives a forced triggering command for some reason the gap function is as follows :

5.1.2.1

Trigatron T1 fires.

5.1.2.2

Voltage divider capacitor C1 discharges via Trigatron T1 within the time constant approx. 10-20 s set by C1 and the damping resistor R. This discharge current goes through the primary winding of the triggering transformer and causes the firing of Trigatron T2 also voltage divider capacitor C2 discharges via Trigatron T2 within the time constant approx 10-20 s set by C2 and the damping resistor R.

5.1.2.3

After discharge of the capacitors C1 and C2 the whole ignition voltage tends to affect over the upper housing (C3 and C4) and the main ignition electrodes in the upper housing will flash over.

5.1.2.4

After discharge of the capacitors C3 and C4 the whole ignition voltage tends to affect over the lower housing (C1 and C2) and the main ignition electrodes in the lower housing will flash over.

5.1.2.5

Now the whole spark gap is conducting and the arcs in the lower and upper housings will move to the final arcing positions.

5.2

SETTING OF THE SPARK GAP

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5.2.1

General Trigatrons together with the voltage divider capacitors determines the ignition voltage of the whole spark gap when used as a conventional spark gap. With forced triggering the spark gap can be triggered in the specified range of voltages across spark gap.

Calibration curves of different Trigatron units slightly differ from each other. To reach a required accuracy in the ignition voltage of the whole spark gap, the setting of Trigatrons must be specially chosen for each spark gap system. The self ignition voltage of Trigatron is got by multiplying the ignition voltage of the whole spark gap with the voltage divider ratio shown below :

U1/U = 1/C1 / 1/C1+1/C2+1/C3+1/C4

U2/U = 1/C2 / 1/C1+1/C2+1/C3+1/C4

5.2.2

Setting of Trigatron

The self ignition is set slightly higher (say 10%) than the maximum forced triggering level (i.e. protection level). The corresponding setting of the

micrometer screw can be found from the self ignition curve. A typical self ignition curve is shown in Fig. 5.2.

110.0

y = 0.0032 x2 + 2.7408 x x 32.127

100.0

90.0

80.0

70.0

60.0

50.0

40.0

30.0

20.0

0

5

10

15

20

25

Micrometer screw setting (mm)

Fig. 5.2

30

5.2.3

Adjustment of the micrometer screw The manufacturers instructions should be followed. For example the spark gaps supplied by M/s Nokian Capacitors for Ballabhgarh (Ballabhgarh-Kanpur Line), the following procedure is prescribed by the manufacturer.

The micrometer screw is placed on top, inside a corona shielf. For the adjustment, lift the cover off and continue as follows :

5.2.3.1

There are two M4 locking screws in the microscrew device;

Do not touch (loose) the upper one that holds the vertical scale shaft.

Loose the lower M4 screw that holds the lock nut.

5.2.3.2

Unscrew approx. ¼ of a turn of the lock nut Turn the micrometer screw until you get the required reading on the scale.

5.2.3.3

Tighten the lock nut

5.2.3.4

Tighten the lower M4 locking screw.

5.2.4

Setting of the main gaps Use a gauge for adjustment of space ‘a’ of main gaps and adjust within allowable limits for spark gap supplied by Nokian capacitors the following procedure is given for Ballabgarh S/S Auxiliary electrode so that spaces a’ are set to the given value.

5.3

INSTALLATION

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The installation is generally done as follows :

The internal components and insulators are fixed at ground level. All components shall be bolted to the enclosures. The mounting goes simply as follows :

A crane is needed for lifting the enclosures when the post insulators are installed at ground level.

After this the enclosures are laid down smoothly on the ground, where all the inner components are bolted on their places. Voltage divider capacitors C1, C2, C3 and C4 are fastened on their places. The locations of trigatrons and voltage divider capacitors are defined by the manufacturer. The enclosures can be lifted equipped on the platform. The location of each component is shown in the manufacturers.

Use contact grease in all electric connections (AL-3, Penetrox or equal), and make sure that also the upper electrode shaft is greased by contact grease.

Check that all graphite electrodes are installed tightly.

Install Cu-Al plates so that the aluminium side is against aluminium and copper side against copper or graphite.

When the spark gap enclosures are lifted on the platform, check that all insulators are touching the steel feet, if not, use shim plates to fill the gap.

All wiring is made by using 50 mm2 copper wires as per manufacturers advice connecting the damping resistor. The spark gap arc distances must be adjusted exactly. The whole installation of spark gap system is done by two or three men and a crane.

The tightness should be checked by wrenches as per manufactures recommendations.

5.4

MAINTENANCE

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5.4.1

Annual maintenance

Main gaps

The enclosure, support insulators, bushing insulators, electrodes, and other parts are visually inspected.

If there are burning marks on the surface of the graphite electrodes, they are smoothed by using a file and a grinding cloth. The copper electrodes, support and bushing insulators are cleaned by wiping with a cloth. It must be checked that lower, upper and auxiliary electrodes and that all graphite electrodes are installed tight.

The clearance between lower and auxiliary electrodes are measured and checked that they are in accordance with the original setting. We recommend that you keep a written record of this checking.

Trigatron

Trigatron needs no real annual maintenance.

The insulator must be wiped clean if it is dusty.

Voltage divider capacitors.

It must be checked that there exist no oil leakages in the voltage divider capacitor. The voltage divider ratio ought to be checked. It can easily be reached by measuring the capacitances :

Trigatron T1

Trigatron T2

U1 1/C1 ____ = ____________________ = U

1/C1+1/C2+1/C3+1/C4

U2 ______ U

=1 = ___________________ 1/C1+1/C2+1/C3+1/C4

Chapter-6 CONCLUSIONS

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CHAPTER SIX ___________________________________________________________________________ CONCLUSIONS Back to contents page 6.1

Conclusions

6.1.1

It can be easily concluded that the Series compensation and FACTS prove very useful in long lines where the line reactance is high and also cost of construction of additional line is high. The enhancement of power transmission capability of the line increases by 50 to 80% by providing Series Compensation and Thyrister Controlled Series Compensation. This way the cost can be saved substantially.

6.1.2

The problems of obtaining Right-of-way are becoming more and more difficult as well as time consuming. On the other hand, Series Compensation and FACTS is generally provided at the substation where land is generally available in existing substation or can be acquired comparatively easily. The provision can be made by proper planning the new substations.

6.1.3

As the system is new in Indian conditions, the operation of the system is likely to be stabilized in near future and teething problems are likely to be overcome.

6.1.4

This system is likely to prove very useful in India where long lines are existing and are required in future also.