Substation Engineering
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WELCOME TO PRESENTATION ON
CONCEPT OF SUB-STATION ENGINERING
Contents of Presentation PURPOSE CLASSIFICATIONS VOLTAGE
CLASS & RATINGS
PLANNING
OF SUB STATION INSTALLATION
SUB-STATION SUBSTATION
ENGINEERING
EQUIPMENTS
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1.0 PURPOSE OF ESTABLISHING A SUBSTATION
1.1 The substations are very much essential to • • •
Evacuate power from generating stations. Transmit to the load centers. Distribute to the utilities & ultimate consumers.
1.2. The Electrical power generation from Hydel, Thermal, Nuclear and
other generating stations has to be evacuated to load centers. • The generation voltage is limited to 15/18 KV due to the limitation of • •
1.3
the rotating machinery. This bulk power has to be stepped up to higher voltages depending on quantum of power generated and distance to the load centers. Again the power has to be stepped down to different lower voltages for transmission and distribution.
In between the power houses and ultimate consumers a number of Transformation and switching stations have to be created. These are generally known as sub-stations 3
2.0 CLASSIFICATIONS Accordingly the substations are classified as a) Generating substations called as step up substations b) Grid substations c) Switching stations d) Secondary substations. 2.1. The generating substations are step up stations as the generation voltage
needs to be stepped up to the primary transmission voltage so that huge blocks of power can be transmitted over long distances to load centers. 2.2 The grid substations are created at suitable load centers along the primary transmission lines. 2.3 Switching stations are provided in between lengthy primary transmission lines • To avoid switching surges. • For easy segregation of faulty zones. • For providing effective protection to the system in the A.C. network. • The switching stations also required wherever the EHT line are to be tapped and line to be extended to different load centers without any step down facility at the switching stations. • The number of outgoing lines will be more than the incoming lines, depending on the load points. 4
2.4.
Secondary substations are located at actual load points along the secondary transmission lines where the voltage is further stepped down to sub transmission & primary distribution voltage.
2.5. Distribution substations are created where the sub-transmission voltage and primary distribution voltage are stepped down to supply voltage and feed the actual consumers through a network of distribution and service lines. 3.0. VOLTAGE CLASS AND RATINGS. Generally the following voltage class substations prevailing in India • 6.6 KV, 11 KV, 22KV, 33 KV ---------- High Voltage 66KV, 110/132KV, 400 KV and above 220/230KV ---------- Extra high Voltage 3.1 Sub station rating is defined as the capacity of power transformers installed. 5
4.0 PLANNING OF SUBSTATION INSTALLATION The process of planning sub-station installations consists in • Establishing the boundary conditions. • Defining the plant concept, type, & Planning principles. 4.1 The boundary conditions are governed by following environmental
circumstances & availability of the land in the required place. • Local climatic factors • Influence of environment • The overall power system voltage level • Short circuit rating • Arrangement of neutral point • The frequency of operation • The required availability or reliability • Safety requirements • Specific operating conditions 6
4.2. Boundary conditions The following boundary conditions influence the design concept and measures to be considered for different parts of substation installations.
Boundary conditions
Environment, climate conditions
Net work data / Net work form
Concept and measures Outdoor / indoor Conventional / GIS Equipment utilization Construction Protection class of enclosures Creepage, arcing distances Corrosion protection Earthquake immunity Short circuit loadings Protection concept Lightning protection Neutral point arrangement Insulation coordination 7
Boundary conditions
Availability and abundance of power supply
Power balance
Ease of operation
Safety requirements
Concept and measures Bus-bar concept Multiple in-feed Branch configuration Standby facilities Un-interruptable supplies Fixed/draw out apparatus Choice of equipment Network layout Scope for expansion Equipment utilization Instrument transformer design Automatic/conventional control Remote/local control Construction/configuration Network layout Arcing fault immunity Lightning protection Earthing Touch protection Step protection Fire protection 8
4.4. Type of sub-stations 4.4.1. The types of Sub Stations depends upon: •
The availability of the land in the required place.
• Environmental conditions. 4.4.2. Sub-Station types are: •
Out door
• In door • Compressed Air insulated • GIS 9
4.5 Sub-Station Engineering • The
Sub Station Engineering comprises:
Sub-station site selection
Switching scheme. Bus-Bar. Safety clearances. Phase to phase clearances. Phase to ground clearances. Sectional clearance. Ground clearance. 10
4.5 Sub-Station Engineering(Contd) Yard levels. Single line diagram & Layout. Bus levels. First level ---- Equipment interconnection level. Second level ---- Bus levels. Third level ---- Cross Bus / Jack Bus level.
Bay widths Lightning protection. Earth mat. Civil Engineering works. Electrical Installation works. Main electrical equipments. Auxiliary supplies
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4.5.1. Sub station site selection • The aspects are to be considered for site selection Fairly level ground Right of way around the sub station yard for incoming & out going transmission & distribution lines Preferably of soil strata having low earth resistance values Easy approach & accessibility from main roads for Heavy equipment transportation and routine
O & M of sub station
Economy / Cost
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4.5.2. Switching schemes: • The factors considered for selection of switching schemes
Reliability factor Availability of the space Economics (project cost) There can be several combinations in which the equipments, busbars, structures etc. can be arranged to achieve a particular switching scheme.
The switching schemes can be made more flexible by making minor modifications like providing sectionalisers using bye-pass path etc.
The various types of switching schemes along with its advantages and disadvantages are:
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Switching Schemes Switching Scheme
Advantages
Disadvantages Bus fault or breaker failure causes station outage
Single busbar
Least cost
Maintenance is difficult No station extension works without complete shutdown
For use only where loads can be disconnected or supplied from another substation. Single busbar with sectionaliser Single main and transfer bus
Shut down on the part of the Bus can be availed Higher flexibility as compared to single bus One breaker can be taken for maintenance at a time High flexibility with two busbars of equal merit
Double main busbar
Each busbar can be isolated for maintenance Each branch can be connected to either of the bus with bus tie breaker The two buses can be individually operated in case of island operations
Aditional cost for the isolator Maintenance of main bus will involve outage of substation.
Additional cost for the Transfer Bus & Breaker Expensive for additional bus and BC breaker and associated equipments and also extra space is required One Breaker maintenance possible at a time. There will be a time delay for restoration of the circuit in case of breaker outage
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Lines Lines
Lines
Lines
Main Bus1 Main Bus Transfer Bus
Transformer
Main Bus2
Transformer
Transformer
Transformer
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Switching Schemes (contd) Switching Scheme
Advantages
Disadvantages
Double main bus with transfer bus
High flexibility with 3 buses and 2 tie breakers One breaker is available at a time for maintenance No time delay for restoration of the circuit in case of breaker outage.
Expensive consequent to additional two buses and two breakers with associated equipments and additional space is required.
Greatest operational flexibility High reliability Breaker fault on the busbar side disconnects only one branch 1½ Breaker system
2 breaker system
Ring bus
Three circuit-breakers with associated equipments required for two branches
Each main bus can be isolated at any time
Greater outlay for protection and auto-reclosure, as the middle breaker must respond independently All switching operations in the direction of both feeders executed with circuit-breakers Bus fault does not lead to branch disconnections Greatest operational flexibility Each branch has two circuit breakers Connection possible to either Most expensive method bus bar Each breaker can be serviced without completely disconnecting the branch High reliability Flexibility for breaker Breaker maintenance and any faults interrupt the maintenance ring Each breaker removable without disconnecting load Only one breaker needed per branch Auto-reclosure and protection fairly complicated Each branch connected to network by two breakers All change-over switching done with circuit-breakers & hence Area required will be more flexible
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Line
Line
Main Bus1
Main Bus2
Transfer Bus
Transformer
Transformer
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4.5.3.
Bus - Bars
• Selection of bus-bars Type of Bus Bar Sizes of Bus Bar
• Types of Bus –Bars Strung Bus / Flexible Bus
Rigid Tubular Bus • Strung Bus: The various Types of conductors used for Strung Bus are
All Aluminum conductor (AAC)
All Aluminum alloy conductor (AAAC) Aluminum conductor with aluminum alloy reinforced (ACAR) Aluminum conductor with steel reinforced (ACSR 22
• RIGID TUBULAR BUS. Rigid tubular conductors are also used in substations. Rigid tubular buses are more advantageous than the flexible conductors. • Sizes of Bus Bar The factors to be considered for selection of the Bus-Bar sizes are: Normal current carrying capability Short circuit heating with stand capability Surface gradient Corona free performance
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• Selection Criteria for Bus Sizes Electrical & Mechanical Stresses: The bus-bars must be designed for: • The operating current. • To withstand short-circuit fault currents. • The anticipated stresses on the bus-bars and their supports in the event of a short circuit must therefore be calculated.
Thermal stresses Bus bars including clamps and connectors are also stressed thermally under short circuit conditions.
The bus bar conductors/tubes are suitably sized / designed to with stand the short circuit currents not only mechanically, but also thermally.
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Case – Study for 1000 MVA 400/220 S/S • REQUIREMENTS: Normal full load current for 1000 MVA (2 X 500) capacity.
400 KV -- 1445 amps 220 KV -- 2625 amps. Short circuit heating withstanding capability:
Minimum cross sectional aluminum area required to with stand one KA for one second is 15.29 . For 40 KA for 1 sec For 31.5 KA for 1 sec
--- 610.7 sq mm --- 481 sq mm
Maximum Permissible conductor surface gradient -21 KV/cm. Permissible radio interference level --- 40 to 50 db
Considering the Example of Moose A.C.S.R. Characteristics. The characteristics of the ACSR Moose conductor are as follows. Normal Short circuit heating Current withstand capacity Conductor Surface carrying for 1 sec having gradient at KV/CM Sl Voltage system in capacity cross sectional area at 85 C of 529 mm Rph Yph Bph no KV 1. Single Moose 830 Amps *34.6 KA Aluminum area for with standing 1 KA /sec is 15.29 sq mm Kv system 2. 400 ph–ph 7 mtrs Ph–gr 8.0 mtrs a) Single moose
For 1000 MVA Transformer 1445 Amps Tw in moose is required
For 40 KA S.C.w ith standing capacity for 1 sec tw in moose is required for 400KV
34.54
b) Tw in moose w ith 450 mm conductor spacing
3. 220 KV system Ph-ph 5.0 mtrs. Ph-gr 5.5 mtrs a) Single Moose
15.82 For 1000 MVA Transformer 2665 Amps four moose is required
Radio interfere nce level db
41.34 20.81
24.4 14.2
139.11 47.84
For 31.5 KA S.C.w ith standing capacity for 1 sec single moose is required for 220 KV
11.94
14.16
13.32
38.76 26
•
REQUIREMENTS Normal full load current for 1000 MVA ( 2 X 500 )capacity 400 KV -- 1445 amps 220 KV -- 2625 amps
• Short circuit heating withstanding capability Minimum cross sectional aluminum area required to with stand one KA for one second is 15.29 sq mm. For 40 KA for 1 sec
---
610.7 sq mm
For 31.5 KA for 1 sec
--- 481 sq mm
• Maximum Permissible conductor surface gradient --- 21 KV/cm • Permissible radio interference level --- 40 to 50 db By the above it is found
Twin moose conductor is required for 400 KV.
Quadruple moose conductor is required for 220 KV main bus, bus coupler bay.
Twin moose conductor is required for 220 KV transfer bus, transformer & line bays.
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• Rigid conductor selection. Rigid conductors are selected based on the following criteria.
Normal current carrying capacity Short circuit heating withstand capability Surface voltage gradient Fiber stress in tube &Vertical deflection
The characteristics of 100 mm and 75 mm IPS aluminum tube are as follows:
Surface voltage Normal current Sl. Size of Outer dia. Internal dia. Aluminium carrying capacity gradient KV rms/cm mm area sq mm No. IPS mm at 850C *400 KV **220 KV 1 100 mm
114.2
97.18
2825
2665
18.08
11.63
2
88.9
77.93
1428
1775
21.89
13.98
75 mm
* 400 KV system: conductor height 8 mtrs, phase to phase spacing 7 mtrs. ** 220 KV system: Conductor height 5.5 mtrs, phase to phase spacing 4.5 mtrs. By the above it is observed that For 400 KV system 100 mm IPS tubes are required For 220 KV system 100mm IPS tubes are required 28
4.5.3.4 Fiber stress in tube & Vertical deflection
•
Aluminum tube should be capable to with stand the gravitational wind & short circuit forces.
•
The vibrations in aluminum tube are caused due to study wind blowing across the bus at right angles to aluminum tube span.
•
The fiber stress/bending stress of Aluminum tube depends upon the span of the Aluminum tube between two supports.
•
The vertical deflection also depends upon the span of tube and type of supports [i.e. Whether two ends are pinned (simple supported) or fixed, or whether one end is fixed and other is pinned].
•
The safe vertical deflection should be less than the half of the outer dia. of Aluminum tube.
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4.5.3.5 The maximum allowable span lengths are as follows
Size of Aluminum Two ends pinned or Both ends Fixed tube. simply supported. permissible Span length Permissible Span in mtrs 100 mm 11 **12.5 75 mm 9 **12.5 ** Maximum permissible to limit the fibre stress. The adequacy of span of Aluminium tubes has to be verified depending upon sub-station layout arrangement.
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4.5.3.5 The standard sizes of aluminum tubular bus and conductors generally used for different substations are as follows SL. No.
Voltage reference
Al. Tube
ACSR conductor
1
33 KV
50 mm
Coyote / Drake
2
66 KV
63 mm
Falcon / Twin Drake
3
110KV
75 mm
Falcon / Twin Drake
4
220 KV
100/75 mm
Single / Twin Falcon Twin / Quadruple Moose
5
400 KV
100 mm
Quadruple MOOSE
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4.5.4 Electrical safety clearances. The electrical , safety clearances to be adopted in substation are governed by following parameters. • Basic Impulse Insulation levels (BIL). • Basic Switching Impulse level (BSL). • IE Rules. • Allowances in tolerance in dimensions of structural work. • Safety margins for unforeseen errors. Based on the above, certain minimum clearances are defined for a given voltage class and the same are applied in substation
• The various clearances need to be defined.
Phase-to-earth clearance. Phase-to-phase clearance. Sectional clearance. Ground clearance. Equipment to equipment spacing 32
4.5.4.1. Phase to earth and phase to phase clearances The minimum phase to phase and phase to earth clearance for 400 KV, 220 KV and other voltage classes are based on the BIL & BSL values. BSL
Phase to phase clearance in mm
Phase to ground clearance in mm
1425 KVp
1050 KVp
4000
3500
220 KV
1050 KVp
460 KVp
2400
2100
3.
110 KV
550 KVp
230 KVp
1100
1100
4.
66 KV
325 KVp
140 KVp
630
630
5.
33 KV
170 KVp
90 KVp
320
320
Sl. No.
Voltage class
BIL
1.
400 KV
2.
The above mentioned clearances do not include clearance between the live and ground parts of equipments including bus post insulators for which insulation is prescribed as per relevant standards and guaranteed by the manufacturers and confirmed by type tests. 33
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4.5.4.2
Section clearance • Section clearance is the distance between two sections of substation, which
enables a person to work on one section
of a substation in a safe manner, while the other section is charged.
•
Section clearance is chosen in such a manner that phase to earth clearance is maintained between the live point and the approach of the working personnel with adequate margin.
•
In case of 400 KV: The phase to earth clearance of 3.5 meters. The approach of man is considered as 2.5 meters. Margin of 0.5 meters for unforeseen reasons like errors in erections, dimensions of tools and platforms etc.
Thus the section clearance is taken as 6.5 meters.
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The section clearance for all voltage classes shall be:
Voltage class in KV
Highest system voltage in KV
Minimum safety working clearance followed as per the design
Minimum safety working clearance as per rule no 64 of I.E.Rule1956
400
420
6500 mm
6000 mm
220
245
5000 mm
4300 mm
110
123
4000 mm
3500 mm
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72.5
3500 mm
3000 mm
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36
3000 mm
2800 mm 36
4.5.4.3. Ground
•
clearance
The ground clearance is the distance between ground level and bottom of any insulator in an out door substation.
• • •
•
This ensures that any person working in the area cannot touch or damage the insulators accidentally. This clearance is kept as 2.5 meters for all voltage levels. However in cases, where the vehicles and cranes are allowed inside a substation, the ground clearance for the equipment falling on both sides of the road are to be enhanced as the vehicles and cranes height is generally 3.5 meters. The minimum ground clearances between the live point & ground at the substation for the different voltage classes as per rule no 64 of I.E.Rule 1956 are as follows 400 KV 220 KV 110 KV 66 KV
8000 mm 5500 mm 4600 mm 4000 mm
33 KV
3700 mm
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4.5.4.4. Bus
levels
Generally in all the substations two / three level bus arrangements are necessary.
The first level is the equipment interconnection.
The second level is main buses 1 & 2, which may be Rigid / Strung Bus.
The third level cross bus / Jack Bus , required in few large sub stations.
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4.5.4.5. First
level : Equipment inter connection level
The first level height is fixed based on the following considerations • The bottom part of the insulator or top most part of the earth metal position should have the minimum ground clearance i.e. the height of the man standing on the ground with shoes on holding the tools and extending the arms upwards, which is already prescribed as 2.5 meters in all voltage class substations. • The insulator height / length as per I.E.C / I.S.S i.e., phase to earth clearances as prescribed for different voltage classes. • Live metal part height of the various equipments. • Maximum value of electrical field at a height of 1.8 meters i.e. height of an average person level.
The electrical field is the deciding factor not only for the height of the bus level but also for conductor configuration and phase spacing. It is generally considered that 10 KV per meter as safe design value of an electrical field for a period of 180 seconds. The effect of electrical field reduces with increase in bus level. 41
• 400 KV
System.
The calculated electrical field at a level of 1.8 meters from ground level with a bus level of 7 meters height is 12.11 KV per meter. The calculated electrical field at a level of 1.8 meters from ground level with a bus level of 8 meters height is 9.4 KV per meter.
• 220 KV System.
The calculated electrical field at a level of 1.8 meters from ground level with a bus level of 5.5 meters height, is 9.4 KV per meter.. The minimum bus level height for 400 KV is calculated as
•
As per IEC & ISS ---------- 2500 + 3500 = 6000 mm.
As per live metal part Ht of eqpt --- less than 6000 mm
As per electrical field ------------ 8000 mm
As per I.E. Rule also -------------- 8000 mm
• For 400 KV minimum first bus level shall be 8000 mm • For 220 KV minimum first bus level shall be 5500 mm. 42
4.5.4.6. Second
level: • Cross Bus levels are generally called second levels in a substation switchyard. • The height of this bus is decided / designed based on the following. Height of the equipment inter connection level i.e., first level.
The extension of top level live metal part of Bus post insulator / isolator with an expansion clamp.
The maximum sag of the conductor if it is a strung bus. Phase to phase clearance. Half of conductor / rigid bus diameter. Some minimum factor of safety ie some adequate margin
particularly to maintain minimum phase to phase clearance between main bus & cross bus. 43
4.5.4 Standard Bus levels (First levels) / Equipment inter connection level and second level ie cross Bus / main Bus levels for different voltage classes in a sub-station designed as per above principles are follows. Sl. Sl. No.
Voltage class
First level mm
Second level mm
1.
400 KV
8000
15000/22000 *
2.
220 KV
5500
11000
3.
110 KV
4500
9000
4.
66 KV
4250
8500
5.
33 KV
4000
8000
* Third level or Jack bus level in 400 KV stations 44
4.5.4.5
Equipment to equipment spacing.
The equipment to equipment spacing is decided based upon following factors.
•
Adequate clearances (phase to earth, phase to phase, section and ground clearances).
• •
Convenience of erection and security.
• •
Adjacent equipments should not foul physically while installing terminal clamps. Equipment foundations should not foul with each other and cable trenches. Technical requirements.
Location of surge arrestors with respect to protected equipments such as transformer and reactors.
Position of CVT, wave-trap and shunt reactor approaching from line side.
Maintenance flexibility
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4.5.5
Bay widths
The bay widths are chosen in such a way that the minimum clearances are maintained even when the isolator is kept under fully open condition with one end energised. The different types of the isolators like horizontal center brake, horizontal double brake, pantograph and vertical break has a great impact in deciding the Bay widths.
• Vertical brake isolator The bay width can be reduced, Hence this type of isolator is not generally used.
but
the
bus
height
increases.
• Pantograph isolator It requires fine adjustment of sag and too expensive. Bay widths & Lay out sizes can be reduced considerably. These type of isolators will be used in critical lay outs where space is criteria.
• Horizontal center break isolator This type is most commonly used isolator due to it’s low cost, and it will never be placed under the gantry as it will intrinsically demands higher clearances and the bay width has to be increased beyond the rational value.
• Horizontal double break center rotating isolator It is very rigid, good performance, less bay widths and lay out sizes can be reduced. But costlier compared to H.C .B.
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The factor considered for computing bay widths are • Phase to Ground / Earth clearance The distance between nearest point of tower from extreme phases Minimum Phase to earth clearance: as per IE Rule Maximum horizontally protruding live metal part from the center of the equipment Tolerances for execution Length of the isolator blade The movement due to swing of conductor and insulator string in case of strung 4.5.5.1
bus only and not applicable in case of rigid bus.
• Phase to phase clearance Length of the isolator blade Horizontally protruding live metal part of adjacent equipment Phase to phase clearance as per IE rule Tolerances for execution The movement due to swing of conductor and insulator string in case of strung bus only and not applicable for rigid bus system.
• Considering
tolerances for execution etc, the phase to phase and phase to
ground clearances for 400 KV is as follows
Phase to ground ---- 6500 mm Phase to phase ----- 7000 mm Bay width will be 6.5 + 7.0 + 7.0 + 6.5 = 27 meters 50
4.5.5. The standard bay widths, ground & sectional clearances based on above analogy for rigid and strung buses for different voltage classes of substations are as follows
Voltage class
Sl. No. 1. 2.
KV 33 66
3. 4. 5.
110 220 400
Rigid bus Strung bus Phase Phase to Phase to Phase phase to earth phase to earth Bay clearan clearan Bay clearan clearan Ground width ce in ce in width ce in ce in clearanc in mtrs. mtrs mtrs. in mtrs. mtrs mtrs. e in mtrs. 1.25 1 1.5 1.25 4.5 5.5 3.8 2 1.8 2.3 2 7.6 8.6 4 8.2 14 27
2.1 3.65 7
2 3.35 6.5
10.5 17 27
2.75 5 7
2.5 3.5 6.5
4.6 5.5 8
Sectio nal clearan ce in mtrs. 2.5 3 3.5 4.3 51 6.5
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4.6 Yard leveling • Complete switchyard shall be generally maintained at the same level
• • • •
to have sufficient ground clearances and easy for execution, operation & maintenance. But in some cases leveling of the complete switchyard is too expensive. There will be huge cutting and filling if there are large undulations of the site. In such cases 2 to 3 different levels can be maintained for different voltage classes viz. 400 KV, 220 KV, control room etc. Generally the levels of the switchyard will be mainly decided by balancing volume of earth cutting and volume of earth filling for economical considerations.
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4.7. Single
• •
line diagram & lay out design.
Draw a single line diagram also called key diagram, before the locations of various equipments in the substation are decided. This diagram indicates the proposed bus bar arrangement and relative positions of various equipments. There are numerous variations of bus bar arrangement.
• •
The choice of a particular arrangement depends on various factors viz. System voltage, position of the substation in the system, flexibility, expected reliability of power supply and cost. The following technical consideration must be borne in mind while deciding upon any one arrangement. Simplicity is the key note of a dependable system Maintenance should be easy with minimum interruption of supply Safety to the operating personnel Alternative arrangement should be available in the event of an outage on any of the equipments or sections of sub station The layout should not hinder for expansion and/or augmentation at a later date, to meet the future load growth The installation should be as economical as possible keeping in view of the requirements and continuity of supply 54
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4. 8. LAYOUT DESIGN The first task which a substation designer has to undertake after finalizing the single line diagram, bus switching scheme, bay widths, section & ground clearances, is to translate the selected scheme into a layout so as to physically achieve the feeder switching required for ease in erection and maintenance. 4.8.1. BROAD PARAMETERS Following are the broad parameters, which change from one substation to another. a) b) c) d) e) f)
Nature of bus bars i.e. Rigid or flexible ( Strung Bus) Orientation of bus bar Location of equipments Manner of inter-connections Structural arrangement Direct stroke lightning Protection
4.8.2. FACTORS INFLUENCING THE CHOICE The factors need to be considered while choosing a type of layout a) b) c) d) e) f) g) h)
Reliability Ease of construction and provision for extension Ease of operation and maintenance Safety of operating personnel Land requirement Safety of Equipment and installation Aesthetic look. Economy 60
4.9. SAFETY MEASURES: 4.9.1.Power Distribution. • The distribution of power and current is checked and the currents occurring in the various parts of the station under normal and short circuit conditions are determined. • The power flow to be balanced to an extent possible by properly locating th incoming / out going lines & Transformer bays.
4.9.2 Safety Measures The safety respect of a) b) c) d)
measures for the substation and its components are to be designed in Insulation co-ordination. Lightning Protection system. Safe Clearance. Thermal and mechanical stresses
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4.9.3.INSULATION – CO-ORDINATION Insulation coordination is the total of all measures taken to restrict flash over or break down of the insulation caused by over voltages at places with in an installation at which the resulting damage is as slight as possible. This is achieved by using lightning arresters to limit over voltages. The equipments are also to be designed to withstand lightning and switching surges. The nominal lightning impulse withstand voltage and power frequency withstand voltage for various voltage classes are as follows.
Maximum voltage of equipments
Nominal lightning impulse Peak value
Nominal power frequency withstand voltage RMS value
36 KV
170 KVP
90 KV
72.5 KV
325 KVP
140 KV
123 KV
550 KVP
230 KV
245 KV 1050 KVP 460 KV 420 KV 1425 KVP 1050 KVP* * Nominal Switching impulse with stand voltage 62
4.9.4
LIGHTNING PROTECTION:
In H.V.& EHV substations, the protection from the lightning is done either by shield wire or lightning mast (high lattice structure with a spike on top) and sometimes combinations of both depending upon type of layout of substation.
• Shield wire Shield wire lightning protection system will be generally used in smaller sub stations of: Lower voltage class, where number of bays are less, area of the substation is
•
small, & height of the main structures are of normal height. The major disadvantage of shield wire type lightning protection is, that it causes short circuit in the substation or may even damage the costly equipments in case of its failure (snapping ). Lightning masts (LM) This type of protection will be generally used in large, extra high voltage sub stations where number of bays are more. It has the advantages,
It reduces the height of main structures, as peaks for shield wire are not required It removes the possibility of any back flashover with the near by equipments/structure, etc.during discharge of lightning strokes Provides facility for holding the lightning fixtures in the substation for illumination purposes Aesthetic look.
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4.9.4.1 PROTECTION ZONE BY SHIELD WIRE. • The height of equipment to be protected by shield wires depends upon the height of the earth wire and distance between them.
•
•
As per experiment it has been found that, the displacement of the electrode from shield wire at a distance of “B”=2h where ‘h’ is the height of the shield wire, all the discharges will strike the shield wire, and protects all the equipments from lightning discharges in the zone. There fore for two shield wires at a distance of “S”= 4 x h between them (“h” is the height of the shield wire), the point situated on ground surface mid way will not be struck by lightning.
•
Similarly for protection of any equipment of height “h0”, the distance between shield wire “S”, shall not be more than 4 times effective height ( hh0 ), i.e., the difference of height between. shield wire and the object to be protected
“S” = or < 4(h-h0) ho = or < (h-S/4) ho the height of equipments shall be = or < h – S/4
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• 400 KV switch yard. 400 KV bay width = 27 mtrs. Hence Shield wire distance ‘S” is 27 mtrs, apart.. Height of shield wire “h” = 23.5 mtrs. Maximum height of equipments which can be protected by these two shield wires are (23.5 – 27/4) = 16.75 Mtrs The height of the main bus level in the 400 KV station is15.00 Mtrs and all the equipments will be with this level only. Hence the shield wires provided on the peaks of the bus structures will protect all the equipments in the respective bays.
• 220 KV switchyard. Bay width ------ 17 Mtrs. Height of shield wires ------ 19 Mtrs Maximum height of equipments which can be protected by these two shield wires are (19 – 17/4) = 14.75 Mtrs The height of the main bus level in the 200 KV station is13.50 Mtrs and all the equipments will be with in this level only. Hence protects all the equipments in the bay. 65
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4.9.4.2 SELECTION OF LM HEIGHT The factors to be considered
•
The height of the LM will be decided, depending upon the height of equipment to be protected
•
The protection zone or coverage area of LM increases with the increase of its height
•
Hence LM’s height depends upon the height of equipment to be protected
•
The protection zone of same LM would be more if the equipment height to be protected is less
•
The numbers of lightning masts in substation can be reduced by increasing the height of LM, but this will cause increase in cost of structure and civil foundations.
•
The detailed analysis and experience revealed that 30 mtr. LM height is economical proposition & hence to be limited to this height.
69
4.9.4.3. LOCATION & NUMBER OF LIGHTNING MASTS The exact number and locations of LMs will be calculated for complete protection of equipment in substation, by considering the following aspects.
• • •
•
The protection zone of one LM is very limited. In case of two LMs, the protective zone is considerably more than the sum of protective zones of two single LMs. A point of height ho situated midway between two lightning masts of height h can be protected if the distance ‘a’ between LMs is not more than a seven times of active height ( i.e. difference of height between a LM height h and height of equipments to be protected ho ) or a = or < 7(h-ho). In case of 3 LMs forming triangle or 4 LMs forming rectangle, of height h can protect the object of height ho situated inside the triangle or rectangle if diameter D of the circle passing through the tips of LMs is not more than 8 times the active height i.e. D < = 8 (h-ho). 70
71
4.10.
EARTH MAT REQUIREMENT The main objectives of earthing system in the substation are: • To ensure that a person in the vicinity of substation is not exposed to danger of electrical shock • To provide easy path for fault currents into earth under fault condition without affecting the continuity of service • Hence intentional earthing system is created by laying earthing rod of mild steel in the soil of substation area. • All equipments/structures which are not meant to carry the currents for normal operating system are connected with main earth mat • The earthing system in a substation serves :
Protects the life and property from over-voltage To limit step & touch potential to the working staff in substation Provides low impedance path to fault currents to ensure prompt and consistent operation of protective device
Stabilizes the circuit potentials with respect to ground and limit the
overall potential rise Keeps the maximum voltage gradients within safe limit during ground fault condition inside and around substation
72
4.10.1 SELECTION
OF EARTHING CONDUCTOR SIZE FOR MAIN EARTH MAT
The selection of earthing conductor is based on
•
The thermal stability criteria
•
Jointing method
Welded with maximum temperature rise of 6200C Bolted with maximum temperature rise of 3100C
•
Magnitude of short circuit fault current & its duration
73
4.10.2. FORMATION
OF SUBSTATION EARTHING:
•
The main earth mat shall be laid horizontally at a regular spacing in both X & Y direction based upon soil resistivity value and substation layout arrangement.
•
The main earth mat shall be designed to limit the following;
Touch Potential – The potential difference between two points, one on
the ground where a man may stand and any other point which can be simultaneously touched by either hand.
Step Potential
– The potential difference between any two points on ground surface which can be simultaneously touched by feet.
Maximum
ground mat resistance shall be less than 1.0 ohm for substations of 220kV class and below, and shall be 0.5 ohms for 400kV and above voltage class.
The
earth rods shall be capable of with standing short circuit current for specified period.
For I KA SC current for 1 second the minimum cross sectional area of M.S. Rod / Flat shall be 12.16 sq mm with welded joints. 74
•
The crushed rock (Gravel) of 15 mm to 20 mm size shall be used as a surface layer of 150 mm in the substation for the following reasons: To provide high resistivity for working personnel To minimize hazards from reptiles To discourage growth of weed To maintain the resistivity of soil at lower value by retaining moisture in the under laying soil To prevent substation surface muddy and water logged. • The main earth mat shall be laid at a depth of 600 mm from ground. • The earth mat shall be connected to the following in substation
i. ii. iii. iv. v. vi. i.
Lightning down conductor, peak of lightning mast Earth point of S A, CVT Neutral point of power Transformer and Reactor Equipment framework and other non-current carrying parts. Metallic frames not associated with equipments Cable racks, cable trays and cable armour
75
5.0. INSULATORS.
Types
•
Disc type
Post type.
a) Disc type These are required for stringing the ACSR conductor for main bus and jack bus/cross bus. The individual units are rated for 11 KV and string of these units will be used for deferent voltage classes. The number of units per string depends on following parameters. i) System voltage.
ii) iii) iv) v)
Insulation level Power frequency withstand level Tensile strength Purpose – tension string or suspension string.
76
Typical details of string used for various system voltages
No. of units per string System Tension Suspen voltage strength Tension sion in KV in KN string string 400 160 25 400 120 23 220 120 16 220 90 14 110 90 8 8 66 90 5 5 33 70 2 2 77
b)
Post type insulators These are used for supporting ACSR conductor or Rigid Aluminum tube for connecting main bus to equipment or forming main bus. These are also used as supporting insulators for isolators. There are two types i. Pedestal post or stacking type ii. Solid core type The solid core type are preferred The design considerations are, i) The phase to earth clearance which determines the height
ii) Insulation level iii) Power frequency withstand level iv) Mechanical strength i.e., mainly cantilever strength v) Minimum creepage dimensions
78
Typical parameters for various voltage levels
Minimum creepage System voltage in Height of stack Nol of units per dimensioninmm Cantilever at 25 mm/KV strength KN KV (mm) stack 400 3650 3 10500 8 220 2300 2 6125 6 110 1220 1 3075 4.5 66 770 1 1815 4.5 33 380 1 900 4 79
6.0 Steel structures:
i) Towers and Beams: Required for stringing the ACSR conductor for main bus and cross bus/jack bus
ii) Lightning masts: Required for providing protection against lightning and installing the luminaries fitting for illumination of switchyard. iii) Support structures These are required for supporting the equipments and post insulators to maintain the live point heights and other clearances as per the statuary clearance requirements. 80
6.1. The steel structures can be classified broadly into two groups i) Lattice type Formed by mild steel angle sections/plate sections etc by fastening the various sections by bolts, nuts or by welding. ii) Tubular type Formed by using mild steel pipes. These are preferable for support structures for lightning arrestors, post insulators, & instrument transformers. etc. 6.2
Protection against corrosion The steel structures are generally made of mild steel, which are galvanised / painted to protect against corrosion. Galvanising by applying zinc coating is preferable as the protection achieved is superior to painting & maintenance free. The coastal areas where due to saline weather conditions – corrosion phenomenon occurs very fast and hence only galvanising is recommended.
6.3
Design considerations a)Towers i) Wind load ii) Reactions of loads on the beams to which conductor is (along with insulator) strung iii) Vertical loads on the beams like under strung isolators iv) Tension of conductor (if strung directly ) and ground wire. v) Short circuit forces vi) Type of foundation – Stub type or anchor bolt type 81
b) Beams i) Tension of conductor ii) Wind load / weight of conductor and insulator string iii) Vertical loads due to under strung isolators /post insulator etc. iv) Short circuit forces v) Configuration of conductors.
c) Lightning masts (25.0 to 30.0 meters height) i) Wind load ii) Weight of luminaries fittings.
d) Support structure i)
Weight of equipment/post insulator
ii) Wind load on conductor iii) Load due to aluminum pipe and cantilever strength of post insulator
82
7.0.
ILLUMINATION The indoor & out door areas of sub station are to be properly illuminated. The minimum lux levels to be maintained in the different areas are follows.
Sl No
Location in sub station
1
Control Room
Minimum lux levels to be provided 350
2
L.T.Room.
150
3
CableGallery
150
4
Battery Room
100
5
Entrance Lobby
150
6
Corridor Landing
150
7 8
Conference Room Display Room Rest Room
9
Out Door Switch Yard
& 300 250 Main Equipment -- 50 Balance area. -- 30
10
Street / Road
30 83
7.1.
The aspects to be considered are
•
The illumination design to be done by using the software program
to achieve
specified levels of illumination most
economically
•
The energy conservation methods are to followed by using CFL fittings etc.
where ever feasible with out compromising on
required illumination levels
84
8.0 Classification
of the works to be executed in a sub-station
a)
Civil engineering works
b)
Electrical works
8.1. Civil engineering works: The Civil works comprise of 1. Buildings
i. Residential ii. Non Residential – Office, control room, repair bay etc. 2.
Design & construction of foundations for structures and equipment structures and transformer plinth.
3. Cable trenches 4. Fencing around switchyard 5. Water supply 6. Drainage & Sewerage 7. Roads & paths 85
8.2.
Electrical works comprise of: a) Choice of: i. Switching schemes. ii. Bus bars. iii. Preparation of key diagram / single line diagram. iv. Preparation of Lay outs
b) Design & layout of earthing grids and protection against direct lightning strokes.
c) Auxiliaries i. D.C. supply • Battery set. • Battery Charger • D.C. Panel ii. A.C. supply • Auxiliary Distribution Transformer.
• •
Diesel Generator set. A.C.Panel.
iii. Control cable & power cable schedule. iv. Switchyard lighting v. Fire fighting equipment.
86
Major Sub-station Equipments a) b) c)
Power Transformers. Circuit breakers. Instrument Transformers: i. ii. iii.
d) e) f) g) h)
Current Transformers. Voltage transformers. Capacitor voltage transformers
Isolators / Disconnects. Lightning Arrestors. Control & Relay panels. Shunt Capacitor Banks. Reactors.
Technical parameters a)The
principle points to be considered for selecting sub-station equipments are • Standards. • Principle parameters • Ratings & their choice. • Technical requirements. • Tests.
Standards - Power Transformers Sl.No
Standards
Title Code of practice for selection, installation & maintenance of transformers (P1:1993), (P3:1991)
1.
IS – 10028 (Part 2 & 3)
2.
IS – 2026
3.
IEC –76 (Part1 to Part 5)
4.
Dimensions for porcelain IS-3347 (Part 1 to Part 8) transformer bushings for use in lightly polluted atmospheres.
5.
IS-3639(1991)
Fittings and Accessories for power transformers
6.
IS – 6600 (1991)
Guide for loading of oil immersed transformers
7.
IEC-354 (1991)
Loading guide for oil immersed power transformers
8.
IEC-214 (1989)
On-load tap – chargers
9.
NEMA – TR – 1
10.
Power transformers
Transformers, Regulators and reactors CBIP Manual on Transformers
Principle Parameters -Power Transformers Sl. No. 1.
Item
Examples of Technical requirements
Type of transformer/installation
power 3 phase, auto winding 3 phase core type winding interconnecting transformer transformer suitable for out suitable for outdoor installation. door installation -----On wheel mounted on rails----
2.
Type of mounting
3. 4.
Suitable for frequency Rated voltage
5.
Voltage ratio HV/IV/LV No. of phases
6.
No. of windings
7.
Type of cooling
8.
Maximum rating MVA 220 KV winding HV 110/66 KV winding IV 11 KV tertiary winding LV
9.
10.
11.
rated
system
50 Hz
50Hz
245 KV class
245 KV class
220/110/11 KV Three
220/66/11 KV Three
Auto Tr. With tertiary
Three winding with tertiary
OFAF
OFAF
100 100
100 100
60 80 100 HV Star
60 80 100 HV Star
IV Auto
IV Star
LV Delta
LV Delta
YNaod 11
YNynod 11
MVA rating corresponding to cooling system. ONAN cooling a) b) ONAF cooling c) OFAF cooling Winding connection
Connection symbol (vector group)
Principle Parameters -Power Transformers Sl. No. 1.
Item Type of transformer/installation
Examples of Technical requirements power 3 phase, auto winding 3 phase core type interconnecting transformer transformer suitable suitable for outdoor installation. door installation -----Effectively solidly earthed-----
12.
System earthing
13.
Percentage impedance voltage on normal tap and at rated MVA tolerance as per IS-2026. HV-IV a) b) HV-LV c) IV-LV
14.
15.
16.
Anticipated continuous loading of windings a) HV and IV b) Tertiary Tap changing gear : a) Type b) Provided on
10 10 The tertiary winding is for stabilising purpose without loading. The impedance shall be designed in confirmity with BIS, to with stand the short circuit currents for a specified period.
------ Not to exceed 110 % of its rated capacity --------------------------- Un loaded teritiary ---------------On load, suitable for bi-directional power flow --------Neutral end of HV winding -------
c)
Tap range
+5% to –15% +5% to –15%
d)
Step voltage
1.25% of 220 KV
e)
No. of steps
Over voltage operating capability & duration
18. 19.
i) ii)
Max. flux density in any part of core and yoke at rated MVA, frequency and normal voltage (tesla) Current density of HV/IV/LV winding Insulation levels for windings: a) 1.2/50 micro-second wave shape impulse withstand (KVP)
b) power frequency voltage withstand (KV rms) 20.
Type of winding insulation: a) HV winding b) IV winding c)
LV winding
1.25% of 220 KV
16 16
iii) 17.
winding for out
115% of rated voltage continuously 125% of rated voltage for 60 seconds 140% of rated voltage for 5 seconds
----------------1.60---------------------------not exceeding 3 Amps per sq. mm----------
HV
HV
IV
LV
950
950
325
170
HV
HV
IV
LV
395
395
140
70
Graded
Graded
Graded
Graded
Full
Full
Principle Parameters -Power Transformers Sl. No. 1.
21.
22. 23.
24.
Item
power 3 phase, auto winding 3 phase core type winding interconnecting transformer transformer suitable for out suitable for outdoor installation. door installation System short circuit level & duration for which the transformer shall be capable to withstand thermal and dynamic -------------40 KA for 3 seconds -----------stress (KA rms/sec) Noise level at rated voltage and Less than 83 db frequency ---- as per table 01 of latest NEMA std. TR-1-----Permissible temperatures rise over ambient temp. of 500 C i) Of top oil measured by 50 0C 50 0C thermometer ii) Of winding measured by 55 0C 55 0C resistance method Minimum clearance in air (mm). a) H.V. 2000 2000 HVPhase to Phase i) ii) Phase to ground 1820 1820 b) i) ii)
I.V.
c) i) ii) 25.
Examples of Technical requirements
Type of transformer/installation
Phase to Phase Phase to ground
1430 1270
700 660
L.V. Phase to Phase
350
350
Phase to ground
320
320
245 KV class OIP condenser
245 KV class OIP condenser 72.5 KV class OIP condenser bushing
Bushings a) HV winding Line end b) IV winding line end c) HV/IV winding neutral end (for solid grounding) d) LV winding
145 KV class OIP condenser bushing --------36 KV porcelain bushing-------
--------36 KV porcelain bushing-------
Principle Parameters -Power Transformers Sl. No. 1.
Item Type of transformer/installation
Examples of Technical requirements power 3 phase, auto winding 3 phase core type winding interconnecting transformer transformer suitable for out suitable for outdoor installation. door installation OIL OIL
26.
Insulating medium
27.
Terminal current rating HV
800 Amp
800 Amp
IV
800 Amp
1250 Amp
LV
2000 Amp
2000 Amp
800 Amp
1250 Amp
HV/IV Winding neutral 28.
Max. Radio interference voltage level at 1 MHz and 1.1 times max. rms phase to ground voltage for HV winding
29.
Cooling equipments. a) b)
Number of Banks No. of pumps
c) No. of fans 30.
------------ 5000 Micro volts-------------Two nos. of 50% Bank Two nos.of 50% Bank One 100% pump & one 100% One 100% pump & one 100% standby pump in each bank standby pump in each bank Adequate number of fans 18”/24” sweep with one stand by fan in each group
Insulation level of bushings:
HV
a) Lightning impulse withstand (KVP) b) 1 minute power frequency
1050
IV
LV
550
170
HV
IV
LV
460
230
HV 1050 HV
IV
LV
325
170 IV
LV
withstand voltage (KV rms) c) Creepage distance (mm) 31.
a) Bushing current transformers for tertiary provided in each phase i) Current Ratio (A/A) ii) Accuracy class iii) VA Burden
70
25 mm per KV system voltage
of highest
460 140 70 25 mm per KV of highest system voltage
--------- To be provided for LV bushings------1000/1
1000/1
-----------5P – 20------------ ------------15--------------
Standard Rating -Power Transformers Sl. No.
KV CLASS
RATING
1
33/11 KV
5 MVA
2
66/11KV
8 MVA 12.50MVA
3
a.
110/11 KV
b. 110/33-11KV 4
a.
5
b. 220/66/11 KV a. 400 / 220 /33 KV
220/110/11 KV
b. 400 / 220 / 33 KV 3 Units of single phase transformers.166 MVA
16/20 / 31.5 MVA 10 MVA 16/ 20 / 31.5 MVA 10 MVA 16/20 / 31.5 MVA 100 / 150 MVA 100 / 150 MVA 315 MVA 500 MVA
Tests -Power Transformers TESTS: a) Type tests. b) Routine / Acceptance Tests
Type Tests i.Temperature rise test ii. Vacuum test on transformer tank iii.Relief device test iv.Short circuit test v Impulse test on principle tap. vi. IP-55 test for OLTC cabinet and cooler control cabinet.
Tests -Power Transformers Routine tests: i.Operation and dielectric test of OLTC ii.Magnetic circuit test iii OC & SC tests iv. Oil leakage test on transformer tank after complete assembly. v. Measurement of zero sequence and reactance vi Measurement of acoustic noise level vii Measurement of power consumption by fans and oil pumps viii. Measurement of harmonic level on no-load current ix.Measurement of capacitance and tan-delta to determine capacitance between winding and earth before and after series of di-electric tests. x. Insulation resistance test xi. Ratio and polarity test xii.Di-electric and PPM test on oil xiii Tan-delta test on bushing xiv. Measurement of copper and iron losses. xv Leakage tests for radiators / cooler tanks xvi Weld test
Discussions
97
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