SWITCHYARDOF

A 500MW POWER PLANT

EARTH

NGT & NGR21KV / 220 V175 KVA0.212 OHMS 800 A

GCB

3 NOS.GEN TRF 1 BANK400 / 21 kV 200 MVA

UNIT TRF21 / 11.5 KV

50 MVA

LINECT’s

NEUTRALCT’s

A UT - B

400 KV

21 KV

11 KV

11 kV 1 BA SWGR 11 kV 1 BB SWGR

ISOLATED PHASE BUSDUCT FOR GENERATOR AND TRFS.

LAVT 4

SCVT 1,2,3

EARTH SWITCH OF GENERATOR

CT

GENERATOR

Salient Features of the Project

•Total Capacity - 500MW•Generating Voltage - 21kV•Transmission Voltage - - 400kV & 220kV•400kV Switchyard - One & Half Breaker

Scheme•220kV Switchyard - Two Main one Transfer

Bus Scheme

What is a Switchyard ?It is a switching station which has the following credits : (i) Main link between Generating plant and

Transmission system, which has a large influence on the security of the

supply. (ii) Step-up and/or Step-down the voltage levels

depending upon the Network Node. (iii) Switching ON/OFF Reactive Power Control

devices, which has effect on Quality of power.

Switchyard TypeConventional Air Insulated Type.Gas Insulated type.Outdoor Gas Insulated type.

Switchyard layout Objective: Substation layout consists essentially in arranging

a number of switchgear components in an orderly pattern governed by their function and rules of spatial separation as described in electrical single line diagram.

Pre-requisites: 1) single line diagram 2) general layout plan of power plant 3) orientation of line evacuation 4) control room building

LAYOUT CONTD… Options / Alternatives The layout will vary for the following:1) Switching schemes 2) Type of insulation - Air Insulated/Gas

Insulated.

SWITCHYARD EQUIPMENTSEquipments commonly found in switchyard :1. Lightening arrestor2. Current transformer3. Voltage transformer4. Power transformers / I.C.T.5. Bus bar and clamp fittings6. Support structure7. Isolators8. Circuit Breaker9. Wave traps10. Earthing switch

Functions of various equipment :

* Transformers : - Transforms the voltage levels from higher to

lower level or vice versa, keeping the power constant. * Circuit breakers : - Makes or automatically breaks the electrical

circuits under Loaded condition. * Isolators : - Opens or closes the electrical circuits under

No-load conditions. * Instrument transformers : - For stepping-down the electrical parameter

(Voltage or Current) to a lower and safe value for Metering and Protection logics.

* Earth switch : - Used to connect the charged body to ground to

discharge the trapped charge to have a safe maintenance zone.

* Lightning arrestors : - Safe guards the equipment by discharging the high

currents due to Lightning. * Overhead earth wire : - Protects the O/H transmission line from Lightning

strokes. * Bus bar : - Conductors to which a number of circuits are

connected. * Wave Traps/Line traps : - Used in PLCC circuits for Communication and

telemetering. * Reactive Power control devices : - Controls the reactive power imbalance in the grid by

switching ON/OFF the Shunt Reactors, Shunt Capacitors etc., * Current Limiting Reactors : - Limits the Short circuit currents in case of faulty

conditions.

EXECUTION SEQUENCE:- Execution sequence for a substation

Tower foundation Equipment foundation Laying of Cable trench Laying of Earthmat Support structure installation High level stringing Equipment installation Equipment interconnection Cabling layout

Commissioning

TABLE I: INSULATION LEVELS & CLEARANCE REQUIREMENTS AT DIFFERENT VOLTAGE LEVELS

NOMINAL SYSTEM VOLTAGEKV

INSULATION LEVELS HIGHEST SYSTEM VOLTAGE KV

MINIMUM CLEARANCE GROUND CLEARANCE(MM)

SECTIONAL CLEARANCE (MM)

HEIGHT OF SUPPORTS (mm)LIGHTNING

IMPULSE LEVEL(kVp)

SWITCHING SURGE LEVEL(kVp)

POWER FREQUENCY IMPULSE LEVEL(kVrms)

BETWEEN PHASE AND EARTH(MM)

BETWEEN PHASES(MM)

3366

132220400765

170325650

105014252100

----

10501550

275460630830

3672.5145245420800

320630

130021003500

--

320630

130021004000

--

37004000460055008000

--

28003000350043006500

10300

250025002500250025002500

Clearance contd…5) Equipment spacing

a) Ease of maintenance/removal of equipment.

b) Equipment foundation & their cable trenches.

c) Distance between LA and equipment based on the

protection reach of LA. d)The spacings are generally kept

in order to achieve various clearances specified at Table-

I.

Clearance contd…6) Bus bars:

The bus bars of 400 kV switchyard are generally made up 4 “IPS aluminum tube or Quad Moose rated for 3000 A”.

The bus bars of 220/132kV switchyard are generally made up of 3 “IPS aluminum tube or quad/ twin moose conductor”. Bus bars are placed at right angles to the feeders for tapping the power.

7) Equipment Interconnection

8) Spacer spans and locations

9) Connection Level

10) Land & Road Layout

11) Sequence and mounting of line traps

Clearance contd….12) Control Room Layout

13) Lighting System

14) Cabling Philosophy

15) Gravel Filling

16) Earthing System

17) Lightning Protection System

Selection of Bus Switching Scheme PRE-REQUISITES

1)System security2)Operational flexibility3)Simplicity of protection arrangements4)Ability to limit short circuit levels (ease

of sectionalizing)5)Maintenance – Its effect on system security 6)Ease of extension7)Total land area8)cost

EVOLVING A SUBSTATION LAYOUT

LAYING OUT A SUBSTATION INVOLVES STEP-BY-STEP PROCEDURE. MOST IMPORTANT POINTS TO BE CONSIDERED ARE BRIEFLY DESCRIBED BELOW:

THE IMPORTANT ELECTRICAL PARAMETERS ARE ESTABLISHED BY THE SYSTEM DESIGN. THE MAIN PARAMETERS ARE:

1) THE VOLTAGE AND BASIC INSULATION LEVEL OR SWITCHING SURGE LEVEL., THE SITE AND

CLIMATIC CONDITIONS, THE METHOD OF CIRCUIT CONNECTION, AND SWITCHING OVER-VOLTAGE CONDITIONS.

2) THE BUS BAR SYSTEM DIAGRAM, THE NUMBER OF CIRCUITS AND THEIR PURPOSE I.E. THE CONTROL OF GENERATORS, TRANSFORMERS, FEEDERS, ETC.

THE DIAGRAM SHOULD INCLUDE DETAILS OF EXTENSIONS AND FUTURE CONVERSION TO A DIFFERENT BUS BAR SYSTEM, IF INTENDED.

EVOLVING A SUBSTATION LAYOUT

1) THE CONTINUOUS CURRENT RATING OF THE BUS BARS AND CIRCUITS.

2) THE SHORT CIRCUIT RATING OF BUS BARS AND EQUIPMENTS.

3) PARTICULARS OF REACTORS, NEUTRAL EARTHING EQUIPMENT AND REACTING, Interconnecting Transformers REQUIRED.

4) METHOD OF CONNECTION OF CIRCUITS, WHETHER BY OVERHEAD LINES OR BY CABLES.

5) DETAILS OF LIGHTNING PROTECTION EQUIPMENT.

6) DETAILS OF PROTECTIVE EQUIPMENT, DETERMINING THE INSTRUMENT TRANSFORMERS REQUIREMENTS, CARRIER CURRENT EQUIPMENT ETC.

EVOLVING A SUBSTATION LAYOUT

THE EXTENT TO WHICH CIRCUIT AND BUSBAR OUTAGES FOR MAINTENANCE WILL BE POSSIBLE.

SOME PARAMETERS WHICH INFLUENCE THE FORM OF THE LAYOUT ARE DETERMINED BY THE LOCAL CONDITIONS. THESE ARE:

1) THE AVAILABLE LAND AREA, SITE AND CLIMATE CONDITIONS, PLANNING AUTHORITY REQUIREMENTS AND AESTHETIC CONSIDERATIONS DETERMINE THE TYPE OF SUBSTATION.

2) THE DIRECTION OF OVERHEAD LINE ENTIRES POSITION AVAILABLE FOR TERMINAL TOWERS, LOCATION OF TRANSFORMERS AND REACTORS, ETC.

3) THE AVAILABILITY OF MATERIALS AND THE TRANSPORT AND ACCESS FACILITIES.

4) THE CAPABILITY AND SKILL OF THE MAINTENANCE STAFF DETERMINES THE IMPORTANCE OF CLARITY OF LAYOUT AND SIMPLICITY OF MAINTENANCE ZONING.

GUIDELINES FOR MAINTENANCE OF OIL PIT FOR TRANSFORMERS AND REACTORS

1.0 INTRODUCTION: The layout for a transformer or reactor is planned in such away that there is adequate

oil drainage facilities from underneath the equipment. This is essential in order to prevent catastrophic damage to nearby building/ equipments, if the transformer fire takes place and the oil is accumulated below the equipment due to explosion of the transformer/ reactor tank. The oil pit needs to be cleaned at a regular interval so that the oil drainage path is not blocked and in case of explosion, the oil is freely drained to the main oil pit. This regular cleaning is essential because at one of the site, although the oil pit was there below the transformer tank, its drainage was chocked and the transformer fire was accelerated since the accumulated oil in the pit also caught fire.

2.0 TYPES OF OIL PIT: There are two types of oil pits in practice which are made below transformers/ reactors

depending upon location, size and oil quantity etc. These are: Soak Oil pit. Drain and Retention Oil Pit. SOAK OIL PIT: If the oil pit provided below the transformer/reactor is not connected with the oil pit of

any other equipment or main oil pit. it is classified as soak oil pit. The total volume of this individual soak oil pit is designed in such a way that volume of soak oil pit up to gravel filling level minus the volume of gravels should at least be equal to the oil volume in the transformer. The details of this type of soak oil pit are shown in figure-1.

SOAK OIL PIT

DRAIN AND RETENTION OIL PIT:

For the transformer or reactors located in the transformer yard i. e. unit aux. transformers/station aux. transformers/generator transformers and other transformers of 25 MVA and above rating, individual oil drain pits are provided and these individual oil pits are connected to one common retention oil pit for oil/water separation as these transformers are provided with mulsifire system and in case of fire, the mulsifire system will spray water, which will occupy the empty volume available in retention oil pi t. Also the transformers separated by fire walls and having oil quantity of more than 5000 liters are provided with individual oil drain pits which in turn are connected to one common retention oil pit. The drain oil pit is shown in figure-2.

RETENTION OIL PITThe retention oil pit has two interconnecting

chambers. The first chamber is called main chamber and the second chamber is called separation chamber. These two chambers are interconnected with a pipe. In case of fire, the oil-water mixture comes to the main chamber where the pipes from drain oil pit of individual transformers are connected. From here, by virtue of difference in specific gravity, the water is separated and flows to separation chamber. The separation chamber is connected to surface water drain to which the water is drained. The arrangement is shown in figure-3.

RETENTION OIL PIT

After the water is removed, the waste oil is pumped out to waste oil tankers. Under normal conditions, the retention oil pit is filled with water.

The total volume of main and separation chamber is sized to contain total oil volume of the largest transformer plus the volume of water sprayed during 10 minutes of mulsifire operation.

RECOMMENDATIONS:Due to oil seepage and accumulation of dust in the oil

pit, it becomes sticky substance like paste and has a tendency to clog the loose gravels in such a way that the oil/water cannot flow freely from the drain oil pit to retention oil pit. Further, due to such clogging, the designed volume of oil pit reduces, endangering into catastrophic fire. Under these circumstances, the purpose of oil pit for which it is designed is lost.

Design Philosophy / Practice For designing a switchyard layout, various aspects are

considered which are described here under: Space around the switchyard. Adequate space should be

provided for extension of the switchyard facilities when generating units or transmission lines are added in the future. The immediate surroundings should permit the routing of lines to the switchyard area from at least one direction without the need for heavy dead-end structures in the yard.

Switchyard location. The switchyard should be sited as near to the main plant as space permits, in order to minimize the length of control circuits and power feeders and also to enable use of service facilities located in the main plant.

Switchyard fencing. A chain link woven wire fence not less than 3.0 m height above toe wall, with lockable gates, should be provided to enclose the entire yard from safety point of view.

CLEARANCE:- Clearance. The placement of equipment in EHV switchyard is

greatly affected by the air clearances to be adopted. They are as follows:

Earth Clearance: this is the clearance between live parts and earthed structures, walls, screens and ground. A minimum height of live conductors above ground must also be maintained as per IE rules. Also there should be a certain minimum height of supports of various equipment (depending upon the fact that the bottom of any insulator has to be 2.5 meters above the ground level as statutory clearance).

Phase Clearance: this is the clearance between live parts of different phases.

Isolating Distance: this is the clearance between the terminals of an isolator and the connections thereto.

Section Clearance: this is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform where a personnel has to carry out work.

Mounting of Line traps:- Sequence and mounting of line traps The sequence of installation of line traps,

lightning arresters, coupling capacitors is given below for any line feeder:-

From line end: BPI, LA, CVT, Line Trap.

A bus post insulator is installed at line end to avoid mechanical forces on LA.

Mounting of the line trap shall be of pedestal type

Control room layout&lighting Control Room Layout A Control room building has to be built in the switchyard. Its location

should be such as to minimize control cables between switchyard and main plant. Also it should have easy approach from main road and switchyard view should be possible from the control room. The control of the switchyard is either form the switchyard control room or the main plant control room. Al the panels for the purpose are to be located in this control room. For control of the switchyard please refer separate guidelines.

Lighting System Switchyard lighting consist of outdoor lighting of the

yard and lighting of Switchyard control room building. The outdoor lighting is to be done with 400W HPSV lamps. The various lighting fixtures complete with lamps and accessories shall be mounted only on Lightning Masts or Lighting poles. There should be no Lamp suspended on gantries or any live structure of the switchyard as per IE rules. The lighting along the road shall be achieved by providing suitable fixtures on lighting poles. Different illumination levels and type of lighting fixures and lighting design in Switchyard shall be as specified. Refer guideline on Lighting for more details

cable trenches. All cables used in switchyard are armoured type due to induced voltages. There are two types of cables used in a Switchyard viz. Power cables and control cables. The sizes for power cables vary from 3X 35mm2 to 2cX 6 mm2 and for control cables it varies from 5c X2.5 mm2 to 27cX2.5 mm2. Refer guideline on cable routing in switchyard for more details.

Gravel Filling Gravel or surface material coverings, usually

upto 150mm in depth, are useful in retarding the evaporation of moisture and thus in limiting the drying of topsoil layers during prolonged dry weather periods. Also covering the surface with gravel increase the surface resistance and thus reduces shock currents.

GRAVELS IN SWITCHYARD1.The gravels with voids from a good

insulating layer above the soil(Earthing) so that step potential is easily achieved.

It slows weeding in the soilGravels acts as a flame retardant in case of

flaming oil being dropped from CT/CVTIt avoids snakes and other raptiles.

PREPARATION OF BASIC LAYOUT

WHILE MEETING ALL THE NEEDS ESTABLISHED THE FOLLOWING IDEALS SHOULD BE AIMED AT IN MAKING THE BASIC CIRCUIT LAYOUT.

MINIMUM GROUND AREA

MINIMUM QUANTITIES OF CONDUCTOR, JOINTS AND STRUCTURE

MINIMUM NUMBER OF INDEPENDENT INSULATORS, ESPECIALLY IN THE BUS BAR ZONE.

AFTER HAVING DETERMINED THE ELECTRICAL CLEARANCE BE USED A ROUGH CIRCUIT LAYOUT IS MADE. SEVERAL POSSIBLE ALTERNATIVES ARE PREPARED FROM WHICH THE MOST SUITABLE ONE WILL BE SELECTED. SOME VARIATION IS NEEDED, TO MEET THE REQUIREMENTS OF DIFFERENT TYPES OF CIRCUIT.

IT IS ALSO NECESSARY TO CALCULATE SHORT CIRCUIT AND ATMOSPHERIC FORCES TO DETERMINE THE STRESSES IN CONDUCTORS, INSULATORS AND STRUCTURES. THESE HELD IN DECIDING THE MOST OPTIMUM DIMENSIONS.

OPTIONS/ALTERNATIVES

OPTIONS/ALTERNATIVES

1)Single sectionalised bus2)Main and transfer bus3)Sectionalised Main bus with transfer bus4)Sectionalised double main and transfer bus5)Double Bus Scheme6)Ring bus7)One and a half breaker8)Double bus, double breaker

Single Sectionalized

Bus-bar system

I/C Feeders

O/G Feeders

Bus-bar

CB

Isolators

Double main bus & transfer bus system

Merits Demerits Remarks

1. Most flexible in operation

2. Highly reliable

High cost due to

three buses

Preferred by

some utilities for

400kV and

220kV

important

substationsEither main bus can be taken out

of service at any time for

maintenance

ONE & HALF BREAKER DESCRIPTIONONE & HALF BREAKER DESCRIPTIONBUS-1

BUS-2

BUS-1 BUS-2

1-52 CB

2-52 CB

3-52 CB

1-52 CB

2-52 CB

3-52 CB

3. THEY ARE DESIGNATED AS 1-52 CB, 2-52 CB, 3-52 CB.

ONE & HALF BREAKER DESCRIPTIONONE & HALF BREAKER DESCRIPTIONBUS-1

BUS-2

BUS-1 BUS-2

1-52 CB

2-52 CB

3-52 CB

1-52 CB

2-52 CB

3-52 CB

LINE-1

LINE-2

LINE-1 LINE-2

4. LINE - 1 IS CONNECTED IN BETWEEN 1-52 CB & 2-52 CB.5. LINE - 2 IS CONNECTED IN BETWEEN 3-52 CB & 2-52 CB.

400KV One and half Breaker Scheme

400 KV BUS SECTIONALIZER

220KV two main Bus one transfer Bus Scheme

One & half breaker schemeMerits Demerits Remarks

1. Flexible operation for breaker in breaker

maintenance

1)One and half

breakers per circuit,

hence higher cost

2) Protection and

auto-reclosing more

complex since

middle breaker must

be responsive to

both associated circuits

1. Used for 400kV

& 220kV

substations2. Any breaker can be

removed

from maintenance without

interruption of load.

3. Each circuit fed by two

4. All switching by breaker.

5. Selective tripping

Imp. considerations in substation design Safety of personnel and equipment Reliability and Security Adherence to Statutory obligations– I.E. rules, Environmental aspects Electrical design considerations Structural design considerations Ease of maintenance Possibility to Expand