POWERGRID Practices in EHV Substation Layout - Kksarkar

POWERGRID Practices in EHV Substation Layout

K K Sarkar Dy. Chief Design Engineer (Engg-s/s)

Power Grid Corporation of India 1

Important considerations in layout.. ¾

Reliability and Security -

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Selection of Bus Scheme Ease of Maintenance Operational Flexibility

Short Circuit Level Shape of the land Altitude of the land above mean sea level Feeder orientation Safety of Equipment and personnel Possibility of future expansion Cost 2

Bus Switching Schemes… ¾

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Single Main Scheme Double Main Scheme Single Main & Transfer Scheme Double Main with by-pass isolator scheme Double Main & Transfer Scheme One & Half Breaker Scheme Double breaker Scheme Ring Bus Scheme

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SINGLE BUS SCHEME ¾ ¾

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Simplest and cheapest bus bar scheme Maintenance and extensions of busbars are not possible without shutdown of the substation. Operation & maintenance of bus bar is easy. easy

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SINGLE MAIN AND TRANSFER SCHEME ¾ Individual CB can be taken out for maintenance on-load at a time. ¾ The transfer bus coupler acts as the breaker for the circuit under by pass. ¾ Individual circuits have a bypass isolator to connect to the transfer bus and this isolator will be closed during bypass operation of that particular circuit. 5

DOUBLE BUS SCHEME ¾ Load will be distributed on both the buses and the bus coupler shall be normally closed. ¾ For maintenance & extension of any one of the buses the entire load will be transferred to the other bus. ¾ On load transfer of a circuit from one bus to the other bus is possible through bus isolators provided the bus coupler is closed and thereby two buses are at the same potential. ¾ On load bypassing of any circuit for breaker maintenance is not possible. possible 6

DOUBLE BUS WITH BY-PASS SCHEME ¾ This bus arrangement provides the facilities of a double bus arrangement & a main and transfer bus arrangement. ¾ The bus to which the transfer bus isolator is connected can be used as a transfer bus also. ¾ During the time a circuit is under bypass, the bus coupler will act as the breaker for the bypassed circuit. 7

DOUBLE MAIN AND TRANSFER SCHEME ¾ In this bus scheme, in addition to the two main buses there will be a separate transfer bus also. ¾ Since separate transfer bus is available there will be no need of transferring the load from one bus to the other bus unlike in a double main cum transfer bus arrangement. ¾ Other features are similar to the one described in double bus with by pass arrangement.

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BREAKER AND HALF SCHEME ¾ In this scheme, two circuit have three breakers, the middle breaker ties the two circuits and hence is called the tie breaker. ¾ Breaker or bus maintenance is possible without any shut down of the feeder ¾ Even if both the buses are out of service, power can be transferred from one feeder to another feeder through tie breaker

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DOUBLE BUS TWO BREAKER SCHEME ¾ Each feeder is controlled by two breakers. ¾ This arrangement is comparatively costlier than other scheme and hence followed in very important circuit only. ¾ In this arrangement breaker maintenance for any feeder circuit is easily possible without any shutdown. 10

RING BUS SCHEME ¾ As long as the ring is closed load has two sources of supply and any circuit breaker can be taken out of service without affecting the supply. ¾ Extension of ring scheme is difficult. ¾ No bus bar protection required.

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Selection of Switching Schemes… ¾

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No reliability in Single Main, Double Main, Single Main & Transfer Scheme w.r.t bus fault, feeder fault & breaker maintenance Double Main & Transfer Scheme, One & Half Breaker Scheme & Double breaker Scheme are

characterized by reliable and interruption free supply. ¾

One & half breaker scheme can be selected for EHV substations due high reliability, operational flexibility, ease of maintenance, ease of expansion, due consideration of cost 12

Bus Bar Design, Selection of conductor levels & Bay width calculation.. ¾ ¾

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Selection of conductor (AAC, ACSR, Tube) Current Carrying capacity with temperature rise of 35 deg.C over ambient of 50deg.C ambient (IEEE-738) Temperature Rise during short circuit Stresses in tubular bus Cantilever Strength of post insulator Deflection of the tube Natural frequency of tubular bus bar Aeolian Vibration 13

Bus Bar Design & Selection of conductor levels.. ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾

Electrical Clearances (IEC-60071) Corona Electric Field (10kV/m)& Magnetic Field (500μT) Short Circuit Forces (IEC-60865) Sag-Tension Calculation Normal Tension (Factor of safety 2.0) and Short Circuit Tension (Factor of Safety 1.5) Height of conductor levels Bay width & Phase to Phase spacing 14

Minimum Clearances for Layout (at altitude