Main-Tie-Main

Load Configuration.

Both Bus#1 & Bus#2 are supplying normal loads that mean interruption for PT1 or PT2 is accepted for fault located between M2-PT2 and main supply.

No critical load (instantaneous interruption is not allowed) connected on Bus #1 and Bus #2. It shall be supplied from UPS.

Load on Bus #1 has a standby load on Bus#2 or vice versa, so if the bus #2 fail, load on bus #1 is operated.

Basic Operation.

This diagram may be useful for our discussion. The basic M1-T-M2 configuration is shown. During normal operation M1 & M2 breakers are closed and tie breaker T is opened. Supply coming from PT-1 and PT-2.

This drawing indicate when M2 open, T and M1 CBs are closed (abnormal condition). This condition is done for maintenance purpose for equipment located between M2 to upstream (main source). Load transfer from bus #2 to bus #1 can be carried out without interruption done by ATS scheme.

If fault located on bus #2 to tie breaker T or bus #1 to tie breaker T, load transfer is prohibited by ATS scheme. But for fault located from M2 to upstream load transfer is allowed with deenergizing bus #2 loads first, then tie-breaker T closed by ATS scheme. Loads may be in service after this tranfer, if the loads is set in auto position.

We cannot maintain the supply on fault bus (e.g. bus #2 or bus #1) before correction is made.

A redundant bus tie or switch isolator acting as maintenance bypass to ATS operation.

Based on discussion above, I do not know, where we have to install those equipment to maintain supply for fault on respectively bus.

Normally Closed Tie Breaker Operation.

It is possible to operate tie breaker in closed position, but we have to consider a short-circuit level on that bus. By calculation (Short-circuit study), a fault on bus #1 or bus #2 the magnitude become double. So, we have to ensure the equipment s.c. rating (buse, breakers, feeder loads, feeder breakers, and etc) meet the requirements for tie-in in closed position.

Note:

Temporary closing three breakers for maintenance purpose is allowed within 3 cycles to 1 (one) second is accepted.

Relay application.

1). Bus differential for bus#1 and bus #2 may be applied (we apply on 4.16 kV systems).

2). Directional relay may be applied on incoming breaker M1 & M2 if the NC for tie breaker T is applied.

3). Restrictive earth fault is applied for transformer with low resistance grounding.

4). Please consider to provide better coordination for instantaneous relay between incoming breaker and load breakers as well as ground fault protection.

5). Syncheck relay is required for synchronising bus # & bus #2 before closing tie breaker T. We provide permissive closed for ATS schecme. ATS can only be operated if the upstream system is in synchronising condition (Generating buses are in remote but located closed to each other).

Conclusions.

1). We cannot maintain load on bus faulted before repairing is made.

2). I do not know the location for instaling redudant bus tie breaker or isolator to prevent faulted bus total failure.

3). Comprehensive study shall be caried out to operate tie breaker in NC. Especially in selecting electrical equipment and relay coordination.

4). Pay more attention on safety aspect in establishing the ATS scheme.