Main Tie Main, redundant Bus tie breaker

Dear Sir, I would appreciate if could please give the SLD of your system. how many transformers are there.. how many emergency DG sets are there... the fact is that the configuration depends upon the criticality and application. my mail id is shanbhag@sequentindia.com

Dear All,

May be the information below able to response the subject, even with minimum information what we have.

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 (buses, 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 bus is 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 installing redudant bus tie breaker or isolator to prevent faulted bus total failure.

3). Comprehensive study shall be carried 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.

Regards,

Dear Rasam,

If you have a faulted bus, unless the fault is cleared, there is no way you can maintain the load on that bus. However, if you have a faulted source, one can try and avoid load getting affected by fast bus transfer means.

Please elaborate what you mean by:-

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

Amit.

Dear Amit Raje,

Thanks for your attention.

Faulted is cleared by protection, but we cannot renergize the bus before investigating and following the reenergizing procedure. We have to make sure that system is OK by physical inspection and testing. We never know the exact condition before checking and testing. during investigation the bus should be out of service. So, we cannot maintain the supply while checking and testing on the faulted bus. How long the time is required, it will depend how long the repairing, checking and testing is done. It is the basic philosophy during designing secondary selective double ended substation.

Yes we can use transfer scheme as explained above if the fault located between incoming breaker and upstream system (out going breaker in generating bus).

I hope you understand with the clarification above.

Regards,

Dear Sir,

Please be clear on what is the scheme of operation is required.

1) Whether the 2 transformer are operated in parallel? if so, great care must be taken while paralleling 2 transformers since the transformers are connected to infinite bus. We don't know the generating sources from which the transformers are fed. it can be quite risky at the source end as well as the load end. because then the fault level will become infinite.(limited by transformer, cables impedance and fault level decided by the system voltage). So please check the above factors in case you need to parallel the transformers.

2) Whether the sources feeding the transformer are known and separate(in case of DG sets)? then paralleling of transformer becomes a bit easier.

in both of the above cases.. the short circuit withstand , SC breaking capacity of breakers and busbars should be the vector sum of the individual SC withstand level of all the paralleled transformers. Please ensure that the SC level of busbars is type tested for the same.

3) in case the transformers are not parallel... to ensure the continuity.. ring main scheme can be designed. But this system can be costly from the procurement and maintainence point of view.

Please appreciate that .. whatever scheme we consider.. the busbar system inside the panel should be in working condition as Mr. Amit rightly said.. in case the tie breaker fails.. then a separate breaker should be kept as a spare..

Your comment :

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 (buses, breakers, feeder loads, feeder breakers, and etc) meet the requirements for tie-in in closed position.

my opinion : Whenever we are designing the switch board electrically, we have to consider a common short circuit level for the entire Busbar structure as well as the switchgear. the Sc level of 2 incomers and tie breaker should be the same. Even the outgoing beakers should have the same SC level. to clear short circuit faults on the busbar. you can consider Instantaneous short circuit release for tie breaker. Tie breaker only acts as a switch and preferable should not have LSG protection features.

While Transferring the LOAD-2 to M1 breaker.. please ensure that transformer PT1 can take the load of Load-2.. otherwise.. if overload occurs then breaker M1 will trip causing entire black-out of the system.

Please give your feedback on the above and state whether i have understood your problem

Dear Sir,

It is not Mr. Guest/Electra system, this system that I have ever known for discussion purpose only, probably it will give a new idea for Mr.Guest or others.

This diagram is a part of ring bus system, indicate two generating buses with two generators respectively, and each bus also connected to three winding transformer (delta side). The star side of the transformer is connected to the ring bus system (ring bus system is not shown in this simplified diagram).

There is no public electricity available for this plant. The left side indicate the systems are in normal condition (tie breaker open). While right hand side indicates the abnormal condition (three tie breakers are closed).

Please be clear on what is the scheme of operation is required.

Reply: I didn't know what is the single line diagram of Mr. Guest/Electra proposed, so I gave an example of Main-Tie-main as shown on comment #2. So, more detail of the scheme of operation as shown in simplified SLD above.

1) Whether the 2 transformer are operated in parallel? ……… because then the fault level will become infinite ….

Reply : Yes you right, I am with you. The system as in my mind (see comment #2) as shown on simplified one line diagram above. No public electricity available in this plant.

2) ………. in both of the above cases.. the short circuit withstand , SC breaking capacity of breakers and busbars ……….

Reply: Yes I agree with you. The scenario related to the possible operation shall be done on Electrical Power System Study (S.C, L.F, Stability, Relay Coordination, Motor starting Studies, and etc.) from normal operation to the worst case operating condition.

3) in case the transformers are not parallel... to ensure the continuity.. ring main scheme can be designed. But this system can be costly from the procurement and maintenance point of view.

Reply:Yes I agree with you.

.... the busbar system inside the panel should be in working condition as Mr. Amit rightly said.. in case the tie breaker fails.. then a separate breaker should be kept as a spare..

Reply: Yes it is possible by applying two tie breakers supplying one bus load, and tie breaker failed is fail to operate not blown up or s.c on bus load. Very often bus fault is located in the outgoing breakers, but rarely fault in incoming breaker. I have seen both. If it is happened the service load cannot be maintained.

my opinion : Whenever we are designing the switch board electrically, we have to consider a common short circuit level for the entire Busbar structure as well as the switchgear.

Reply: Yes I agree with you, the S.C level of 2 incomers and tie breaker including outgoing feeder breakers should be the same. To clear short circuit faults on the busbar we implement bus differential relay for 4.16 kV, 13.8 kV and 34.5 kV systems, except for LV systems.

What is the LSG protection features? Please clarify it.

While Transferring the LOAD-2 to M1 breaker.. …….

Reply: During normal operation (tie breaker open), each double ended substation transformer is loaded less than 50% as per design basis of double ended substation. The transformer rating is referred on "oil natural" ON, even the fans are installed.

Please give your feedback on the above and state whether I have understood your problem

Your understanding is excellent, and I appreciate with your comprehensive comment. The subject I proposed is the system that I know and already run since 1985 (ring bus system).

Mr. Guest it is look like understand with your explanation, I cannot read comment #8.

Regards,

Thank you for all your expert opinions, actually, the design concept start with dual feed utility power forces MV switchgear (M-T-M) feeding 4-1000 KVA transformers and a emergency paralleling switchgear with 4-1000KVA generating set both side have each 4-ATS with maintenance bypassed switch connecting to two (2) main tie main (M-T-M) power switchgears1 &2, 3&4 at the secondary Voltage.

The main objective is to have a concurrently maintainable system with redundant and continuous operation of the electrical system. My main concern is that in the event of a failure of the bus tie breaker total power interruption will be felt by the secondary equipment in M-T-M switchgear either the bus tie is connected in normally open (M-T-M) or closed system(spot network system). or bus tie my mechanical means(manually operated switch breaker will address it).

The way I look at it, designer cannot avoid power shutdown such as the failure in the bus tie unless as I mentioned that the tie has bypassed like the ATS. Designing redundancy by autothrow transfer 2 pair breaker at the main would have the same result.

Electra

Dear Sir, I now feel that i am understanding your problem in a better manner.

Ok.. If you want to maintain uninterrupted supply and Cost factor is not the most deciding factor for you/ your client.. then following are the suggestions :

1) Consider double busbar system( 1 on M1 and 1 on M2) i.e which will avoid Tie breaker. then consider FP breakers each in the outgoing side and electrically interlock the same so that whenever 1 source fails. the outgoing will be fed instantaneously from the other source. You can consider the controller as suggested by Mr. Amit for faster transfer time. Yes.. the cost will considerably increase.. but the down time will reduce and avoid the need for Tie breaker.

Sounds Complicated ???

2) Another method to avoid failure of tie breaker...choose current rating of tie breaker 1 step higher than the incomer breakers(M1 & M2) or may be 2 steps higher.. because then the breaker contacts will withstand higher currents..

In my total experience.. i have never heard that buscoupler to be failed since the buscoupler will carry only half the current compared to its actual rating and most of the time the buscoupler will be off in case interlocking scheme is provided.

In case the two transformers are to be paralled, please check that the outgoing feeder breakers of load1 & 2 are in off condition. Please parallel the 2 transformers first and then switch on the respective outgoing loads. this will prevent overloading of the tie breaker and increase in life. you can also consider using motor operated breakers in outgoing and use PLC for giving commands to the outgoing breakers which will operate in a sequence

There are many methods to maintain the system continous.. but please appreciate that even the machines have certain operating life.. and there is bound to have a failure .. which may happen because of oversight.. But please dont worry.. i just wanted to know .. what type of load is being used.. whether for industry or building segment or software park etc... then we will be able to know the probability of tie breaker failing..

Thank you for your feedback