Over/Under voltage and frequency protection 1

[sabap]

Dear All,

At our power plant, we want to install the subject protection relay(s) in our 132 KV switchyard. The purpose is to open the 132 KV line breakers in event of under/over voltage and frequency to keep the power plant running in island mode. We have a single busbar scheme and two lines in parallel going to same grid station at remote end.

Should we conenct our new relay using Busbar PT or two relays on transmission line using line PTs.

Please suggest the pros and cons of both.

Thanks in advance.

 

[cuky2000]

Consider a multifunctional relay with undervoltage (27), overvoltage (59), Under/over frequency (87), etc. See if the enclose guides could help you

http://www.basler.com/downloads/genprotguidC.pdf

 

[rbulsara]

I am not HV expert, but principal of protection is not much different so my two cents:

It appears that the 132kV lines are 'loads' to your plant.
You want to protect your plant so if the freqency go under on your bus, shed (trip) some load.

I would think you want to conncet to the bus.

Shedding the load will help under frequency, not sure it helps over frequency condition.

 

[stevenal]

A voltage or frequency event will affect the bus and any of the lines in service equally. Of the three locations, the bus is least likely to be out of service. For restoration you'll need to see that the line side has recovered and that both sides of the breaker are in sync.

 

[sjvanhecke]

It would seem to me that when you have a fault that will bring your voltage and frequencies down the bus and line would both see it, but I would tend to agree with stevenal in that you should look to the line PT's when you are going to restore the system.

 

[davidbeach]

If you hope to ever resynchronize and close back onto the 132kV line, you are going to need VTs on both sides of your disconnecting means. Which you use for determining when to seperate would be a matter of preference since while connected both sets should track each other.

 

[sabap]

Thanks for all of you guys for answering.

I would like to add further to my scenario as per some of the queries in the discussion:

We are supplying power to our national grid and are connected to a grid station via two lines in parallel. We are concerned only with system faults.

The actual problem we face it that in event of fault in national grid that is not in Zone I of distance protection. Our bus protection , overcurrent acts first. Although line protection, distance and overcurrent should clear the fault first and isolate us from grid. Otherwise our steam turbine / generator trips. The apparent reason for line protection failure to act is " the line protection settings are based on 100 % load on per line basis; whereas normally both lines carry 50 % load" Now the bus overcurrent has 100 % settings with 100 % load so it trips first. Correct me if i am wrong."

We are also looking into the possibility of having two sets of tripping parameters in our line protection relays so that they should distinguish between one line in service and both lines in service. We have Siemens distance 7SA511 and overcurrent relay 7SJ511 installed there. I will check if 7SJ511 provides the two settings option. We can use binary input from line breaker contact to switch select the settings.

It is also true that we have to resynchronize on 132 KV breaker. We have the facility available with us.

 

[busbar]

One reference may be IEEE Std C37.106-2003 …Abnormal Frequency Protection for Power Generating Plants

 

[RRaghunath]

I didn't understand what sabap means by 'bus overcurrent protection. Neverthless, to me it appears that this is problem of overcurrent coordination from line overcurrent relay to the generator overcurrent relay.
Generators are not normally provided with a simple overcurrent protection precisely to avoid maloperations such as above, rather voltage restrained or voltage controlled over current protection is provided. Sabap may like to check the type of protection existing for their generator. The setting ensures that the relay doesn't operate with any speed unless the overcurrent is coupled with drop in generator terminal voltage (signalling the fault is close to generator terminals).
Distance protection in lines should operate for faults in the line irrespective of the load carried by the line. The relay setting is in terms of line impedance and the relay operation is not affected by the magnitude of current in the line (generally speaking).

 

[sabap]

Thanks rraghunath for correcting me.

Overcurrent on trafo bay tripped the 132 KV trafo breaker. Generator protection did not operate and neither the line overcurrent protection.

I was talking of changing the setting of line overcurrent relays as lines carry half the full load current in parallel.

Best Regards,
Sabap.

 

[powercomplex]

You can apply protection to your bus, for under/over voltage and frequency simply.
Only in condition of fault at one line your system load will shift to other line, the system should not trip in this case.
Tell your operational requirement to the protection engineer. or you can call me at US$1000/ day.

 

[RRaghunath]

Why didn't the line distance protection operate.

May be, my comment with regard to overcurrent coordination is still valid. You may like to verify.

The overcurrent protection on the 132kV side of generator transformer is normally directionalised, made to look in to the transformer. If so, the operation of GT 132kV side OC protection for line fault seems not correct.

 

[sabap]

On of the two distance line protections, the fault was found to be in Zone 3 which has 700 ms time delay before tripping. Line overcurrent did not operate due to the reason i have stated earlier. Trafo overcurrent is not directional at our plant and it tripped the trafo breaker in less than 700 ms, time delay in distance for zone 3.

The fault ideally should have been cleared from grid side but somehow it did not trip the breakers at national utility substation.

 

[digitrex]

Make it simple. Let the protection relays located on the feeder panel trip their own breaker. If you use busbar PT for undervoltage trip of feeder breakers, it means an undervoltage relay located at one panel tripping a circuit breaker of another panel.

Other issues that may influence your choice:
1. Do you need to energize the dead line from your power plant?
2. Do you need to energize your dead busbars from the grid?

I suggest one idea that had been successfully implemented at my site. You can make the undervoltage trip to be more intelligent to make decision on external fault, i.e. by wiring undervoltage relay output in series with the feeder's directional overcurrent relay(or reactive power relay). In this way you can set directional overcurrent relay to very low value(20%) and very fast operaton(eg 200ms).

 

[buchholz]

In case of 81O/U, and 27 we will just trip the trafo breaker in our compound or both line breakers.

But for 59, overvooltage, we would like to trip all four breakers on line our side and as well other side to protect equipment like CT, PT, line trap etc.

What are the typical settings for these functions and how should we coordinate them? I mean like time delays, etc.

 

[sabap]

Would like to add that buchholz also works at our plant with me to avoid any confusion here.

sabap.

 

[mc5w]

You might also want to think about a valve that can dump some of the live steam from the steam generator to to condenser as well as a high speed throttle that slams shut and then very quickly cracks open enough to supply the off grid needs of your main turbine. If you are a nuclear plant both devices would help avoiding having to scram the reactor.

You might also want to consider an auxiliary steam turbine to operate the plant auxiliaries off grid during demanding conditions such as peak periods or lightning storms. An alternate wise investment would be a gas turbine generator for the same purpose and for black starting the plant.

You might also want to think about extra hydraulic accumulators for the turbine governor and extra direct current battery supported hydraulic pumps for the governor. Also some extra battery supported pumps for lubrication with hydraulic accumulators to supply oil while the extra lubrication pumps are starting. The system disturbances on 14 August 2003 caused a lot of turbine governors to run out of hydraulic pressure which tripped off the boiler or scrammed the reactor. Fossil fuel and nuclear plants were equally affected by loss of hydraulic pressure as the governors tried to keep up with swings in frequency.

Mike Cole, mc5w@earthlink.net