Protective Relaying

[Pete2]

We are designing a service entrance to a sewage treatment plant in Souther California. Power from the utility's 2000/2576 kva transformer enters at 480 volts through a 3200 amp main circuit breaker. Back-up power is from a 1500kw diesel generator through a 2500 amp circuit breaker to the main switchgear bus.

The circuit breakers are electrically operated and controlled by an automatic throw-over control system in the switchgear. Trouble is detected on the line side of the main breaker by devices 27, 47, 59, and 81 O/U (undervoltage, neg. sequence voltage, overvoltage, and frequency relays).

A comment from the client states that relays 59, 47 and 81 are not required for sensing availability of normal power. In fact they may interfere with our attempts to load shed at the generator bus level with frequency relays, they say.

I don't understand how relays 59, 47 and 81 on the incoming line will interfere with generator load shedding. Can anyone shed some light on this issue?

 

[electricpete]

Apparently they will monitor the normal power an initiate source transfer at a point in time when the relaying indicates the normal power is in danger. If your relaying should trip the normal power earlier then they sense a loss of power, then I would think that would initiate their source transfer.

I assume their sequence is to verify all loads tripped (shedded) so the diesel won't assume a large load when it reenergizes the bus. The problem escapes me also.

 

[jbartos]

Suggestions:
1. 47 relay - is a phase sequence or phase-balance voltage relay. It functions upon a predetermined value of polyphase voltage in the desired phase sequence, when the polyphase voltages are unbalanced, or when the negative phase-sequence voltage exceeds a given amount.
This relay will potentially trip when the load shedding is not balanced from a three-phase standpoint. However, it also protects the equipment. Therefore, some trade-off is necessary.

2. 59 relay - is an overvoltage relay. It operates when its input voltage is more than a predetermined value.
The generator may experience some overvoltage during load shedding. This essentially depends on the voltage regulator speed; how high voltage will develop and potentially for how, long if it is voltage transient.

3. 81 relay - is a frequency relay. It responds to the frequency of electrical quantity, operating when the frequency or rate of change of frequency exceeds or is less than a predetermined value.
It can potentially interfere with load shedding, depending on the load block to be load shed. But at the same time it may protect your equipment. Therefore, some analysis is justified for the proper setting of this relay.

Assessing the above pros and cons, any flat statement about the above relays is not very accurate. Some more systematic analysis will be beneficial. The equipment manufacturer recommended protection schemes should not be altered without more thorough analysis.

 

[rhatcher]

(disclaimer: I am not a relay expert)

First, the use of "devices 27, 47, 59, and 81 O/U (undervoltage, neg. sequence voltage, overvoltage, and frequency relays)" to detect problems with the incoming power will provide pretty comprehensive protection for a wide variety of faults (and accidents). The statement by your client that "relays 59, 47 and 81 are not required for sensing availability of normal power" is based on one of three philosophies. I have amused myself with my pet names for the cases. I hope you share the fun...haven't you faced these people before?
1)"accountants versus engineers": they believe that cost saving during capital aquisition is more important than long term system reliability and don't understand where the payoff is.
2)"the faithful few": they believe that the utility power is infallible....well, almost infallible in that you may experience low voltage or a loss of voltage but never single phasing, voltage inbalance, frequency instability, negative sequence....
3)"regular guy (could be you or me)": they mean well but in this case truly don't understand the purpose for those relays and the type of scenarios that they are designed to protect against for which the U/V relay provides no protection.

I hope for you that they are in the "regular guy" group as you can (usually) convince an engineer what is right but almost never an "accountant" or a "faithful few".

Second, with respect to the interference question: I am under the impression that the referenced relays were on the utility side of the main circuit breaker and that the question is whether these relays will interfere with operation of the frequency relays applied to the emergency power generator for load shedding. My answer to that question is that it is not obvious how this would be the case as the utility side relays would be electrically isolated from those applied to the emergency generator.

Specifically, the CT's and PT's for the listed utility side relays would be on the incoming utility bus on the line side of the main breaker and would operate a shunt trip in the main breaker. Of course, this would be the start of a sequence that would end with the emergency generator breaker closing onto the switchgear bus.

The frequency relays used for e-generator load shedding would be installed on the line side of the e-generator breaker and would ultimately (ie. through a load shed controller) be tied to shunt trips on the feeder breakers.

Since the main breaker and the e-generator breaker will never be closed at the same time and since the utility relays and the generator relays are connected to the trips of different breakers, I do not see any method for interference. If anyone else knows about this please speak up.