Setting a 51V-C element below IEEE recommended current pickup settings.

[Distribution73]

Hi,
I would appreciate it very much to have your view on the following: I am looking at the settings of a 51V-C (Voltage Controlled) element used for system backup protection in a Small Synchoronous Generator of a Hydro installation. Several sources I have been looking at (most importantly IEEE C37.102 "Guide for AC Generator Protection")recommend a pickup value for this element of 50% of generator full load current, enabled / controlled by a voltage element set at 75% Vrated. However, this element will have to be coordinated with the protection of the DNO and it might be required to operate during the "synchronous" period of the fault. Looking at the Xd impedance of the generator and impedance to the fault, the fault level could be around that 50% full load current or maybe a bit lower. My question is: What problems could we expect if we set the pickup current of the element below the 50% full load current recommended? Say for example 30%.
Thank you very much in advance for your help

 

[dpc]

It could work if the voltage setting is appropriate. In a voltage-controlled (as opposed to a voltage-restrained) OC relay, it will never pickup until the voltage is below the voltage setpoint. So there will be a point down the line looking into the grid where the element won't respond to faults. In most cases, this element is backup protection and the fault will almost always be cleared before this unit would operate.

Also, the generator decrement curves generally assume NO CHANGE in excitation. If your small generator has some type of excitation support (current boost) such as a PMG or static exciter, the sustained short circuit contribution could be quite a bit higher than the synchronous reactance would lead you to believe. At least for a while.

Cheers,

Dave

 

[Distribution73]

Thank you very much Dave. The reply helps a lot.

 

[dpc]

I'll just add that if this small hydro generator connects to a typical utility distribution feeder, the undervoltage element will be the most reliable indicator of utility-side faults close to the generator.

Good luck.

Dave

 

[RRaghunath]

There is no harm even if you select 30% as pickup except that you don't want this to be < rating of any of the outgoing feeders. Another thing is pickup during fault (when the voltage dips below 75%) falls to (K x the pickup selected) K being the factor you select which can be 50% or 30% or something else.
What ever pickup and TMS you select, the coordination with downstream outgoing feeders needs to be ensured.