Coordination Plots

[timm33333]

I have question about the protective device coordination for arc flash study. We have a generator, then a high voltage circuit breaker interlocked with 50/51/51N relays, then 13.8/4.16kV transformer, then a high voltage circuit breaker interlocked with 50/51/51N relays, and then the switchgear.

My question is that while performing the coordination study, should I plot the TCC (time current curves) for only the relays, or for relays as well as breakers?

As the relays are already interlocked to breakers, so it makes sense to plot the TCC curves for only the relays and not for the breakers. But also there is an instantaneous element of the breakers and it might be required to plot the TCC for instantaneous element of breakers along with the TCC of the relays on the same log-log paper.

 

[jghrist]

I'm not sure what you mean by "interlocked" and TCC curves for breakers. Normally, 13.8 kV and 4.16 kV breakers use relays for protection and do not have TCC curves themselves. TCC curves for breakers are only for low voltage breakers with integral trip units.

 

[davidbeach]

For the 13.8kV and 4.16kV breakers, you just add the breaker interrupting time to the relay time.

 

[MCF59s]

At medium voltage, the sensing/processing (relay) and current interruption (circuit breaker) components of circuit protection are separate devices, while at low voltage they're usually integral parts of the same device. So your typical "circuit breaker curve" wouldn't apply in this case.

What's important is remembering to leave some "daylight" between the relay curves during your coordination exercise to account for processing time and breaker opening time. I believe the incident most energy calculation software out there (SKM PTW, ETAP, etc.) automatically include the relay pickup and delay values and breaker opening times in the calculation.

 

[timm33333]

Yes thanks that makes sense. A couple of more questions: the relays have two settings: overcurrent setting and short time setting. Is overcurrent setting of relay something similar to the thermal setting of low-voltage breaker, and short time setting of relay something similar to the instantaneous setting of the low-voltage breaker. Also, can I use the same relay for different functions (50, 51, 51G, 1N, etc.)?

 

[MCF59s]

timm33333
Device 50 is the "instantaneous overcurrent" designation according to the ANSI/IEEE Standard C37.2, while Device 51 is the "AC time overcurrent" designation. The "G" suffix typically denotes "Ground" or "Generator" (depends on the manufacturer's nomenclature) and "N" denotes "Neutral".

As far as the relay settings being analogous to those for low voltage circuit breakers, a better comparison would be to LVPCBs with fully adjustable pickup and delay values (Long Time, Short Time, Instantaneous and Ground). A low voltage breaker with a thermal magnetic trip unit has a thermal portion (fixed Long Time pickup and inverse-time delay) and an instantaneous, magnetic portion (usually with limited variability on the pickup). My guess is that the Overcurrent setting corresponds to A pickup value (Long time, short time, or instantaneous - no way to tell definitively) and the short time setting corresponds to the short time delay. If those are the only settings available, you might be missing something. Ultimately, you need to consult the manufacturer's published data for the specific relay(s) you're using. That'll describe every protective function and setting in detail.

Can you use the same relay for different functions? If these are discrete electro-mechanical relays then you'll probably have to model them as multiple devices with intersecting curves. If it is a multi-function solid state relay available as part of the standard offering from well known manufacturers (Schweitzer Engineering Labs, GE Multilin, etc) your modeling software should let you model it as a single device with multiple protective functions (just as you would, a LVPCB with the entire suite of LSIG functions)

Just out of curiosity, what relay are you using (Manufacturer and model)? And what modelling software? I use SKM PTW.

 

[timm33333]

I am using ETAP. I don’t use SKM because the reliability module of SKM has given me wrong results in past and put me in serious trouble. Adding a generator should improve availability but SKM has given me opposite results. So now before starting a new job I tell them that I will not use SKM.

I am using Alstom P139 relay. The OCR tab of relay editor of ETAP is the same for all relays and it only has two sections (overcurrent and instantaneous). It does not have an option to choose 50, 51, 51N etc. Only available option is to choose the directional element (#67).

So do we use CT for all overcurrent relays and PT for voltage relays, or do we also use a combination of CT and PT in some kind of relays? Should the CT of relay be always put on the incoming side of the breaker? Thanks for help!

 

[timm33333]

Is there some document that describes the construction of different relays? For example what is the difference between the construction of electromechanical overcurrent relay and electromechanical loss of field relay.