minimum operating time delay in overcurrent instantanous relay (function 50) 6

[Ali-rashid]

Hi everyone,

I'm working as the technical director in a power equipment-supplying company. We work with different distribution protection relays. Some of these relays are FANOX relays which provide 50:instantaneous over-current function. The minimum operating time of 50p function in these relays is 0.02 seconds (Fig 1).

fig 1

The relay uses RMS measurements and we investigated the reason behind this minimum value of 0.02s (while we need a relay with zero operating time) and some engineers believe that the minimum operating time that can theoretically be implemented in overcurrent relays is 0.02, reasoning that every relay needs at least a cycle of the current waveform (the frequency is 50Hz, 1 cycle = 1/50= 0.02s) to be able to detect a fault.
This is while, in similar overcurrent relays of other brands like ABB and Arcteq( to be specific, arcteq mk220), the operating time minimum limit is 0. Fig 2 shows the minimum operating time indicated in a red circle from Arcteq mk2200 relay user manual.

fig2

I want to know which one is true and what value can the minimum operating time of a relay be set to.
Thanks in advance

 

[HamburgerHelper]

I don't know about those relays but SEL relays have elements that pickup in 1/4 cycle.

The second figure is intentionally added delay. Your total operating time will be Intentional delay + relay operation time + breaker operating time = clearing time

If the operating time of the relay is 20ms +/- 30 ms, don't you plan on it operating in 50ms? Maybe, I am not reading that right. I would contact the vendor. Maybe, preset = 0 ms for instantaneous tripping and it might operate at 30 ms.

I don't know what breakers you are using but from what I see, the breaker operating makes up a far larger portion of the total operating time than the relay. SEL is putting out traveling wave relays that can operate ridiculously fast but you are still stuck with how slow your breakers are.

 

[davidbeach]

Pickup time of an instantaneous element depends on the digital filtering process in the relay and the extent to which the current exceeds the pickup point.

Just above the pickup point it may take a whole cycle for the relay to recognize that level. Way above the pickup point and it may only take a quarter cycle. It all just depends.

Instantaneous does not mean zero time; instantaneous means without any intentional delay - as soon as the relay recognizes that the condition exists.

The Definite Time Setting in your second example is intentional delay added after the relay has recognized that the current is above the pickup. That chart does not address the time necessary to recognize the condition. The first relay might well have definite time settings that could also extend the trip time.

A zero operate time, as you seem to be defining it, is not possible in the real world. The best you could achieve, using relays I'm aware of, would be 2-3 milliseconds using traveling wave relays. At that point you're talking about $12k-$15k per relay. Then you still have breaker time.

If you think you can't live with a 1 cycle recognition time, you're probably already into a situation where you're not going to get the performance you need. Perhaps it's time to take a step back and see what assumptions you can change to work better with relays that are actually available and for a cost that is reasonable for the application.

 

[Ali-rashid]

Thanks for your instructions

what is the typical range of breaker operating time?
could you please provide me with the SEL overcurrent relay manuals? i was not able to download it form their site as they have not responded to my manual request.

 

[NCTHAI]

The reality of electrical protection is it works in sequence of FAULT - SENSING - RELAYING & FAULT ISOLATION. The technology acts laggingly and not proactively. First fault has to happen, electrical quantities has to change and then only protection relays act. The technology is not proactive - it does not act on the verge of fault occurring and prevent it from happening. Even if zero time delay set on relay, there will be some time lag between fault occurring and fault clearing.

Davidbeach has very nicely and correctly said - "Instantaneous does not mean zero time; instantaneous means without any intentional delay - as soon as the relay recognizes that the condition exists."

NC

 

[HamburgerHelper]

Ali,

You gotta register with SEL to get the manuals. I think their only restriction is that you need to use a company email. You can't use gmail or yahoo or whatever. I think they use it to track people in the industry. I believe I have had to create new accounts every time I have switched companies.

 

[JG2828]

Ali,
Here are some screenshots from the SEL 751 manual.
Instantaneous 50 settings:
Intentional time delay: 0 to 400 seconds
50 element Pickup time: < 1.5 cycles
Standard contact closure time: <= 8ms
Optional Hybrid high speed contact closure time: < 50us

So you would need to add these times together along with your breaker operating time to figure out the overall clearing time.

 

[Ali-rashid]

Thank you all for your comprehensive information.
JG2828, I appreciate your effort for providing screenshots, it was really helpful.

 

[RRaghunath]

Just to add what Mr. Davidbeach has said -
'Instantaneous' element these days is known as 'Highset' element and is more appropriate as the element generally is accompanied by settable definite time.
Further, many vendors offer option of 'peak' and 'RMS' sensing for the element.
'Peak' sensing is obviously faster as there is no processing when compared to 'RMS' sensing.

Rompicherla Raghunath