Issue with low fault level on protection relays

[dfdt]

We all know that protection relays at time may not work satisfactorily under very low fault level on the network. What is your opinion or practise in dealing with this issue?

 

[davidbeach]

I guess we don’t all know. What is the issue/question?

 

[dpc]

Install downstream protective devices with lower rating.

 

[dfdt]

Hi Davidbeach
This is about the practices being followed by utilities when setting the current based protection elements/schemes to ensure that they still operate with certainty under minimum fault conditions. Some utility set overcurrent operating level below 50% of minimum fault current. hence I am asking/looking for experiences in dealing with this issue.. that's all

 

[davidbeach]

Perhaps you could provide an example. Minimum fault level where? At the substation? At the most remote location on the mainline? At the most remote tap end? There’s a variety of other low fault conditions that can be encountered. Different conditions have different solutions.

 

[dfdt]

I was talking about the minimum fault level at the bus in a substation(say 1000A). For example an over current relay with high set element set at 900A may under some condition, will fail to pickup

 

[RRaghunath]

Where fault levels are low, distance protection relays are recommended. If overcurrent relays are employed Voltage controlled overcurrent relay (ANSI device no. 51V) can be the option.
The 51V allows pickup to be set more than the full load current to provide overload protection during healthy conditions and will lower the relay current pickup threshold (to below the load current so that the protection can be faster) when the increased current is associated with voltage drop that signals fault condition.

 

[davidbeach]

If you're trying to run a high-set instantaneous, which we do on our system, you live with the possibility of certain system configurations producing a fault current below the setting. If there are a couple of common configurations with very different available fault currents you need different high-set elements.

As RRagjunath mentions, a distance element does a great job with changing source impedance (available fault current), but the minimum reach may be way too far.

One possible solution is to set the element lower, and a communicating fault indicator at the point you want reach. If the fault indicator sees the fault it's beyond that point and you can reclose; if the fault indicator doesn't see the fault it's close-in and don't reclose.

 

[thermionic1]

@davidbeach is this what you had in mind? I had no idea these were so attractively priced.


SEL Communicating Fault Indicator

 

[FreddyNurk]

How do distance relays go in terms of supplying feeders with lots of connections on them (say, radial 22kV feed with a heap of smaller step down supply transformers hanging off them)? I assume the general use for them would be feeders with very few connections, such as a sub-transmission line.

I am familiar with 51V for use on feeders with multiple connections on them, generally used due to soft sources that decay quite quickly, but haven't had much to do with distance relaying.

EDMS Australia