CT powered differential relays for small transformers

[submonkey]

Hello,

Where I work, there is a policy of protecting distribution
transformers larger than 1.5 MVA with differential protection.
In some other cases, small transformers need differential
protection to provide discrimination when multiple units are
connected in parallel.

The distribution substations are generally fitted with tripping
batteries only. These batteries are fitted with trickle chargers
that are not able to support a standing DC load. All the relays
therefore have to be CT powered (electromechanical, or CT powered
electronic like the Schneider P115).

The current means of providing differential protection is via
a "fuse wire diff" relay. The relay has three pieces of fuse
wire which are connected in the differential spill circuit.
The fuse wire holds a mechanical trip contact open. If significant
spill current occurs, the fuse wire blows and trips the transformer
breakers (high and low).

The fuse wire differential relays have an excellent service record,
but do have some limitations. I would like to understand what
alternatives exist, and their pros and cons.

My questions are:

1. What do other companies/utilities provide for small transformers,
and what is the reasoning? I gather that in the US fuse protection
is common for transformers even as large as 10 MVA.

2. Has anyone used an MCB in a spill circuit for providing transformer
differential protection? If so, how successful was it?

3. Does anyone make CT powered transformer differential relays anymore?
They used to be common for larger transformers (duo-bias 2C21, etc),
but everything on the market today seems to require an auxiliary supply.

Thanks in advance for your comments!

Submonkey

 

[dpc]

Rather than trying to apply a CT-powered relay, I would find an ac source, such as a PT or small CPT and then install a small UPS, such as the SEL-UPS. Or use capacitive trip breakers operating on ac control power.

The "standing load" of a microprocessor relay is very low, BTW. I have a hard time believing your battery charger would have much problem.

 

[submonkey]

Hi DPC,

Thanks for the response - the SEL-UPS is an interesting suggestion.

The battery chargers provide a few mA, and were initially introduced
to prevent some kind of battery corrosion problem (I don't have the
details to hand). For many years there was no charger at all, and the
batteries (flooded NiCd) were charged periodically as a maintenance task.

A bigger problem than charger output is how to deal with a long outage,
where having a standing load results in a lack of protection when the sub
is re-energised.

Thanks,
Submonkey