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Protection riddle no.3 – Short line between power plant and substation
The high voltage substation of power plant is usually built near it and the generated electrical energy in each power generation unit is transferred to substation by one short transmission line.
The many different protection methods may be considered for that mentioned line and the best way to choose main protection system is related to the length of line and purposed protection system reliability. The longitudinal differential relay and low or high impedance differential relay may be applied for this purpose.
In one of the projects, the substation contractor proposes a new economical opinion to reduce protection system costs. The representative of contractor says that protection zone of power plant transformer can be extended to within the high voltage substation to cover the transmission line; because the length of line is very short.
The related power transformer protective relay is a standard low impedance differential relay with three analog input currents which can support one electrical zone with three branches. Two input currents of relay is specified to low voltage side of main power transformer (low voltage side contains generator and unit transformer branches) and the other is usually connected to high voltage BCT of main transformer.

In deed, the technical proposal of contractor is replacement of connection of relay current input from main transformer BCT to substation current transformers. In the other hand, the primary scheme of substation is "one and half "; therefore the input of relay shall be fed by two similar sets of substation current transformers. The representative of client answers that it is not possible; because just one input can be specified for high voltage side connection.
But the representative of the contractor says; we can sum the secondary currents of two quite similar sets of current transformers in one nod then apply their summation currents to the differential relay.

Is the technical proposal of contractor correct? Is applying secondary current summations to the relay suitable solution in this case?

Author : Hamid
Thu, September 13, 2007 - 11:29

Yes, but its to be ensured by contactor that the diff relay doesn't operate normal operation. Care should be taken for long CT cable, vector group of ICT, relay character etc,. During seting of relay all these considerations also to be taken care also during testing and commissioning.

Author : biswanath.das
Thu, September 13, 2007 - 11:29

The precise way to perform this protection would be to install two different protections: one for the short line and another to the power transformer.

But presently an overall solution already exists.

One manufacturer is employing this solution in a plant I am working now.

A conception of IEDs is adopted and a line differential is located in the substation and connected to the local currrent transformers.

A fiber optic is deployed between the substation and the power transformer.

The power transformer has a secondary BCTs set connected to a regular differential relay and the primary to HV BCTs set, The fiber optics is connected to this differential transformer and the line will be protected and the power transformer as well 
Author : elbf2801
Thu, September 13, 2007 - 11:30
Yes, the IEDs conception is very good technical solution.

In deed, in that protection conception, several protection zones that are separated by suitable current transformers are supported by one intelligent digital system.

But in our protection system that subjected in this thread, the secondary current of one set of HV BCT is replaced by summation currents of two sets of substation current transformers.

Is this correct?

Author : Hamid
Thu, September 13, 2007 - 11:31

Summation Of CT secondaries is the only way in view of the circuit requirement. The need is to input relay with the correct secondary current. Low impedance relay requires less stringent requirement on CTs.

Author : Chris Chew
Thu, September 13, 2007 - 11:31
When all individual secondary through fault currents of differential protection system are not applied to bias element of relay directly, we can not ensure about protection stability ,because the bias characteristics of relay can not active against all current elements that involved to protection system.

Author : Hamid
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