lz5pl
Electrical
- Feb 6, 2007
- 313
I have difficulties to determine relay protection settings for the following case:
We have block "110 kV cable - 110/20kV transformer". Cable is 240 sq. mm, 700 m long (I suppose it's impedance could be neglected). Transformer is 10 MVA, Uk=11%. CT's on supply side (110 kV) are 400/1 A, on remote end (20kV) are 800/1 A. Relay protection is RED670, interconnection via FO-cable, and it covers both line and trafo.
My problem is to deside about Id> and Id>> values. Nominal current of the transformer is very small ( only 52 A), compared with CT's. But CT's are chosen because of other considerations, mainly high short-circuit level in 110 kV switchgear - it is 19 kA, and necessary connection to existing electromechanical busbar differential protection. (BB-protection is on a separate core, but anyway it is difficult to obtain CT's with so different cores, especially when one of them to be with low CT-ratio and high saturation level.)
So, what to choose as a reference for tresholds calculation ? In Application Manual for this relay it is suggested to use as IBase (defined by ABB as "Base (refferent) current of the differential protection") the nominal value of CT's - in my case 400 A. But if I choose standard (and minimum possible for setting) value Id> = 0,2 IB, it will be 80 A primary, more than the nominal current of the transformer. Usually for standard 87T function we use 0,2 In trafo.
I hope somebody here already has faced similar problem - what is your advice?
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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
We have block "110 kV cable - 110/20kV transformer". Cable is 240 sq. mm, 700 m long (I suppose it's impedance could be neglected). Transformer is 10 MVA, Uk=11%. CT's on supply side (110 kV) are 400/1 A, on remote end (20kV) are 800/1 A. Relay protection is RED670, interconnection via FO-cable, and it covers both line and trafo.
My problem is to deside about Id> and Id>> values. Nominal current of the transformer is very small ( only 52 A), compared with CT's. But CT's are chosen because of other considerations, mainly high short-circuit level in 110 kV switchgear - it is 19 kA, and necessary connection to existing electromechanical busbar differential protection. (BB-protection is on a separate core, but anyway it is difficult to obtain CT's with so different cores, especially when one of them to be with low CT-ratio and high saturation level.)
So, what to choose as a reference for tresholds calculation ? In Application Manual for this relay it is suggested to use as IBase (defined by ABB as "Base (refferent) current of the differential protection") the nominal value of CT's - in my case 400 A. But if I choose standard (and minimum possible for setting) value Id> = 0,2 IB, it will be 80 A primary, more than the nominal current of the transformer. Usually for standard 87T function we use 0,2 In trafo.
I hope somebody here already has faced similar problem - what is your advice?
------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant