overflux relay 1

[144x]

from which side an over excitation relay should
be fed?the HV side CVT of the transformer or the LV side CVT
thanx.

 

[redtrumpet]

HV.

 

[jwerthman]

I disagree. Ideally, the overvoltage and overflux detection should be on the secondary side of the transformer because the transformer capabilities are defined by ANSI from the secondary side. ANSI permits 105% continuous full load V/Hz on the secondary side, which could be 110% or more on the primary side, depending on the impedance.

Many times the voltage is not available at the secondary in a location that would always be energized whenever the transformer primary is energized. So it is quite common to connect the V/Hz detection to a primary VT. But the secondary voltage is still a better correlation to the defined capabilities.

 

[redtrumpet]

I admit my response was hasty. However, if the intent is to identify overexcitation of the transformer, I would want to know what the primary voltage was. I have worked with transformers with +/-15% range on the secondary OLTC. In other words, primary voltage could be 115% of rated and the secondary voltage measurement (load side of OLTC) could indicate rated secondary voltage and no problem.

 

[redtrumpet]

jwerthman - okay, that last post didn't really make sense. Obviously, the transformer should be built to withstand the excitation associated with operation at the tap extremes. However, considering that the transformer receives its excitation from the primary, that's where I would want to measure voltage - for the reason you gave, since the line voltage is more likely to rise with the load off (ie. transformer secondary breaker open), and the secondary VTs may or may not be located on the line side of the secondary breaker.

However, your point is well-taken and measurement on the secondary probably is more in line with the transformer capability. You have given me food for thought.

 

[144x]

jwerthman
I'll give you 2 stars .one for your good answer and
the other for not giving one word answers.

 

[electricpete]

It sounds like the question has been answered well.

just thinking out loud a little bit:

Primary leakage reactance would represent flux linking primary coil but not secondary coils or core.

Secondary leakage reactance would represent flux linking secondary coils but not primary coils or core.

SO... current flow thru the primary leakage reactance causes a drop in voltage from primary terminals to the voltage associated with core dPhi/dt.

Current flow thru the seoncary leakage reactance causes a drop in voltage from the voltage associated with core dPhi/dt to the voltage at the secondary terminals.

So by my way of thinking the per-unit voltage which will represent the core flux condition is somewhere between that on the primary and that on the secondary.

I believe for core form construction geometry with lv coil inside of hv coil the lv leakage reactance would be very small and therefore secondary side more representative.

Of course the standard is what's important.. I'm just thinking out loud. Does it sound reasonable?

 

[jwerthman]

rt - In the case of a OLTC transformer, I completely agree that measuring the V/Hz on the secondary side would be meaningless.

ep - I think your logic is correct, but obviously no practical way to really measure the V/Hz at that point. In spite of the ANSI capability definition, I would say that most of the V/Hz protection I have seen (and applied myself) have been on the primary side.

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 242-2001 Buff Book
Section 11.7.8 Overexcitation Protection indicates:
Direct-connected generator transformers are subjected to a wide range of frequency during the acceleration and deceleration of the turbine. Under these conditions the ratio of the actual generator terminal voltage to the actual frequency shall not exceed 1.1 times the ratio of
transformer rated voltage to the rated frequency on a sustained basis:
(generator terminal voltage)/(actual frequency) ≤ 1.1 x (transformer rated voltage)/(transformer rated frequency)
This implies that the overexcitation protection is on the generator side of transformer, which is usually LV side of the transformer; however, in general it could also be the high voltage of the transformer, which would be a rare case.

 

[arminalidoosti]

well I faced to this problem as a young engineer and after discusion with experts I came to this conclusion finally:


1- POWER PLANT SWITCHGEAR: in this case the transformer is directly fed by synchroneous genrator. in this case you should protect the transformer from overflux due to unwqanted voltage and frequency fluctuation from the generator so position the relay on the LV side (generator side).

2- TRANSMISSION NETWORK: position the overflux protection on the HV side sine the frequency fluctuation due to feroresonant is common on HV side .

3- DISTRIBUTION NETWORK: no need to overflux protection since you dont expect any frequency fluctuation on distribution network ,just voltage fluctuation so the overflux is used as overvoltage on the secondary (LV) side.

 

[arminalidoosti]

well I faced to this problem and after discusion with experts finally I came to following conclusion:

over flux relay positioning is depend on the voltage level and application of the tranformer (step up or step down )

1- POWER PLANT SWITCHGEAR: in this case the transformer is directly fed by synchroneous genrator. in this case you should protect the transformer from overflux due to unwqanted voltage and frequency fluctuation from the generator so position the relay on the LV side (generator side).

2- TRANSMISSION NETWORK: position the overflux protection on the HV side sine the frequency fluctuation due to feroresonant is common on HV side .

3- DISTRIBUTION NETWORK: no need to overflux protection since you dont expect any frequency fluctuation on distribution network ,just voltage fluctuation so the overflux is used as overvoltage on the secondary (LV) side.