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Where there are several generators connected on a common bus, one method of system earthing would be to provide a separate earthing transformer and resistor. This ensures that the system earth fault current is the same no matter how many generators are on line. In order to protect the...

Generally, that would be a winding that has been treated to be resistant to fungus/mildew that would otherwise present a problem under hot & moist tropical conditions.

I had a rude awakening when I read the following in the IEEE PES PRSC report on Wide Area Protection and Emergency Control (talking about a WSCC disturbance on 10 August 1996)- "The conditions leading to this incident built up over a period of 1 1/2 hours before the disturbance started...

Here's a suggestion for you - read the energy meter at the start and end of each "overtime" period. This will give you the actual kWH used to provide the service, which should be a reasonable basis for the charge. Of course, if you have more than one tenant working overtime at any...

I fully agree that the system should conform to established standards for harmonic content for normal operation. However, what we are discussing here is abnormal operation with faults on the system - harmonics here are a more random function, depending on the type of fault or system abnormal...

Sounds similar to the case I mentioned above. In my case, the harmonic restraint was set too high and we could see from the fault oscillogram that the trip command was issued at the same time that the harmonic restraint reset. I played back the fault recording Comtrade file on a Pulsar test set...

Four years in service at a client's plant, no real problems. There was a case of tripping on through fault (these are quite frequent on this system) - turned out to be harmonic restraint setting problem. I managed to reproduce the fault from the recording (on a Pulsar test set) and proved the...

You are right, Pentatek - this is a separate question. Of course, operation of the overcurrent relays will be affected by any saturation of the feeder CTs - you have to evaluate the CT requirements for each case. In general, follow the manufacturer's specified CT requirements - my experience...

Pentatek - Recall that the high impedance scheme is ancient (somewhat like myself), tried and true. The scheme is based on allowing for complete saturation of the offending CT - no output at all. In the practical case, there will always be some output, even if the level of saturation is...

Pentatek - What type of busbar protection are you specifying? I would expect it to be a high impedance protection scheme, which has well developed criteria for CT selection. You would select the scheme setting voltage based on the assumption that one set of CTs has in fact saturated - the...

David707 - The crux of the visualization problem is that you are drawing the wrong vectors. For your scenario, you need to draw the currents in phase with V12 and V32 respectively (60 degrees apart) - the way the loads are connected, NOT V1 and V3 (120 degrees apart). Having done that, I think...

Instantaneous trips have to coordinate with downstream protections. Unless there are transformers downstream which limit the current to secondary faults, instantaneous settings may not be possible - some time delay is required. A coordination study is the only real way to answer the question.

OK, here goes - -Draw the equilateral voltage triangle 1-2-3 -As the load is resistive, the current vectors I12 and I32 are in phase with the respective voltages V12 and V32 -Add the vectors I12 and I32; the resultant is a vector with magnitude {sqrt(3)*I12}, at an angle leading V12 by 30...

Oops! Vector [b]SUM[\b] of the two is in fact sqrt(3)*I1 - I managed to confuse myself and subtracted the vectors. This corresponds with the readings that you took (10.4*1.73 = 18A).

The line 2 current is the vector sum of the line 1 and line 3 currents. Draw the circuit and the vectors and you will clearly see the conditions. In other words, line 2 current magnitude is the same as the other two lines.

Question on question 2 - Are these perhaps Completely Self Protected (CSP) transformers, with integral breakers for transformer protection? Otherwise, sounds like the principle of the TV picture tube self-destructing while protecting the 10 cent fuse.

PowerSim - Hopefully, the LV system switchgear bus is braced to withstand the maximum asymmetrical short circuit current forces. Also, close and latch capability of the breaker used for switching is sufficient to allow closing on to a fault. Interruption isn't the only consideration ... This...

RonShap - Please take into account DanDel & my comments re safety factors. "Poor but acceptable terminations/splices" are NOT significant impedances in the overall scheme of things (per Jghrist's post above) and will certainly not constitute an adequate safety margin. Bottom line -...

The relay measures the circuit voltage and current, as well as the angle between them. When the current lags the voltage by more than +/- 90 degrees, the real power flow is in the reverse direction - the reverse power relay is sensitive to the real component of the current (kW).

In either case, the subject impedances are essentially resistive and would have potentially minor impact on the system impedance, which is primarily inductive. This is a problem that many face, where electrical systems have been added to over the years and have outgrown their rating. Some...