Multiple Resistor Grounding 1

[joan271273]

When you have several generators in paralell that are grounded via resistors the "brief in nature" information I have indicate that sizing shall account for circulating currents.This is also stated when discussing having all generators grounded through a single resistor.

Can someone provide input to the following:

1.- Circulating currents in the situations described above.

2.- Single or Multiple resistor grounding. Pro's and Con's

3.- Reference Material. [sig][/sig]

 

[jbartos]

Are you sure that they are not using CBs? Please, notice that you have not provided the alternate product commercial reference. Therefore, some traceable evidence it overdue to support your statements.

 

[jack6238]

For joan271273. I want to answer your question concerning how to determine the magnitude of the circulating current in parallelled generators. My review of the previous postings did not reveal anyone providing information on this topic.

I have installed many generator systems and have always used the following method to determine if this is a real or imagined problem. If the problem is real, then how does the circulating current effect the generator and/or the neutral grounding device(s)? First a little background.

In general, only third harmonic voltage can cause circulating currents in the neutral of the power system. Equal third harmonic voltages will exist in all three of the generator phases and these voltages are in phase. Thus these voltages will only be effected by the zero sequence circuit impedances. The magnitude of the circulating current is dependent on the third harmonic voltage of the individual generators and the third harmonic impedance of the zero sequence circuit. Other triplen frequencies are generally so small that only the third harmonic voltage needs to be considered.

The generator(s) zero sequence impedance is usually provided by the manufacturer and this value has an impedance which is three times as large as the fundamental frequency impedance stated on the generator data sheets. Thus, if a generator has X0= .06 per unit on the data sheet it will have a value of 3x0(.18 per unit) in your equivalent circuit. The voltage behind this impedance is the per unit third harmonic voltage. Now the third harmonic voltage is varies with the load on the generator and will be a maximum at full load on the individual machines. I have seen values on American designed machines which vary between .015 to .03 per unit from no-load to full load. Foriegn made units typically have higher third harmonic voltages. I have installed ASEA generators where the third harmonic voltage at full load is nearly .20 per unit. I might also add that the third harmonic voltage is related to the winding pitch and the majority of steam turbine units (3600 RPM) use a winding pitch of 5/6. Units operating at other speeds such as 1800 RPM diesel units may have third harmonic voltages slightly higher than mentioned above.

With multiple generators there are several conditions which should be considered. If the generators are identical, equally loaded and have equal third harmonic voltages and these voltages are in phase, then a third harmonic current will flow between the generators and any other neutral grounding device on the system. This could be a transformer connected to the bus which has a grounding resistor or one that is solidly grounded( I hope you don't have that). Note that no circulating current will flow between the two generators-only between the generators and the other neutral grounding device. If the two(or more) generators have unequal third harmonic voltages, then a circulating harmonic current will flow between the generators. Depending on your single line, both conditions may require evaluation.

Without the presence of a transformer or other neutral path, the zero sequence circuit consist of two third harmonic voltages in series with their individual grounding resistors and the third harmonic value of the zero sequence impedance of the generator. Note that the external resistor will be in the circuit as 3R and it does not require additional modification. If a grounding reactor is used then the correct value to be used in the equivalent is 9 x ( 3 X because it is in the neutral circuit and a multiplier of 3 because it is exposed to the third harmonic voltage). The case which will result in the largest circlating current is one in which the voltages are out of phase.

Next step is to convert the per unit impedances to a common base and calculate the circulating current between the units. Let's assume you have made the calculation and have determined that the circulating current is .10 per unit. Let's further assume that the generators are operating at a full load condition of 1 per unit cuurent. What remains to be calculated is the amount of derating associated with this circulating current. Given a 1 per unit load and a .10 per unit third harmonic current, the equivalent load on the machine is the square root of the sum of the squares. In this case, the effective load on the machine is 1.00498-call it 1.005. This means that the generator output has to be limited to .995 of the machine rating. You must agree that this is not of a derating for the conditions outlined.

Does the circulating current cause any problems with a neutral grounding device? After determing the circulating current, you need to evaluate the effects on the neutral gorunding device. I believe the existing standard for neutral grounding device grants an implicit continous capability which is a function of the short time rating. In addition, I remember the short time rating for the third harmonic current is to 15 % of the implicit continous current rating. Basically this means that a resistor which has a 10 sec rating can have a continous third harmonic rating of .45 %, a 1 minute rating of 1.05 % and a 10 minute rated resistor has a 4.5 % continous third harmonic current rating.

When I started this I said that it was necessary to determine if the problem is real or imaginary. Get the data on the third harmonic voltage from the manufacturer,alos the zer sequence impedance and then make the calculation. If you do not have or cannot get this data, I have it for machines with ratings from 4MW -100MW for American and some foriegn made units. Let me know if you need this and I will send.

In closing I wnat to say a brief word about Beeman. This is a very good book but it is introductory and somewhat out of date. I use it only for a quick reference and then go to my many files or other text books when I need detailed information.

One other word. If you go to a seperate grounding transformer for this application, the smallest KVA for any application is the Zig-Zag followed by the Scott-T and the Wye-Delta is last. The Zag-Zag is 15 % smaller than a Scott-T and 58 % smaller than the equivalent Wye -Delta. I've written a small paper on the size differeces and how to calculate the KVA of each.

Good luck on your application and it really sounds like fun!! _

 

[joan271273]

Thanks jack6238 for the excellent response and I really agree with the statement about Beeman. Hopefully this book would be revised and maybe a second volumen create to include added information.

 

[jbartos]

Suggestions:
References:
1. IEEE Std 142-1991 “IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems,” dated 1991
2. D. Beeman (cited above)
3. D. S. Baker (cited above)
Reference 1 appears to be one of the current authorities on the System Grounding. It refers to “D. Beeman” and “D.S. Baker” references mentioned above in postings among other references. It makes distinction between Resistance Grounding, par. 1.4.3, and Grounding Transformer method in par. 1.4.7 “Obtaining the System Neutral.” The Three-Phase Zigzag Grounding Transformer has its advantages and disadvantages (see D. Beeman page 349) and the more modern disadvantage is in its vulnerability to a voltage harmonic content appearing more and more in power distribution systems. Please notice that in "one parallel generator grounded only" of a group of parallel generators, the circulating currents (differential mode currents) will not propagate to the earth. Only common mode currents flow through the ground.

 

[jbartos]

Suggestion: There is a statement in: Reference:
1. Grigsby L.L. "The Electric Power Engineering Handbook," CRC Press LL, 2001
on page 5-7: "Load break disconnect switches have been furnished in the past, but with improvement and cost reductions of circuit breakers, it is not practical to continue to furnish load break disconnect switches, and a circuit breaker should be used instead."
Reference 1 has just been published; therefore, there is no need for any "underhand actions."