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Problem with delta to wye VT's on HRG system
5

Problem with delta to wye VT's on HRG system

Problem with delta to wye VT's on HRG system

(OP)
I'm involved with the replacement of the diesel genset on a 480V high resistance ground system.

As part of the project, a Schweitzer SEL-547 distributed generator interconnection relay is replacing several individual relays protecting the utility interconnection breaker. The OLD relays were all connected using two VT's in an open delta configuration.

The SEL-547 needs a four wire wye voltage input, and is designed for a direct connection to a 480V system without VT's, which initially sounded quite convenient. However, we were advised against such a connection in an HRG system. Something about the neutral begin unusable.

The recommended solution was to connect three VT's in a delta-to-wye configuration, thus providing a four wire wye output to connect to the relay. In order to supply 480 volts L-L on the secondary, a VT ratio of 1.73:1 was needed. The only VT's readily available at that ratio were actually labeled for 208:120 V service.

This scheme was wired up and the primary 2A fuses were closed with the secondary fuses open. The VT's heated up quickly and either two or three of the primary fuses blew.

Is the problem simply from using the VT's above their design voltage, or is there something more complex at work here?

Thanks,
John

RE: Problem with delta to wye VT's on HRG system

Check if ferroresonance is a possible cause.

RE: Problem with delta to wye VT's on HRG system

Do I understand that you applied 480 volts to a 208 volt rated transformer winding?
respectfully

RE: Problem with delta to wye VT's on HRG system

2
You need to start over and do it right with three VTs in wye from the lines.  That initial open delta configuration blocks the zero-sequence voltage and putting another set of VTs in wye downstream of the delta VTs won't get it back.  If the relay needs a 4 wire VT connection, you need to have a true 4 wire VT connection rather than something that looks like a 4 wire VT connection but is missing critical information.

RE: Problem with delta to wye VT's on HRG system

(OP)
Hello davidbeach,

The original open delta VT's are gone. Three new ones were installed, but their primaries are connected in "full" delta with their secondaries connected in wye. All of this was done to derive a "usable" neutral. With that clarification, please expound a bit on what we should do differently.

Thanks.

RE: Problem with delta to wye VT's on HRG system

You should have connected the primaries in wye with the wye point grounded.  VTs should never be connected delta-wye, and in general I can see no reason to ever connect VTs in any configuration with a delta primary.  All you do with a delta primary is throw away all of the zero-sequence voltage information; information that can never be recovered.

Unless you are using the secondaries of the VTs in a broken delta (not open delta) to directly measure 3V0, the secondaries should also be connected in wye.  If connected to a grounded system, the wye point of the secondary should be grounded.  If connected to an ungrounded system, the grounding of the VT secondary needs to be evaluated and it may be necessary to ground one of the phases (generally B) rather than the wye point.  This is to avoid having a ground fault on the secondary be detected as a ground fault on the primary.  On solidly or low impedance grounded systems the VT secondary ground fault can't draw enough primary current to be an issue.

In the days of electromechanical and solid state relays, the 2-VT open delta configuration probably had a reasonable economic explanation as the V0 (or 3V0) information was generally not used and there was no way of recording it for later analysis.  Now with numeric relays, the oscillography produced during faults can be vastly more informative if the VTs are connected in wye, plus there are things the relay can do with the voltages that used to be much more specialized.  Fault analysis is much easier if you know the phase-to-ground voltages in addition to the phase-to-phase voltages.

RE: Problem with delta to wye VT's on HRG system

(OP)
Thanks for all of the good info, davidbeach.

Like I said in the beginning, the relay is designed to be connected directly without VT's, for systems up to 480 V. Do you think I have any chance of making this work with the three phase voltages connected directly to the relay (through fuses, of course) and the relay's neutral input grounded? This was my plan before I was warned by this other fellow about possible problems in an HRG system.

Thanks

RE: Problem with delta to wye VT's on HRG system

I really can't go into the specifics of how to connect or use an SEL relay as they are the competition.

I doubt that a directly connected relay would impact the system grounding.  If the voltage inputs on the relay (any relay) have a phase-to-ground voltage rating equal to the phase-to-phase voltage of the system it should be possible to connect them to an HRG system.  If the phase-to-ground rating of the relay is less than that, you need the VTs.  On an ungrounded system (capacitively grounded system) you should always have VTs between the system and the relay.

RE: Problem with delta to wye VT's on HRG system

The reason transformers burnt up because, as waross, suspected, you applied 480V to 208V rated transformer windings.

You still need three (3) 480V:480V voltage transfomers with proper VA rating, to follow davidbeach's advice.



RE: Problem with delta to wye VT's on HRG system

I'd suggest you contact SEL - they have great technical support.  The connection of wye connected VT's to a high-impedance grounded system should concern you as the neutral to ground voltage can get very high.  Obviously, the use of 208 volt VT's on the 480 volt system speaks for itself...................

RE: Problem with delta to wye VT's on HRG system

I recently hooked up 3 PT's in a wye-wye configuration on a high resistance grounded system and it did not work well because the voltages on the primary were not perfectly balanced (1% unbalance). The problem is with the neutral floating on the primary side of the PT's the voltages on the secondary become distorted. My power quality analyzer said I had 18% THD.
The NEC in 250.36 does not permit the primary side of the PT's to be grounded on a high resistance grounded system as this would constitute a line to neutral load. Therefore on a high resistance grounded system you are probably better off hooking the PT's in open delta or delta-delta.
I also tried hooking up the PT's in a delta-wye configuration. The problem with this hookup is that the KWH meter and the motor protection relay's are not designed to take into account the 30deg phase shift on the voltage. This is a problem in that the power factor and therefore the power are calculated wrong.

RE: Problem with delta to wye VT's on HRG system

The NEC deals with instrument transformers (VTs and CTs) separately from any type of load and considers them something distinct from any thing else connected to the system.  In this case the VTs need a full 480V rating.  I'm not sure why you (TestBeforeTouch) would have considered your VT installation to have been a failure; if the VT secondary accurately reflected the primary, where was the problem?  Wye connected VTs are regularly used on systems of all grounding types.  250.36 would not apply any way as the VTs are not connected to neutral, the wye point of the VTs is grounded, always, regardless of the system grounding.  The relay needs to know the voltage to ground, voltage to neutral would be of little use in a HRG system.

RE: Problem with delta to wye VT's on HRG system

(OP)
TestBeforeTouch, thanks for your post. Good to know someone else has been in a situation similar to ours. We don't feel quite as crazy for trying the delta-wye connection now. What was your final solution?

As for applying 480V to a 208V VT, another member of the team convinced me that a voltage ratio was a voltage ratio, and as long as it was a 600V class device, we'd be OK. In hindsight, we would never think of doing that with a power transformer, so we shouldn't do it with a VT, either.

davidbeach, from you posts it sounds as if you recommend we should always ground the primary wye point, and either ground the secondary wye point or float the secondary wye point with a phase grounded instead. What factors would you evaluate to determine which secondary node to ground?

If it helps, the grounding resistors on the the generator and the utility transformer are sized for 5 amps.

RE: Problem with delta to wye VT's on HRG system

The problem I had with an ungrounded wye on the primary is that the station KWH meter gave false information saying I had 18% THD when, in fact, I did not. The THD was due only to the ungrounded wye connection on the primary. The secondary line to ground voltages did not reflect the primary accurately because with slightly unbalanced voltages on the primary it caused the neutral point to have a sine wave on it, therefore distorting the secondary voltages.

There is a good chance I am missing the section, but I cannot find where the NEC allows you to hook the primary of the instrument transformers directly to ground.   

RE: Problem with delta to wye VT's on HRG system

I did a search on the word "instrument" in the NEC and there is no prohibition on connecting the VT wye point to ground.  Otherwise there would be no means of measuring phase-to-ground voltage.  Leaving the wye point floating will cause all manner of trouble, might as well not bother with the VTs if you are going to leave the wye point floating.  The NEC is woefully lacking when it comes to use of instrument transformers and relays, tending to concentrate too much on fuses and circuit breakers with internal trip units.

Evaluate a ground fault on the secondary of the VTs.  If the current on the primary of the VT is high enough to cause a zero-sequence current relay to pick up, you should ground a phase of the secondary (usually B) rather than the wye point.  The relay (or meter) will still see accurate voltages, but "ground" reference used by the relay (or meter) won't actually be ground.

RE: Problem with delta to wye VT's on HRG system

I believe the NEC does not allow you to connect the PT primary wye point to ground. The grounding electrode conductor is only allowed to be connected at one point per 250.36 (F).   

If the PT primary is connected to ground then current would normally flow over the grounding equipment (if the voltages are unbalanced) and would cause a voltage to appear on the neutral resistor.

It may be that the only way to correctly connect PT's on a resistance grounded system per the NEC is to use a delta connection. This does not give you phase to ground voltage but phase to ground voltage loads can't be used anyway. An ungrounded wye would be legal but as I have found out does not work correctly.

I believe that if you have a solidly grounded system a wye connection would be allowed, but only if you connected the star point to the neutral conductor and not the ground, otherwise you violate 250.30 (A).

RE: Problem with delta to wye VT's on HRG system

On what grounds do you base your belief that the NEC won't allow a proper connection of a VT?  The VT is not a load and there is essentially no current flow.  I've seen many installations with wye connected VTs on systems required to comply with the NEC and in all cases the wye point of the VTs is connected to ground.  A grounded-wye - grounded-wye transformer does not provide a system grounding point, NEC 250.30(A) does not apply to the VTs.  By your logic, it would be impossible to have any means of detecting ground faults on ungrounded systems and no means of monitoring the ground shift associated with ground faults on HRG systems.

The NEC 250.21 requires ground detectors.  It is impossible to meet the ground detector requirement without connecting something between phase and ground.  Remember this is a code requirement to connect something between phase and ground.  In 480V systems, this is sometimes done using light bulbs (3 - 240V incandescent lamps in series between each phase and ground).  In many 480V systems, and almost universally at higher voltages, the only way to do this is using VTs connected phase to ground.  Again that is to meet a specific requirement of the NEC.

So, once again, what basis do you have for you belief that connecting the wye-point of a set of VTs to ground is a violation of the NEC, particularly when the NEC actually requires this connection?

RE: Problem with delta to wye VT's on HRG system

I am just trying to have an exchage of ideas in a non confrontational way.

It is not necessary to connect something phase to ground to detect a ground fault. A very common connection is  to measure the current through the neutral grounding resistor  with a CT.  A second method is to measure the voltage across the neutral grounding resistor (neutral to ground not phase to ground). A third method is to use a zero sequence CT.

I am aware that this is a common practice (common practices though are not always correct by the code, e.g. the issue of a obtaining a quiet electronic ground ).

But looking at it strickly from what the code book says I can't find a section that says it is allowed. I can find, however, where one could intepret that it is not allowed as I stated in the references above.

I do agree with you that the code is lacking in this area.

RE: Problem with delta to wye VT's on HRG system

Hi folks:
I am wondering what conditions the relay is intended to react to? Am I wrong in thinking that under conditions of ground current flowing, the phase to neutral phase angles and magnitudes will be different from the phase to ground phase angles and magnitudes.
If the meter is intended to monitor power functions such as KW, KVA, KVAR, and power direction, does it not have to have the wye point connected to the neutral to maintain the correct phase angles with the current transformers?
Likewise, if the wye point is grounded, the relay will be able to infer ground faults, but it will not be able to accurately monitor power functions if there is any unbalanced ground current leakage or faults.
In regards to potential transformers connected to ground on HRG systems:
I would assume that the currents drawn by the PTs will be very little.
Can you take the relay burden and divide by the applied voltage to estimate the PT primary current? The fundamental currents will cancel at the wye point and only harmonics will flow in the grounding resistor. Aren't we looking at a small percentage of a small current flowing in the grounding resistor as a result of grounding the wye points. The voltage resulting from the PT current flowing in the grounding resistors would be so low as to never pose a hazard.
davidbeach, I am asking here, and I respect your expertise in this field. If my comments are in error, please edit or red flag this post out rather than let me cause confusion.
thanks
respectfully

RE: Problem with delta to wye VT's on HRG system

I concur with davidbeach. It is wuite common to connect VTs line-to-ground in a HRG system.

As to Waross's question....yes, you can approximate the current flow in the VT winding by taking the applied burden and secondary voltage to derive a secondary winding current then reflect that the the primary side. Of course, you also have to include the excitation current of the VT, which is also very low. With modern electronic relays and meters, the current in the 480V VT's primary winding is probably going to be in  the order of 2-3 mA.

RE: Problem with delta to wye VT's on HRG system

It seems to me that whether you use line-to-neutral or line-to-ground depends on the purpose of the measurement.  If you need to measure power or voltage to neutral, I don't think the line-to-ground connection will measure the correct value.  According to the SEL-547 manual:

Quote:

The connection is made directly—there are no interposing voltage
transformers. These voltages are used in the following protection and control
elements:
- Undervoltage tripping (device 27)
- Overvoltage tripping (device 59)
- Over- and underfrequency tripping (device 81)
- Reverse-phase-sequence (i.e., negative-sequence) alarming
(device 47)
- Directional power tripping (device 32)
- Synchronism check close supervision (device 25)
The manual does say connect to the neutral, not the ground, although the relay is meant for direct connection, not through VTs.  The voltage inputs are rated 600 volts L-L.  Why not follow the instruction manual and direct connect the 480 volt inputs?  The OP indicated that they were advised against using direct connection on a HRG system.  Was this advise by Schweitzer?  If not, what is Schweitzer's advice?

RE: Problem with delta to wye VT's on HRG system

It can get it bit tricky.  As I stated earlier, I won't/can't address the specifics of an SEL relay, but I can discuss the general issues.  If you want to discuss a certain, different, Intertie Protection System I would be glad to go into specifics of the relay.

In an HRG system, the neutral does not leave the location of the grounding resistor and is not available for connection to VTs or for direct connection to the relay.  Ground should be available everywhere in the system.

I'm not familiar with "power to neutral"; as far as I know, power is power and it can be calculated using phase-to-phase voltages in a manner such that voltage to ground or voltage to neutral is irrelevant.  In a three wire system, which all HRG systems are, you can measure power, real, reactive, and apparent, using the two wattmeter method and have no reference to ground or to neutral.  This all that would be available if the VTs were connected open delta.

If the relay is attempting to monitor for ground faults on an ungrounded or HRG system (remember the neutral does not existing in an ungrounded system and is not available in an HRG system) the relay needs to know the individual phase-to-ground voltages.  This can only be achieved if there is a ground connection on the primary of the VT.

If the relay is attempting to provide a complete protection package in compliance with IEEE Std 1547, there is no way that it could depend on any measurements from the neutral grounding resistor in an HRG system.  There are no provisions in IEEE Std 1547.1 (the testing standard for compliance with 1547) for testing a system that would require inputs from a CT on a neutral grounding resistor or voltage across the neutral grounding resistor.

The current into the voltage terminals of a relay is somewhere close to zero, on the order of a few milliamperes.  A circuit of sufficient size to warrant relay protection is very likely to have more current to ground through the system capacitances than through the VT.  As you move into the higher voltages, the current into the primary of the VT is reduced by the ratio of the transformers, so that very small value becomes even smaller, and the high voltage increase the leakage through the system capacitance.  The VT primary current is essentially noise lost in the leakage currents.  The current necessary to feed the losses of the VT are likely to be greater than the current being transformed to the secondary.

jghrist mentions a line-line voltage rating of the relay in question that would allow for direct connection, but for connection to an HRG system it would also need the same line-ground voltage rating.  I would not direct connect any relay to an ungrounded system and would apply surge arrestors of the VT secondary if applied to an ungrounded system.

If the relay is given each phase-to-ground voltage, it can calculate the corresponding phase-to-neutral voltage.  If the relay is given each phase-to-neutral voltage, it knows nothing of phase-to-ground voltages and can not calculate them.

This is pretty much all settled state of the art, and it goes back many years.

RE: Problem with delta to wye VT's on HRG system

I agree with your analysis. The ground current is very small, probably insignificant compared to charging current, I think the code needs to be revised to clarify this situation.

I am not familiar with IEEE 1547 or the term complete protection package. Does IEEE 1547 require voltage detectors? The NEC only requires ground detection without saying how it is to be done. One manufacturer,Startco, uses a CT on their SE-330 to measure ground current through the NGR.

In addition it is common practice to use a single phase transformer in the neutral of the transformer with a resistor on the secondary of the single phase transformer and then measure the secondary current or voltage to detect a ground. Does this violate IEEE 1547?

RE: Problem with delta to wye VT's on HRG system

Gotta disagree with DavidBeach on the power. Blondel says count the conductors and subtract one. Since ground can be part of this circuit, I count four. It is true that under un-faulted conditions two element metering will suffice, but once that ground resistor starts conducting the metering will be incorrect. Might not be a huge error, but it is still an error. Unlike LRG systems, HRG systems can operate continuously with a ground fault, so the energy error can add up over time.

RE: Problem with delta to wye VT's on HRG system

OK, stevenal, you're right.  I was only thinking of unfaulted conditions.  You are right, a standing fault could mess things up.  To me, as a relay guy, metering takes second place to having the protection work correctly.

On the other hand, I believe that with all three voltages to ground and all relevant currents (including neutral on systems with phase-to-neutral loads) the relay can do sufficient mathematics to accurately (within the limitations of the instrument transformers used) calculate power values.

I think that, to me, the important point is that there is valuable protection information available if the VTs are connected in wye and the wye point of the primary is grounded.  Information that would not be available to the relay if the wye point is connected to neutral or if the VTs are connected in delta.

RE: Problem with delta to wye VT's on HRG system

(OP)
Guys,

Thanks for the great discussion. I'm going to print all of this out and sit down with our team to figure out how to best apply it to our situation.

John

RE: Problem with delta to wye VT's on HRG system

Three element metering using line to ground voltage has the same limitation as two element metering. Consider a grounded phase; the phase to ground measurement is zero volts on that phase, so no power is measured on that leg. The three element metering is essentially two, and the power dissipated in the grounding resistor is not measured. You could capture it with a fourth element picking up neutral to ground voltage and neutral current. Or if the IED in question allowed you to input the resistor size, you could simply measure the neutral current. If you are satisfied with just measuring the power going to the load, the phase to ground voltage measurement will suffice. If you want the total generator load, though, you'll either need to use three element metering in front of the resistor using line to neutral voltage, or include another element or half as suggested above.

RE: Problem with delta to wye VT's on HRG system

I think that the correct answer may be to "Bite the Bullet" and instal six VTs. Three VTs for power monitoring, connected phase to neutral, and three VTs connected phase to ground for protective relaying. If the instrumentation will not accept six voltage inputs, then decide whether you want power monitoring, or protective relaying, and connect accordingly.
One connection may not be suitable for both jobs.

Power will be correctly monitored with the VTs connected to the Neutral. The phase angles between the voltage and the current will be the same as the phase angles of the loads on the three phases, so PF, KW, KVAR, and KVA will be accurate. Any ground fault will be a single phase load added to the load on the grounded phase and will be properly metered.

If we assume a load current of 1200 amps and ground current limited to 5 amps, then when the VTs are connected line to Ground And there is a Ground fault:
The load currents and the load current phase angles will stay the same. (plus 5 amps on one phase)

While the voltage to the loads remains unchanged, the voltage reported by the VTs will drop to zero on one phase.
The voltage on the other phases will rise to line voltage.
The angular displacement of the non zero VTs will change from 120 degrees to about 60 degrees.

I think it is clear.
For power monitoring, connect the VTs to neutral.
For fault monitoring, connect the VTs to ground.
If you try to serve both functions (power and protection) with one set of VTs, one function will be unsatisfactory.
respectfully

RE: Problem with delta to wye VT's on HRG system

waross, excellent points.  If we check the original post, the question is about a relay, therefore protection.  In an interconnection application it seems to me (a relay guy) that getting the protection right is far more important than getting the metering right.  Yes, modern relays have metering functions, whether the relays I deal with or the relays of those other guys, but none the less they are still relays, and relays are for protection.  Therefore, it is always more important to get the protection correct than to get the metering correct.  No relay (using protection grade instrument transformers) will ever provide revenue grade metering, so if you really care about the metering, use the right instrument transformers and use a dedicated meter.  If you want protection and would like to get some metering information at the same time, use a relay and get metering information from the relay, but remember it is a relay and connect it like a relay.  To my understanding, this thread was never really about metering, but was started with a protection question.  So, if I can get metering that is close enough under non-fault conditions that would seem to be good enough for me, but I absolutely want the protection to work correctly.

RE: Problem with delta to wye VT's on HRG system

Hello davidbeach;
Thank you for your comments and your explanations.
I was trying to resolve the controversy over the correct connection method, and to put in perspective, the comments of the protection camp, and the comments of the metering camp.
Hopefully, jwmccoype will be able to sort the conflicting advice into the "metering solution" file and into the "protection solution" file.
It is not a question of "Which connection is correct?" as it is "What is the intended application?"
I think you are probably correct that this is a protection problem.
Respectfully

RE: Problem with delta to wye VT's on HRG system

Davidbeach-

I agree with your last comments in principle, although, it should be noted that almost all VTs have a metering accuracy of class 0.3 in the US. They can normally be used for protection AND metering.

RE: Problem with delta to wye VT's on HRG system

I don't agree that phase to ground voltage is preferred for protection. From the User Guide: "All voltage pickup settings are in percent of Vnom (Vnom = setting NOMV/sqrt(3) = nominal input voltage, line-to-neutral)."

If you substitute line to ground voltage for the above, undervoltage tripping will occur for line to ground faults. This eliminates the ride-through advantage of having a HRG system.

I need to admit, after browsing the manual, that this particular device makes a really poor meter. Seems it just looks at Ia and Van and multiplies the calculated values by three. One element metering, so much for Blondel.

RE: Problem with delta to wye VT's on HRG system

I think the protection issue was WAY overlooked here.  I have installed / commissioned many of these types of Small/Medium 480 Gensets.  The issue, I believe is providing Ground Fault protection.  Since the system is HRG, measuring the currents in a typical residual connection would be difficult.  Slapping a 50A CT on the HRG and feeding that into a current input would suffice.  This would be the 50/51N Function.  Grounded Wye/Open Delta with a resistor across the open part, feeding a 59G is another method for GF detection.

As someone mentioned earlier, the 547 is intertie protection.  Basically IEEE 1547 requires the following protective functions

32-Reverse Power
27-Under Voltage
59-Over Voltage
81 O/U Frequency.

The protective functions are provide to protect the Utility in the event of down stream faults.  

Connect the relay as SEL recommends.  If there is not adequate protection for the generator, consider additional relaying.

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