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