There is an Industrial Plant with a 5

[electrageek]

There is an Industrial Plant with a Gas Turbine Power Plant connected to the same 138Kv substation. The Utility connects via a 15 mile 138Kv transmission line to the existing substations where there are three 70MVA delta connected transformers. Two of the transformers feed the industrial facility and the third connects to the Power Plant. The customer desires to be able to island from the Utility. The Utility end is a grounded wye 138Kv system. My question is, if we island we lose the ground reference supplied by the Utility, can we run without a solid ground reference? The Lighting Arrestors on the Transformers are 88Kv rated. By the way there are three PT's, connected phase to ground in the substation. One set of windings is connected to monitor the zero sequence voltage and trip the Power Plant offline.

 

[jbartos]

Suggestion to electrageek (Electrical) Jul 20, 2002 marked ///\\There is an Industrial Plant with a Gas Turbine Power Plant connected to the same 138Kv substation. The Utility connects via a 15 mile 138Kv transmission line to the existing substations where there are three 70MVA delta connected transformers. Two of the transformers feed the industrial facility and the third connects to the Power Plant. The customer desires to be able to island from the Utility.
///It is assumed that the Gas Turbine Power Plant will supply all energy for the island.\\ The Utility end is a grounded wye 138Kv system. My question is, if we island we lose the ground reference supplied by the Utility, can we run without a solid ground reference?
///Please, check and post the generator system grounding. Usually, the generators are connected to y-connections and the y-connection neutral provides the system grounding, e.g. high resistance, etc.\\ The Lighting Arrestors on the Transformers are 88Kv rated. By the way there are three PT's, connected phase to ground in the substation.
///Please post where those PTs are located and describe this in more detail.\\ One set of windings is connected to monitor the zero sequence voltage and trip the Power Plant offline.

 

[jwerthman]

It sounds like you want to backfeed one of the transformers from the generator to the 138-kV and feed the power back into the other two transformers from the 138-kV. The answer to your question is yes, that can be done. However, the 138-kV system would be operating ungrounded, which generally isn't a good idea, although it is done occassionally. It sounds like you have zero sequence voltage detection on the 138-kV, which should protect your arrestors and VT's in the event of a line-to-ground fault during this mode of operation. Just make sure that the tripping time is less than the volt-time rating of the arrestors. It would be best if the tripping were nearly instantaneous, but this would probably not be acceptable for normal operation in parallel with the utility. You might need two groups of settings - one for normal parallel operation to coordinate with the utility, and one for island operation that would be very fast.

If this is a rare occurrence, your current arrangement would probably be OK. If this mode is going to be more frequent, I suggest a grounding transformer on the 138-kV to maintain a ground reference when disconnected from the utility.

 

[electrageek]

jbartos:

I apologize for not being clearer. The two Generators are connected through one of the three transformer (13.8Kv Y to 138Kv Delta). The 13.8Kv secondary of the 70MVA transformer is also a low resistance grounded Y. The Generators are resistor grounded (low resistance grounded) The Power Plant will supply all the power for the attached Industrial Facility. We will probably need some sort of load shedding scheme. The 138Kv terminates on a dead end structure and then hits a horizontal bus structure. All three of the transformers are connected through disconnects or circuit switchers to the horizontal bus. The PT's are also connected to the bus. They are connected phase to ground and have two secondary windings (one for protection and metering and the other for the ground detection.

 

[busbar]

In the islanded condition, it sounds like there are three 138kV ∆ windings, ø-g arrestors and ø-g PTs? Offhand, a ground fault on the islanded 138kV system may damage the ground-connected components, and somewhat stress insulation in other equipment. Careful study and application of ANSI C57.13 and C62.22 is warranted.

The zero-sequence/ground-detection on PT secondaries is assumed to be a broken-delta/”59G” arrangement. Is there a circuit switcher between the GSU transformer and the 138kV bus? It would likely need to tripped via the 59G to protect the arrestors and PTs from overvoltage failure.

 

[electrageek]

Busbar

That is my concern. We lose the solid ground reference when the Utiltiy goes away. But I can think of quite a few 13.8Kv delta systems that are ungrounded whenever the generator is offline. The issue would seem to be risk. The 138Kv system is fairly small once the Utility goes away. That is assuming that we actually put in a SF6 breaker to allow islanding. I had thought about putting in a grounding transformer, at our end, but would need to check with the Utility about possible ground current problems. The PT's are connected winding 1 (Y - Y) and winding 2 (Y - broken Delta)

 

[busbar]

eg— The overvoltage ratings of Instrument transformers and surge arresters should be carefully weighed, provided in the first place there are the right circuit-interruption devices there to protect the system. They have ratings for short intervals that need to be well understood to be sure that relaying schemes prevent equipment damage and personnel injury.

The consensus standards for group 3 PTs severely restrict their overvoltage withstand to very specific limits. The “leftovers” from PTs and arrestors that have detonated from overvoltage are not pretty. I have not been in the presence of same while this was happening, but the remains are sobering.

 

[electrageek]

Thanks everyone

I believe I know what problems I am looking for now. I will look into the various ratings of the existing equipment and let the customer know what his risks are. They are planning on adding the additional equipment to make the Plant a combined cycle operation (steam and gas). The additional power will make the islanding possible.

 

[jbartos]

Suggestions:
1. Please, clarify the generator windings, wye or delta.
2. The generator transformer (13.8kV, Wye low resistance ground to 138kV Delta, no grounding) would be better off with the 13.8kV low resistance grounding disconnected and Delta-Wye or Zig-Zag grounding transformer added on in the 138kV side during the islanding condition. This will break the zero sequence current path from generator to the generator transformer, 13.8kV/138kV, which is highly recommended. It is assumed that the generator has the Wye connection with the low resistance grounding (pending clarification of my point 1 above).
3. The three transformer 138kV primary windings connected in Delta would experience some harmonic current circulations and could experience overvoltages caused by arcing since there would not be grounding. The suggested high or medium resistance grounding transformer could supplement the PT open delta protection scheme since an artificial neutral will be created and ground fault current monitored on the 138kV side. It would potentially eliminate overvoltages due to arcing.
4. Nature of load of the industrial facility is a factor since it can be nonlinear, in general, causing harmonics.

 

[electrageek]

jbartos

The Generators are Y grounded (low resistance). The secondary of the step up transformer is also Y connected and it is also grounded. (transformer grounded) I am not sure at this time what the actual loads are in the Industrial Plant. I think it is mostly typical motor loads. They process magnesium ore. What is the general rule in regard to sizing a grounding transformer? I would think it would depend on the amount of ground current to be allowed before tripping the primary power offline.

 

[busbar]

jbartos has a {qualified} good point. Load (‘grounding’) resistor should be sized to dissipate a fraction of the PT-set thermal-VA winding rating, based on 0.577 p-u of summed winding VA. Worst-case resistor-terminal voltage will be Vsecondary-tap x √3 x 1.05 [allowing for ANSI-limit bus-voltage excursions.]

700/1200:1 is the usual PT ratio for 138kV service. 700:1 ratio would allow for greatest watt dissipation of resistor in parallel with 59G coil—as long as ratio error would not decrease security or sensitivity of the ground-protection scheme. This effort would increase ground-fault current to highest possible level without other transformation, to minimize transient-overvoltage stress.

Locating a surplus 3ø transformer configurable to use the low-side broken-delta arrangement may be difficult. A resistor would possibly be most cost effective in 600V class. A resistor placed in the high-side wyepoint to ground would likely be impractical.

A more desirable alternative to a full-blown grounding transformer for the 138kV bus may be to use a grounded-wye/broken delta scheme set up with a set of 50-100kVA auxiliary station-power transformers {avaialbe from at least a couple of HV instrument-transformer producers.}

The charging zero-sequence impedance of connected equipment must be higher than that of the grounding bank/resistor to effectively limit transient-overvoltage stress on the system. It may be worth calculating the charging imbalance if the line section is to be fed from the islanded configuration, depending on where the separation occurs.

 

[jbartos]

Suggestion: Depending on the substation physical dimensions, the charging current may be relatively low, which would permit high resistance grounding (usually used on 4.16kV). The high resistance grounding, if charging current is small, would be preferable, since it can allow the longer time to decide about the fault disposition.

 

[cuky2000]

Jbartos and Busbar:

Thanks for your excellent posts. We encourage this high quality advise and education for the rest of our virtual community.
In apprciation for that, I vote for an star for both of you.

Gongratullation.

 

[busbar]

Salient but by no means comprehensive material at ANSI C57.13 §7 and C62.22 §5.

Indexes at:

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.13-1993_desc.html

and,

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.22-1997_desc.html