Arrester voltage rating 4

[SphincterBoy]

I have a situation in which the substation source (12.47 kV, L-L, grounded-wye) is feeding a 500 KVA, 3-phase padmounted distribution transformer (delta-connected primary windings).

There are surge arresters connected phase-ground at the phase connections on the primary side of the padmounted transformer. The current surge arrester ratings are 9 kV
(> 7.2 kV L-G source voltage).

When the transformer is first energized, it is done so one phase at a time.

When the last and final phase is energized, the surge arrester "blows".

My hunch is the transformer inrush current causes a phase imbalance, resulting in a phase-phase overvoltage at the primary terminals of the padmounted transformer, exceeding the 9 kV phase-ground arrester rating.

Should I be increasing the arrester rating to 12 kV?

 

[dpc]

If the MCOV of the arrester is greater than about 8 kV, I don't think you should be having this problem. I would want to find out why the voltage is going high.

Obvious check you've probably already done is to make sure the transformer neutral is grounded properly.

You could certainly get by a higher rated arrester at this voltage and still protect the transformer. But on the other hand, if the transformer neutral is solidly-grounded, you should be able to use the line-to-ground rated arresters.

 

[SphincterBoy]

The padmounted xformer primary has no neutral.....it's delta-connected.

The substation feeding the padmounted xformer is wye connected, with a solidly-grounded neutral.

 

[dpc]

OK, I missed that in the first post. But as long as the neutral is solidly-grounded somewhere nearby, you should be able to use the line-to-neutral rating for the arrester.

Is this a long run of underground cable?

 

[jghrist]

This is a classic ferroresonance situation. When one or two phases are connected by cable to an ungrounded transformer winding, the transformer winding inductance is placed in series with the open phase cable capacitance to ground. If the cable capacitance is high enough (length is long enough), there will be an overvoltage at the transformer terminals. A slight overvoltage will change the winding inductance because the inductance is nonlinear. The situation can become unstable and result in very high voltages.

Solutions are:

1. Use three-phase switching. This could be a problem if the transformer is protected be fuses at the switching point. Ferroresonance could occur when one fuse blows.

2. Use a grounded wye - grounded wye transformer. This can still result in ferroresonance because of interwinding capacitance, but it is unlikely, particularly at 12.47 kV.

3. Limit the cable length. Depending on the transformer magnetizing impedance and the cable construction, the maximum cable distance for 12.47 kV will be somewhere between 10 and 30 feet for a 500 kVA transformer (shorter for smaller transformers).

 

[SphincterBoy]

To dpc & jgchrist, many thanks for your help!

dpc: The HV primary cable feeding the padmounted xformer is several *thousand* feet.

I think jg is correct about ferroresonance. Very correct.

I think we are going to adopt 3-phase switching (future), and in the mean-time, we'll perform single-phase switching with the elbow arresters *unplugged*, until the resonant condition dies out, then simply plug them (arresters) back in.

I can't remember why (many years ago) we adopted the DELTA-WYE configuration, but it seems to me the WYE primary had third-harmonics, and the DELTA primary did not exhibit this problem (Westinghouse T&D Handbook).

 

[busbar]

There may be unduly excessive resistance in the padmount grounding system, causing excessive neutral shift, and contributing to the undesirable effects of single-pole switching.


Excerpt from IEEE C57.105-1978 Guide for Application of Transformer Connections in Three-Phase Distribution Systems

7. Ferroresonance
7.1 Qualitative Description
Ferroresonance is a phenomenon usually characterized by over-voltages and very irregular wave shapes and is associated with the excitation of one or more saturable inductors through capacitance in series with the inductor. In single- and 3-phase power distribution circuits susceptible to ferroresonance, the capacitance usually is due to presence of shunt capacitor banks, series capacitor banks, cable circuits, overhead lines, and the internal capacitance of transformers and other equipment. The saturable inductor usually is present in the form of a transformer or reactor which utilizes an iron core. Under normal conditions, ferroresonance will not occur in distribution systems, but certain conditions may be established during single-pole switching or the operation of single-pole protective devices which permit ferroresonance to result. The winding connections used for distribution transformer banks are an important factor influencing whether ferroresonance can occur during single-phase conditions.

When ferroresonance is present in a distribution system, it usually causes one or more of the following abnormalities which are easily measured or observed:

1) High voltage phase-to-phase, phase-to-ground, or both with peak voltages which may be five or more times the system normal peak voltage
2) Extremely jagged and irregular voltage and current wave shapes
3) Excessively loud noise in the transformer due primarily to magnetostriction at high flux densities. Frequently the transformer is described as rattling, rumbling, or whining when ferroresonance is present. These noises are considerably different from those which emanate from the transformer when excited from a sinusoidal source at rated voltage and frequency.

The simple 3-phase circuit of Fig 13 will be used to discuss how ferroresonance can occur. A 3-phase effectively grounded source supplies single-conductor shielded cables through three single-pole switches. This type of cable has capacitance from phase to ground C0, but phase-to-phase capacitance essentially is not present. At the end of the cable circuit is an unloaded 3-phase transformer bank with the primary windings connected in D. When the single-pole switch for phase A is closed as shown in Fig 13, two phases of the transformer are energized by a path through the cable capacitances from phases B and C to ground. At the instant the switch in phase A is closed, the capacitance to ground on phases B and C appears as a short circuit, and the transformer windings of legs A-B and A-C start to draw normal inrush or exciting current. The transformer iron during the first cycle of applied voltage may saturate due to closing at or near voltage zero, or due to residual flux in the transformer core or both. Saturation results in a large current pulse through the transformer windings and capacitances of phases B and C. Next the transformer iron drops out of saturation leaving a substantial trapped charge (voltage) on the cable capacitance. In subsequent cycles the transformer iron may go into saturation in the opposite direction, thereby changing the polarity of the trapped charge on the capacitance. If the transformer continues to go into and out of saturation in either a cyclical or random fashion, high sustained overvoltages will occur phase-to-phase and phase-to-ground. These sustained overvoltages can cause over-excitation of the transformer, surge attester failure, and even failure of major insulation in the transformer or system. When the second phase in Fig 13 is closed, the overvoltages may persist or become higher. Closing the third phase restores balanced 3-phase conditions, and ferroresonance will terminate.

A distribution system should be designed and operated so that ferroresonance is unable or very unlikely to occur during single-phase conditions. For a given system and method of operation, the transformer connections and switching arrangements should be selected so that the probability of ferroresonance and the resultant overvoltages is minimized.

 

[SphincterBoy]

Thanks for the great info. Busbar!

 

[busbar]

Gif of C57.105 Figure 13 at:

http://67.115.161.42/imag/etip.c57.105.fig13.gif

Happy I could help. Ferroresonance problems can be a bit spooky and unlpeasant. Good luck and work safe.

 

[busbar]

Shameless huckstering follows. C57.105-1978 {reaffirmed 1999 / ieee cat ‘ss6924’ as pdf] is an oldie but a goodie, and may be well worth obtaining to read cover-to-cover. It is a useful collection of basic transformer information—good reference for convincing upper tiers of odd problems.

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.105-1978_desc.html

is an outline.

 

[SphincterBoy]

Busbar, many thanks for all your suggestions and info.

I think we are going to install 3-phase switching on 500 KVA and greater transformers, to mitigate this problem.

It seems the overvoltage swing can get as high as 4.5pu, therefore changing arrestors will not help (my original question).

 

[jghrist]

Using three-phase switching only for larger transformers is not the solution. Actually, the problem is worse for smaller transformers. You might want to consider using a GrdY - GrdY connection for the smaller transformers.

GrdY - GrdY connections are the most common type of three-phase connections in US utility distribution systems. The possibility of third harmonics in the neutral causing interference with telephone systems was the usual reason for caution with this connection. This has become less of a concern because there are few unshielded open wire telephone lines run on poles with open wire power system neutrals anymore. In your underground system, interference with telephone systems is probably unlikely.

You should use five-legged cores to prevent tank heating from unbalanced loads or single phase low-side faults.

 

[busbar]

Jim, I do not contest popularity of Grd∙Y/Grd∙Y configuration in distribution, with variations chalked up to "regional differences/local customs." My assumption for deviation [id est, ∆-Y] at ~225kVA and up [based on need for NEC 1000A, 480Y/277V ground-fault protection] is that primary-side ground faults can trip low-voltage ground-fault protection, given bank zero-sequence "pass-through" characteistics.

Triplen-harmonic handling is also an issue with some. Agreed that Grd∙Y transformer primaries in 35kV-class {and to a lesser extent, 25kV} are a default arrangement for many.

 

[SphincterBoy]

jg & busbar, do you think I should switch to WYE-WYE?

This would be a radical switch, given the large utility to the north of where I'm located specs DELTA-WYE.

You've got a heck of a great point about improvements in the phone systems, whereas the third harmonic is no longer a problem.

I'd love to give WYE-WYE three-phase padmounted xformers a try, since most of our overhead three-phase banks are WYE-WYE, and we haven't had any complaints about "harmonics".

 

[jghrist]

busbar,

I'm not clear how a primary side ground-fault would trip secondary side ground fault protection. No zero-sequence current will flow in the secondary for a primary fault.

There can be a problem with Grd-Y/Delta transformers acting as a ground source for a primary side ground fault, blowing transformer fuses.

Is the problem caused by ground currents flowing between the service grounding electrode and the transformer grounding electrode? I could see where a ground-fault sensor encircling only the bonding jumper may see some current if there is a potential difference between the ground electrodes. (ref. NEC 2002 Handbook Exhibit 230.29) You might then get come current flowing in the service neutral/ground loop. I don't see how the primary transformer connection would contribute to this. Using a ground-fault-sensor around the three phase wires and neutral on the load side of the neutral ground should eliminate this. (ref. NEC 2002 Handbook Exhibit 230.28)

If this is a problem, it would seem to point toward using Grd-Y/Grd-Y for smaller transformers (where the 1000A service ground-fault protection requirement doesn't apply) and three-phase switching for larger transformers.

 

[Bung]

Not always a practical proposition, but you could try loading the transformer - loaded transformers are less prone to ferroresonance. In the past we used a litle gizmo with some L and R in it connected to the LV busbar for this purpose, in situations where we had a ferroresonance problem that was too expensive to cure by circuit re-arrangement. All new circuit design is done with ferroresonance minimisation in mind.
Bung
Life is non-linear...

 

[busbar]

Jim — After some checking, I find my statement about lo-side GFP trips from primary ground faults is incorrect. My apologies.

 

[jbartos]

Suggestions:
1. A transient voltage suppressor could supplement arrestor. Increasing the arrestor voltage rating is risky since it protects transformer and its insulation. The transient suppressor could be rated below the arrestor voltage rating and dissipate energy caused by the ferroresonance phenomenon.
2. Suggestion to jghrist (Electrical) Oct 10, 2002 marked ///\\Using three-phase switching only for larger transformers is not the solution. Actually, the problem is worse for smaller transformers. You might want to consider using a GrdY - GrdY connection for the smaller transformers.
///y-y transformer connections are used among network transformers. Since this is a distribution transformer transferring energy from primary distribution circuit to a secondary distribution circuit or a consumer's service circuit, the d-y connection is appropriate. It breaks path of zero sequence current. Also, the secondary can have an appropriate system grounding.\\\

 

[jghrist]

In countries where MV distribution systems have three-phase switching and fault protection, where ground faults are limited by resistance grounding at the source, and where very sensitive ground fault protection is used, Grd-Y/Grd-Y transformers from MV to LV would not be appropriate.

In North America, resistance grounding at the HV/MV substation to limit ground faults is not common. Single-phase switching and fault protection on MV distribution is common. Grd-Y/Grd-Y transformers are appropriate and reduce the probability of ferroresonance. The secondary system grounding source is supplied by the primary source through the transformer connection.

Most MV/LV transformers in North America are single-phase, connected line-to-neutral on primary and secondary. Most utility-owned three-phase transformers and banks of single-phase transformers are connected Grd-Y/Grd-Y.

 

[jbartos]

Comment to the previous posting marked ///\\\ Most MV/LV transformers in North America are single-phase, connected line-to-neutral on primary and secondary.
///Please, do you have any reference for this statement?\\ Most utility-owned three-phase transformers and banks of single-phase transformers are connected Grd-Y/Grd-Y.
///Please, do you have any reference for this statement?\\