What is the recommended approach for energizing and de-energizing unloaded power transformers with group-operated or individual disconnect air switches? If the switch has arcing horns, do you de-energize the transformer's excitation current with the switch? What about the inrush current during energization?

The only reference I've found on this topic is IEEE C37.36b-1990. I've attached a screenshot from it. It states the maximum "Resistive or Transformer Excitation Current" based on voltage class. I'm confused by this wording since it seems like reactive excitation current would cause much higher voltage spikes than resistive current, but they're grouped together as a single rating. I assumed that the no load excitation current is very reactive.

http://files.engineering.com/getfile.aspx?folder=a51173e7-9f35-45b9-b361-0e

Great post!ReportRE: Transformer Switching with Air Switchesdavidbeach (Electrical) 22 Jun 16 00:30 That's why they build circuit breakers and circuit switchers. Can't imagine why anybody would want to close into a transformer with a switch. Great post!ReportRE: Transformer Switching with Air Switchesmagoo2 (Electrical) 22 Jun 16 01:34 I agree with David. Years ago, when we routinely used motor-operated air-break (MOAB) switches instead of circuit switchers to energize our distribution substation transformers, we had GE do a TNA study on the effect of the energizing transients using MOABs. The conclusion was the relatively long closing span between poles of the MOAB switch led to ferroresonance. These were mostly delta wye connected transformers and were mostly 230 kV and 115 kV high side transformers ranging in size from about 20 MVA to 100 MVA. Transformer engineers at Pittsfield were concerned about possible damage to these transformers. As a result, we stopped this practice.

As David suggests, why would you want to do this? Great post!ReportRE: Transformer Switching with Air Switches111R (Electrical) (OP)22 Jun 16 13:13 Thanks. I'm arguing against it, but trying to develop a solid argument on the reasons why and the calculations behind it.

What is the recommended practice on a fused transformer on a tapped line with other tapped loads? I've heard of these being re-fused manually which seems risky. Is it common to use circuit interrupters to de-energize the entire line and remove all loads (if bypass switch not available), re-fuse, and then re-energize the entire circuit? Great post!ReportRE: Transformer Switching with Air Switchescuky2000 (Electrical) 22 Jun 16 13:43 Quote:Disconnectors do not have interrupting current ratings but, given that they have a contact breaking arrangement, they have a certain current interrupting capability. Standards recognize this fact and the IEC disconnector standard defines a negligible current interrupting capability at 0.5 A and a further bus-transfer (loop) switching capability of up to 1600 A for some disconnectors. Present IEEE standards have a similar negligible current definition but do not recognize bus-transfer as a switching duty. In an earlier version of the IEEE standard, transformer magnetizing and capacitive currents, and small load currents were identified in this context

QUESTION 1: What is the recommended approach for energizing and de-energizing unloaded power transformers with

a) Group-operated disconnect switch: for HV medium size and large power transformer is a risky option to use disconnect switch even for switching unloaded transformers. If the budget allow, circuit switcher or better, circuit breaker is recommended. For MV applications, there is 3 pole load break disconnect switches available in the market.

b) Individual disconnect air switches: This is an standard utility practice for small MV distribution transformer using fuse cut-out. Occasionally this individual switching operation could create unwanted ferroresonance issues.

QUESTION 2:If the switch has arcing horns, do you de-energize the transformer's excitation current with the switch? For HV application some US manufacturer have design with high speed whip that allow to interrupt transformer magnetizing and line charging current. Arcing horn are primarily designed to control the arc reach and somehow help with the Interruption of Inductive and resistive current.

Great post!ReportRE: Transformer Switching with Air Switchescrshears (Electrical) 28 Jun 16 15:27 "My" utility has been taking this risk for many, many years as both a capital and OM&A cost-saving measure; switching 115 kV and 230 kV transformers on and off potential with ganged disconnect switches is routine. 500 kV transformer disconnect switches, on the other hand, not being ganged but individually motor-operated due to physical distance considerations, could experience appreciable time displacements between phases and therefore cause cumulative single-phasing over the life of the transformer, and as such are always operated off potential only, with the transformer itself being switched on and off pot with breakers.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

Great post!ReportRE: Transformer Switching with Air SwitchesMbrooke (Electrical) 29 Jun 16 01:19 I know of several US utilities that used MODs for energizing and denergizing transformers (and still do in legacy substations). The scheme was usually accompanied with transfer tripping or suicide tripping (deliberate shorting via MO ground blade) to remove a faulted transformer or secondary side fault. In some cases the MOD would open after the remote breakers tripped for faulted transformer and then reclose. The key however being that all maintenance switching was done with no load on the secondary. This practice however is being phased out via circuit switchers in North America. Whether the MODs had no arcing horns or other special requirements I am unaware of. I know in loop splitting arcing horns are not recommended because the high current can over heat them.

As for the IEC (outside of North America) the trend has always been to use a circuit breaker.

Personally I would never use a MOD to protect a transformer. Not so much what the magnetizing make or break will do to the switch, but rather the fact you remove other

elements (like transmission lines) out of service for transformer faults. Multiple transformer failures at a substation can remove multiple transmission lines with it. I believe this practice was routine in North America to save on breakers while still being able to have a ring buss, but personally its no longer worth it.

Quote:Disconnectors do not have interrupting current ratings but, given that they have a contact breaking arrangement, they have a certain current interrupting capability. Standards recognize this fact and the IEC disconnector standard defines a negligible current interrupting capability at 0.5 A and a further bus-transfer (loop) switching capability of up to 1600 A for some disconnectors. Present IEEE standards have a similar negligible current definition but do not recognize bus-transfer as a switching duty. In an earlier version of the IEEE standard, transformer magnetizing and capacitive currents, and small load currents were identified in this context

Question, not to side track, but why do IEEE disconnects not recognize loop splitting? Does the arc break/distorting mechanism play a role? And thank you regarding the change. Any particular reason why magnetizing currents are no longer identified in this context?