GSU Transformer energization

[radug]

Hello,

I have a ST Generator (15.75kV) connected directly to a GSU Transformer by IPBs, and then in the HV side of the GSU Transformer is a circuit-breaker (33 kV).

As usually transformer manufacturers specify in their commissioning manuals that the transformer must be energized with no load and kept it unloaded during around 24 hours prior to connecting load, I wanted to know what would be the procedure in my situation. How can I do the transformer energization? Can it be done starting-up the generator with the HV circuit-breaker openned?

 

[7anoter4]

From: VIRGINIA TRANSFORMER CORP. TRANSFORMER INSTALLATION MANUAL:
“For start-up temperatures below –20°C (–4°F), it is recommended to energize the transformer, hold at no-load for
at least 2 hours and slowly increase the load in 25% increments, allowing a minimum of 30 minutes between each
increase. In cases of directly connected generator step-up (GSU) transformers, where energized operation at noload
may not be reasonable because of turbine or steam conditions or fuel cost considerations, other means of
heating the oil may be a more reasonable approach.”

http://www.vatransformer.com/Resources/files/Installation Manual.pdf

 

[rcw retired EE]

Disconnect the generator leads, but leave any surge suppression and surge capacitors and VT's connected on the generator side and leave all transformer protection in service. Close the HV side breaker to energize the transformer for the "soak".

While it is energized, verify phase rotation and voltages for synchronizing and metering. Since the synchronizing breaker is closed, all voltages should be in synch and of the proper rotation.

Record all values. After ther transformer intial soak, denergize and return the system to normal. When the generator starts, turn on excitaiton and verify all the voltages and phase rotation again and you will be ready to synchronize.

 

[radug]

Thank you both for your comments.

A further question: once I have commissioned the transformer for the first time and checked that everything is ok following the procedures you explained, every time I open the circuit breaker, shall I have to repeat the commissioning procedure, or I can energize the transformer directly loaded? I mean, can I start the generator and close the circuit breaker when synchronizing is possible without caring for an unloaded energization of the transformer during some hours?

 

[ScottyUK]

Dismantling an IPB to warm a GSU transformer? Wow. You must like hard work!

In the absence of a breaker with synchronising capability on the LV side I'd be tempted to close it onto the system and leave the turbine at minimum stable load until the transformer warmed up a bit. The transformer would likely have less current flowing in the HV winding with the generator running than with it disconnected because the reactive load would be met by the generator. If there are other undisclosed factors such as the transformer being located inside the Arctic Circle and the oil being being at -40°C then maybe it would be a greater risk.

 

[DTR2011]

I have seen IPB feeding Station Aux Xfmrs that have a cover available to access the jumpers that feed Aux Xfmr. If I understand correctly, the GSU is brand new and mgf. calls out for a 24 hour soak period at no load. That is fairly common and I am assuming you will energize from HV side. I am also assuming this is an all new installation and perhaps the Station Aux feeds loads such as GT starting.

I have commissioned many such setups and we removed the jumpers to the 13.8/4.16 kV and 13.8/.480 kV Xfmrs, as there was no other disconnecting means for this very purpose. It maybe took 1 hour to remove the jumper leads to the Aux transformers. I would imagine your IPB has such provisions as you would not want to high potential test the aux transformers as part of the IPB High Potential Test.

 

[ScottyUK]

A brand new installation? If that's the case then I missed it. Sorry.

 

[radug]

Yes the power plant is new, it has a GT and ST. The GT has a GCB between the GTG and the corresponding GSUT, but the ST no, it is directly connected to its GSUT through an IPB, so synchronization will be done by closing a circuit-breaker that is at different voltage than the generator voltage. So, if to synchronize I have to previously have the GSUT energized under no load and the HV circuit-breaker is opened then the only way I see of doing it is to energize the GSUT first closing the circuit-breaker but with the ST generator connections disconnected as you all suggested, then open the circuit-breaker, after that connect the IPB to the generator and then do the synchronization. During this hour or two that the operation can take, I have deenergized the GSUT because I see no other way. As this is a first for me, I am asking for advices and procedures in similar designs. I have seen SLDs with this design for example in IEEE 666, and even I know that inserting a GCB is the preferred option nowadays, it seems that the design I am talking about is or was not uncommon.

 

[rcw retired EE]

The design like yours is very common. Many plants that are dispatched daily energize the STG GSU transformer with the generator and load it up based on the thermal capability of the steam turbine. I do not recall a plant that has a limitation for loading the transformer, or that does special preheat of the transformers prior to loading.

The energization sequence I described above only applies to the first time the transformer is commissioned. Once the plant is in operation, the GSU will probably still be warm from the last run and not require any special care or loading sequence.

But it is probably best to get information from your transformer supplier.

I am interested in learning if my experience is contrary to any plant operators' experience where they bring up the generator based on transformer loading limitations.

 

[crshears]

Based on my experience all the advice so far is spot on. Once STG GSU has had an on-potential 'soak' from the HV side for the period specified by manufacturer's recommendation, there shouldn't be any issue with loading limitations.

One point I do recall is that during normal starts field was typically not applied to the generator until it had been accelerated to near-synchronous speed; the reason was to preclude exciting the GSU transformer at other than design frequencies [on one pair of units there was a special protection scheme installed both to prevent closing of the field breaker [hence blocking energization of the turbine/transformer 'line'] and to trip the entire unit for out-of-tolerance V/Hz conditions].

CR