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Commentator

Join Date: Dec 2011
Posts: 82
Good Answers: 2

11kV Generator Plant Protections

08/27/2017 8:12 AM

Dear all, I request your kind advice for the below,

We are using 11kV generators for the back up supply of 415V system. There is a group of 6nos. 2.75MVA 11kV gensets synchronized then it is distributed by 12nos. outgoing VCBs of the synchronization panel to each building's 11kv/415V transformers. There is total 12nos. of 1600kVA transformers are connected to the panel. (The 11kV switchgear consists of 6nos. incomer VCBs + 12nos. outgoing VCBs, total separate 3groups for the project ie 18gensets)

Its a hospital so generator room is kept away from the patient buildings so the 11kV generator is adopted for the ease of distribution. The generators are inside the building not outdoor. The outgoing feeder cables are lay through tunnels using cable trays. My question is

1. Is it need to install a surge arrestor in the parallelling switchgear or is there any chance of voltage surge during transformer switching. If required is it sufficient if we provide only one number in the main bus of the parallelling switchgear?

2. Secondary side of the transformers are connected to inductive loads also (HVAC/VFDs) and each transformers are provided with capacitor banks for the power factor improvement. There will be harmonics THD% generated in the secondary side of the transformer within the limit of 415V system, does it make any impact in the primary sided of the transformer (11kV generators)?.

The 6nos. generator neutral is earthed using a common NGR where only one generator neutral will be closed at time to avoid circulating current. Is it needed to provide any harmonic filters in the parallellling switchgear busbars?

Thank you

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Guru

Join Date: Dec 2010
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#1

Re: 11kV Generator Plant Protections

08/27/2017 3:17 PM

Arrestors are usually necessary for outdoor equipment subject to lightning. You have one transfo per load breaker so advise the transfo supplier and switchgear supplier and cable supplier of the connected equipment and circumstances and ask if they think any overvoltage surge problems are likely.

2/ Both transformers and generators could be damaged by harmonics. First you get the harmonic currents specified by the VF drive supplier (tell them the transfo rating & connection), then you ask the tro ansfo maker what that level of harmonic will do to the transfo. N.B. transfos in star-delta connection do suppress some harmonics. It is always best to filter at the VFD. You also get the generator maker to specify permitted harmonic currents. It is best to get levels of each harmonic rather than just total - which can hide a problem with some individual harmonic amplitudes.

3/ Harmonics should be filtered at the loads. Busbar filters are a "last choice" remedy because they take away responsibility from the harmonic makers.

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Commentator

Join Date: Dec 2011
Posts: 82
Good Answers: 2
#2
In reply to #1

Re: 11kV Generator Plant Protections

08/28/2017 7:01 AM

Thank you

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