Power Plant Gen Set

Hi,

Pls. check. You can have delta for 3.3 kV and star for 11 kV with solid neutral. This will also save transformer cost.

Having delta for 11kV means, you cannot detect / isolate the genset for any earth fault on grid side.

hi,

Thanks,

but unfotnuatlly we already purchase transformer star at primary (3.3 KV) and delta at 11 KV side, its do by mistake.

now please advice me what we do?

Now basically you have a power network with grounded neutral, since at least the generator is grounded. This affects the value of the ground fault current, but it also enables you to detect the faulty phase faster. The thing is that the grounding of the neutral helps you control both the overvoltage and the short-circuit current in a network. When having a single phase to ground fault the value of the fault current depends on the grounding reactance - the more places (ie. generator, transformer 1, transformer 2 ,...) you ground the neutral then the larger the short-circuit current will be. On the other side if you have an isolated neutral than you may have solved the overcurrent problem but another problem is born- the problem with the overvoltage. When there is a ground fault in a network having isolated neutral, the "healthy" phases will suffer from overvoltage- the insulation could burst. In my country the neutral is insulatet for the 20 kV (or treated neutral) network but is grounded for 110 kV, 220 kV and 400 kV. The reasons for this are that : first: the insulation for the 20 kV is strong enough to withstand the temporary overvoltage for a couple of hours. second:The 110 to 400 kV networks need to maintain the "juice flow" so in you have a phase to ground fault the faulty phase is de-energized- the circuit braker uses an autorecloser for single phase faults on overhead power lines. third: the insulation for 110-400 kV power lines and equipment is expensive so the idea is to minimize the temporary overvoltage. Changing the insulation for a 400 kV power line is very expensive and the line is being shut down until repaires if there is punced insulation. fourth:The circuit breakers for the 110-400 kV networks are monopole- so as to trip the faulty phase only. The 20 kV network has three pole breakers so the fault on one phase takes out the entire powerline. In your case I suppose the circuit breaker is a three pole. The generator is grounded so in the case of a short you'll have a three-phase trip no matter what the fault is, and even without grounded neutral for the transformer. More over be aware that the 3.3/11 kV transformer becomes a source feeding the faults in your network so consider installing appropriate circuit breakers and protections. To be able to give you a reliable advice I need to know the electrical scheme and what are the working scenarios for your units.

Thanks,

In my scheme, 5 Mw genset star winding with NGR , of 3.3 KV connected to 3.3 kv bus. there i have 3 transformer of 2MVA each (3.3 KV / 0.4 KV star/delta connection). these transformer use for my plant load, and subject transformer 4 MVA (3.3 KV /11 KV star/delta connection for power export to national grid.

considering the scheme you described if between the transformer and the 3.3kV bus is a feeder, you should ground the transformer neutral to be able to detect and take out feeder faults without tripping the genset. If not than it does not matter because a bus fault trips the bus and a transformer fault trips the transformer.

Remember that in case of a fault the 4 MVA transformer becomes a source feeding your fault so treat it as one.

Another possible solution: have differential transformer protection and there is no need to ground the 4 MVA transformer neutral. If there is a feeder between 3.3 kV bus ant the transformer just extent the transformer differential to protect the feeder also. In this case you have the greatest selectivity and sensibility, and the genset should work even though there might be a fault affecting the 4 MVA transformer or the 11 kV network. For faults in the 11 kV network close to you use feeder protection to trip the 4 MVA transformer-or the 11 kV breaker if you have one installed.

isn't possible to isolate the genset using overvoltage protection?

yes it is possible i connect over voltage protection to trip the breaker of transformer primary (3.3 KV). and for tranformer can i use Diff. protection to trip 3.3 kv or 11 Kv feeders of Transformer (both side we have breakers)