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Inverters as Feeder Circuit Source

Inverters as Feeder Circuit Source

Inverters as Feeder Circuit Source

(OP)
We are working with an experimental 5MVA battery storage system that is to be connected to one of our 13kV feeder circuits. While it will generally be in parallel with the normal source, it may be possible to island the circuit and supply it from the batteries.

Does anybody have any experience with things of this type? In particular I'm concerned about what happens when there is a fault out on the circuit while the inverters are the only source. The inverters won't produce enough fault current to blow a tap or transformer fuse, nor will they produce enough current for an overcurrent relay at the point of connection to detect the fault and separate the inverters from the system. The inverters should shut down, but controls aren't protection.

If anybody's worked with something similar, what have you done to detect and clear faults?

RE: Inverters as Feeder Circuit Source

It's an interesting problem as the common utility approach is to trip off islanded distributed generation. The industry in the UK is asking manufacturers of small scale inverters to provide short circuit test data at 10% of nominal voltage.As the output from an Inverter reduces to zero when a short circuit is applied to its terminals, a short circuit test does not represent the worst case scenario; in most cases, as you know, the voltage will not collapse to zero for a network fault.
The test ensures that at least 10% of nominal voltage remains and which allows the inverter to feed into a load with an X to R ratio of 2.5. The onus would be on the utility to protect itself and other customers from the detrimental effects of islanding.
Do you have enough oomph to operate the relays if you apply dual settings groups which invoke alternative settings or functions in the event of an islanding condition? If not then the only safe approach may be to accept that the operation of an island is likely to be only an occasional event, and the likelihood of occurrence of a subsequent fault within the island is very remote, and consider it appropriate that the entire island can be tripped on fault, without need for normal discrimination.
Regards
Marmite

RE: Inverters as Feeder Circuit Source

The description Marmite gives of the inverter's behaviour during a fault is similar to how a synchronous generator with a bus-fed exciter behaves during a fault close to the machine terminals. A voltage-controlled or voltage-restrained overcurrent relay might be worth considering if the inverter's behaviour duirng a fault is predictable, which I expect it is.

RE: Inverters as Feeder Circuit Source

(OP)
Hadn't thought about the voltage controlled/restrained overcurrent. Question would be whether or not the inverter would stay on long enough. Inverter shutdown is good, but I really want to get a breaker open too.

RE: Inverters as Feeder Circuit Source

As such kind of systems as the one you are describing are quite new, I'm afraid you will rarely find a standard dealing exactly with this application. However, the requirements you are describing are also prevalent in other applications too.

For example a commerical UPS should not shut down in case of overload, but supply sufficient fault current to blow fuses or trip breakers in downstream distribution. Take a look into the related IEC Standard IEC 62040.

Also inveters for feeding renewable energy to the grid are required to stay on the grid during a fault and to contribute to fault current (islanding is usually not desired in this case). This requirement is usually referred as LVRT (Low voltage ride through).

As these converters all contain output filters, these requirements can be met by properly sized semiconductors (usually IGBT-modules) and suitable control and modulation.

RE: Inverters as Feeder Circuit Source

David,

Let us know what you learn. We have a similar situation with a wind farm that has generators with full inverters. We've made repeated requests to the manufacturer for information on fault performance of the generators, but so far, they have not come up with any meaningful data.

dpc

RE: Inverters as Feeder Circuit Source

I do not think that full inverter wind turbines and similar applications were designed with fault behaviour in mind at all. There are many data that the wind turbine manufacturers do not make available. I think it is because those questions never occured to them during design and that the answers may need to be withheld from public.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Inverters as Feeder Circuit Source

FWIW, we did get some information from the wind turbine manufacturer indicating that these full inverter wind turbines would have a maximum fault current output of only about 1.05 X full load rating. The current output is severely limited to protect the IGBTs in the inverter. Something may have been lost in translation, but they seemed pretty clear on this.



RE: Inverters as Feeder Circuit Source

We perform network impact studies and distributed generation (DG) modeling and so far, most DC coupled generation we worked with, had fault around 1.05-1.1 current pu. I think there was one manufacturer that mentioned 1.2 p.u. but it was never confirmed. For a manufacturer view it makes sense since cost is related to current carrying capacity even for a short duration. For network people, it is a challenge.

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