Reverse power from utility

Re: Reverse power from utility

zamri wrote: »

Hi i'm newbies here.

i have a system with 990kWp and connect to the utility grid trough two nos. of inverter (GT500E) and one no. transformer 11kV/2 x 315V. The no load losses for step up transformer and switchgear is about 5kW and for inverter auxiliary supply is about 2.6kW (both are including spare factor of 20%). So the total no load losses is about 8kW.

Based on the parameters stated above, the Reverse power Relay (utility requirement) has been set so that the reverse power will not flow more than the setting value (during night time). The total no load losses mentioned above is totally 8kW, but i set it higher i.e 10kW so based on calculation it is 0.01p.u (setting value of RPR).

Considering the inrush current of the transformer 0.5 sec and starting current of all motor not more than 7 sec, the time allowed for reverse power flow is 10 sec.

Setting at RPR
1) Setting active power value = 0.01p.u
2) Time allowed for reverse power = 10 sec

with the setting mentioned above, the system was tripped at utility 11kV switchgear. Do you have any idea on this? or is it there is another condition other than inrush current / starting current need to be consider?

thanks

So it is only tripping at night when the GTIs are offline?

How is your 11Kw transformer wired?

One possibility is high harmonic content circulating currents if an unbalanced load is presented or if you have high non-linear loads like a VFD motor driver or other high wattage electronics.

A high base load of current through the RPR may be biasing the RPR so that it does not give you the full combination of power and hold time that you need for motor starting.

The inrush current of the transformer should only be a factor to consider if you are switching the primary connection on and off.

Re: Reverse power from utility

zamri wrote: »

thanks inetdog

Exactly it is tripped at night when the inverter shut off.

The transformer is YNd11d11 11kV/2 x 315V 1000kVA.

Do you know, how to calculate the harmonic current for inverter and how long it will happen.

thanks a lot

No, I do not. It will depend on your loads. Am I correct that the transformer is always connected to the utility on the primary side, and the the inverters either feed it or do not, while the loads remain connected at all times?

It is possible that when the inverters are shut off they introduce a momentary current spike into the network. If the Grid-tie inverters are working properly, there should not be any phase shift in the power to your loads or the voltage in the transformer, but the secondary current will suddenly reverse direction, which may well cause a large transient on the utility side.

Are you shutting the inverters off or just waiting for them to stop when the solar power goes low enough? One thing you should consider it to try is to deliberately shut one inverter off before the other, with at least ten seconds delay and see if that makes a difference. I do not know exactly how the RPR works, so I do not know all of the things that might disturb it.

Do you have a reference URL for information about the GT500E? And is it single phase or three phase power?

For a system of this size, I would expect that you would want to find a local electrical engineer who could give you better suggestions. It is not the sort of problem that usually comes up on this forum.