Asymmetry voltage level IEC60034-1

[npowerthes]

Hello to all. I want to ask about the voltage level of assymetry at 400kV grid which is specified on IEC 60034-1 standard and for which a generator should be kept synchronized to the grid.

 

[ScottyUK]

What do you want to ask about it?

Voltage assymetry will manifest itself as negative phase sequence currents in the generator stator, which in turn lead to high frequency currents in the rotor forging causing the rotor to overheat. Each machine is different in its ability to handle NPS currents. Machines connected to a large system will usually be required to remain in service with a certain level of voltage assymetry; the grid operator will specify the requirements which may or may not reference the IEC standard.

Per the UK's grid code:

CC.6.1.5

.......

(b)
Under Planned Outage conditions, the maximum Phase (Voltage)
Unbalance on the GB Transmission System should remain, in England and Wales, below 1%, and in Scotland, below 2%, unless abnormal conditions prevail.

CC.6.1.6

In England and Wales, under the Planned Outage conditions stated in CC.6.1.5(b) infrequent short duration peaks with a maximum value of 2% are permitted for Phase (Voltage) Unbalance, subject to the prior agreement of NGET under the
Bilateral Agreement. NGET will only agree following a specific assessment of the impact of these levels on Transmission Apparatus and other Users Apparatus with which it is satisfied.

Other systems will have similar requirements.

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