On-load tap-changer control method, excitation control system carrying out said control method and power excitation chain
The present invention relates to an on-load tap changer control method for a power transformer in a power system. The power transformer has a primary side for a connection to a first grid in which electric power is generated, and a secondary side for connection to a second grid in which electrical power is consumed, the power transformer being equipped with an on-load tap changer. The method includes measuring the voltage and current at least on the primary side (u 1 , i 1 ) or on the secondary side (u 2 i 2 ) of the power transformer, processing said measured voltages (u 1 ; u 2 ) and currents (i 1 ; i 2 ) in order to derive prospective reactive power at the output of the power transformer after prospective tap-change, comparing prospective reactive power to a predefined set-point, and—initiating tap-change of on-load tap changer if prospective reactive power is closer to said predefined set-point than actual reactive power.
1. An on-load tap changer control method for a power transformer in a power system, wherein the power transformer has a primary side for a connection to a first grid in which electric power is generated, and a secondary side for connection to a second grid in which electrical power is consumed, the power transformer being equipped with an on-load tap changer,
said method comprising:
measuring the voltage and current at least on the primary side (u 1 , i 1 ) or on the secondary side (u 2 , i 2 ) of the power transformer,
processing said measured voltages (u 1 ; u 2 ) and currents (i 1 ; i 2 ) in order to derive prospective reactive power at the output of the power transformer after prospective tap-change,
comparing prospective reactive power to a predefined set-point,
controlling tap-change of on-load tap changer if prospective reactive power is closer to said predefined set-point than actual reactive power.
2. The on-load tap changer control method according to claim 1 , wherein processing said measured voltages (u 1 ; u 2 ) and currents (i 1 ; i 2 ) in order to derive prospective reactive power at the output of the power transformer after prospective tap-change takes into account the grid reactance (x Q ) and the internal grid voltage (u Q ) of said grid where electrical power is consumed.
3. An on-load tap changer control method according to claim 1 , further comprising a further step of detection of change of an operation point of the power transformer, and wherein the detection of a change of operation point initiates the calculation of said parameters representing the grid condition (x Q , u Q ), based on a set of actual and historical electrical quantities, wherein the historical quantities are stored in memory.
4. An on-load tap changer control method according to claim 3 , wherein detection of change of an operation point of the power transformer comprises to detect a variation beyond a predefined range of the the internal electromotive force (e T ) of the power transformer.
5. An on-load tap changer control method according to claim 4 , wherein said change of the internal electro-motive force (e T ) of the power transformer is detected by detecting stepping of the OLTC.
6. An on-load tap changer control method according to claim 4 , wherein the said primary grid comprises a synchronous generator and an excitation system with an automatic voltage regulator (AVR G ), where said change of the internal electro-motive force (e.m.f.) of the power transformer is detected by detecting a change in the reference value of the automatic voltage regulator of the generator (AVR G ).
7. An on-load tap changer control method according to claim 6 , further comprising altering temporarily and slightly the reference voltage of the generator automatic voltage regulator (AVR G ) for purposely producing a change of internal electromotive force e.m.f. of the power transformer.
8. The on-load tap changer control method according to claim 7 , wherein altering the reference voltage of the generator automatic voltage regulator (AVR G ) lasts at most for some seconds and where said alteration of the reference voltage is less than 1% of the rated value, preferentially 0.5%.
9. The on-load tap changer control method according to claim 1 , wherein the first grid comprises a power excitation chain having a generator (G), and where the power transformer is a step-up transformer equipped with an on-load tap changer and connected on the one hand to the output of said generator (G) and on the other hand to a transmission bus, where said measured voltage and current are respectively the generator voltage (u G ) and generator current (i G ).
10. An on-load tap changer control unit for a power transformer in a power system, wherein the power transformer has a primary side for a connection to a first grid in which electric power is generated, and a secondary side for connection to a second grid in which electrical power is consumed, the power transformer being equipped with an on-load tap changer, wherein said unit comprising
means to measure the voltage and current at least on the primary side (u 1 , i 1 ) or on the secondary side (u 2 i 2 ) of the power transformer,
means to process said measured voltages (u 1 ; u 2 ) and currents (i 1 ; i 2 ) in order to derive prospective reactive power at the output of the power transformer after prospective tap-change,
means to compare prospective reactive power to a predefined set-point,
initiate tap-change of on-load tap changer if prospective reactive power is closer to said predefined set-point than actual reactive power.