Calculation of transfer functions in grid-controlled convertor systems. With special reference to h.v. d.c. transmissions

Abstract
In high-power grid-controlled convertor systems large parallel capacitor batteries are usually installed and connected to the a.c. busbar at the same point as the convertor. They can be used either for filtering harmonics, e.g. as parts of tuned filters, or for the generation of reactive power, or both. The capacitive components introduced complicate the dynamic behaviour of the convertors considerably, and thus the system cannot be described by simple convertor theory. In the paper, the frequency response of such a system is deduced, using the concept of ‘conversion functions’ introduced to systematise and simplify the calculations. The frequency response for the current-control loop is deduced with reference to h.v. d.c. transmissions, but the results are applicable to any kind of rectifier load. The numerical calculations are laborious and require the use of a digital computer. The comparison between calculated and simulator-measured frequency responses for two practical h.v. d.c. transmissions shows satisfactory agreement. The calculated transfer functions are useful for the synthesis of the current-control system and allow an inexpensive study of the effects of system changes on the achievable speed of response.

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