Control of HVDC Converter and System

An ideal control system for the HVDC converter has the following requirements.

(a) Symmetry of valve turn-on angles during steady state operation.

(b) Ability to determine the turn-on angle using the commutation voltage and margin minimum reactive power consumption by the converter without incurring commutation failure.

(c) Insensitivity to the variations in voltage and frequency of the AC transmission system.

(d) Ability to predict the optimal turn-on time based on the actual system voltage and the DC current without causing commutation failure.

(e) Current control scheme with a sufficient margin of speed and stability to handle either a change in the reference value or a disturbance.

There are many methods for controlling the firing angle of a thyristor. The individual phase control (IPC) method and the equidistant pulse control (EPC) method are the most commonly used. The IPC control method was frequently used as a turn-on method for thyristors in the earlier days when HVDC systems were first introduced but it has been replaced with the EPC method.