Control method for a grid following voltage source converter without phase locked loop

A method of controlling a voltage sourced converter (VSC) electrically connected to an alternating current (AC) electrical power system to operate in a grid-following mode with respect to the AC electrical power system without a phase locked loop operatively connected between the VSC and the AC electrical power system, wherein the VSC and the AC electrical power system are electrically interconnected by an AC bus bar, wherein the AC electrical power system has three phases, the method comprising:

• forming, for each phase of the AC electrical power system, direct-axis and quadrature-axis voltage signals based on a corresponding phase voltage measured at the AC bus bar, wherein the direct-axis reference voltage signal comprises a sinusoidal wave with a magnitude of one and in phase with a positive sequence component of the corresponding phase voltage at the AC bus bar and the quadrature-axis reference voltage signal comprises a sinusoidal wave with a magnitude of one and 90 degrees out of phase with the positive sequence component of said corresponding phase voltage;
• using the direct-axis and quadrature-axis voltage signals associated with each phase of the AC electrical power system, determining, for each phase of the AC electrical power system, a reference voltage signal for output to a firing pulse modulator of the VSC to form an output voltage of the VSC associated with the phase, wherein determining the reference voltage signal for the phase comprises:
• forming an instantaneous reference current signal for the phase based on multiplicative products of (i) the direct-axis voltage signal and a direct-axis reference current magnitude, and (ii) the quadrature-axis voltage signal and a quadrature-axis reference current magnitude;
• forming a phase current error signal for the phase based on the instantaneous reference current signal, with a gain factor applied thereto, and an actual instantaneous phase current signal for the phase;
• and
• forming the reference voltage signal based on the phase current error signal and a proportional gain factor;
• and
• for each phase, outputting the reference voltage signal to the firing pulse modulator of the VSC;
• wherein, when the direct-axis and quadrature-axis reference current magnitudes respectively comprise a first component based on a positive sequence current signal and a second component based on a negative sequence current signal:
• (a) the instantaneous reference current signal of a first one of the phases is based on a sum of multiplicative products of (i) the direct-axis voltage signal of the first phase and the first and second components of the direct-axis reference current magnitude, and (ii) the quadrature-axis voltage signal of the first phase and the first and second components of the quadrature-axis reference current magnitude;
• (b) the instantaneous reference current signal of a second one of the phases is based on a sum of multiplicative products of (i) the direct-axis voltage signal of the second phase and the first component of the direct-axis reference current magnitude, (ii) the quadrature-axis voltage signal of the second phase and the first component of the quadrature-axis reference current magnitude, (iii) the direct-axis voltage signal of a third one of the phases and the second component of the direct-axis reference current magnitude, and (iv) the quadrature-axis voltage signal of the third phase and the second component of the quadrature-axis reference current magnitude;
• and
• (c) the instantaneous reference current signal of the third phase is based on a sum of multiplicative products of (i) the direct-axis voltage signal of the third phase and the first component of the direct-axis reference current magnitude, (ii) the quadrature-axis voltage signal of the third phase and the first component of the quadrature-axis reference current magnitude, (iii) the direct-axis voltage signal of the second phase and the second component of the direct-axis reference current magnitude, and (iv) the quadrature-axis voltage signal of the second phase and the second component of the quadrature-axis reference current magnitude.