The Sensitivity of Grid-Connected Synchronverters With Respect to Measurement Errors

Abstract

The synchronverter algorithm is a way to control a switched mode power converter that connects a DC energy source to the AC power grid. The main features of this algorithm are frequency and voltage droops as well as synthetic inertia, so that the inverter resembles a synchronous generator (SG). Many versions of this algorithm have been proposed and tested, but all share the same "basic control algorithm", which is based on the equations of a SG. We analyze the sensitivity of the output currents of a synchronverter, with respect to the measurement errors. We show that some of the sensitivity functions exhibit high gains at the relevant frequencies, leading to distorted grid currents, which makes the use of this inverter control algorithm problematic. We then do a similar analysis assuming that we have controlled current sources available at the grid output of the converter, that we control using virtual currents generated in the algorithm. The virtual currents are flowing through virtual output inductors, that we can choose to be significantly larger than the actual output inductors. We show that using the current sources reduces the sensitivity considerably, thus indicating a better approach to synchronverter design.

The equilibrium points and stability of grid-connected synchronverters

“…We omit the superscript e to indicate the equilibrium point values. If the system is at an equilibrium point, then from (11) we see immediately that ω = ω g , from (9) we see that T e = Tm and from (16) we see that Q = Q . Thus, we have proved all the parts of (42).…”

“…The fourth order grid-connected synchronverter model, which considers i f as a given parameter, can be constructed by combining the equations ( 7)- (11). Its state vector is…”

“…An extension of the model (20), to include the effect of measurement errors, has been derived in [11]. This was needed for an analysis of the sensitivity of the currents i d , i q with respect to the measurement errors.…”

Synchronverters with fast current loops

“…Here V is the rms line to line voltage. Assuming that the line voltage V and frequency ω g are equal to their nominal values V r and ω n , the nominal active torque T m needed to achieve a desired active power P set and reactive power Q set at equilibrium is computed based on formula (22) in [16]:…”

“…Our conference paper [22] contains the main ideas of the sensitivity analysis published later in [16], and also a preliminary version of the current loops that we propose here, without experimental results. Since presenting the conference paper, we have significantly improved the design of the current loops, by adding virtual capacitors in the current loops and compensating the delays occuring in this fast control sub-system by correcting the angle in the Park transformation.…”