Multi‐agent voltage balancing in modular motor drives with series‐connected power electronic converters

Abstract

Modular motor drives can be considered as multi‐agent systems, in which the agents can work together to reach a common goal. One agent in such a modular motor drive consists of only a subset of the machine phases and power electronic converter modules, and is equipped with a dedicated controller. When the dc‐links of the different agents are connected in series to a single voltage source, giving rise to a so‐called stacked polyphase bridges converter, the power electronic converter components can have low voltage ratings. However, a voltage balancing controller must ensure that the total dc‐link voltage is distributed evenly among the agents when working in motoring mode. The goal of this research is to propose a multi‐agent voltage balancing algorithm based on dynamic average consensus, which depends solely on local computations, local measurements, and neighbour‐to‐neighbour communication. The scalability and reliability of the modular hardware are hence extended towards the control. Simulations and experimental results on a 4 kW modular axial‐flux PMSM demonstrate the feasibility of the concept.

Dynamic multi‐agent dc‐bus reconfiguration in modular motor drives with a stacked polyphase bridge converter

“…In this work, a completely decentralised multi‐agent voltage balancing algorithm is used for this purpose, which is based on the algorithm presented in Ref. [13]. This algorithm avoids the need for a central voltage sensor to measure the total dc‐bus voltage E dc , as this is an additional voltage sensor and a single point of failure.…”

“…At each discrete time instant k (with the voltage balancer's update period T vb = 1/ f vb between subsequent time instants), all the agents are assumed to have exchanged their variables p x and $${\overline{v}}_{\text{dc},x}$$ with their two closest neighbours, and to update their variables as follows [13, 28, 29]: $$\begin{align*}\hfill {q}_{x}(k+1)& =\rho {q}_{x}(k)+{k}_{\mathrm{p}}\left\{\left[{\overline{v}}_{\text{dc},x}(k)+{p}_{x}(k)\right]\right.\hfill \\ \hfill & \quad -\frac{1}{2}\left[{\overline{v}}_{\text{dc},x-1}(k)+{p}_{x-1}(k)\right]\hfill \\ \hfill & \quad \left.-\frac{1}{2}\left[{\overline{v}}_{\text{dc},x+1}(k)+{p}_{x+1}(k)\right]\right\},\hfill \end{align*}$$ …”

“…The authors of Refs. [13, 14] propose to stabilise the dc‐link voltages by adding a stabilisation term with a gain g to the original current component references i d ,ref and i q ,ref at each update period T vb : $$${i}_{d,x}^{\ast }(k)={i}_{d,\mathrm{r}\mathrm{e}\mathrm{f}}(k)+g{i}_{d,\mathrm{r}\mathrm{e}\mathrm{f}}(k)\left[{v}_{\text{dc},x}(k)-{\overline{v}}_{\text{dc},x}(k)\right],$$$ $$${i}_{q,x}^{\ast }(k)={i}_{q,\mathrm{r}\mathrm{e}\mathrm{f}}(k)+g{i}_{q,\mathrm{r}\mathrm{e}\mathrm{f}}(k)\left[{v}_{\text{dc},x}(k)-{\overline{v}}_{\text{dc},x}(k)\right].$$$ …”

“…[14] have proven that the voltages v dc,x over the dc-link capacitors of the agents only remain stable in generating mode. In motoring mode, an active voltage balancing algorithm [13,14,20] is required to ensure that the total dc-link voltage E dc is divided equally among the agents of the converter, and thus to avoid that the breakdown voltages of the semiconductor power switches are exceeded.…”

“…The agents can be fed by multiple dc power sources [11], or they can be connected in series [12,13] or parallel [9,10] to a single dc power source. The specific multi-three-phase machine topology that will be studied in this paper, is the so-called stacked polyphase bridge converter [14,15], depicted in Figure 1.…”