Different Load Types and their Challenges to the Stability of DC Microgrids

  Picture showing load types Copyright: © RWTH Aachen

The DC microgrid is, from a technology standpoint, an interesting case, as all distributed generation and storage devices are normally interfaced through an inverter, and the majority of the AC loads are also connected to the grid via a power electronics interface. The direct interface of a load through a converter introduces challenges which lie in the dynamic stability of the system, due to the nonlinear behavior. From the point of view of the DC bus, such converters exhibit a Constant Power Load (CPL) behavior, as they tend to maintain the load power constant under fast current and voltage disturbances.

Cascading two dc-dc converters means that the downstream converter now referred as point of load converter (POL) acts as load of the upstream converter also called line regulating converter (LRC). Cascading also means that the output voltage of LRC is equal to the input voltage of POL and the input current of the POL equals the output current of the LRC.

An important consequence of the converter’s tight control is that they make the load appear to have a negative incremental resistance at the DC bus connection. This negative damping, in certain conditions, may cause instability of the bus voltage, a phenomenon broadly investigated. In the related stability analysis though, commonly the load is assumed to exhibit idealized constant power behavior. This assumption has limitations when the input characteristic of the load converter deviates from the constant power load (CPL) behavior. If the control bandwidth of the load is sufficiently high, the load behaves like a constant power load, and the system stability margin decreases with the increase in output power. However, in a practical range, with a lower control bandwidth, the system stability margin is influenced critically by the converter’s characteristic impedance, as well as its output power. Under these conditions, the minimum stability margin may occur at a low power range. Consequences are that the assumed overshot is assumed too optimistic (i.e. less than observed), so that the system appears more stable which could lead to lesser security margins. Furthermore, the assumption of an ideal CPL neglects the influence of the controller design on the behavior of the load as for example voltage controlled mode offers other dynamic properties than current controlled operation.

In this project we investigated how different converter technologies affect the load behavior so that it deviated from ideal CPL. The main influence factors considered in this study are the load characteristics (e.g resistive, inductive, capacitive, non-phase minimal behavior, nonlinear behavior), switching frequency and control bandwidth. The approach consists in defining a general load impedance model, including sensing and actuation. The main goal is to determine under which operating conditions the ideal CPL representation is faithful, and under when it may produce misleading results. The determination of the idealized assumption has to be taken into account especially in the following cases: first order lag, non-minimum phase or unstable behavior. Converters supplying loads that show these load characteristics may be destabilizing for the yielding a different dynamic behavior in contrast to the idealized CPL assumption, and therefore the stability analysis of DC microgrids needs to be extended to include these cases and also the stabilizing control which resides in the line regulating converters needs to be adapted to properly handle these loads.

A better dynamic model of the load and the converter with which it is interfaced to the bus facilitates a better design approach for a stabilizing control which resides in the line regulating converters.