Control of Parallel Operating Battery Inverters

The approach of modular systems technology has now become generally accepted for the design of island grids with different renewable energy converters. This technology is characterised by a stipulated energy coupling (ac-bus bar 230/400 V, 50 Hz), a standardised information exchange and a supervisory control. These preconditions allow an adaptable and expandable system structure to be set up, thus covering almost every supply situation; but mean that the generators integrated into the systems have to be equipped with special control features. The core of a modular designed island grid is the grid forming unit. Because of its control and storage properties a battery storage unit should be chosen. The systems mostly run in master/slave operation with one battery inverter as the master. Such energy supply systems are only expandable as long as the original battery inverter has been oversized or is replaced. It would be preferable to install further inverters, i.e. parallel operation. It will be shown that voltage source inverters are preferable for distributed renewable energy systems. The synchronisation of the inverters can be handled by introducing static properties (droops) according to the conventional approach of the utilities. This way only slow communication for system optimisation is necessary. Some simulation and experimental results will be presented proving the feasibility. 1 MODULAR AC-HYBRID SYSTEMS 1.1 Systems technology Most promising opportunities for renewable energies are stand-alone power ac-supply systems for decentralised electrification. Figure 1 shows a general diagram of an isolated grid supplied mainly with renewable energy sources like PV generators and wind energy converters but also with fossil fuel and biomass-fueled aggregates. The central component of the system is a battery storage unit with a bi-directional inverter, capable of forming the island grid. For reasons of stable operation the integration of a short-term storage with rotating mass (synchronous generator with increased inertia) might be considered. Standard loads like ohmic loads and induction machines, e. g. pumps can be supplied. In order to achieve modularly expandable grids which are suitable for hybrid systems, the energy coupling and communication of the components have to be standardised. Field and installation busses are suitable for the communication purposes. The standard AC bus (230V / 400V; 50Hz) is the obvious choice for energy coupling and allows Figure 1: General modular hybrid system the direct supply of standard loads. The guarantee of a constant power supply in systems with renewable energy sources is aggravated by the fluctuations of wind and solar radiation. In order to maximise the use of renewable energies and to increase the systems' performance, adequate storage units (battery, rotating mass) and an intelligent supervisory control are required. 1.2 Categories of power units The components of such a modular system can be distinguished by their function as either a grid forming unit, a grid supporting unit (controllable generators) or a grid parallel unit (uncontrollable generators and loads). Grid forming unit: The grid forming unit controls grid voltage and frequency by balancing the power of generators and loads. Standard systems contain just one grid forming unit as a master which can be a diesel aggregate or a battery inverter. Grid supporting unit: Being similar to traditional electrical supply systems, the grid supporting unit’s active and reactive power is determined by voltage and frequency characteristics which allow primary control and power distribution. Grid parallel unit: These units comprise loads and uncontrollable generators. Uncontrollable generators are e. g. wind energy converters without control or PVinverters for grid connection. Both devices are designed to feed as much power into the grid (island grid) as possible. 1.3 Sizing of power units The size of common grid forming units is determined by the overall maximum apparent demand or supply of power. These systems mostly contain one grid forming unit (master), loads and several uncontrollable generators designed as current sources (slaves). In case of dominating loads the power of the grid forming unit is simply determined by: