Minimizing the impact of resonances in low voltage grids by power electronics based distributed generators

Today’s Distributed Generators (DG) and load appliances are increasingly build up with power electronics. This trend is expected to grow further in the future. Also developments are ongoing to improve the performance and efficiency of grid components by means of power electronics and several grid components might be replaced by power electronics based versions in the future. One of the nice properties of power electronics based grid components is the possibility of voltage control. This feature is very welcome to keep voltage levels within prescribed limits while implementing large numbers of DG. Load appliances often use internal circuits that work on a controlled direct voltage level and this is achieved by using power electronics. In this way the performance of such appliances is much higher today than in the past. Besides this and other advantages, disadvantages will also show up. The disadvantages that are studied in this thesis are the increased harmonics caused by resonances and oscillatory voltages caused by non-linear constant power loads. Beside existing resonances in the grid, new ones will be added by large numbers of capacitances used in Electro Magnetic Interference (EMI) filters of power electronics based appliances. These capacitances can interact with inductances in the grid and bring significant resonances that can amplify harmonic currents and voltages to a high level, even in the lower harmonic frequency range. Oscillatory voltages can be caused by improper stabilization actions. Stabilization of a voltage level on a load in an active way, thus by making use of a power electronics converter, brings a constant power load to the grid. This kind of load behaves as a negative differential impedance which can contribute to an oscillatory grid voltage. The proposed solution in this thesis for minimizing the impact of resonances caused by parallel capacitances in the grid is a combination of two so called ancillary services, namely Virtual Parallel Capacitance Reduction (VPCR) and Virtual Resistive Harmonic Damping (VRHD). VPCR is an ancillary service that let a power electronics converter generate a current to compensate currents through the capacitances placed in parallel with the grid, for a frequency range that includes the fundamental and a number of harmonics. VRHD is an ancillary service that gives a power electronics converter a resistive behavior for a number of harmonics. This action will bring extra damping to resonances in the grid. Especially the combination of both VPCR and VRHD is an approach that is very effective for minimizing the impact of resonances in the Low Voltage (LV) distribution grid. This combination provides two measures. Firstly VPCR with its compensation current causes the effect that a resonance is virtually shifted towards a higher frequency value, in the range where the propagation is limited. Secondly VRHD damps the resonance peak to a lower level. VPCR and VRHD can be implemented in appliances with a power electronics interface directly coupled to the grid and in inverters for DG. They do not need a series regulator, because these services are shunt based and acting as active shunt filters. This thesis studies the implementation of VPCR and VRHD in inverters for DG. It has been noticed in various situations in the Netherlands that concentrations of large numbers of EMI filter capacitors of Photovoltaic (PV) systems can bring problematic resonances to the distribution grid. In these cases, large numbers of small inverters for PV systems were connected to the grid which resulted in a high level of harmonic voltage distortion caused by these grid resonances. Studies showed that the parallel output capacitor of inverters for PV systems is relatively high and on an average can triple the total parallel capacitance value at the Point of Connection (PoC) of a dwelling. Another development that increases the total number of capacitors connected to the grid is that appliances are not galvanically isolated from the grid anymore in the switched-off mode. In this mode the appliance goes to an idle state and the EMI filter capacitor remains connected. To master the transition towards a more decentralized generation, knowledge about the harmonic interactions and minimizing the impact of resonances is very important. Research on this is needed to separate causes and effect in situations of insufficient quality of the grid voltage, and to come to the right measures to handle these problems. The research in this thesis deals with the harmonic interaction and minimizing of the impact of resonances in a future situation with large numbers of power electronics based load appliances and DG. Based on the problem definition above, the general objective of this thesis is defined as: Investigate the possibilities to minimize the impact of resonances and harmonic distortions by using ancillary functionalities of the power electronics inverters of DG that are connected to the LV distribution grid. The research work is performed by means of computer simulations and laboratory validation. The most important contribution of the work is the development of a control strategy for the grid connected DG inverters which minimizes harmonic voltage pollution. An important element of the work was the building and programming of a versatile inverter with a Digital Signal Processor (DSP) structure, used for validation of the laboratory set-ups. With this versatile inverter, various control strategies could be implemented to minimize harmonic voltage pollution. The contributions of this thesis can be summarized as follows: ¿ a detailed description of the basic concept of harmonic interaction and grid resonances to separate cause and effect, ¿ the development of a grid impedance spectrum measurement system for the estimation of grid resonances, ¿ the development of computer models and simulations of a small grid and inverters for DG with the ancillary service functions VPCR and VRHD to study grid resonances, ¿ a versatile hardware model of an inverter with a DSP control is build and the ancillary service functions VPCR and VRHD to minimize the effect of grid resonances are implemented, ¿ laboratory validation is performed of computer model simulations of inverter hardware with the ancillary services, ¿ computer simulations are done of a distribution system with grid resonances and inverters with ancillary services. The main conclusions are described below. Oscillatory voltages caused by the interaction between voltage control systems and constant power loads are only expected at sub harmonic frequencies. A possible solution for these oscillations can be found in adjusting the parameters of voltage control systems. Results of the validated simulations show that the studied ancillary services perform as expected, especially the combination of two described ancillary services VPCR and VRHD is a strong measure to minimize the impact of resonances in the harmonic frequency range, in the LV distribution grid. These services produces the effect of a virtual resonance shift towards a higher harmonic frequency range where the propagation is limited, and damp resonance peaks to a lower level. Results from simulated and practical measurements show that the grid impedance spectrum measurement system can work well by injecting a very low measurement current. The system is capable of operating under polluted grid voltages