Multilevel converter-based grid-connected energy storage systems
Date of Issue2018-09-11
School of Electrical and Electronic Engineering
Singapore enjoys more solar irradiance than other temperate countries, thus the solar Photovoltaic (PV) is one of the most promising renewable energy sources for the electricity generation in Singapore. The growing capacity of grid-connected renewable energy sources introduces intermittency problem which poses threat on the stability of power system. As for the solar PV, the output is greatly relevant to the irradiance which can vary rapidly with the weather condition. The solar irradiance change caused by the cloud passing can lead to hundreds of MW level power fluctuation in seconds. The dramatic power fluctuation can’t be mitigated by the traditional power system operating reserves whose response time is relatively long. Grid-connected energy storage systems are more suitable to solve the intermittency problem in this case. In this project, the battery energy storage system (BESS) is considered. BESSs are more feasible than operating reserves and have less response time, thus they can react quickly when there is a sudden output power fluctuation from the solar PV. Ramp rate control is applied in this project to generate corresponding power reference for BESSs. The moving average filter and the first order filter are compared. The first order filter need less storage space and have smoother output when the maximum ramp rate is same. So, first order filters perform better than moving average filters in ramp rate control. The BESS is connected to the grid based on the cascaded H bridge (CHB) multiphase multilevel converter. The multiphase multilevel converter can output voltage steps more than 3 levels, thus it has higher voltage and power rate and smoother output than the ordinary multiphase inverters. There are many multiphase multilevel converter topologies, e.g. the CHB converter, the Neutral Point Clamped (NPC) converter, Flying Capacitor (FC) converter and other derivatives. Among the classic ones, CHB converters enjoys high level of modularity than other multiphase multilevel converters and need less capacitors and diodes. So, it is more compatible with battery packages than other multiphase multilevel converter topologies. The control methods of the multiphase multilevel converter are generally similar to the control methods of ordinary multiphase inverters. Based on the control method of ordinary multiphase inverters, the multiphase multilevel converter needs more efforts in modulation, voltage balancing and power distribution among modules. The multiphase multilevel converter modulation methods can be classified into high switching frequency ones such as the Sinusoidal Pulse Width Modulation (SPWM) and the Space Vector Modulation (SVM) and low frequency ones with specific modulation objective. The SPWM and the SVM are most popular ones. The SPWM has many derivative forms e.g. phase shift, phase disposition and phase opposition disposition. Different objectives such as reduce output waveform harmonics, elevate equivalent switching frequency and reduce common mode voltage can be realized. The phase shift modulation is selected in this project. The SVM can reach the highest modulation index and is more precise in control the output magnetic linkage, thus it is preferred in the motor control. The unbalancing of the State of Charge (SoC) of batteries may cause the overcharge or over discharge problems and may influence the variance in batteries life time. Therefore, the SoC balancing control is also indispensable. The zero-sequence voltage injection can change the output power among phases without introducing other undesired phase to the phase wave form changes to the circuit. Therefore, the zero-sequence voltage injection is applied in this project to fully utilize the batteries and balance the dc voltage among phases. In the simulation model, the power reference from the grid side and the solar PV ramp rate control should be given, and the simulation model is to build a battery supplied cascaded H bridge multilevel converter to output the required power to the grid side and the load. The SoC balancing control among phases will be also applied to the simulation model.
DRNTU::Engineering::Electrical and electronic engineering