APPLICATIONS OF DMC BASED SST FOR RENEWABLE ENERGY SYSTEM

Abstract

The consumption of electrical energy has been increasing day-by-day due to the development in infrastructure and transportation system along with the increased living standards. Therefore, to meet the extra demand of energy, renewable energy sources (RES) are used along with conventional energy sources. Fossil fuels-based conventional energy sources are the primary cause of the global warming problem, and they are limited in nature, and eventually, they will get exhausted soon. Due to faster depletion of fossil fuel based resources, the use of clean energy-based RES is increased. Wind power is one of the most prominent RES over solar, biomass, geothermal, wave & tidal, etc. because of clean energy and simple economics for round-the-clock generations. Generators are required to convert the wind energy into electrical energy. Permanent Magnet Synchronous Generators (PMSG's) are mostly used to harness wind power because PMSG's are high power density machines without slip rings and brushes. Moreover, owing to the presence of permanent magnets, they have high efficiency and low maintenance as well as they provide decent voltage and power capabilities. However, the risk of demagnetization under high temperatures due to high currents operation and gearbox need for constant drive operation under variable wind speed are downsides of PMSG. The power generated from wind energy is intermittent in nature and relay on environmental conditions such as wind speed, air density, temperature, etc. Therefore, power electronics find a viable solution for providing the regulated power/voltage/frequency to the local load/grid from PMSG based variable speed - wind energy conversion system (VS-WECS). In general, two stages (AC-DC-AC) based rectifier-inverter pair are used for wind power conversion and regulation. However, the intermediate DC-link with capacitor banks increased the overall system losses, size, weight, and costs and decreased the lifetime of the system. As a result, direct AC-AC conversion is gaining more interest in the research field. Recently Solid State Transformers (SSTs) are in the limelight for such works. A Direct Matrix Converters (DMC) based SST are single-stage forced commutated AC-AC bi-directional power flow converters without DC-link or any large energy storage element. DMC based SSTs are capable of converting variable voltage and variable frequency into fixed voltage and fixed frequency or vice versa at nearly unitary input power factor. Due to the absence of the DC link capacitor, the system's size and weight decrease, as well as the transient response, is extremely fast compared to conventional multistage converters. The use of PMSG equipped with DMC based SST makes the WECS gearless, compact, and reliable as well as this system can be directly synchronized with the grid or local load. There are various modulation techniques available in the literature for DMC based SST, such as Scalar techniques, Pulse Width Modulation (PWM) techniques, Model Predictive Control (MPC), and many others. Venturini based control algorithms were the first modulation technique for matrix converter, and they are based on the direct transfer function approach. The output voltages are obtained by the multiplication of the transfer matrix or modulation matrix with input voltages. The SVM and DTC are the most popular and robust control techniques for drive applications, but they are complex and not instinctive. In recent years MPC has gained popularity for the control of matrix converter of it is fast and reliable under both steady-state and dynamic conditions as well as easy tuning of the control if the system model is known. Venturini based Control algorithm is sufficient to regulate the voltage and frequency of DMC based SST with power factor control but unable to regulate power exchange between source and load. Various control techniques mentioned in the literature to regulate the power exchange between the source and the load and voltage-oriented control (VOC) are among them. VOC scheme is based on instantaneous p–q theory in the process error between the active and reactive power components of the line currents are compared with its reference values and fed to a proportional-integral (PI) based controller. The output of the PI is then used to generate the reference voltage for the converter. In the proposed work, the MPPT control strategy is applied at the turbine side of the PMSG based WECS for extracting maximum power. In addition, venturini based control algorithm and VOC scheme are implemented on DMC based SST to regulate the voltage, frequency, and active and reactive power of the system. However, this scheme has a limitation of voltage transfer ratio less than one and cannot regulate and deliver constant power at constant efficacy to the grid under variable wind speed conditions. The use of energy storage systems can overcome the drawback of intermittency of WECS. The intermittent nature of wind power creates a difference between supply and demand. This mismatch can be overcome by integrating a Battery Energy Storage System (BESS) with WECS The excessive energy during high wind intervals or light load periods is stored in BESS and can be used under heavy load periods or wind power shortage intervals. Hence, the fluctuating power from the WECS can be smoothened and stabilize by the use of BESS. Thus the reliability of power to the consumer/utility can be maximized. Hence, in the proposed work's primary attention is interfacing a variable speed PMSG based WECS with standalone load and grid using DMC-based SST with BESS. The performance of the proposed system has been investigated and developed in stages. In the first stage, Simulink studies have been carried out for finding out a better control strategy for DMC based SST. After the selection of control strategy, extensive simulation studies have been implemented for PMSG based VS-WECS connected to standalone load and grid using DMC based SST. The integration of the BESS system has mitigated the shortcomings of this system. At the final stage, the experimental prototype has been developed for the validation of the proposed work.