Research On The Key Techniques Of Hybrid Controller For Power Quality And Fault Current

The power system is crucial for the economic boom and people’s daily life. In recent years, our recognization about the power production, transmission and consumption are deepening. With the rapid development of the grid, the short circuit capacity is also increasing and the power quality becomes more and more important to those customers owning sensitive loads. Moreover, the growing nonlinear loads exacerbate the power quality problem. The renewable energy generation(e.g., wind energy, photovoltaic) has changed the traditional distribution network profoundly. It not only makes the power quality problem become more complicated, but also increases the short-circuit current to some extent. Therefore, the smart grid construction becomes an effective way to energy substitution, power quality control,and reliability improvement of the power grid.Traditional series power quality controller may be in danger during short-circuit faults. Also, a traditional fault current limiter(FCL) is single functional, rarely used,namely a long-term idle FCL and low using efficiency. Thus, the paper is funded by the National Natural Science Foundation of China “Key technology of novel hybrid controller for power quality and fault current”, and the key scientific projects of China Southern Power Grid “Novel fault current limiter with power quality control function”, It carries out these researches on hybrid controller for power quality and fault current(HPQFC) and its control method. It researches the cooperative methods between the HPQFC and distribution network relay protection, improves the low voltage ride through(LVRT) of the wind farm, and elaborates its engineering application. It has solved the key problems and provided a relatively complete basic theory. It lays good foundation for the promotion of the HPQFC. The main innovation points are as follows:(1) It presents a new topology of the hybrid controller for power quality and fault current. HPQFC can operate at the fault current limiting mode when the fault occurred,and it also switch to the power quality control mode during normal condition. Based on theoretical analysis, different mathematical models and comprehensive model of the HPQFC are established. It also proposes synthetic control strategy of the HPQFC,such as the synthetic control strategy at the parallel side, series side and fault current limiting control strategy. The proposed method can ensure the smooth switchingamong different modes during different operation states. It presents the relationship between the number of output level and the modulation ratio of the single carrier phase-shifted sinusoidal pulse width modulation. And the dc side voltage selection method of the expectation level output from the inverter module is introduced at the specific depth compensation. Because of the fact that the HPQFC with fault current limiting and power quality control functions, it can improve the utilization rate thus reduce investment of power grid.(2) It analyzes the key operation technology of the HPQFC in order to ensure the switching reliability between different functions. It analyzes two dynamic processes in detail, namely from the compensation mode to current-limiting mode and current-limiting mode to the compensation mode. The switching effect among different modes is also proposed. Meanwhile, a control technology of the short-circuit fault current is presented by the match of series transformer and adjusting firing angles of the anti-parallel thyristors. It presents the relationship of trigger,transformer ratio and limiting impedance. In order to avoid dc-side voltage shocks during the switching processes, it proposes a suppression technique of dc-side voltage by the varistors and ensures the safe operation of the HPQFC.(3) It proposes a cooperative method between the HPQFC and distribution network relay protection. Based on the relationship of the HPQFC and the relay protection,two control methods are proposed respectively, i.e the time adjustment method of the HPQFC considering sensitivity checking and the current adjustment method of the HPQFC considering the selective checking. Thus, it can achieve a right fit between the HPQFC and the relay. Moreover, it puts forward the operation strategies under the transient and permanent short-circuit fault by action timing analysis respectively in order to ensure the correct match for the reclosing protection.(4) It proposes a method to improve the LVRT ability of the wind farm. Based on the current research situation, it analysis the operation characteristics of the squirrel-cage induction generators during and after the fault. Integrated control method to improve the ability of the wind farm is raised and two control schemes are applied: a DVR mode and a multifunctional mode. The HPQFC can operate in the DVR mode to compensate the slight voltage fluctuations/imbalances at the point of common coupling. When serious voltage sags occur in the line, yet the HPQFC controller deactivates the faulty phase inverter and activates its corresponding thyristors to insert the equivalent impedance to maintain the voltage level during the fault. Then, it switches to DVR mode to improve the recovery time after the fault. Anoperational algorithm is applied to deal with different voltage sag depth caused by various fault conditions(both symmetrical and asymmetrical faults).(5) It proposes the basic principles of the development and engineering applications of the HPQFC. Based on the project of power quality compensation and fault current limiting in a 110 k V substation of the China Southern Power Grid, an integrated design idea is brought forward. Some key technical problems of the HPQFC are raised to ensure its control performance and long-term operation security. The HPQFC system for engineering use is developed. The installation progress of the HPQFC is elaborate at project site, and the protocol scheme is introduced in detail as well.Viewing from the point of the demand that power quality control and fault current limiting, the paper proposes a HPQFC with its control scheme as well as engineering applications and many more in detail. The HPQFC can overcome the problems which the traditional series power quality control device are unable to realize, low using efficiency and just being a single functional device. It achieves the aim that a single device with multi-function. The research on HPQFC is very practical for other research of the hybrid systems. The overall program design method and some key technical experience can be extended to other power quality device and fault current limiter. It also provides a useful practical reference for the development of the power electronic devices.