Power electronic technology and harmonic control in power system

The application of power electronic technology in the power system involves major issues such as improving transmission capacity, improving power quality, improving grid operation stability, reliability, control flexibility and reducing losses. However, while promoting the development of power systems, power electronic technology has become the most important source of harmonics in power systems, and the harmonic pollution generated by power electronic devices has become a major obstacle to the development of power electronic technologies. It forces researchers in the field of power electronics to conduct more effective research on harmonics to control harmonic pollution.

2 Application Status and Development Forecast of Power Electronic Technology in Power System There are many power semiconductor devices in the power system, ranging from rectification and inverter devices for DC transmission to TV power supplies and battery chargers, including frequency conversion and chopper (DC voltage regulation) and AC voltage regulation devices, etc., their applications are all over the various voltage levels of the power system. This article introduces the main aspects and development of power electronic technology in power system applications.

2.1 The high-voltage direct current transmission technology (HVDC) has a backup capacity of more than 36GW. The new generation of HVDC technology is considering the use of GTO, IGBT and other switchable devices, and pulse width modulation (PWM) and other technologies. Based on the construction of domestic high-voltage transmission projects and the introduction and absorption of foreign equipment and technologies, it will be feasible and necessary to design and manufacture HVDC projects based on domestic small capacity.

2.2 Static var compensator (SVC) SVC is a solid-state switch that uses thyristors as the basic components to replace electrical switches, to achieve fast and frequent changes in the admittance of the transmission system by controlling reactors and capacitors. SVC can have different loop structures, which are called thyristor switching capacitor (TSC), thyristor switching capacitor (SR) or thyristor control reactor (TCR) according to the controlled objects and control methods.

The SVC capacity of five 500kV substations in China's power transmission system is 105 ~ 170Mvar, all of which are imported equipment. The type is TCR plus TSC or mechanical switching capacitor bank. There are about 20 sets of TCR devices used in domestic industry, with a capacity of 10 ~ 55Mvar, and half of them are domestic equipment. The low-voltage 380V power supply system has various types of TSC-type domestic reactive power compensation equipment in operation, but so far there is still no set of domestic SVC in China's transmission and transformation system.

23 Flexible AC transmission system (FACTS) In recent years, FACTS technology has received extensive attention. Electric power research departments have conducted feasibility studies on FACTS applications, and manufacturers have invested heavily in development research. The table lists some FACTS controllers that have been put into operation.

Table has been put into operation FACTS controller (to the beginning of 1998) English abbreviation controller power flow suppression system voltage commissioning first commissioning year name control system oscillation support project number (implementation site) thyristor series influence influence capacitance compensator size static synchronization influence The compensator is small, medium and large (Japanese Inuyama). The unified tidal current influence controller is large. In addition to the above-mentioned FACTS equipment, there are many other examples, but they are basically in the stage of research and development or preliminary application. ), Thyristor controlled phase shifter (TCPS), interphase power controller (IPC), superconducting energy storage 2.4 user power technology (CP) CP technology and FACTS technology are rapidly developing sister-type new power electronic technology. The core of adopting FACTS is to strengthen the controllability of the AC transmission system and increase its power transmission capacity; the purpose of developing CP is to strengthen the reliability of power supply and improve the quality of power supply in the distribution system. The common basic technology of CP and FACTS is that the respective controllers of power electronics technology are also the same in structure and function. The difference is only the rated electrical value. The fusion of the two is a trend. Typical user power technology products are: dynamic voltage restorer (DVR), solid state circuit breaker (SSCB), fault current limiter (FCL), unified power quality regulator (PQC), etc.

2.5 Frequency Conversion Speed ​​Regulation Technology (SFC) With the rapid development of power electronic technology, SFC technology has entered a stage comparable to and competitive with DC speed regulation, and there is a tendency to replace it. The application of SFC technology in the power system mainly has two aspects: ① The fan and water pump of the power plant use variable frequency speed control, which will have very large power saving benefits. ②The use of SFC technology in pumped storage units can reduce the impact of the unit's starting process on the power grid, and when the unit is running at a low head, it can also improve the unit's efficiency.

3 Application of harmonic suppression of power electronic technology There are two main ways to solve the harmonic pollution of power electronic equipment: implementing harmonic compensation on the power grid and improving the power electronic equipment itself. There are the following two commonly used filters.

3.1 The basic working principle of active power filter (APF) APF is to detect the current and voltage of the compensation object. The harmonic and reactive current detection circuit calculates the compensation current command signal, which is amplified by the compensation current generation circuit. The compensation current is obtained, and the compensation current cancels the harmonics and reactive currents to be compensated in the load current, and finally the desired power supply current is obtained.

There are two factors driving the breakthrough of active filter research: one is the development and application of high-power turn-off devices, such as high-power gate turn-off thyristors (GTO) and insulated gate bipolar transistors (IGBT ) And the gradual application of devices such as field-controlled thyristors (MCT), making it possible for the inverter to generate high-power current and voltage; second, the instantaneous reactive power theory provides a theoretical basis for real-time detection of distortion current in three-phase systems.

Due to the high cost and limited capacity of the APF device, the current research focus of APF has turned to a hybrid filter system (HPFS) composed of active power filters and passive power filters. In HPFS, harmonic currents are mainly filtered by passive power filters, and active power filters are mainly used to improve the filtering effect of passive filters and suppress the occurrence of series resonance. Compared with the APF system used alone, the required APF capacity is greatly reduced, therefore, HPFS can improve the filtering performance of the entire system.

3.2 Active Power Factor Corrector (PFC) The basic idea of ​​power factor correction is to let the input current of the rectifier track the input sinusoidal voltage. To achieve this goal, both passive and active circuits can be used. The passive method is the simplest, but the current harmonics are still large and require reactive loads. Although the active mode requires a dedicated control circuit, the controllable switch makes the input current a sine wave and completely eliminates harmonics. It is currently used in large quantities. Among the power conversion circuits used for power factor correction, the boost chopper circuit and the buck-boost type circuit are the most used. PFC is divided into discontinuous conduction mode and continuous conduction mode according to whether the inductor current is continuous. Active power factor correction technology has been widely used in small power switching power supplies and uninterruptible power supplies (UPS). However, the three-phase active power factor correction technology is far less mature than the single-phase active power factor correction technology, and the engineering technology community is currently working on this issue.