Wind power grid-connected reactive voltage control development trends

background

Wind power generation is one of the most mature technologies in the development and utilization of renewable energy in the world. It is one of the power generation methods for the development of large-scale development and commercialization, as it is reducing environmental pollution, adjusting energy structure, and solving power consumption problems in remote areas. The prominent role of the parties has been increasingly valued by all countries in the world and has been extensively developed and utilized.

According to China's wind power development plan, China will establish 70 million kilowatts of wind power bases in Gansu, Inner Mongolia, Xinjiang, Hebei, Jilin and Jiangsu. It is estimated that 58.08 million kilowatts will be completed by 2015 and 90.17 million kilowatts will be completed by 2020, accounting for Total installed capacity of wind power is 78%. As China's onshore wind energy resources are mainly concentrated in the “Three North” region, in addition to the common problems of conventional wind power generation, there are still many special personality issues for wind power bases located at the end of the grid, including system stability, transmission capacity, and frequency modulation. Peak regulation and power consumption, among which the reactive voltage problem is one of the main concerns of wind farms in grid-connected operation, and measures need to be taken to effectively regulate the reactive voltage of the wind farm.

Development status

In the early days, wind turbine generators mainly used asynchronous generators. They did not have the ability to maintain and regulate the voltage level at the terminal. In operation, they also needed to absorb reactive power from the system. Accordingly, the wind farms need to be installed with fixed capacitors to compensate. With the development of power electronics technology, dynamic reactive power compensation devices such as SVC and STATCOM have emerged, and the wind farm adopts a fixed capacitor + dynamic reactive power compensation device to control reactive power.

In recent years, reactive power compensation for voltage stabilization of wind farms has been a hot issue for power companies and related research institutions. Under this background, domestic researches on wind farm reactive power control technology have been gradually carried out, including research on wind turbine reactive power control technology, wind farm reactive power compensation device research, FACTS device coordination control and so on.

(1) Research status of reactive power control technology for wind turbines With the development of wind power technology, wind turbines have evolved from the original without reactive power control capability to capable of outputting certain reactive power. At present, doubly-fed asynchronous wind turbines and permanent-magnet direct-drive wind turbines are the mainstream models. Double-fed asynchronous wind turbines achieve decoupling of active and reactive power through control, and have certain dynamic reactive output adjustments. Capability; The permanent-magnet direct-drive wind turbine can flexibly control reactive power because it is connected to the grid through a full-capacity frequency converter. Both of these wind turbines have the ability to operate in constant voltage mode, which can achieve reactive power and voltage control to some extent.

(2) Research status of reactive power compensation devices for wind farms To adapt to the needs of different occasions, reactive power compensation devices for wind farms have been developed in many types, their required costs are not the same, and they affect the transient characteristics of grid voltages. Not the same.

1 Shunt Capacitor Shunt Capacitor compensation can be used to connect certain nodes of the power system with a circuit breaker. The shunt capacitor can only supply capacitive reactive power to the system. Shunt capacitors have the advantages of investment, economical operation, simple structure, convenient maintenance, arbitrarily selectable capacity, and strong practicality; the disadvantages are: (1) Shunt capacitor compensation is achieved through the switching of capacitors, because the adjustment is not smooth and is stepwise The optimal compensation status cannot be achieved during system operation. When using a capacitor group switching mode, the effect of reactive power compensation is limited by the number of capacitor component groups and the capacity of each capacitor group. (2) The switching of the capacitor is mainly realized by a vacuum circuit breaker, and its switching and switching response is slow, and it is not suitable for frequent operation. Therefore, fast tracking compensation of reactive power cannot be performed. If a thyristor-switched capacitor bank is used instead of switching the capacitor bank with a vacuum switch, the problem of slow switching response and high inrush current during closing is solved, but the inconsistency of reactive power regulation and the inconsistency of the capacitor group cannot be solved. The use of high-power power electronics has also greatly increased the cost of the system. (3) As the switched capacitor is grading compensation, over-compensation and under-compensation are inevitable. According to the relationship between reactive power and voltage, over-compensation will cause voltage increase, and under-compensation will cause voltage drop due to inductive load. (4) A sharp drop in voltage drop is not conducive to voltage stability, and spike voltage pulses are generated when the voltage is dropped. The reactive power generated by the capacitor is proportional to the square of the voltage, and the reactive power output at low voltage is reduced. Obviously more reactive power is required at this time. If the reactive power cannot be supplied in time, the voltage level of the system will be reduced.

2 On-load tap-changer The on-load tap-changer (OLTC) can not only change taps under load, but also has a large adjustment range. Usually it can be UN±3×2.5% or UN±4×2. 0%, there are 7 to 9 taps to choose from. Therefore, the on-load voltage regulator OLTC is an important voltage regulation means in the power system. In the operation of the system, the tap can be automatically changed and the ratio can be adjusted to maintain the voltage level in the load region. However, the transformer cannot be used as a reactive power source. Instead, it consumes the reactive power in the power grid and belongs to reactive power. The adjustment of the transformer tap (tap) not only changes the voltage status of the transformer on each side, but also on each side of the transformer. The distribution of power is affected. It has been pointed out that under certain circumstances, when the OLTC changes the tap according to its lifting logic, it will not only improve the voltage condition, but will worsen it, and it is even considered to be one of the important causes of voltage collapse. Therefore, the use of this equipment must be carefully considered when the wind farm is connected to the grid.

3 Static Var Compensator The Static Var Compensator (SVC) usually consists of a parallel capacitor bank (or filter) and an inductance element with adjustable inductance. The difference between the SVC and the common shunt capacitor compensation device is that it can track reactive power fluctuations of the power grid or load and perform real-time compensation for reactive power to maintain voltage stability. SVC is completely static, but its compensation is dynamic, that is, it automatically tracks the compensation according to the reactive demand or voltage change. Static var compensation systems are reactive components (capacitors and reactors) that generate reactive power and adjust capacitive or inductive currents as needed. The static compensator can increase the voltage stability limit value, and the SVC installed on the middle node of the system has a good effect and is often the preferred solution in the technical and economic comparison. Some literature has applied the FACTS equipment to the wind farm to improve the voltage stability of its operation, indicating the excellent performance of SVC in reactive power compensation of wind farms.

4 Static Synchronous Compensator (STATCOM)

The Static Synchronous Compensator (STATCOM) is also called Static Var Generator (SVG). The basic circuit is divided into voltage type bridge circuit and current type bridge circuit. In the voltage bridge circuit, the DC side uses a capacitor as the energy storage element, while the AC side is connected to the grid through a series reactor: the current bridge circuit, the DC side uses the inductor as the energy storage element, and the AC side connects after the capacitor. Grid. In fact, due to operational efficiency, most of the STATCOMs that have been put into use today use voltage-based bridge circuits. The basic working principle of STATCOM is to directly connect the bridge-type converter circuits in parallel or through reactors on the power grid, and appropriately adjust the phase and amplitude of the output voltage of the AC side of the bridge-type inverter circuit or directly control the AC-side current of the bridge-type converter circuit. Absorb or deliver reactive currents that meet the requirements to achieve dynamic reactive power compensation. Compared with SVC, STATCOM has five advantages: fast adjustment speed, wide operating range, wide adjustment range, small component capacity, and small harmonic content.

latest progress

With the development of wind power technology, power electronics technology and control technology, future wind farm reactive power control technology will be based on "closed-loop" control, through the coordinated operation of wind turbine generators, reactive power compensation devices, and power grids, to achieve no wind farms Effective control of the work.

For product applications, GE Wind has developed a closed-loop wind farm voltage control called WindVAR. Dynamic reactive power control can provide reactive power and stable voltage to the grid. With a fan with dynamic reactive power control, voltage control and regulation are performed by power electronic devices mounted on the fan.

European related power companies and technical institutes and the American Wind Energy Association (AWEA) have developed relevant guidelines for wind power generation and IEEE-1547 (distributed power supply and power system access standards), including voltage stability control/reactive power compensation The content requires to ensure that the bus voltage of the wind farm is stable within a certain range and to ensure that the power quality is qualified.

At present, in the forthcoming national wind power grid new national standard, not only does the wind turbine require reactive voltage regulation, but it also requires low voltage ride through capability. The promulgation of the new national standard will promote the development of wind power-related industrial technologies in a more grid-friendly manner, and realize the smooth and large-scale adoption of wind power.