Improvement measures for power quality

Improvement measures for power quality

(1) Steady-state voltage control. For the steady-state or quasi-steady-state power quality problems where the voltage deviates from the ideal state, such as voltage deviation, conventional voltage control methods or the installation of reactive power compensation equipment can be used to solve the problem. For example, by adjusting the excitation current and other means, change the power factor of the generator set, control the reactive power output by the generator set to control the voltage; change the tap position of the transformer, thereby changing the transformation ratio of the transformer to adjust the voltage; change the line parameters to adjust the voltage.

(2) Dynamic voltage control. For dynamic or transient power quality problems such as voltage fluctuations, flicker, and voltage sags, reactive power compensation equipment with fast response characteristics should be used, including static var compensator (SVC), static synchronous compensator (STATCOM), dynamic voltage restorer (DVR), power quality unified controller of power distribution system, etc.

(3) Voltage unbalance control. In order to maintain the voltage balance of the three-phase system, all single-phase loads and single-phase distributed power sources should be arranged on different phases as balanced as possible. In addition, selecting a transformer with a connection group of Dynll can reduce the zero-sequence impedance and help reduce the influence of three-phase load imbalance. For the occasions where it is determined that there will be a three-phase voltage unbalance problem, a reactive power compensation device with split-phase control, such as STATCOM, DVR, etc., can also be used to compensate for the unbalanced state.

(4) Suppression of power grid harmonics. There are two main measures for suppressing harmonic current: ① suppressing the amount of harmonic current generated by the harmonic source; ② absorbing or canceling the harmonic current in the vicinity of the harmonic source.

1) Limit the amount of harmonic current generated by the distributed power supply. Considering the grid-connected requirements of distributed power sources, the distributed generator set itself and the grid-connected inlet can be designed and transformed so that no harmonics are generated or the harmonics generated are within the acceptable range of relevant standards. This is one of the most important ways to solve the harmonic problem of distributed power.

2) Use the filter to absorb the harmonic current from the harmonic source in situ. The use of power filters to absorb the harmonic current generated by the harmonic source on site is an effective measure to suppress harmonic pollution. According to the filtering principle, power filters can be divided into passive filters, active filters, and a combination of the two—hybrid filters.

Passive filters are simple in technology, reliable in operation, convenient in maintenance and low in cost, so they have been widely used in power systems and are still the most common way to suppress harmonics so far. The main disadvantage is that the compensation characteristics are affected by the grid impedance and operating state, and it is easy to generate parallel resonance with the system, causing the LC filter to be overloaded or even burned out. Active filter devices are an important trend in harmonic suppression research. Active power filter (APF) is a new type of power electronic compensator for dynamic reactive power compensation and harmonic suppression. It is composed of static power converters and has the high controllability and fast response of power electronic converters. More importantly, it can actively inject compensation current into the AC grid, thereby canceling the harmonic current generated by the harmonic source. APF can track both frequency and amplitude, and can compensate harmonics in real time, and the compensation characteristics are not affected by grid impedance, so it has received extensive attention and more and more applications.

(5) Suppression of DC components. At present, the measures to suppress the DC component mainly include current limiting and DC isolation. The main methods include the neutral point injection of reverse DC current, the neutral point series resistance, the neutral point series capacitor, the AC transmission line series capacitor, and the improvement of the DC current distribution in the power grid.