In recent years, the power quality control technology of power distribution system has made great strides. Among them, the most influential ones are DSTATCOM, APF and power system data Monitoring (SCADA), Dynamic Voltage Regulator (DVR).

(A) distribution system static var compensation (DSTATCOM)

There are many fast impact load current in the electric system, such as high-frequency electric furnace, high-capacity large-capacity induction motor load suddenly started, will cause voltage flicker, causing the distribution system current and voltage imbalance, the traditional static reactive Compensator (SVC) to suppress voltage flicker, but the SVC's response speed is slow (tens of milliseconds), the flicker rate is difficult to achieve more than 50%, compared with the PWM control of the power system in parallel with the voltage source converter DSTATCOM has the advantages of fast dynamic response, independent compensation of system current, high harmonic rejection, good suppression of voltage flicker and low active power loss, so DSTATCOM devices gradually replace SVC device is widely used.

(B) Active Power Filter (APF)

In the power system, the uncontrolled harmonic voltage and harmonic current are the main factors affecting the power quality. First of all, with accurate root mean square value of the instrumentation on the circuit and the neutral line regular measurement, harmonic investigation. Strengthen the measurement and inspection of the power quality of the transmission lines of the users, and regularly test and compare the transmission and distribution system, especially the medium and low voltage systems, and find the law of the generation and development of harmonics. Harmonic pollution suppression and control measures according to the different working principle of the device is divided into passive filters and power electronic devices composed of active filters. Passive filter device consists of capacitors, reactors, and sometimes also includes passive components such as resistors to form a low-impedance path for a certain harmonic or its sub-harmonics, in order to achieve the effect of suppressing higher harmonics.

(C) timely monitoring of power system data (SCADA)

In the power system, the dynamic power quality problem is exposed in recent years with the development of new and high technology. Studying the power user's sensitivity and stress to the power quality can provide an important basis for choosing the appropriate compensation mode, and also for the power quality Standard formulation provides important reference. The SCADA system which monitors the quality of on-site dynamic data is the research direction of the distribution management system. Field data includes not only measurement data of power, voltage and current, but also event data such as opening and closing, overcurrent, quick break and other operations and accidents. When the accident caused the trip, but also record the scene fault recorder data, we can see that the amount of data that needs to be communicated is unmatched in the general industrial control. As the power system data changes very fast, an overcurrent may only maintain a dozen milliseconds, the data fleeting, so the real-time data, communication speed requirements are very high.

Monitoring system underlying data Reliable and efficient communication is the key to system reliability and is the focus of designing monitoring software. Some of the existing software data communications, processing and monitoring are done in a software, although it is intuitive and compact, but the system upgrade is very inconvenient, a minor changes to the entire system to be reorganized, so take the module Structure is a better choice.

In a large-scale power monitoring system design, hardware and communication stations and monitoring stations separate the independent way, the software will be the underlying communication software from the monitoring software separated out in the communication station to work independently, the communication station is responsible for the underlying real-time data on-site Of the collection and monitoring stations and the upper two-way data communications. Due to the independence of the communication station, the task of the upper monitoring station is greatly reduced, which not only improves the communication speed at the bottom but also accelerates the data refresh speed of the monitoring interface. If the lower number of instruments, you can use multi-channel dual-port RAM on the communication station intelligent communication card, and expanded to multiple serial ports to further improve the underlying communication speed. In this way, in order to effectively improve the quality of power transmission.

(D) Dynamic Voltage Regulator (DVR)

The DVR is equivalent to a dynamically controlled voltage source connected in series in a power distribution system. With appropriate control methods, this voltage source output can counteract the adverse effects of power system disturbances on the load voltage such as voltage sags, voltage imbalances and harmonics Wait. When the DC side energy is obtained from the system rectification, even if a single-phase fault occurs on the system side, the other two phases can still provide the power to maintain the normal operation of the DVR and it is also possible to compensate for the long-term voltage sag.

If the DC side of the capacitor in parallel across the battery, or the use of large-capacity capacitor energy storage, the device can also play the role of UPS, short-term failure in the system side can provide a load of time power. With a suitable topology, DVRs can be used to comprehensively address dynamic voltage quality issues in power distribution systems such as drop, surge and steady-state voltage quality problems (such as harmonics, fluctuations, three-phase imbalance) and are multi-target Voltage quality control device.

Further, the power distribution system as well as quality control techniques uninterruptible power supply (the UPS), Unified Power Quality Controller (of UPQC), solid state switch (the SSTS), distributed generation (DG) and the like.

Conclusion

With the development of high-tech industries and the application of information technology in various fields of society, power quality problems are aroused widespread concern. Achieving intelligent, informative and hierarchical interactive management of the entire power generation industry such as power generation, transmission, power supply, electricity, customer electricity sales, power grid grading and dispatching, and integrated services will become the development trend of grid quality improvement in the future.

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