Is there any problem related to power quality in the process of power generation, power generation, distribution, and power consumption? How do we define the three-phase imbalance? What kind of harm does the grid imbalance cause to the grid? These answers will be revealed to you here.
In the power grid, we usually say that the three-phase voltage unbalance refers to the degree of imbalance of the three-phase voltage in the three-phase system, and is expressed by the rms percentage of the voltage or current negative sequence component and the positive sequence component. In the power quality test, we can not only observe the voltage and current value of the three phases through the vector diagram, but also evaluate the three-phase unbalance. Just like the tripod shown below, you can control the length of each bracket to maintain a good balance.
Figure 1 Balance bracket
China's current implementation of GB/T 15543-2008 "three-phase voltage allowable imbalance" stipulates that when the grid is in normal operation, the negative sequence voltage imbalance does not exceed 2%, and short-term does not exceed 4%. Each user connected to the common connection point causes the negative sequence voltage imbalance value of the point to be generally 1.3%, and the short-term does not exceed 2.6%. The actual situation of the electric energy can be visually reflected in the specific test items.
Figure 2 Unbalanced interface
Here are the test items we need to monitor: positive sequence voltage, current, negative sequence voltage, current, zero sequence voltage, current, and voltage and current negative sequence imbalance, voltage and current need to be unbalanced. In particular, the most obvious manifestation of the three-phase imbalance of the motor is the most obvious. The effect picture is shown in Figure 3.
Figure 3 Hazardous motor
In a circuit with a large motor load, the input voltage signal of the three-phase motor must meet a certain degree of balance. If the three phases are unbalanced, it may cause relay protection misoperation, additional vibration torque and heat of the motor, and uneven motor speed. The electric control equipment is disordered and the neutral current is too large, which may cause damage to the motor. Due to heat generation, the life of the motor insulation will be reduced by half. If the voltage of a phase is higher than the rated voltage, the operating life will be more severe.
Figure 4 E8000 power distribution cabinet
For this reason, in the operation of the three-phase four-wire low-voltage network, the three-phase load should be measured frequently and adjusted to balance it. This is an effective measure for reducing energy loss. For distribution lines with a long transmission distance. The effect is particularly significant. The E8000 series power quality online monitoring device of Guangzhou Zhiyuan Electronics Co., Ltd., for the power system, the online monitoring of power quality in the network is of great significance, which enables the power company to understand the power generation, power transformation, power distribution and power consumption process in real time. Detailed power quality information in the system, timely detection of hidden troubles, advance management and rectification, optimize grid and equipment operation, and reduce maintenance costs. The series of power quality online monitoring devices independently developed by Zhiyuan Electronics rely on excellent performance and have passed the authority certification of Guangdong, Guangxi, East China and Sichuan.
l Passed the authority certification of the National Electric Power Academy
l Fully compliant with IEC61000-4-30A standard
l Fully meet the collection and recording of all power quality parameters
l Support IEC61850, MODBUS, 103 communication protocol
l Support PQDIF, Comtrade standardized data format
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