What is power quality?
Power quality mainly refers to the voltage quality and frequency of alternating current. grid frequency
The nominal frequency of China's power system is 50Hz. GB/T15945-2008 "Power Quality - Frequency Deviation of Power Systems" stipulates that the frequency deviation limit of the power system under normal operating conditions is ± 0.2Hz. When the system capacity is small, the deviation limit can be relaxed to ± 0.5Hz. The standard does not specify the limit of the system capacity size. In the "National Electricity Supply and Consumption Rules", it is stipulated that the allowable deviation of power supply frequency for power supply bureaus is ± 0.2HZ for power grid capacity of 3 million kilowatts and above, and ± 0.5HZ for power grid capacity of 3 million kilowatts and below. In actual operation, it is maintained within the range of no more than ± 0.1HZ from the operation of major power systems across the country.

voltage deviation
According to GB/T 12325-2008 "Power Quality - Supply Voltage Deviation", the sum of the positive and negative deviations of the supply voltage of 35kV and above shall not exceed 10% of the nominal voltage; The deviation of three-phase power supply voltage of 20kV and below is ± 7% of the nominal voltage; The voltage deviation of 220V single-phase power supply is+7% and -10% of the nominal voltage.
Three phase voltage imbalance
GB/T15543-2008 "Electricity Quality - Three phase Voltage Imbalance" stipulates that the voltage imbalance limit at the common connection point of the power system is: during normal operation of the power grid, the negative sequence voltage imbalance shall not exceed 2%, and shall not exceed 4% in a short period of time; There is currently no regulation on the zero sequence voltage limit of low-voltage systems, but the voltage of each phase must meet the requirements of GB/T 12325. The allowable value for the negative sequence voltage imbalance caused by each user connected to the public connection point is generally 1.3%, and should not exceed 2.6% in a short period of time.
Harmonics in the public power grid
According to GB/T14549-93 "Electric Energy Quality - Harmonics in Public Power Grids", the total harmonic distortion rate of voltage (phase voltage) in public power grids at all levels of 6-220kV is 0.38kV, 5.0%; 6-10kV, 4.0%; 35-66kV, 3.0%; and 110kV, 2.0%; The allowable value of harmonic current injected into the power grid by users should ensure that the harmonic voltage of all levels of the power grid is within the limit range. Therefore, the national standard stipulates that the total harmonic distortion rate of the voltage generated by harmonic sources at all levels of the power grid is: 2.6% for 0.38kV, 2.2% for 6-10kV, 1.9% for 35-66kV, and 1.5% for 110kV. For the 220kV power grid and its power supply users, refer to this standard for 110kV implementation.
Harmonics between public power grids
GB/T 24337-2009 "Electric Energy Quality - Inter Harmonics in Public Power Grids" stipulates that the voltage content of inter harmonics is 0.2% for voltages below 1000V and<100Hz, 0.5% for voltages between 100-800Hz, 0.16% for voltages above 1000V and<100Hz, 0.4% for voltages between 100-800Hz, and under study for voltages above 800Hz. The harmonic content rate between single users is 0.16% for 1000V and below<100Hz, 0.4% for 100-800Hz, 0.13% for 1000V and above<100Hz, and 0.32% for 100-800Hz.
Fluctuations and flicker
GB/T 12326-2008 "Power Quality - Voltage Fluctuations and Flicker" stipulates that at the common connection point of the power system, in a small mode of system operation, with a measurement cycle of one week (168 hours), all long-term flicker values Plt meet the following requirements: ≤ 110kV, Plt=1; >110kV，Plt=0.8。 And the relevant regulations for individual users.
Voltage sag and short-term interruption
GB/T 20137-2013 "Power Quality - Voltage Transient and Short term Interruption" defines voltage transient as the phenomenon in which the root mean square value of the power frequency voltage at a certain point in the power system suddenly drops to 0.1p.u.~0.9p.u. and returns to normal after a brief duration of 10ms~1min; Short term interruption refers to the phenomenon where the root mean square value of the power frequency voltage at a certain point in the power system suddenly drops below 0.1 p.u. and then returns to normal after a brief duration of 10 ms to 1 min.