As a product, electric energy has been widely used at present, and the quality of electric energy is directly related to people's lives and safety. So what is power quality? What does power quality include?
1. What is power quality?
Power quality refers to the quality of electric energy in the power system. The ideal electrical energy should be a perfectly symmetrical sine wave. Several factors can cause the waveform to deviate from the symmetrical sinusoidal shape, thus causing power quality problems. On the one hand, we study what influencing factors will cause power quality problems. On the other hand, we study what problems these factors will cause. Finally, we study how to eliminate these factors so that the electric energy can be as close to a sine wave as possible.
2. Main indicators for measuring power quality
Marks for measuring power quality include: voltage deviation, frequency deviation, voltage three-phase unbalance, harmonics and inter-harmonics, voltage fluctuations and voltage flicker.
a. Voltage deviation: Voltage deviation refers to the instability of the power supply voltage, with voltage rising or falling.
b. Frequency deviation: All power grids have the same frequency requirements for the power grid, which will not change due to different power users. Each country has corresponding regulations on frequency deviation.
c. The three-phase voltage is unbalanced: the value of the three-phase voltage exceeds the specified standard.
d. Harmonics and interharmonics: Currents or voltages whose frequency is an integer multiple of the fundamental wave and behave as sinusoids are called harmonics. Non-integer multiples are collectively called interharmonics.
3. Specific indicators of power quality in China
a. Grid frequency
The nominal frequency of my country's power system is 50Hz. GB/T15945-2008 "Power Quality Power System Frequency Deviation" stipulates that the frequency deviation limit under normal operating conditions of the power system is ±0.2Hz. When the system capacity is small, the deviation limit It can be relaxed to ±0.5Hz. The standard does not specify the limit of system capacity. The "National Power Supply and Consumption Rules" stipulates that "the allowable deviation of the power supply frequency of the power supply bureau: ±0.2HZ for grid capacity of 3 million kilowatts and above; ±0.5HZ for grid capacity of less than 3 million kilowatts. In actual operation , judging from the operation of major power systems across the country, they are maintained within the range of no more than ±0.1HZ.
b. Voltage deviation
GB/T12325-2008 "Power Quality Supply Voltage Deviation" stipulates that the sum of the absolute values of positive and negative deviations of power supply voltages of 35kV and above shall not exceed 10% of the nominal voltage; the deviation of three-phase power supply voltages of 20kV and below shall be the nominal voltage ±7%; the voltage deviation of 220V single-phase power supply is +7% and -10% of the nominal voltage.
c. Three-phase voltage imbalance
GB/T15543-2008 "Power Quality Three-Phase Voltage Imbalance" stipulates that the voltage imbalance limit at the public connection point of the power system is: During normal operation of the power grid, the negative sequence voltage imbalance does not exceed 2%, and it shall not exceed 2% for a short period of time. 4%; the zero-sequence voltage limit of low-voltage systems is not specified for the time being, but the voltage of each phase must meet the requirements of GB/T12325. The allowable value of the negative sequence voltage imbalance caused by each user connected to the public connection point is generally 1.3%, and shall not exceed 2.6% in the short term.
d. Public grid harmonics
GB/T14549--93 "Harmonics of Public Power Grids for Power Quality" stipulates that the total harmonic distortion rate of public grid voltages (phase voltages) at all levels from 6 to 220kV is 0.38kV, 5.0%, 6~10kV, 4.0%, 35~ 66kV is 3.0%, 110kV is 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 power grid is within the limit range, so the national standard stipulates the total harmonic distortion rate of the voltage generated by harmonic sources of power grids at all levels Yes: 0.38kV is 2.6%, 6~10kV is 2.2%, 35~66kV is 1.9%, and 110kV is 1.5%. For the 220kV power grid and its power supply users, refer to this standard for 110kV.
e. Harmonics between public grids
GB/T24337-2009 "Interharmonics in Public Power Grids for Power Quality" stipulates that the interharmonic voltage content rate is 1000V and below (0.2% for 100Hz, 0.5% for 100~800Hz), and 0.16% for 100V and above (100Hz, 100~ 800Hz is 0.4%, and above 800Hz is under research. The harmonic content rate between a single user is 0.16% for 1000V and below (100Hz, 0.4% for 100~800Hz), and 0.13% for 1000V and above (100Hz, 0.32% for 100~800Hz).
f. Fluctuation and flicker
GB/T12326-2008 "Power Quality Voltage Fluctuation and Flicker" stipulates that at the public connection point of the power system, when the system is operating in a small mode, with one week (168h) as the measurement period, all long-term flicker values Plt satisfy: ≤ 110kV, Plt=1;》110kV, Plt=0.8. and relevant regulations for individual users.
g. Voltage sags and short interruptions
GB/T30137-2013 "Power Quality Voltage Sag and Short-term Interruption" definition: Voltage sag refers to a sudden decrease in the root mean square value of the power frequency voltage at a certain point in the power system to 0.1p.u.~0.9p.u., and continues briefly for 10ms~1min. The phenomenon of returning to normal; short-term interruption refers to the phenomenon that the root mean square value of the power frequency voltage at a certain point in the power system suddenly drops below 0.1p.u., and returns to normal after a short duration of 10ms~1min.
4. How to improve power quality effectively?
Capacitor Bank
Static Var Generator(SVG)
Active Harmonic Filter(AHF)
Statcom
