**Abstract:** Electrical energy has become a vital product in modern life, and its quality directly affects people's daily lives and safety. But what exactly is power quality? What are the factors that affect it, and how can we improve it? This article will explore these questions in detail, providing you with a comprehensive understanding of power quality.
**1. What is Power Quality?**
Power quality refers to the condition of electrical energy supplied to consumers within a power system. Ideally, the voltage and current should be perfect sine waves. However, various factors can cause deviations from this ideal waveform, leading to power quality issues. These issues can affect the performance of electrical equipment and even cause damage. Therefore, understanding the causes of power quality problems and how to mitigate them is essential for maintaining a stable and efficient power supply.
**2. Main Indicators of Power Quality**
Several key indicators are used to assess power quality, including:
- **Voltage Deviation**: Fluctuations in the supply voltage, either rising or falling.
- **Frequency Deviation**: Variations in the grid frequency, which should remain stable.
- **Three-Phase Voltage Unbalance**: Differences in the three-phase voltages exceeding acceptable limits.
- **Harmonics and Interharmonics**: Distortions in the waveform caused by non-linear loads.
- **Voltage Fluctuations and Flicker**: Rapid changes in voltage levels that can affect lighting and sensitive equipment.
These indicators help identify and quantify power quality problems, allowing engineers to take corrective actions.
**3. Specific Power Quality Indicators**
**Grid Frequency**
In China, the standard grid frequency is 50Hz. According to GB/T 15945-2008, the allowable frequency deviation under normal conditions is ±0.2Hz. For smaller systems, it may be relaxed to ±0.5Hz. In practice, most major grids operate within ±0.1Hz.
**Voltage Deviation**
GB/T 12325-2008 specifies that for 35kV and above, the total absolute deviation of the supply voltage should not exceed 10% of the nominal value. For 20kV and below, the deviation is ±7%, and for 220V single-phase supply, it is +7% to -10%.
**Three-Phase Voltage Unbalance**
According to GB/T 15543-2008, the negative sequence voltage unbalance at the common coupling point should not exceed 2% under normal operation, with short-term peaks up to 4%. Each user connected to the grid should not cause more than 1.3% unbalance.
**Harmonics in the Utility Grid**
GB/T 14549-93 sets limits on harmonic distortion in the utility grid. For example, the total harmonic distortion (THD) for 0.38kV is 5.0%, while for 110kV it is 2.0%. The standard also regulates the harmonic currents injected by users to ensure grid stability.
**Interharmonics**
GB/T 24337-2009 defines interharmonic limits, such as 0.2% for frequencies below 1000V and 100Hz, and 0.16% for higher voltages. These limits help prevent interference with other equipment.
**Voltage Fluctuation and Flicker**
GB/T 12326-2008 outlines the maximum allowable flicker values for different voltage levels. For instance, at 110kV, the long-term flicker value (Plt) should not exceed 0.8, while at lower voltages, it is typically 1.0.
**Voltage Sags and Short Interruptions**
GB/T 30137-2013 defines voltage sags as a sudden drop in RMS voltage to between 0.1pu and 0.9pu, lasting between 10ms and 1 minute. A short interruption occurs when the voltage drops below 0.1pu for a similar duration.
Understanding these standards helps in identifying and addressing power quality issues effectively. By monitoring and controlling these parameters, utilities can ensure a reliable and safe power supply for all users.
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