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Voltage and frequency are important indicators of power quality, and reactive power directly affects voltage quality. In a power system, various electrical devices can only operate safely and achieve optimal economic performance when the voltage is at the rated value. However, during normal operations, the load and system operation modes frequently change, leading to voltage fluctuations and inevitable voltage deviations.
Static electric fields generated by charges alone cannot maintain a steady current. However, with the help of a DC power source, non-electrostatic forces (referred to as "non-electrostatic forces") allow positive charges to return from the negative terminal (low potential) through the internal power source to the positive terminal (high potential), thereby maintaining a potential difference between the two electrodes and forming a steady current.
The power factor depends on the phase relationship between voltage and current. If voltage and current are in phase, the power factor is 1. The greater the phase difference between voltage and current (within the range of -90° to +90°), the lower the power factor.
Power factor = Active power / Apparent power; Apparent power = Voltage (RMS) × Current (RMS).
In a sinusoidal steady-state circuit, for a one-port circuit with U, I, and Φ:
The power factor is related to the nature of the load, as in an AC circuit, the cosine of the phase difference (Φ) between voltage and current is called the power factor, represented by the symbol cosΦ. Numerically, the power factor is the ratio of active power to apparent power.
Power factor and the phase of voltage:
In the absence of harmonics, or when the influence of harmonics is negligible, the power factor is the cosine of the phase angle difference between voltage and current. This conclusion is derived from the definition of the power factor and can be used for data calculations but should not be taken as a definition itself.
Power factor and voltage magnitude:
When viewed from the power source’s output side, with a constant load, an increase in power factor causes the output voltage to rise and the output current to decrease. Although the calculation is complex, this qualitative description suffices. The exact amount by which voltage increases and current decreases depends on numerous parameters, making it difficult to obtain precise values.
Power factor and harmonics in voltage:
The more harmonics present in the voltage and current, the lower the power factor. This is why simply compensating reactive power with capacitors alone is often insufficient when harmonics are present.
The main factor causing voltage deviation is the voltage loss in the system due to the system’s lagging reactive load. Therefore, when the load changes, adjusting the capacitor bank accordingly can change the system's voltage loss, thereby reducing the range of voltage deviation. The change in voltage loss after adjusting reactive power can be calculated using the following formula:
The amount of shunt reactor injection can be considered equivalent to the removal of parallel capacitors, and the same calculation applies.
Shunt reactors are sometimes installed in substations with voltages above 35kV or in large enterprise substations, compensating for the excess leading current formed by the excessive insertion of shunt capacitors and cable capacitance. This can effectively control high voltage during light load conditions. Medium- and small-sized enterprise substations usually do not have this device.
Similarly, the principle of adjusting capacitor and reactor capacity applies to adjusting the excitation current of a synchronous motor. By allowing the synchronous motor to operate in leading or lagging mode, the reactive power generated or consumed by the motor can be changed, achieving voltage regulation.
Reference Standards:
GB 12326 - Power Quality: Voltage Fluctuation and Flicker
GB/T 14549 - Power Quality: Public Power Network Harmonics
GB/T 15543 - Power Quality: Allowable Imbalance of Three-Phase Voltage
