1. Generation of harmonics

In the distribution system, electrical waves with a frequency that is an integer multiple of the fundamental frequency 50Hz are all higher-order harmonics. High order harmonics can cause distortion of the fundamental waveform of the system. The equipment and loads that generate higher-order harmonics are called higher-order harmonic sources or harmonic sources, and all nonlinear loads are harmonic sources. When the distribution network supplies power to nonlinear loads, these devices transmit (distribution transformers), transform (silicon rectifier and converter devices), absorb the fundamental energy supplied by the (arc furnace) power grid, and at the same time convert some of the fundamental energy into harmonic energy, sending a large amount of higher-order harmonics back to the system, causing distortion of the sinusoidal waveform of the system.

2. The Harm of Harmonics in Power Plants

The harmonics in the distribution network cause a series of hazards to the distribution system. Its hazards in power plants include increasing additional losses and local heating of rotating motors, causing shaft vibration, reducing output and electromagnetic torque of motors, and accelerating insulation aging; In addition, certain harmonics produce distortion at the zero crossing point of the current waveform, which can interfere or cause instability in the generator control circuit; The harmonic torque generated by harmonics causes noise in induction motors. Causes the distribution transformer to overheat and even burn out. Causing the arc extinguishing coil to fail. Causing abnormal operation of circuit breaker equipment.

Causes the fuse to overheat, and high harmonic current can cause the fuse to overheat, causing the time characteristic curve of the fuse current to shift. This phenomenon is particularly evident during low current faults. Causing mechanical resonance in the distribution cabinet. Causing overheating of the busbar and wiring board. Danger of overheating, insulation damage, short circuit, or circuit interruption in lighting circuits. Harmonics generate a "reverse power factor", and nonlinear loads generate harmonic currents that are added to the fundamental wave. At this time, the apparent power is significantly higher than the active power, reducing the power factor.

3. Standard limits

Harmonic voltage and current: The national standard limit for the total distortion rate of harmonic voltage at 110kV public connection points is 2%, the national standard limit for the total distortion rate of harmonic voltage at 35kV public connection points is 3%, and the national standard limit for the total distortion rate of harmonic voltage at 10kV and 6kV public connection points is 4%.

Voltage flicker limit: The short-term voltage flicker limit P of 110kV public connection point is 0.8, and the long-term voltage flicker limit is 0.6. The short-term voltage flicker limit Pst of 10k and 6kV common connection points is 0.9, and the long-term voltage flicker limit is 0.7.

Three-phase voltage imbalance limit: The allowable value of normal voltage imbalance at the public connection point of the power system is 2%, and it should not exceed 4% in a short time. The allowable value of normal voltage imbalance caused by each user connected to a public connection point is 1.3%.

Generator negative sequence current limit: According to the unbalanced operating conditions of the generator, it is usually specified that I2/IN should be less than 0.08. Under long-term operation of the generator, the ratio of the negative sequence current injected into the generator to the rated current of the generator should be less than 8%.

Power quality monitoringdeviceThe application of power grid should further supervise the pollution sources of auxiliary power, strengthen the analysis and research of harmonic sources and their hazards in the distribution network, propose comprehensive treatment measures, and pay attention to the pollution of harmonics on the power grid just like paying attention to natural environmental pollution, and improve the modern technological level of harmonic control in power plants.