The frequency of alternating current send from the power plant is 50Hz; the waveform is a sine wave. It is usually named as power frequency.
    The oscillogram as follows: Chart one, normal voltage and current waveform----- sine wave In the electrical power system, the harmonic is refers to how many time the waveform which leads to the power frequency, is called “times”, it’s scope is from 2 to 30 times, such as the frequency of 5 times harmonic voltage (current) is 250Hz; the frequency of 7 times harmonic voltage (current) is 350Hz; the frequency surpasses 13 times harmonic is called the high harmonic. Chart two A, the voltage waveform includes the Harmonic Chart two B, the current waveform includes the Harmonic The voltage waveform from the chart two has the aperture, the current waveform is the square-wave, and it has the big difference with the chart one. The electric power harmonic is very dangerous to the electric power network, it is one kind of electric power pollution, one kind of pollution that people cannot see, smell and touch (with five senses direct feeling not through instrument).
     Therefore it often does not pay attention by the people.
★ There are several harms to the electrical power system by harmonic as follows:
    1. Harm to the power equipment
    （1）Accelerates the process of power transformer getting older, reduces the transformer’s service life.
    （2）Makes the switch (circuit breaker) to overload and break frequently. Because the harmonic current flows in the conductor surface caused the conductor get hot, then reduce the switch actual capacity.
   （3）Damages the reactive compensate equipment, causes it unable to carry on the reactive compensate, increase the circuitry loss and reduces the transformer rated capacity.
     2. Disturbs the transformer substations relay protection, create the misoperation protection easily, causes the regional power cut accident.
     3. Disturbs and affects the communication network. Communication system use a great deal computer is interference by harmonic easily.
     4. Harm to the electric appliance, such as air conditioning microwave oven, television and so on.