1 harmonic
       The non-sinusoidal periodic electricity Fourier series decomposition, with the power grid, apart from being the same fundamental frequency component, also received a series of fundamental grid frequency greater than the weight of this portion of electricity known as harmonics. Fundamental frequency harmonic Motor capacitor frequency and the ratio of (n = fn/f1) called harmonic frequency. Harmonic is a fact of interference to power grids by the "pollution." Its frequency range is generally 2 ≤ n ≤ 40.
      
2 harmonic source
       The harmonic current injection utility power grid or the Internet in public electricity generated harmonic voltage electrical equipment known as harmonic sources. With nonlinear properties of electrical equipment is the main harmonic sources, in view of the Tianjin Port this particular power supply environment, via Tianjin, ASTRI testing, the main source is used harmonic pay - Direct - TAC and the VVVF Pier machinery, equipment access to the current non-sinusoidal shape, and its harmonic components to the Department of EC produced sinusoidal voltage distortion. The amount depends on the harmonic current harmonic source equipment itself and the characteristics of working conditions, and has nothing to do with the power grid parameters, it can be regarded as constant current source. Various thyristor circuit with the frequency of the harmonics of the circuit, the characteristics of the circuit known as harmonic. In addition harmonic characteristics, in addition to three-phase voltage imbalance, trigger pulse stability asymmetric or non-working state, the circuit will have a non-harmonic characteristics. A harmonic analysis and calculation of the most significant characteristic is the Motor capacitor harmonic, such as 5,7,11,13 inferior.
       When the power grid to a number of harmonic sources, as all the same harmonic source harmonic current phase of the different components, and each component will be less than the arithmetic and. Exciting Current transformers containing 3,5,7, and so the harmonic component. As the former vice-transformer windings in the total angular access for a group of law, provide for the 3rd harmonic pathway, it is not the 3rd harmonic current inflow grid. But when the magnetizing current phase imbalance, the 3rd harmonic enable the residual component (up to 20%) access to the grid.
3 harmonic transmission
       Multi-voltage grading for the power grid, is characterized by its harmonic harmonic current flows from the high-pressure side of low pressure side, the size basically has nothing to do with the parameters of high-pressure side, can be regarded as constant current source. Harmonic voltage transmission from the high-pressure side to the low-pressure side, can be regarded as constant voltage source. In harmonic analysis, the structure is the basis of this principle of the harmonic grid equivalent circuit.
       3.1 The frequency of grid components
       In harmonic frequency range, due to eddy current and leakage magnetic field, electrical network components harmonic parameters to consider long-term effects, that is the equivalent transformer and the wire resistor R frequency increased with the increasing frequency of the equivalent inductance L with a Motor capacitor lower rise. Cable, wire and the capacitor with capacitance C basically does not maintain a constant frequency changes. Load impedance and frequency according to the relationship between the different load varies.
       3.2 Equivalent circuit power grids
       Network components can be different from the harmonic grid parameters Rn, In and Cn component equivalent network. Equivalent symmetrical three-phase power system commonly used single-phase circuit said. According to the equivalent circuit of frequency admittance matrix of nodes Yn, calculated impedance Zn, calculated harmonic voltage Un = ZnIn.
      
4 harmonic limits
       To maintain harmonic voltage power grids in the following allowable value, we must limit the power grid into the harmonic source harmonic CURRENT. Most industrialized countries have formulated a harmonic grid management standards or requirements. Harmonic management standards are based on the formulation of the principle of electromagnetic compatibility, that is a common electromagnetic environment, electrical equipment both normal work, but it also may not excessive disruption to the environment.

       In 1993 China promulgated the restriction of the power Motor capacitor system harmonics national standard "power quality: utility grid harmonic" (GB/T14549-93), provides a common power grid harmonic voltage limits to the utility grid and users harmonic current injection of the allowable value.
      
5 Harmonic dangerous
       Harmonic electrical equipment increased thermal loss, disrupted its function and may even trigger a fault. In addition to the harmonic frequency coupling information system to produce interference.
       5.1 Motor
       Harmonic voltage electrical short-circuit impedance Motor capacitor as a result of harmonic currents and electrical current negative sequence fundamental to the equipment I have additional heat loss, and motor starter when easily develop into a disturbing torque. Harmonic currents and negative sequence current RMS of fundamental and general not to be more than the motor rated current Ie 5% ~ 10%.
       5.2 capacitors
       Capacitors can harmonic flow fever. The capacitor provides long-term work order shall not be more than 1.3 times the current rated current (Ic = CUn). Located near the harmonic source capacitor filter capacitor or higher usually by the RMS current special manufacture.
       5.3 electronic devices
       Harmonic voltage thyristor trigger devices can trigger error occurred, or even lead to equipment failure. Harmonic will be on the grid audio control systems and computer Motor capacitorgenerated effects.