Harmonic Measurements, Analysis and Filter Design

Harmonics in power systems can result in undesirable influence such as Capacitor heating/failure, Telephone interference, Rotating equipment heating, Relay mis-operation, Transformer heating, Switchgear failure, Fuse blowing. The main sources of harmonics in power system are static power converters, arc furnaces, discharge lighting and any other load that requires non-sinusoidal current. In order to limit the harmonic current propagation in to the network, harmonic filters are placed close to the source of the harmonic currents. Harmonic filters provide low impedance paths to harmonic currents and thus prevent them from flowing into the power network. Harmonic analysis program computes indices such as total voltage harmonic distortion factor at system buses to evaluate the effect of the harmonic sources and to evaluate the effectiveness of the harmonic filters. Also, driving point impedance plots of the buses of interest are generated to identify whether series or parallel resonance phenomenon occurs at any harmonic frequency of interest.

Approach to Harmonic Analysis

1. In the first step, existing and functional networks harmonic current measurements are performed at selected points to identify the harmonic currents injected into the network by the harmonic sources. These measurements reflect harmonic currents for one operating configuration and the loads prevailing at the time of measurements only. These may not represent conservative estimates of harmonic currents available.
2. In the second step, the measurement information of the first step will be used along with design data of harmonics (where available) from non-linear loads generating harmonic currents. A computer network model will be prepared as per IEEE standards and the effect of various harmonic sources at various harmonic orders will be examined. Various harmonic distortion factors will be computed as outlined in relevant IEEE standards. The advantage of computer model and simulation is that it can take care of large number of operating configurations and conservative estimates of harmonic currents, which cannot be covered by field measurements. Field measurements of the first step can however be used to validate the computer model developed. 
3. In the third step, harmonic driving point impedances of all buses of interest will be generated at various harmonic orders and plots of the driving point impedances will be generated with respect to a range of harmonic orders [orders 1 through 50]. These plots indicate series and parallel resonance conditions in network. 
4. In the fourth step, analysis of results of the first 3 steps will be carried out and solutions needed to solve any harmonic related problems will be obtained. These solutions are verified by using the computer model developed. The problems that might arise could be excessive harmonic distortion factors beyond relevant IEEE specified standards, existence of resonance conditions close to an exciting harmonic frequency. Where these problems are encountered, solutions will be provided by introduction of harmonic filters and its design will be verified again by using the computer model developed. Recommendations include specifications on sizing of individual components of the harmonic filters. 

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