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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Power Flow Studies

Power flow/Load flow calculations provide active & reactive power flows, bus voltage magnitude and their phase angle at all the buses for a specified power system and operating conditions. These values are typically subject to various factors like regulating capability of generators, synchronous condensers, static VAR compensator, HVDC controls, FACTS controllers, tap changing under-load transformers and specified net interchange between individual operating systems (utilities). Power flow information is essential for the continuous evaluation of the current performance of a power system and for analyzing the effectiveness of alternative plans for system expansion to meet increased load demand. These analyses require the calculation of numerous power flow cases for both normal, and emergency (contingency) operating conditions.

Applications of Power Flow Study and Analysis

• Transmission expansion planning ,operation planning 
• Distribution expansion planning , operation planning 
• Industrial/Commercial distribution system planning, operational planning 
• Network interconnection, Grid interconnection studies 
• Evaluation of energy transactions between various stake holders 
• Energy audit to accurately determine network losses and estimate billing losses if any 
• Sizing of transformers, cables, overhead lines, transformer tap ranges, shunt capacitors, shunt reactors, reactive power management, FACTS devices, HVDC operation 
• System security assessment via static contingency studies 
• Decision making tool in operation planning and operation of the system in load dispatch center 
• Motor starting studies using load flow type analysis, where the starting impedance of the Induction motor is modeled as constant impedance model with starting impedance. 
• Evaluation of static voltage stability using load flow technique

The following general criteria of acceptability of design is used in power flow studies

1.  Voltage Drop at all buses should be within +/- 5% of the nominal rating for all operating conditions considered 
2. No over load conditions of any electrical circuits for all operating conditions considered 
3. Reactive power generation/import/export to be within specified limits for all operating conditions considered 
4. Ensuring quality power supply to all loads, under specified contingency conditions, as per design philosophy adopted.

The following study cases/ power flow outputs are generally considered in power flow studies

1. Extreme operating conditions of maximum and minimum loading conditions will be considered to check the adequacy of the network, even though some of these conditions may not exist during normal operation 
2. Contingency conditions such as outage of lines, transformers and generators will be considered and network adequacy for power evacuation will be assessed 
3. Operating solutions such as transformer taps, generator excitation, shunt reactive power compensations will be provided as needed. 
4. Recommendations for strengthening and equipment upgradations will be provided to meet specific operating requirements. 
5. Summary of load flow studies and concise reports in tabular formats and single line diagram formats will be provided, along with the summary of recommendations

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