Harmonic studies are performed to determine harmonic distortion levels and filtering requirements within a facility. Field measurements and computer simulations are used to characterize adjustable-speed drives (ASDs) and other nonlinear loads and simulations are then performed to determine the filter specifications and effectiveness. The application of harmonic filters will significantly alter the frequency response of the power system. An evaluation of the harmonic voltage and current limits, (e.g., IEEE Std. 519) is completed to determine the effectiveness of the proposed filter installation.

The potential for harmonic distortion problems is dependent on two important factors:

- The level of harmonic generation which can be associated with loads in the plant. Harmonic currents are generated by loads which have nonlinear voltage-current characteristics. The number and sizes of these devices at a given bus determines the level of harmonic current generation.
- The system frequency response characteristics. The frequency response at a given bus is dominated by the application of capacitors at that bus. Series reactors for transient control or harmonic control significantly change the frequency response. Problems occur when the system response exhibits a parallel resonance near one of the harmonic components generated by the loads on the system (usually the 5th or 7th harmonic). Resistive load provides damping near these resonant frequencies.

The combination of these two factors determines whether or not a harmonic problem will exist at a particular bus. It is also possible for harmonic problems to occur at buses remote from the harmonic source if local resonances exist. If capacitors are applied at any locations that have large adjustable-speed drives, the potential for resonance problems must be considered carefully. A harmonic study evaluates these concerns as described in the following sections:

The first step in a harmonic study is to develop a system model to be used for the analysis. The model is developed from the oneline diagrams, the electrical equipment data (transformers, cables, machines, etc.), the utility system characteristics, and the load information. The result is a database that includes the following elements:

- Representation of the utility system supplying the facility. This system can be represented as a simple equivalent as long as there are no switched capacitors. However, it is quite likely that the utility does have switched capacitors on the supply system and these must be represented.
- Step-down transformers (ratings and nameplate impedances).
- Important low voltage circuits (specifically ASDs).
- Load data for each bus (kW, kVAr, kVA).
- Capacitor data (level of compensation, kVAr).

The electrical database developed at this stage is used for the development of the harmonic analysis model of the system. The model must include important connected capacitors, cable capacitances, transformer characteristics, reactor values, motor representations, and an equivalent representation for the utility supply system.

Harmonic measurements are very useful in that they provide information necessary to characterize the loads as well as provide a means for verifying the harmonic model. Measured harmonic currents are used as input to the model and simulated harmonic voltage distortion levels are then compared with measured values to determine the accuracy of the model.

The model is developed for the SuperHarm® computer program used by Electrotek for harmonic analysis. This program permits convenient analysis of system frequency response characteristics as well as direct representation of important harmonic sources in order to simulate system harmonic levels.

Harmonic measurements are an important part of the overall investigation for a number of reasons. Most importantly, the measurements must be used to characterize the level of harmonic generation for the existing nonlinear loads. Voltage and current harmonic levels are measured at multiple sites to accomplish this. It is important to accurately document system conditions at the time of the measurements so that the results can be used to verify analytical results.

The specific objectives of the measurements include:

- Determine the harmonic generation characteristics of the nonlinear loads (e.g., dc drive waveform below). This is done by performing current measurements at a variety of locations within the facility. Three-phase measurements are made so that characteristic and non-characteristic (triplen) harmonic components can be determined.
- Determine system response characteristics for particular conditions. Voltage measurements are used in conjunction with the current measurements to characterize system response for specific system conditions. These conditions are then be the basis for verifying the analytical models.
- Determine the background harmonic voltage and current levels.

The measurements typically are performed over a period of 1-5 days in order to assure that adequate data is collected to characterize the system operation and for verification of the analytical models.

dc Drive Current Waveform

A list of nonlinear loads is compiled and representations of these loads as harmonic generating devices is developed. Loads at the individual buses are categorized as follows:

- ac Motor Loads
- Resistive Loads
- Adjustable-Speed Drives
- SCR Bridge Rectifiers (furnaces)
- dc Motor Drives
- Welding Loads
- Other Loads

The motor loads and resistive loads have important impacts on the system frequency response but they are not sources of harmonic distortion. These representations are used in conjunction with the system model to estimate harmonic levels throughout the facility.

Simulations (frequency scans) are performed to determine the frequency response characteristics looking from the 480 volt buses. Output consisting of magnitude and phase angle for the driving point impedance is produced. The effect of important system parameters (capacitors, loads, transformer sizes) is evaluated and the potential for problem resonance conditions is determined (5th or 7th harmonic resonance is the most important). Tabular and graphical results (e.g., scan figure below) of various switching conditions is prepared so harmonic resonance conditions can easily be identified.

Frequency Response Characteristic

If problem conditions are identified at a given bus, filter designs are developed to alleviate the resonance problems. These filter designs should be coordinated closely with the transient analysis.

The frequency scan cases identify system conditions that can cause harmonic problems due to resonance conditions. These system conditions are emphasized when evaluating the response to estimated harmonic current injection levels. The simulations to estimate actual harmonic distortion levels include representations of the harmonic generating devices and the important system conditions. The output for these cases consists of individual harmonic levels (harmonic spectrums), bus voltage distortion levels, current distortion levels, RMS voltage and current levels, and important waveforms.

Severe Secondary Bus Voltage Distortion

Expected harmonic voltage distortion levels are evaluated based on recommended limits outlined in IEEE Std. 519. This standard states that the bus voltage distortion level should be limited to 5%. This limitation should prevent any harmonic problems with process controls, capacitors, transformers, or adjustable-speed drive controls.

One of the most important impacts of the harmonic currents caused by nonlinear loads and system resonances is the increased heating in system equipment. Transformers are the most important devices affected but cable ratings could also be impacted. Spreadsheets are developed during the study to evaluate the transformer and cable derating required to accommodate harmonic currents. Recommendations for derating factors as a function of the harmonic current distortion level are presented.

Motors in the plant can be adversely impacted by the voltage distortion levels at the various buses. Motors and controls are the primary reasons for the 5% limit in IEEE Std. 519. A more detailed evaluation of the impact of harmonic voltages on motors can be performed by evaluating the individual frequencies involved. A spreadsheet for this purpose is developed and motor heating concerns are identified.

Harmonic voltage levels determined through both simulation and measurement are evaluated with respect to recommended limits. If harmonic voltage distortion levels are not within acceptable limits the frequency response characteristics of the facility or system can be altered by changing capacitor sizes and/or locations, or be installing harmonic filters. In many instances, harmonic filters are an excellent solution because they can be designed to provide power factor correction at the fundamental frequency and a low impedance path for harmonic currents. Filter components must be designed to withstand both harmonic and fundamental frequency voltages and currents.

A filter design spreadsheet is completed for each filter installation. The information provided can be used to develop specifications for power factor correction/harmonic filter equipment.

Harmonic current and voltage levels determined through both simulation and measurement are evaluated with respect to recommended limits, such as those presented in IEEE Std. 519. If harmonic distortion levels are not within the acceptable limits the impact of harmonic filters is evaluated.

One possible harmonic current limitation is given in Table 10.2 of IEEE Std. 519. Many utilities will require their industrial customers to meet a guideline such as this. This will make the evaluation of power factor correction / harmonic filters even more important.

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