Harmonic Analysis

The Harmonic Study is provided using SKM Power Tools for Windows software. The power system model and the initial input data is derived from the short circuit and load flow study results. Ideally the analysis includes Power Monitoring and Measurements to validate the model, and to simulate the actual operating conditions in the study model. The motor and panel loads in the model are adjusted to match the actual running loads and to generate a load flow that matches the recorded measurement. The load flow provides a close approximation of the bus voltages, the current flowing in branch circuits, and significant power values including kW and KVA demand, reactive power (kVAR), and power factor (PF). Low Power Factor (PF) is often the result of a significant amount of induction motor loads. The PF can also be affected by non-linear loads (i.e. variable speed drives, power converters, UPS, harmonic, etc.) When the PF falls below the minimum level required under the utility rate schedule, the electric demand is increased and the utility company has to supply the additional reactive power. The increased reactive power reduces the total capacity of the electrical system. As an incentive to correct a low PF to a higher level, the utility company may add a penalty to the electric bill when the PF falls below the required minimum level. To avoid a penalty charge, PF Correction Capacitors are installed at one or more strategic locations in the electrical system. This increases the PF by supplying the additional reactive power locally instead of drawing it from the utility source. When capacitors are installed on a system that has nonlinear loads, the natural resonant frequency of the system may become tuned near one of the harmonic frequencies, and cause a large impedance to the flow of harmonic current. When the Total Harmonic Distortion Current (THDi) encounters system impedance it can cause an increase in the Total Harmonic Distortion Voltage (THDv) and cause power disturbance or damage to electrical or mechanical equipment. The harmonic current sources are identified and Case Studies are developed to compare the results from different operating scenarios. The THD voltage and current waveforms are analyzed in time domain and frequency response to determine the impact to the bus voltage at specific locations in the power system and to predict the system response to the addition of PFC capacitors. The worst case results are compared to the recommended IEEE limits. For branch circuits that have a significant amount of THD current, an analysis is conducted as follows:
• Identify dominant individual harmonics that can cause an increase in the THD voltage under certain conditions. Identify the resonant frequency and the conditions that can change or increase the system impedance.
• Determine the directional flow of the harmonic current, and isolate the harmonic current sources that can cause an increase in the THD voltage at a particular bus.
• Place power factor correction (PFC) capacitors in service and determine if the system is tuned to a significant harmonic order that can cause a harmonic resonant condition.
• Study the results and determine if the predicted increase in THD voltage will exceed the recommended IEEE limits or has the potential to effect critical equipment. Develop Case Scenarios to test Harmonic Filter Solution to reduce THD voltage.
• Based on the findings provide recommendations and mitigating solutions to eliminate excess THD levels and related power disturbance.

Harmonic Case Studies
Power monitors are used to measure the actual load conditions, and capture the voltage and current waveforms to identify any abnormalities and to confirm that the system equipment and components are operating within the specified tolerance. The actual Power Measurements (i.e. bus voltage drop, branch current, kW, kVA, kVAR, PF and THD), in the time domain and frequency response, are compared with Load Flow data generated from the study output. The system loads, in the study model, are adjusted to closely match the actual power measurements. The system model is analyzed prior to adding or energizing any existing power factor correction (PFC) capacitors. Based on the system operating scenarios, case studies are developed to analyze the system response to PFC’s at one or more locations in the electrical system. The typical analysis and approach to solutions include the following:
• Determining the Total Harmonic Distortion Current and Voltage
• Determine the Potential Effects (motors, cable, transformers, generators, etc.)
• Isolate the Harmonic Sources by size and spectrum (i.e. SMPS, SPC, Inverters, ASD, UPS) and capacitors in the System and Model
• Develop study model to predict harmonics (i.e. Short-Circuit-Coordination, Equipment Evaluation, Frequency Scan, Harmonic Load Flow, THDi, THDv, Filter Design and Evaluation)
• Provide Proposed Power Factor Correction
• Provide case study reports
• Compare and satisfy the applicable standards
• Proposed corrective action to minimize the harmonics

Recent Projects Patrick V McNamara (VM) Building in Detroit, Michigan Power Monitoring and Harmonics Analysis for Vector Electric, Inc, Ann Arbor, Michigan Developed system study model and load flow to identify harmonic power disturbance and recommended solution to repair. The power study considered 2-MV Primary Distribution Feeder Circuits at 13.2kV to Main SWGR, including 4-1500kVA unit substations at 13.2kV-to-480V 3-Phase, 4-Wire wye, and 2-2500kVA unit substations at 13.2kV-to-4.16kV, 3-Phase, 4-Wire wye. The unit substations are networked through the common 13.2kV primary feeder circuits. The configuration for the 480V secondary bus includes a breaker-and-a-half scheme for each substation to supply power, lighting and mechanical loads for the building. The 4.16kV-substation buses supply the large motor-drives for the chillers. Determined the locations to install power monitors, record data to develop load profiles for the branch circuits and harmonic source, and validate the load flow. Determined Provided study analysis to determine the resonant frequency, directional flow of the harmonic distortion current that could combine with system characteristics to cause the excessive THD voltage and potentially cause the power disturbance to become manifest in the same way. Provided recommendations to consolidate the power factor correction capacitors and transfer VSD loads from the bus that contains critical loads, to the mechanical bus that is less susceptible to the flow of harmonic current, and provide harmonic filter solution to mitigate the THD voltage to IEEE-519 Standards.

RM Clayton – West Ares CSO Storage Tunnel in Atlanta, Georgia Harmonics Analysis for Hood Patterson, in Atlanta, Georgia Provided major revisions to short circuit and coordination study and developed the harmonic analysis for a 4.16kV double ended substation to four separate unit substations at 4.16kV-to-600V, each feeding three Adjustable Speed Drive units (ASD) to three 400HP motors. Use manufactures data to develop motor components, harmonic spectrum for the ASDs. Develop case studies case studies to verify that the THD voltage was within the limits of IEEE-519 Std. Developed separate case scenarios to evaluate the motor circuit protection and coordination between other protective devices. Verified drive input and output data including the motor protection for the effects transient voltages due to the extremely long feeder cables from the output inverter to the motor. Provided analysis to predict the THD current and voltage and verify the drive model. The use of 24 Pulse Drives did not require the installation of Power Factor Correction capacitors nor was customers system potentially affected by the use of capacitors at the utility substation bus. The THD levels were minimal and within the recommended limits.

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