POWER SYSTEM STUDIES
This webpage describes system analysis performed by NEPSI in relation to the application of medium-voltage harmonic filter banks and capacitor banks. It outlines the typical analysis performed by NEPSI in evaluating the possible system impacts related to the installation and switching of medium-voltage harmonic filter banks and shunt capacitor banks.
NEPSI's engineering evaluation is centered on the design, specification, and system impact of new and/or existing capacitor banks and harmonic filter banks. NEPSI's recommendations will typically be paid for by savings resulting from one or more of the following:
• Increased savings from power factor penalties by optimizing controls and bank size to utility rate structure.
• Reduced installation cost.
• Reduced capacitor bank and harmonic filter bank cost.
• Elimination or reduction of negative system impacts related to capacitor bank installation.
The engineering evaluation described herein, illustrate the engineering capabilities of Northeast Power Systems, Inc. All capacitor/filter bank evaluations will vary in some degree, and is dependent upon system characteristics and the type of equipment to be purchased or analyzed. For a detailed quote for a power system evaluation, contact NEPSI or one of NEPSI's sales representatives. As with all power system evaluations, NEPSI will provide a performance guarantee with a subsequent purchase of either a harmonic filter or capacitor bank. This performance guarantee is unmatched by any engineering consulting firm or manufacturer.
The installation of a large shunt capacitor bank or harmonic filter bank raises concerns primarily in the areas of harmonic distortion, harmonic resonance, switching surges, and over voltage as a result of the installation of power factor correction equipment. It is prudent to perform a capacitor/harmonic filter bank evaluation before equipment is purchased so that any adverse conditions, added costs, and/or cost reductions, can be accounted for and identified in the design stages.
The installation of a large shunt capacitor bank or harmonic filter bank raises some or all of the concerns listed in the Table 1 below. These concerns are evaluated by on-site power system measurements and digital simulations using sophisticated power system software. The measurements provide the ambient distortion levels, operating voltage, and other data required to validate and perform digital simulations. The digital simulations calculate system performance, and are used to predict and mitigate power system problems before they occur, so that special design requirements and cost can be accounted for in the planning stages rather than in the construction or commissioning phase.
|Table 1 - Concerns Relating to the Installation of Power Factor Correction Equipment|
|Harmonic Resonance||Harmonic Distortion|
|Switching Requirements For Breakers/Reactors||Over-Voltage During Light|
|Short Circuit Duty Requirements||Switching Transients|
The scope of work required in an evaluation is divided up as follows:
• Power system measurements and data collection
• Short circuit analysis
• Load flow analysis
• Transient Analysis
• Filter/capacitor bank design and specification
• Development of installation specification
A description of each of these analyses is provided below.
The primary purpose of measurement and data collection is to collect the necessary data to perform the capacitor/filter bank engineering evaluation. Data is collected for normal and abnormal operating conditions, as well as for future system conditions or enhancements. It involves power system measurements, data validation, and discussions with plant personnel. This task may be performed by either NEPSI or plant personal with direction from a NEPSI certified power engineer.
Data Collection and Validation
Data required to perform a capacitor/filter bank engineering evaluation is provided below.
• Impedance data, including one-line diagrams, location of existing and future capacitor/filter banks, generators, motors, and non-linear loads
• Utility system data.
• Digital copy of system data if available.
• Equipment ratings and relay settings where necessary to perform the evaluation.
• Normal and abnormal system conditions and operating practices.
• Future expansion and/or system changes.
The validity of the system data is checked by a NEPSI power engineer. Questionable data will be brought up and checked on-site.
Power System Measurements
Power system measurements may be taken to quantify the existing power factor, load level, operating voltage, harmonic distortion levels and other power quality concerns.
In some cases, capacitor banks and or loads may have to be switched off to collect significant data, such as background voltage distortion levels. If this switching operation is necessary, a plan to do so will be submitted in advance for consideration and approval.
Harmonic analysis involves the use of sophisticated computer programs to identify and predict potential harmonic problems and mitigation techniques. NEPSI's harmonic analysis program provides features that help in the identification of harmonic problems. Significant features include:
• Harmonic Impedance Scans.
• Harmonic Amplification Scans.
• Voltage and current distortion calculations.
• Sensitivity analysis
Harmonic Impedance Scans
Impedance scans are used to determine where resonant conditions exist. They are basically an impedance verses frequency plot of the system looking from the harmonic current source. The Figure below shows two impedance scans with harmonic resonance near the 4th, 7th, order harmonics. The scan show the response of a system when capacitors are installed on electrical system verses a harmonic filter.
Impedance scans are developed for both normal and abnormal operating conditions as well as future expansions.
Current Amplification Scans
Current amplification plots have the same general appearance as impedance scans, but with a very different meaning. These plots show the current magnification/attenuation versus frequency at a given bus for a one-amp injection at another bus in the system. These scans are used for finding localized resonant problems. They aid in the identification of negative interactions that may exist between surrounding electrical equipment, non-linear loads, and the planned and/or existing capacitor/filter bank.
Voltage & Current Distortion Calculations
In addition to the scans above, voltage distortion calculations can be performed throughout the power system to confirm that the system meets IEEE 519 requirements for voltage and current distortion. Other local or international standards or limits may be used at the request of the customer. With a purchased study and harmonic filter or capacitor bank, NEPSI will provide performance guarantees against IEEE-519 limits. This performance guarantee is unmatched by our competitors or consulting engineering firms.
|Maximum Harmonic Distortion in % of IL|
Current Distortion Limits for General Distribution Systems (120 volt thru 69,000 Volts)
Individual Harmonic Order (Odd Harmonic)
* All power generation equipment is limited to these values of current distortion, regardless of actual ISC/IL
Where ISC = Maximum short circuit current at point-of-common-coupling and
IL = Maximum demand load current (fundamental frequency) at point of common coupling.
Even harmonics are limited to 25% of the odd harmonic limits above.
Current distortion that results in a direct current offset, e.g., half wave converters are not allowed.
|Voltage Distortion Limits|
|Bus Voltage at PCC||Individual Voltage Distortion (%)||Total Voltage Distortion THD (%)|
|69 kV and below||3.0||5.0|
|69.001 kV through 161 kV||1.5||2.5|
|161 kV and above||1.0||1.5|
|NOTE: High voltage systems can have up to 2.0% THD where the cause is an HVDC terminal that will attenuate by the time it is tapped for a user.|
Sensitivity analysis is very important when doing harmonic analysis. Variations in capacitor and reactor impedance's, as well as source impedance, can greatly affect voltage and current distortion calculations. The sensitivity analysis feature included in NEPSI's harmonic analysis software is utilized to determine if any adverse system conditions exists due to slight variations in system and/or equipment impedance's. This feature is especially useful in the design of harmonic filters where the worse case harmonic duties must be calculated.
A short circuit analysis is used to calculate system fault current levels to determine the interrupting and withstand adequacy of the power system equipment and associated protective devices. It provides a guide in the selection and rating or setting of protective devices such as direct-acting trips, fuses, and relays and the bases for the short circuit rating required for the capacitor or harmonic filter bank.
The short circuit analysis is typically limited to the major buses (nodes) and equipment connected directly to the capacitor or filter bank. It may be expanded to include other busses at the customers request, in which case a specific work scope would be developed. The short circuit calculation will account for local generation, utility impedance, and short circuit current contributions from motors. The cases selected for the short circuit calculation will depict the power system configuration for which the three phase bolted fault short circuit currents will be at a maximum. All comparisons of interrupting device short circuit ratings or capabilities will be based on this "worst case" condition.
A load flow analysis is conducted to predict power flow magnitudes, power factor, voltage levels and losses in branches of the system based on the specified operating conditions. The results are used to determine one or more of the following:
• Recommended transformer tap settings to maintain proper voltage level.
• Size of capacitor/filter banks to maintain an acceptable power factor and/or voltage level.
• Equipment rating (Ampacity).
• Contingency Analysis
A load flow study will investigate system steady state load performance under normal and abnormal operating conditions. All significant system loads (watt and var components) and power sources (utility, co-gen, etc.) relating to the filter/capacitor bank installation are modeled. Where possible, system equivalents may be developed. The modeling can be expanded to include more of the system at the request of the customer, in which case, a specific work scope will be developed.
Switching surges occur during most switching operations. They occur during the transition when the system is changing from one steady state operating condition to another (this occurs during energization and de-energization of all equipment). The magnitude of the switching transient depends upon switching time and the resistive, capacitive, and inductive characteristics of the system. Capacitor switching and miss-operation of switches due to re-strike and pre-strike generally cause the more severe switching surges.
The Electromagnetic Transients Program (EMTP) is typically used to investigate switching surges and can produce actual waveforms as shown in in the figure below. Transient simulation models require more detail than 60 Hz models used in load flow, short circuit, and harmonic analysis programs. Due to this increase level of detail, the circuit elements of concern are usually modeled in detail, while the remainder of the system is modeled as a lumped circuit parameter.
Where switching surges are found to be a potential problem, recommendations on their mitigation will be included in the final report.
The design and specification of the capacitor/filter bank is based on the analyses provided above. The Capacitor/Filter bank performance is evaluated as part of the load flow and harmonic analyses while the short circuit requirements are evaluated from the short circuit analysis. This task involves the functional specification of the specific components within the filter/capacitor bank to meet the performance requirements imposed. A close interaction with plant/utility engineers is required so the bank can be designed with the desired operating controls and protection system. A guide form specification and functional specification is then provided so that the customer can go out for competitive bid.
The specification will typically contain the following:
• Capacitor ratings
• Reactor ratings
• Filter bank configuration
• Protection requirements
• Switch requirements
• Disconnecting requirements
• Layout requirements
• Warranty requirements
• Transient inrush reactor requirements
• Control requirements
• Approved supplier list