Electrical power quality and system harmonics: Practical options exist
Harmonic distortion resolution of variable frequency drive motor applications will provide the most beneficial actions for IEEE 519 compliance
- Review IEEE 519 requirements.
- Identify practical solutions and practices for harmonic distortion compliance.
- Leverage manufacturer experts as value-added partners.
Consulting and specifying engineers specializing in electrical and power systems seek a practical harmonic distortion design solution for the majority of applications not requiring in-depth modeling and analysis.
Electricity supports our modern productivity and fashionable conveniences on an industrial scale at a sensible cost. Innovation continues to offer new solutions for electrical distribution, control of power and responsible stewardship of the resource.
Most innovative technology leverages switching-type power supplies that introduce nonlinear harmonics on the power system. For example, the industry accepts the benefits of variable frequency drives for speed control, system balancing and energy savings in exchange for harmonic distortion. The end user generally accepts incremental harmonic distortion on our power system with a limited ability to quantify it, understand it or care about power quality issues until there is a significant problem.
Besides, the public cannot see the problem without the aid of special metering equipment and is rarely concerned about it until equipment fails. However, harmonic distortion contributes to additional heating (energy loss), insulation damage and general reduction of equipment life cycle expectations. Harmonic distortion can also have an impact on the efficacy of energy usage and utility costs.
Complicated calculations, system dynamics and vague guidelines further complicate an already intimidating topic. However, employing various tools and commercially available solutions can lead to robust and compliant commercial applications.
Industry guidelines, including IEEE 519
Let’s begin with an investigation of recommended practices and industry standards. IEEE 519 – 2014 Recommended Practice and Requirements for Harmonic Control in Electric Power Systems identifies acceptable levels of harmonic distortion within power distribution systems. The point of common coupling is the location of the power distribution system requiring compliance with these limitations. IEEE 519 defines the PCC as the point on a public power system, electrically nearest to a particular load, at which other loads are or could be, connected.
The PCC is essentially located at the common connection between the utility and the customer, which for most installations is the utility transformer. The primary terminations of the utility transformer represent the closest point another customer may be connected to the utility grid and expect electrical power with recognized compliance of harmonic distortion limits.
However, the secondary terminations are commonly used and industry-accepted because the utility rarely allows nonutility personnel access to the primary side of the transformer. Industrial and campus-based distribution systems often require engineering judgment to select the point of common coupling.
Acceptable voltage distortion limits are based on system voltage. The typical commercial or industrial facility will distribute electrical power below 1,000 volts. IEEE 519 suggests a maximum voltage total harmonic distortion goal below 8% for systems with a minimal voltage of 1,000 volts or less. The current distortion goal is subject to other parameters including the available short circuit current and the maximum demand current at the PCC.
The design professional will calculate short circuit current based on utility information and the impedance of the utility transformer. The maximum demand is directly related to power demand. NFPA 70: National Electrical Code Article 220 provides industry-recognized guidance for power demand with a very conservative result. Realistic demand values are commonly much lower and require reliance from historical data of similar facilities or measured values post construction, arguably not sufficient for design phase decisions.
Therefore, industry-recognized demand values of approximately 35% of connected load are commonly used for preliminary analysis. This value is based on U.S. Department of Energy studies previously conducted and U.S. DOE 2016 regulatory policy identifying minimum efficiency standards for low-voltage dry type transformers.
A summary of the IEEE 519 guidelines for Current THD and Voltage THD are included in Figure 2.
How to use IEEE 519
The IEEE 519 guideline is limited to harmonic distortion evaluation and recommendations at the PCC. The purpose is for responsible facility managers to mitigate their own self-generated harmonics without impacting the utility network. Harmonics are arguably a concern within the internal electrical distribution system and not limited to external interaction with the utility supply.
Cable length, impedance, distribution system configuration and the location of nonlinear loads will influence harmonic distortion levels throughout the system. The designer encounters an important question: What are desirable harmonic distortion limits within the commercial or industrial facility?
Some informed and progressive facility owners adopt a conservative total harmonic distortion – current (THDi) of 5% and total harmonic distortion – voltage (THDv) of 8% to protect their investment of expensive process equipment, sensitive electronics, medical imaging equipment and on-site standby generation. Other facility standards reference the IEEE 519 recommendations as a good practice with a limited understanding of the topic. A majority mitigate harmonic distortion only when their process is impacted or when utility requirements threaten penalties.
The primary sources of harmonic distortion within commercial buildings include switching power supplies, LED lighting drivers, uninterruptible power supplies and heating, ventilation and air conditioning loads with VFD control. Industrial facilities commonly include process applications coupled with VFD controlled motors. We commonly refer to these electrical loads as nonlinear type loads. Reputable LED drivers operate with 10% or less harmonic distortion. Computer power supplies contribute similar harmonic content as LED lighting.
VFDs arguably contribute the most THD for both commercial and industrial buildings.
Domestic water systems commonly provide a visual analogy of electrical power systems in the classroom. The sanitary portion of domestic water systems have similar regulatory contamination limitations before discharge into the public water system. Dilution is the solution to some water contaminant and pollution concerns in lieu of treatment. Harmonics generated by nonlinear loads within the electrical power system are essentially diluted when operated simultaneously with linear loads. The facility operations, processes and user influences will make the combination of linear and nonlinear loads dynamic. The fluctuations can be observable with a harmonic analysis meter.
The modeling of a simple motor and VFD application suggests voltage distortion guidelines are commonly achievable without corrective action. Current distortion guidelines are the challenge. Freeware is available from various manufacturers for preliminary modeling and analysis of harmonic distortion solutions. The simultaneous operation of one times nonlinear load and two times linear loads will commonly dilute harmonics to IEEE 519 acceptable limits.
Examples of linear loads include constant speed motors using across-the-line full voltage starters, resistive electric heat and incandescent lighting. As an example, a 15-horsepower motor with VFD control can commonly be coupled with two 15-horsepower motors without VFD control or a single 40-horsepower motor to dilute harmonics to acceptable levels.
Commercially viable solutions
Contemporary VFDs apply pulse-width modulation with the use of insulated-gate bipolar transistors in a bridge rectifier configuration to replicate a traditional alternating current waveform. The imperfect switching of the IGBTs contribute to harmonic distortion. Additional IGBTs can be added to “smooth” the switching steps and reduce harmonics.
VFDs are available with 6-, 12- or 18-pulse configurations with respectively increasing price tags and reduced availability. The 6-pulse drive is arguably most common for HVAC applications due to low cost. The 12-pulse drive improves harmonic distortion conditions but rarely to acceptable guidelines. The 18-pulse drive will commonly result in harmonic distortion connection between 5% to 8% THDi at a price nearly two-and-a-half times that of a 6-pulse drive with an input line reactor.
Technology advancements offer VFDs with an active front end. The performance characteristics of the active front end drive commonly limit the total current distortion to 5% or less. The technology is available from manufacturers across the range of common motor horsepower ratings.
Input line reactors and direct current chokes offer additional options. These devices are essentially inductors or coils of wire. The electromagnetic properties of an inductor act as a filter and impede quick-changing current typical with harmonics and IGBT switching. Line reactors are commonly applied to the input terminal of the VFD while DC line chokes are installed by the manufacturer on the DC bus serving the IGBT rectifier bridge. Line reactors and DC chokes are commonly available with 3% or 5% impedance values with line reactors imposing a respective voltage drop on the branch circuit. The 3% value is common to improve harmonic distortion and limit the voltage drop burden.
Harmonic filters offer alternative options and are available in passive and active types. Passive filters are applied to the input of the drive, configured with a series inductor (similar to a line reactor) and a parallel capacitor. The capacitor contributes a second benefit by correcting power factor, which needs to be considered when operating the loads on finite power sources like packaged emergency generators. The typical passive filter maintains the capacitor in the circuit when the motor is not operational.
Therefore, the capacitor continues to correct power factor when the motor is operating at low load or off. Multiple passive filters connected to an emergency generator can be a concern. Emergency generators and other finite power sources are commonly limited to a 3% to 5% leading power factor condition before initiating a safety shutdown. Passive filters can be equipped with an auxiliary contact to disconnect the capacitor when the associated motor is not operational.
Active harmonic filters can be applied to the input of the VFD or on a common bus application with multiple VFDs. The active filter monitors the harmonics on the bus, evaluates corrective requirements and applies corrective amps on the common bus to eliminate harmonic distortion. Active filters commonly represent a significant investment and are rated in amps of corrective current. Active filters may be sized based on preliminary model analysis or by evaluating the result of a harmonic analysis study in existing applications.
Practical application and recommendations
Harmonic distortion mitigation is commonly limited in original designs for a variety of reasons including a general lack of understanding, the mystery related to solutions, cost impact and the fact of IEEE 519 being a guideline, not required code. Don’t forget the timeless adage, why fix it before it is broken? In reality, it is arguably broken and the observable consequences are imminent.
Practical applications and recommendations (see Figure 3) should consider redundancy, repeatability and promote harmonic distortion mitigation to be a good neighbor. The mitigation of harmonic distortion inside the customer’s facility arguably provides similar value as harmonic distortion mitigation at the utility system PCC. IEEE 519 compliance at the PCC does not ensure compliance at every subcomponent of the power distribution system.
Most of the electrical loads in the modern commercial building contribute harmonic distortion. LED lighting and energy codes have significantly reduced the portion related to lighting. Computers and other miscellaneous equipment with switching type power supplies also represent a smaller portion. VFD controlled motors represent the largest contributor of harmonic distortion.
Therefore, harmonic distortion resolution of VFD motor applications will practically provide the most beneficial action for IEEE 519 compliance. Harmonic distortion compliance at the input terminations of the VFD, in lieu of at the utility service, adds the benefit of protection of equipment within the facility.
Remember that dilution is a strategy. Larger horsepower motor/VFD combinations have a higher harmonic distortion impact on the overall system when not packaged with input line reactors or filters. In reference to our previous example, it takes two times or more linear type load to properly dilute the harmonic contributions of a VFD-controlled load. As a general rule-of-thumb, 6-pulse VFD motor applications with a horsepower rating equal or less than 15 horsepower and a 3% input line reactor or DC line choke will commonly not require additional correction due to dilution with other linear loads. The 3% and 5% input line reactors will commonly reduce 15 horsepower and lower applications to approximately 35% to 40% total current distortion.
VFD motor applications above 15 horsepower require an additional harmonic distortion corrective plan because a pairing with adequate quantities of linear load is difficult to justify. Input line reactors and DC line chokes will rarely provide adequate correction in combination with 6-pulse drives. Solutions include 18-pulse drives with a 3% input line reactor or 6-pulse drives with an input passive filter. These configurations will commonly reduce current distortion 5% to 8%.
An alternative solution is an active filter. An active filter can be applied to the front end of individual VFDs or applied to a common bus. Some VFD manufacturers offer VFD product lines with an active front end and similar performance as an active filter. Applying an active filter to a common bus serving multiple VFDs offers a dynamic solution to resolve new and existing harmonic distortion conditions. Active filters and active front end drives will commonly reduce current distortion to less than 5%.
Competitive bidding, VFD procurement and submittals
Product procurement commonly occurs through a local factory representative or distribution network. Distribution organizations fill orders with varying technical support capabilities. Representative networks are commonly factory-trained agents capable of providing technical support. Competitive bidding will commonly benefit the purchaser, project budget and bid results. The challenge is a VFD specification allowing manufacturers to offer their best solutions, at the best price and promote IEEE 519 compliance.
The harmonic distortion performance of a VFD will differ between various manufacturers. Therefore, IEEE 519 harmonic distortion studies should be performed with respect to the installed equipment by the manufacturer. Most reputable VFD manufacturers can perform an IEEE 519 study during the shop drawing submittal phase of the project.
The analysis requires utility transformer size, demand current, short circuit current, understanding of the distribution system and a summary of new and existing nonlinear and linear loads to develop a report. Vendors and contractors are commonly motivated to present lower cost options to win project bids. The collection and analysis of the required information for the harmonic distortion reports is commonly not the priority during the bidding phase.
Therefore, studies are commonly not submitted or when submitted, identify harmonic distortion concerns without solutions being financially included in the bid pricing. The financial responsibility for harmonic distortion mitigation is commonly questioned post-bid.
Performance-based harmonic criteria measured at the input terminals of the VFD assembly with a list of acceptable technical solutions offers a practical solution. Therefore, a vendor can submit a 6-pulse drive with filter, VFD with active front end, 18-pulse VFD or any combination of acceptable configurations. This strategy empowers manufacturer representatives to contribute additional technical expertise and offer their best technical solutions at the best price.
Performance-based requirements can be defined for applications exceeding 15 horsepower. The designer can require IEEE 519 compliance at the input terminal of the VFD or specify a specific threshold requirement. Examples include THDv < 8%, THDi < 5% for commercial facilities and THDi < 8% for industrial facilities.
Third-party harmonic analysis tools
Various manufacturers offer harmonic distortion analysis freeware for applications requiring deeper evaluation. The following list is not inclusive but offers examples for consideration:
- ABB Harmonic Distortion Calculations.
- Eaton Harmonics Calculator.
- Schneider Electric Harmonic Calculator.
- Trans-Coil International Harmonic Guard Solutions Center.
- Yaskawa Harmonics Estimator.
Alternative subscription-based modeling programs offer additional features and capabilities.
By considering the practical applications and recommendations and employing the third-party tools at the consulting engineer’s disposal, commercially viable solutions for harmonic distortion can be achieved.