Understanding Harmonic Voltage and Current Distortion Levels at Your Facility
The proliferation of end-use electrical and electronic loads in customer facilities — residential, commercial, and industrial — is not slowing down.
Despite economic pressures, utility customers are still securing new spaces and purchasing/connecting additional loads. Facilities that are now very lightly loaded or empty will eventually be reoccupied (using even newer electronic loads) and be placed back on the grid in the next few years. Increased competition in many business sectors will result in added load as well. Load growth will place older power quality problems back on the table for utility engineers, product designers, and facility engineers to deal with again. New power quality problems will surface as well. One of the traditional power quality problems that engineers have been dealing with for years is the effect of harmonics on utility power distribution systems and on the quality of voltage delivered to facilities.
Levels of harmonic voltages and currents have been showing an ever-increasing trend in the past few decades. Similar to the concept of voltage sags and surges, customers may or may not be the cause of their own voltage distortion problems. One problem site that was investigated in the recent past showed the harmonic voltage distortion (VTHD) at the point-of-common-coupling (PCC) was above the limit prescribed by the IEEE 519 (1992) by more than 4% at the feeder voltage used to power that facility. This facility did not use much non-linear load and was clearly a victim of high VTHD present on the utility power system. Another problem site investigation showed the same type of problem, but at a more complex level — VTHD higher than the IEEE 519 limit by as much as 3%, harmonic current distortion as high as 600%, and true power factor as low as 0.20 at the main feeder to the facility. In this case, the facility was heavily saturated with harmonic-rich non-linear loads, thus adding significantly to any distortion already present on the utility power system. Moreover, the business in this facility was industrial, scientific, and medical (ISM)-based. The customer’s business activities could not tolerate these steady-state power quality problems.
The Figure illustrates an example of a voltage sine wave with a significant amount of harmonic distortion captured at the main utility feeder to a commercial facility. Notice that a lot of the distortion occurs at the maxima and minima of the waveform. This voltage waveform has about 12% distortion. Some of the pieces of electronic equipment in this facility that use electronic power supplies experienced power supply failures caused by this distortion.
The growing trend of high levels of harmonic voltage distortion at facility feeders will continue as a result of the anticipated increase in the penetration of newer harmonic-rich loads. Interestingly, a large percentage of the new loads are energy-efficient technologies, such as emerging electronic lighting and heating, ventilation, and air-conditioning (HVAC) technologies. New lighting technologies include electronic high-intensity discharge (EHID) lighting, electronic-based magnetic induction lighting (MIL), compact fluorescent lamps (CFLs), light-emitting diode (LEDs), and light-emitting plasma (LEP) systems. Many of the energy-efficiency HVAC technologies include systems designed for residential, commercial, and industrial facilities that use adjustable-speed drives and have little or no resistive load typically helpful in providing cancellation of harmonic currents generated by their non-linear load characteristics.
Other rapidly growing energy-efficient technologies include point-of-use on-demand water heating systems, Internet data center equipment, electric vehicle (EV) battery chargers, electronic LED signage and billboards, variable-frequency drive (VFD)-based appliances for home and commercial use, and consumer electronics products, including flat-screen televisions (FSTVs) — liquid crystal display televisions (LCDTVs), plasma-based televisions (PBTVs), LED-based televisions (LEDTVs), and other high-definition displays such as 3-D TVs, etc. Last but not least are the video gaming systems that use high-performance graphics cards requiring higher power levels and large computer power supplies that consume greater amounts of power every year. Inverters in distributed-energy resources (DERs), also known as renewable energy technologies or distributed-generation (DG) technologies (such as photovoltaic (PV) solar panel systems, microturbines, wind mills, fuel cells, and other energy-conversions systems) will be another growing source of harmonic injections in utility and facility power systems.
While end-use loads are the primary concern regarding the generation of harmonic currents (and thus harmonic voltages) in facilities, they will soon take competition from DERs. DERs are typically located on facility electrical systems and directly tied to utility power distribution systems. For example, commercial and industrial facilities are seeing a growing number of PV systems located on their rooftops to reduce their energy consumption. A PV system will use an inverter — an electronic system designed to invert DC power back to AC power — so the power harnessed from the sun can be injected back into the grid. Inverters will generate harmonic-rich currents that will impact the quality of voltage on the grid. Like VFDs, different model inverters use different types of harmonic filtering, so their harmonic current levels will be different among models. Facilities planning for any type of upgrade — whether it is related to end-use equipment as in the application of energy-efficient technologies or related to the use of renewable power — should carefully examine their quality of power beforehand. Examination of power quality at the facility level can be achieved by conducting a power quality audit on the facility. Utility customers should also keep an eye on their voltage distortion level at utility feeders to ensure that it does not cross the limits prescribed by IEEE 519 for their feeder voltage level and type of business. In some cases, a harmonic study may be needed to determine the harmonic voltage and current levels as well as true power factor at the PCC.
Keebler is a principal engineer and partner at KCE Engineering, LLC located in Knoxville, Tenn. He is an 18-year veteran of the Electric Power Research Institute (EPRI) in the Energy Utilization Group of the Power Delivery & Utilization sector. His work at EPRI also involved the development of new standards for ANSI, IEEE, IEC, NEMA, IESNA and CISPR as well as product standards for lighting, building PQ and EMC, including EMC standards on EMI filters; energy-efficiency and PQ standards for CFLs, HID, LED, and MIL. He can be reached at [email protected]. Check out his Power Quality Perspectives blog exclusively on www.ecmweb.com.
