Please click on the questions below for the answers to the FAQ questions.
What are harmonics?
Harmonic currents and voltages are integer multiples of the system’s fundamental frequency. For example, with a fundamental frequency of 60Hz, the 3rd harmonic frequency is 180Hz (3 x 60Hz).
What happens to the fundamental frequency’s sinusoidal waveform when harmonics are present?
The fundamental frequency’s sinusoidal waveform, which is always predominant, becomes distorted by the addition of harmonic sinusoidal waveforms. The measure of distortion is given as Percent Total Harmonic Distortion of the Fundamental Waveform (%THDv [voltage] & %THDI [current]).
What causes harmonic currents?
Nonlinear loads are the source of harmonic currents. That is, the load’s current waveform is non-sinusoidal. As a result, the distorted current waveform is rich in sinusoidal harmonic current waveforms.
Nonlinear loads include electronic devices such as rectifiers, current controllers, AC and DC drives, cyclo-converters and devices with switch-mode power supplies such as computers, monitors, telephone systems, printers, scanners, and electronic lighting ballasts.
What causes harmonic voltages?
The electrical distribution system’s harmonic impedances cause the load-generated harmonic currents to produce harmonic voltages (EH = IH x ZH). Transformers, and feeder and branch circuit impedances will cause maximum voltage distortion at the nonlinear loads.
Is there really a harmonics problem?
Yes, there is a problem. In fact, there are so many significant harmonics problems that hundreds, if not thousands, of theses, papers, articles and case studies have been published on this subject. To define and limit harmonic problems, IEEE has published a standard entitled - ‘Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems’ (ANSI/IEEE Std 519-1992).
What causes harmonic problems?
All electronic loads generate positive- and negative-sequence harmonic currents. Single-phase electronic loads, connected phase-neutral in a three-phase, four-wire distribution system, also generate zero-sequence harmonic currents. Three-phase motor drives and single-phase lighting loads often account for a significant portion of a system’s 480V loads. Single-phase electronic office and data processing equipment typically accounts for more than 95% of a 120/208V power panel’s loads. At these levels, 100% Total Harmonic Distortion of Current is common.
How big are the harmonics problems?
In the early 1980s, the proliferation of personal computers and conversion to electronic lighting ballasts produced harmonic problems in commercial office buildings. Facility managers and designers soon discovered that single-phase electronic loads caused distribution transformers to overheat and ‘shared’ neutral conductors to become overloaded.
Today, more than 95% of all 120/208V power panel loads, in modern office and data center environments, are electronic. Electronic motor drives are now routinely applied to ventilation fans and elevators at the 480V level.
How have some engineers dealt with harmonics in their system designs?
To improve system performance and provide the best possible environment for nonlinear loads, a designer’s options have been limited to over-sizing distribution transformers and ‘shared’ neutral conductors.
As an alternative, branch circuits have been configured with a separate neutral conductor for each phase conductor. In either case, branch circuits have been underutilized and limited in their length as a means of reducing voltage distortion and neutral-ground voltage (common mode noise) at the loads.
As an alternative to over-sizing conventional distribution transformers, many designers have specified K-Rated transformers. Unfortunately, a K-Rated transformer’s higher harmonic impedances cause an increase in voltage distortion.
Who are the ‘stakeholders’ and what are their problems if the harmonic problems are not solved?
The ‘stakeholders’ include the electrical distribution system designer, the facility owner, the tenant(s) and, ultimately, the tenant’s clients or customers. Their potential problems include:
• Harmonics Exceed IEEE Recommendations & Requirements,
• Harmonics Exceed CBEMA / EPRI Recommendations,
• Power System and Loads are Incompatible,
• Project Exceeds Budgetary Goals and
• Loss of Client Confidence.
The Facility Owner
• High Capital Costs,
• Increased Fire & Safety Risks,
• Premature Apparatus Failures and
• Significant Tenant Dissatisfaction.
• Higher Power Cost,
• Premature Office Equipment Failures,
• Data Corruption or Loss,
• Computer and System Lock-Ups,
• Loss of Productivity,
• Reduced Quality Assurance,
• Loss of Customer Confidence and
• Loss of Revenue.
• Higher cost for products and/or services,
• Reduced product and/or service quality,
• Loss of Customer Confidence and
• Loss of Revenue.
What is IEEE Std 519?
ANSI/IEEE Std 519-1992 – Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems is the recognized power quality standard in North America. From this standard we wish to provide the following sections:
Section 10.3 – Limits on Commutation Notching,
states that - ‘The notch depth, the total harmonic distortion factor (THD), and the notch area of the line-to-line voltage at PCC should be limited as shown in Table 10.2.’
Low-Voltage System Classification and Distortion Limits
Special Applications 
Dedicated Systems 
Note: The value AN, for other than 480V systems, should be multiplied by V/480.
 Special applications include hospital and airports.
 A dedicated system is exclusively dedicated to the converter load.
 In volt-microseconds at rated voltage and current.
Section 11.5 – Voltage Distortion Limits,
states that - ‘the limits listed in Table 11.1 should be used as system design values for the “worst case” for normal operation (conditions lasting longer than one hour). For shorter periods, during start-ups or unusual conditions, the limits may be exceeded by 50%.
Voltage Distortion Limits
Bus Voltage at PCC
Individual Voltage Distortion
Total Voltage Distortion
69kV and below
69.001kV through 161kV
161.001kV and above
1] From Section 10.3 above, a THDv of 3% is required for hospitals and airports.
It is important to understand that THDv limits apply at the point of load connection. THDv levels at the loads may be several times higher than those at the distribution transformer’s secondary terminals. It is, after all, the loads that require reasonable levels of voltage distortion.
What will happen to my loads if I allow the electrical system to operate outside IEEE Std 519-1992 guidelines?
Electronic equipment is susceptible to disoperation caused by harmonic distortion. Data in computer ‘networks’ may be corrupted. Harmonic currents may also contaminate audio and video signals in interconnecting shielded cables.
Conventional linear loads can also be affected. For example, motors may act as an unintended harmonic shunt. This will result in loss of torque, overheating and premature failure.
Sections 6.2, 6.6 and 6.9 of the standard detail the probable outcomes if harmonic currents and voltages are not controlled.
What will happen to my electrical system if I allow it to operate outside IEEE Std 519-1996 guidelines?
Harmonic currents, generated by single and three-phase nonlinear electronic loads, will cause additional ‘penalty’ losses throughout the electrical distribution system. These losses result in apparatus overheating and premature apparatus failure. Section 6.0 of the standard details the probable outcomes if harmonic currents and voltages are not controlled.
Will harmonic currents affect my power costs?
Harmonic currents, generated by single and three-phase nonlinear electronic loads, will cause significant ‘penalty’ losses throughout the electrical distribution system. For example, distribution transformers, when supporting 100% THDI nonlinear electronic office loads, will produce approximately 3.25 times higher losses than when supporting linear loads. ‘Penalty’ losses, produced by the other elements of the sub-system, will typically equal and often substantially exceed the transformer’s ‘penalty’ losses.
‘Penalty’ losses result in apparatus overheating, higher air conditioning costs and higher power costs. A reduction in ‘penalty’ losses will produce a very attractive annual saving. An approximate saving may be calculated as follows:
Annual Savings = (Total kW Savings ? $/kWh ? hrs/day ? days/year) +
(Total kW Savings ? $/kW Demand Charge/month ? 12 months) +
(Total PF Penalty Savings)
Can I purchase a harmonic mitigating transformer that will solve all my harmonic problems?
No! Since the vast majority of harmonic mitigating distribution transformers are used to supply switch-mode power supply loads, it is likely that these transformers will have secondary windings with ultra-low zero-sequence impedance. This characteristic normally allows for the cancellation of zero-sequence fluxes by the secondary windings, a reduced core cross-section, avoidance of zero-sequence current in the primary ‘delta’ connected windings, and relatively low losses and high efficiency. The transformer also provides a reduction in THDv at its secondary terminals. These are the benefits.
However, in reducing the distribution transformer’s Zero-sequence harmonic impedances (Z0), load-generated zero-sequence harmonic currents will increase. zero-sequence harmonic currents are normally limited by the conventional transformers relatively high zero-sequence impedances. For example, a conventional or K-Rated transformer may have a Z0 of 0.1O. By contrast, a harmonic mitigating transformer, with ultra-low zero-sequence impedance, may have a Z0 of less than 0.005O, 20 times less than a conventional or K-Rated Transformer.
In this scenario, zero-sequence harmonic currents will increase in the phase and neutral conductors. This, in turn, will cause even higher ‘penalty’ losses in the 120/208V feeder and branch circuits and, most importantly, higher neutral to ground (CMN) voltages at the loads.
High CMN is normally the most significant ‘power quality’ problem since it causes data corruption in computer ‘networks’. In addition, high voltage distortion (THDv) will cause switch-mode power supplies to overheat and reduce their life expectancy.
All issues considered, and as a ‘rule-of-thumb’, it is often inappropriate to apply ultra-low zero-sequence harmonic mitigating transformers in ratings above 75kVA. With higher ratings, circuit lengths tend to be longer and, consequently, THDv and CMN levels will be unacceptable.
Manufacturers who recommend the application of these types of harmonic mitigating transformers, even with improved efficiencies, while ignoring the critical ‘power quality’ issues, do a disservice to their customers, and the utilities that may subsidize the application of their products.
An engineered system solution may provide the only acceptable alternatives if a sub-system’s capacity exceeds 75kVA. An engineered solution will always provide the best power cost reduction.
Should I consider harmonic filters?
With reference to the preceding question, the only alternative, in solving all the harmonic problems, may be the application of zero-sequence harmonic filters, particularly if the transformer’s rating is higher than 75kVA.
Where is the best place to remove harmonic currents?
The best place to remove harmonic currents is often at the nonlinear loads that generate harmonic currents or, alternately, at the sub-panels that supply the branch circuits. The object is to reduce harmonic current at a point that is as-close-as-possible to the harmonic generating nonlinear loads. The reduction of harmonic current will result in the reduction of ‘penalty’ losses. The zero-sequence harmonic filters may be used for this purpose.
What will happen if I install a 180Hz zero-sequence current high impedance blocking filter at the ‘load-side’ of a distribution transformer?
In most applications, this type of current blocking filter will substantially increase voltage distortion (THDv) at the loads.
Several independent NETA companies reported THDv values in excess of 20%. One UPS/PDU manufacturer, which was obliged to install this type of filter at the output of its PDU, reported a 15% THDv increase after its installation. In this instance, a system that was marginal with a 4% THDv, before the installation of the filter, rose to 19% after the installation.
A designer or facility manager, who specifies this type of filter, assumes that the load-generated zero-sequence harmonic currents have no alternative parallel zero-sequence path than the secondary windings of the distribution transformer. This assumption has caused serious apparatus and system failures.
In addition, some sensitive electronic loads and distribution system devices will reject the highly distorted voltage as being unsuitable. For example, small single-phase UPSs may transfer their loads to the battery backup, until they are discharged, then remain unavailable when a system ‘re-closure’ or ‘outage’ occurs.
One independent testing company has also reported an increase in transformer core losses and temperatures after a blocking filter installation. This test was conducted as part of a general assessment of the filter’s characteristics and benefits. In this case, the purpose in adding the filter was to reduce the distribution transformer’s high operating temperature. The filter produced the opposite result.
What does K-Rating really mean?
K-Factor rating, applied to transformers, is an index of the transformer's ability to supply harmonic content (%THDI) in its load current while operating within its temperature limits. K-Rated transformers are only intended to survive in a harmonic rich environment. They do not mitigate harmonic currents or voltages. In fact, they normally cause higher levels of voltage distortion because of their higher harmonic impedances.
Should I apply a transformer that is only capable of supporting a sub-system with less than 50% nonlinear electronic loads?
No! Most power panels in a modern commercial facility support more than 95% nonlinear electronic loads. On this basis, current distortion will probably approach or even exceed 100% THDI.
A transformer that is only designed to support a mix of 50% nonlinear loads and 50% linear loads will, under full nonlinear load conditions, exceed its design losses and temperature rise if loaded to much more than 75% of its nameplate rating. To meet the 80% NEC requirement, this transformer, under these conditions, should not be loaded to more than 60% of its nameplate rating.
We doubt that a professional engineer would want to take the risk in applying this type of transformer since, at least in the longer term, he has no control of the type of loads that will be connected to the system. Because the load mix is becoming more nonlinear, the risk increases over time.
Essentially the same logic applies to retro applications. There is no guarantee that the transformer might not eventually become overloaded.
This potential problem is not merely a reliability issue. The higher losses will reduce the transformer’s efficiency, and increase air conditioning and power costs.
Does the Department of Energy’s Oak Ridge National Laboratory validate and endorse the performance of harmonic mitigating transformers.
The Department of Energy’s Oak Ridge National Laboratory does not evaluate or endorse the performance of any manufacturer’s transformers because: (i) they lack the statutory authority to do so and (ii) there are no established guidelines or standards.
Are manufacturers’ published transformer efficiency claims, under nonlinear load conditions, legitimate?
A number of manufacturers now claim transformer efficiencies that meet or exceed the requirements of NEMA TP 1-2002, under severe nonlinear load conditions. One manufacturer has even published their test methods. At best, these claims are misleading since:
There is no recognized standard guide for determining the energy efficiency of distribution transformers or a standard test method for measuring their energy consumption,
The manufacturers that make these efficiency claims are basing them on the ‘Power In – Power Out’ Measurement Method. Based on the manufacturer’s published test method, which utilizes +/-0.3% revenue class instrumentation accuracy, the measurement error will be +/-1.5%, when measuring the efficiency of a transformer. As a result, a claimed efficiency of 98%, for a 75kVA transformer, may, in fact, be only 96.5%. There are a number of IEEE transactions that confirm that this test method will yield inaccurate results.
What is the source of Power Quality problems?
Most people believe that all power quality problems can be traced to the power coming from the utility. The reality is more complex. The source of a power quality problem can be the utility, but it also can be the facility or even the equipment inside the facility. Utility Power quality problems that begin with the utility often have the greatest impact on a facility’s operation. Typical utility-generated events range from a breaker clearing, which can produce sag, under voltage or outage, to arcing contactors, which may generate impulse. Stopping or limiting the impacts of utility-generated events must be done where electrical service enters the facility. Facility – The building typically produces the majority of power quality problems, partly because the normal use of energy creates power-line events that can affect the facility’s equipment. Typical facility problems include loose connections, overloaded circuits and transformers, ground loops and wiring errors. Beyond comprehensive plant maintenance, addressing these problems may include the use of transformers with some output filtering. Equipment— particularly the new generation of automated and computer-based technologies — can produce power quality impacts through normal operation. The impacts of routine activities such as equipment turn-on/off can include impulse, sag, surge, voltage distortion and repetitive disturbances. Mitigation equipment between the load and the facility wiring can correct the problems.
How can Power Quality affect the facility electrical distribution system?
The facility electrical distribution system has more influence on the quality of power than any other single factor, and it is an area that can be positively impacted by good electrical facility design, installation and maintenance. Those facilities with protective maintenance programs have fewer power quality problems than their counterparts without such programs. Monitoring may provide advance warning of capacity or general power-related problems before they affect building operations. In addition, monitoring helps in planning for new equipment and in rapidly identifying the source of problems. The facility electrical distribution system has several components, each one capable of affecting power quality. Transformers are used to “step up” and “step down” voltage to meet load requirements. The introduction of non-sinusoidal loads that draw their energy in short, current pulses every half cycle. These return currents are rich in odd harmonics, especially third-order, and can cause overheating in distribution transformers not designed for this type of load. Panel boards and Circuit Breakers may be unable to withstand high peak inrush currents of some loads, causing nuisance tripping, while the use of fuses in feeder circuit for supplemental protection can cause overheating and damage to the load. Feeders run between service equipment, panel boards and transformers, while the branch circuit is a set of conductors between the final overcurrent protection device and the point of connection for the equipment being used. With many newer electronic loads, there is little, if any, return current cancellation, and the resulting neutral currents can be as high as 1.7 times the phase current. An undersized neutral and high-return currents overload circuits and cause heating at connection points. Some symptoms of a high-impedance neutral problem are high failure rates of power supplies, erratic equipment operation and system crashing, and load crashing when one load is turned off. Receptacles are contact devices mounted in outlet boxes and come in multiple configurations and quality grades. Incorrectly wired receptacles are not unusual and can impact power quality.
What are high-frequency events?
Power quality disturbances generally are classified into broad categories of high frequency, voltage, distortion and fundamental frequency variations. High frequency events or disturbances refer to voltages with frequency components significantly higher than the nominal frequency line of 60 hertz. Important characteristics of a high-frequency event include maximum voltage level, energy content, rise time, phase angle and frequency of occurrence. High frequency events occur in several distinct varieties, each with characteristics that may help identify the source of the disturbance and the relative distance from the monitoring station. A unidirectional impulse is a high frequency, transient wave of current, voltage or power of unidirectional polarity. Impulses of a purely unidirectional nature generally are generated within a facility, or close by, without passing through a transformer. An oscillatoy impulse has both positive and negative polarity and poses two problems: first is the impulse with its associated rise time and peak voltage amplitude; second is the secondary frequency of the decaying waveform. One of the most common oscillatory impulse events is caused when power factor capacitors are switched on to the power line. A repetitive event is a series of events that occur at regular intervals. These can be unidirectional, oscillatory or a combination. A common repetitive event is the impulse caused by phase angle controlled loads (SCR). Although individual events pose no special problem for equipment, the concentration of events may stress filter circuit components and cause premature failure. Common and normal-mode represent the two ways high-frequency events can occur. Common mode events have no magnetic path through the transformer and must be coupled through capacitive paths. Normal mode events are magnetically coupled through a transformer. Equipment generally is more sensitive to common mode events.
What kinds of in-plant systems or equipment are vulnerable to power quality disturbances?
Numerous systems and types of equipment in a typical business facility are vulnerable to power quality disturbances. Among them:
Computer EquipmentUnlike many electrical loads, computers use ground as a reference for all operations, operate at very low voltage levels, and contain data circuits that connect to loads through a facility. Variables that directly the sensitivity of computer equipment include grounding, system design, operating speed, data links to other equipment, and the number of devices in an immediate area. Note that even the physical movement of a computer from one area to another area can have an impact on power quality and computer performance.
Telecommunications EquipmentIncreasingly sensitive to power disturbances, telecommunications equipment use ground as a reference, operate with several voltages, connect to equipment throughout a facility, and eventually connect to external phone lines. Power disturbances can scramble programs, change address information, drop calls and damage circuit components. Common problems for DC-powered telephone systems are a lack of AC synchronization to standby power generators, common mode interference paths into the system, and failure of the rectifier and internal circuit components from large impulses.
Process ControlComputer-based process control systems range in size and include programmable controllers, adaptive/proportional controllers, and numeric controllers. Disturbances can affect specific components of process control systems in distinct ways. Computer control can be affected by line variations or transients, producing memory scramble, program loss and semiconductor failure. Remote, digital-to-analog (D/A) controls may be disabled, blocked or forced into false operation. Feedback controls may be activated by false operation of D/A controller or electrical interference, and watchdog circuitry may be confused by harmonic distortion of a voltage waveform forcing false shutdown.
What are voltage events?
Power quality disturbances generally are classified into broad categories of high frequency, voltage, distortion and fundamental frequency variation. Voltage, or low- frequency, events are variations of voltage amplitude and occur at or near power line frequencies of 60 hertz. Among the types of voltage events are the following:
Sags are short-term cases of under voltage in which the voltage fluctuation exceeds the allowable threshold for at least one cycle (16.7 milliseconds). Sags commonly are caused when heavy loads, such as motors, are switched onto the line, drawing heavy inrush currents that drop the voltage for short periods. Sags of sufficient magnitude can cause serious impacts to sensitive electronic equipment. When fluctuations of the RMS voltage occur over an appreciable time interval, they are known as under voltage conditions.
Surges are the opposite of sags, often resulting from the disconnection of heavy loads from the line. A surge, according to the IEEE dictionary, is “…a transient wave of current, potential or power in an electric circuit.” Also known as a swell, a surge of sufficient magnitude can have substantial effects on the power system, particularly electronic equipment. A variation of a surge is known as an overvoltage condition, which occurs when fluctuations of the RUM voltage occur over a long period of time.
Outages and line interruptions occur when the voltage drops to a level at which devices cannot perform their intended function. Duration may range from one cycle to several hours or more. Short-term outages often are caused by mundane events such as a utility breaker tripping to clear a fault and then re-closing automatically, while long-term outages typically result from accidents to power lines or utility transformer failure.
Neutral-to-Ground voltage is inherent in a facility’s electrical distribution system, and equipment manufacturers sometimes specify acceptable limits for neutral-to-ground voltage. A frequent cause of neutral-to-ground voltage is an illegal neutral-to-ground bond in a panel board or other location, which represents a safety hazard and diverts part of the return current flow through the grounding conductor.
What is distortion?
Distortion is one of the four main categories of power disturbance events. It is a deviation from an ideal reference waveform, which for commercial power is a pure 60 Hz sinusoidal waveform. Two causes of voltage distortion are large amounts of harmonic current from nonlinear loads and power sources with non-sinusoidal voltage characteristics. Voltage distortion causes increased heat in motors and transformers, and extreme levels of harmonic distortion may decrease filter capacitor life in power supplies. Voltage distortion has a range of potential causes, including SCR controlled loads, large UPS systems, variable speed drives, switch-mode power supplies and high-impedance electrical wiring. Symptoms of voltage distortion to equipment include excessive heat, lack of phase synchronization, under voltage circuit activation, motor failure and nuisance tripping. Non-sinusoidal and nonlinear phase currents have an adverse impact on facility power distribution systems. The amplitude of peak currents and concentrations of harmonic currents can cause heating and may force breaker operation. In high impedance power distribution systems, voltage distortion increases significantly with nonlinear current. Typical causes of non-sinusoidal phase current include computers, electronic ballasts, electronic phone systems, UPS and variable speed drives. Equipment symptoms range from circuit breaker tripping to excessive heat in wiring and transformers. Solutions to the problem of distortion will depend on the specific source of the disturbance, but may include adding harmonic filters, decreasing the non-sinusoidal load, moving or rewiring problem loads and decreasing the impedance of the power source.
What are some typical power quality events experienced by manufacturing operations?
Manufacturers are confronted by a broad range of potential power quality disturbances, many of them specific to the production process, product line and industry represented. Notwithstanding the individuality of each manufacturing operation, a number of potential power quality problems are possible. Among them are the following examples of typical problems and solutions.
Severe Voltage SagA manufacturing process shutdown at a large industrial manufacturing plant was caused by the failure of a company-owned 2,000 kVA transformer in another part of the plant. The failure shorted the line for five cycles before the fault reclose restored normal voltage. Although the incident was isolated, it affected the entire facility. The solution centers on supplying power to the controller through a UPS or a motor/generator with sufficient flywheel inertia to rid through the sag.
Failure of Variable Speed DrivesIntermittent shutdowns of 5hp variable-speed drives at a manufacturing plant resulted in productivity drops. The problem was identified as disturbances on the line caused when power factor capacitors were switched into the utility power grid without a corresponding increase in RMS voltage. Because the protective circuitry of the drive was sensitive to extremely short periods of overvoltage, an increase to 800 volts peak for as little as 40 microseconds would cause shutdown. The solution involved the installation of transient suppressors at the inputs to the drives.
Rectifier SpikesRectifier spikes disturbed control circuits at a printed circuit-board manufacturing plant with its own automated waste treatment process. Each of several treatment vats was outfitted with a single-phase, 0.5 hp, rectifier supplied, DC-motor-driven metering pump. The pumps locked up, shutting down the treatment process, or failed for no apparent reason. Investigation uncovered a 480-volt, six-pulse, phase-controlled rectifier for an electroplating cell. Because of the high source impedance of the step-up transformer, disturbances on the 480-volt line were reflected back into the 120-volt pump feed line and the pump control electronics. A separate feed for the waste treatment system was brought directly from the facility’s main service entrance to resolve the problem.
PC Software LockupsIn the final assembly and test area of a computer system hardware manufacturing operation, software for the 30 PCs controlling the test procedures locked up, shutting down the test procedures once or twice each day. Neutral-to-ground voltage disturbances were traced to an isolated grounding conductor within the transformer enclosure. The conductor had been cut, possibly to eliminate the ground look created through the static discharge ground rods after circuit board assemblers complained about minor shocks while working on the bench. To address the lock-up problem, the ground system was unified by rebounding the isolated ground conductor to the ground buss bar in the main service entrance and reconnecting wrist straps to the same isolated ground. Neutral-to-ground impulses and computer lockouts stopped, as did shocks to operators.
What are some common power quality problems confronting medical facilities?
Medical facilities, including hospitals and laboratories, face unique power quality needs because of their reliance on highly specialized and precise diagnostic and treatment equipment. These concerns extend beyond the routine power quality issues common to any operation employing electronics equipment. Three examples of specific power quality issues for medical facilities include:
Computed tomography (CT Scan) system lockup and component failure was a repetitive problem for a medical clinic. Power to the CT Scan unit was supplied from 480-volt service fed to a 480-to-208 volt isolation transformer. Investigations identified the cause of the disturbance as a utility power factor correction capacitor bank located a block away from the clinic. The attenuation effect of the isolation transformer was not enough to protect the CT Scan system from such a severe transient. An active-tracing filter specifically designed for this type of disturbance was installed on the 480-volt line to protect all downstream equipment.
Imaging problems and software lockups on the computer driving the magnetic resonance system at a hospital were traced to impulses caused by contact bounce. The source was identified as the contactor on an infrared heater in the humidifier of the air handling system. When the heater was turned off, the impulses stopped. A UPS was installed to isolate the magnetic resonance system and protect against power outages.
Computer failures and data error sat a laboratory typically began at 10:00 each morning. An event summary of the RMS voltage revealed a pattern of repetitive sags on the RMS voltage beginning just after 10:00 a.m. The regular repetition of the voltage sags indicated automatic switching of another load on the circuit, eventually identified as a laser printer in a nearby office. Moving the printer to another branch circuit removed the source of the computer interference at the laboratory.
What kinds of power quality problems are likely to affect office buildings?
Modern office buildings rely on a range of automated control technologies to provide facility management functions extending from temperature control to security and power use. While the control technologies typically improve building performance and tenant satisfaction, they are vulnerable to power quality problems. Among the types of issues confronting office building managers are the following:
Harmonic DistortionRepeated failure of electrical distribution equipment severely affected tenants of an office building with personal computers, terminals, copiers and other electronic office equipment. Circuit breakers were tripping, electrical connectors were burning out, and a distribution transformer overheated and failed. Although these problems were symptomatic of overload conditions, initial measurements showed current readings that did not exceed equipment ratings. Further investigation showed severe current distortions caused by the typical switching mode power supplies used by the majority of modern office automation equipment. The effect on facility wiring is that the common neutral conductor frequently carried current beyond its rated capacity. In the short term, these problems can be addressed by oversizing neutral conductors and derating transformers to a conservative value of 60 percent.
Switchgear Problems At a computer center using supercomputers, new automatic switchgears to control two incoming 13.2 kV utility feeders caused power outages resulting in computer shutdowns. Although the timing of the new switchgear was supposed to accommodate the three-cycle gap allowed by other relay-sensing equipment, existing instrumentation could not measure whether the specification was being met. Resetting the switchgear timing relays improved performance enough to bring the switching gap within acceptable limits.
Ground LoopsAt a company’s administration building, a UPS-fed computer room contains the mainframe and several connected t4erminals that are, in turn, connected to numerous other terminals outside the computer room via data link. A diesel-driven generator protects against utility power failure, but when power was transferred from the utility to the generator, the data link terminal boards in the exterior terminals would burn up. Investigation showed a transfer voltage gap when the system switched from utility to diesel power — but it also showed a high current surge of 42 amps in the neutral conductor. Ground loops caused by dual earth ground points were identified as the problem. The solution centered on unifying and improving the grounding system, which resolved the data link terminal board burnout problem.
Why should I monitor power if I have a UPS?
Uninterruptible power supply (UPS) systems have become critical for virtually all factories, industrial facilities, offices, medical operations and even retail establishments. With the integration of computers into modern American life and their corresponding need for extremely high-quality power, UPS systems provide a vital “ride-through” in the event of power disturbances. That ability effectively hides power quality disturbances — but it does not address the source of the disturbance. If left unattended, “hidden” disturbances can produce very visible problems. The importance of monitoring is demonstrated in a case study of a major customer service center in the southwestern United States. The service center serves more than 50% of the US for one of the nation’s largest air transportation companies. To fulfill the company’s commitment to customer service, the center and its extensive arsenal of computer equipment must be on-line 24/7. To ensure that reliability, the company selected a Toshiba 7000 series UPS system that included three 300 kVA parallel redundant units. The UPS system also was equipped with a Signature System™ power monitoring system. During the first six months of the facility’s operation, the Signature System confirmed the expected performance of the UPS, detecting no power quality events generated within the facility. Routine monitoring of the supply from the utility documented far different results, however. In just the first three months of operation, 50 disturbances in the supply from the utility were documented. Although the UPS successfully mitigated the disturbances before their impacts reached the facility’s equipment or systems, these disturbances included sags and transients that could have threatened unprotected loads. Even with the performance of the UPS, if the disturbances had remained unidentified and unresolved, they eventually could have compromised the longevity of the UPS — and the safety of the facility. Beyond verifying the UPS performance, the Encore System provided trends of power reliability and quality, delivered enterprise-wide scalability, and gave company personnel access to all power quality monitoring information from anywhere with computer access.
What are some practical uses of power quality monitoring data?
There are numerous uses of power quality monitoring data, all of them dependent on the depth, range and accuracy of the generated data. The Signature System™ captures critical power events that typically are missed by other monitoring systems, addressing both power cost and quality. In addition, its analytic capabilities provide answers — not just data — to address key power quality issues such as what triggered a disturbance, what the impacts of the disturbance are, and what needs to be done to prevent future occurrences. Beyond the expected uses of identifying power quality disturbances, the data generated by the Encore System can be used for a range of other uses. Among them:
By “seeing” the continuous performance of equipment and systems, cost-efficient maintenance procedures can be developed and implemented.
Cost ManagementPower use can be adjusted quickly and loads can be shifted based on data that correlates power use and cost and that provides the information to reduce energy use, prevent peak demand and avoid power-factor penalty charges.
Energy ManagementPower costs can be allocated to individual product lines, buildings, tenants or products, while historical trends of power use and equipment performance assist in identifying and correcting problems, anticipating load efficiencies and optimizing power supply contracts.
Capital InvestmentsPlanning capital investments in building or equipment upgrades is facilitated by documenting power use and performance of targeted systems or processes and facilitating investment decisions.
Quality ControlPower quality — and, consequently, the performance of linked process and production systems — is confirmed for regulatory agencies, internal quality control programs and industry-specific quality standards.
How can I recognize high frequency events?
High-frequency event” refers to voltage or corresponding current changes with frequency components substantially higher than the nominal power-line frequency of 60 hertz. The frequency of a high-frequency disturbance can vary from several hundred hertz to more than one million hertz. There are numerous causes of high-frequency events, since such an event is generated any time a current-carrying inductive circuit is abruptly interrupted. Power line switching, arcing to ground, load switching, power factor correction capacitor switching and lightning can produce high-frequency disturbances. They can cause the largest voltage swings of any power line event, with maximum amplitudes approaching 6,000 volts — the flashover point of a standard 120-volt receptacle. Important characteristics of high-frequency events include the maximum voltage level, the energy content, the rise time, the phase angle, and the frequency of occurrence. Because high-frequency events occur in several distinct varieties, the unique characteristics of each event are critical to understanding the type of disturbance, the source of the interference, and the relative distance from the monitoring location. The distinct types of high-frequency events are:
• Unidirectional Impulse
• Oscillatory Impulse
• Repetitive Even
• Common and Normal Mode Event
What are the effects of and to power quality from alternative power sources?
The increased reliance on alternative power sources — both as backup in the event of an emergency and as a primary power source — exposes a number of vulnerabilities between power quality and alternative power systems and their components. Alternative power sources typically are used to change voltage levels and frequency, isolate critical equipment, provide voltage regulation, and maintain power to a load during utility power interruptions. They can cause several types of disturbance. Peak currents or harmonic currents generated by the load can interact with the impedance of the alternative power source, causing voltage instability and distortion. Off-line UPS de4signs may pass common mode disturbances through to the protected load. On-line UPS designs may have bypass circuitry which allows the pass through of common-mode disturbances to the protected load. And SCR-controlled battery chargers may add impulses back into the incoming power line. Conversely, power disturbances can affect alternative power sources in several ways. Input SCRs and controller networks may be damaged by surges. Voltage distortion and dropouts may force continuous battery operation in the UPS system, and multiple cycle outages may trip input circuit breakers. Standby power generators, widely used to supply power during a utility outage or to supply power to the UPS equipment, present their own unique challenges. Standby systems use an engine, an electrical generator, a transfer mechanism and a controller to start the generator and transfer the electrical load to and from the generator. The standby generator may be a source of power disturbances if power transfer to and from utility power is not synchronous, mechanical or fuel problems induce generator instability, and peak current or harmonic currents from the load interact with generator impedance and cause voltage instability and voltage distortion. Alternately, power disturbances also can affect standby generators, with high-frequency impulses potentially damaging the controller, voltage distortion preventing synchronous transfer to and from utility power, and voltage distortion forcing false operation.
What are the symptoms, causes and solutions of impulse events?
A voltage impulse is a high-frequency voltage wave of positive or negative amplitude. An impulse that is measurable between current-carrying conductors is a normal-mode event; an impulse common to all current-carrying conductors and measurable with reference to ground is a common-mode event. Common symptoms of equipment suffering impulse events range from parity errors and component failure to hard-disk crashes, lock-up, memory scramble, SCR failure and power supply failure. Factors that influence the ability of an impulse to disturb a load include impulse amplitude, duration and frequency; system filter and bypass capacitors; semiconductor design and operating speed; and grounding. There are multiple causes for impulse events. Those potential causes include external sources, such as lightning, and internal sources, such as faulty wiring and circuit breakers, contact and relay closure, load startup or disconnect, SCR controlled loads and variable speed drives. Even photocopiers can be the source of impulse events. Solutions to impulse problems depend on the source of the event, but may include replacement of faulty breakers or wiring, the addition of snubbers to contacts and relays, the physical relocation of the sensitive load, or the use of power treatment devices.
What are the symptoms, causes and solutions to neutral-to-ground voltage events?
Neutral-to-ground voltage is any voltage measurable between the neutral conductor and the grounding conductor, usually reflecting voltage losses in the neutral conductor due to neutral return current. Neutral-to-ground voltage can affect electronic equipment if the amplitude of the voltage exceeds the withstand capability of the load. Typical symptoms of neutral-to-ground voltage events include parity errors, poor resolution, erratic equipment operation, the need to reset/reboot equipment and, for telecommunications systems, dropped calls. There are numerous causes of neutral-to-ground voltage events. Common causes are large-equipment startup, loose neutral wiring and grounding wires, lose or missing neutral-to-ground bond, excessive ground and neutral current. Solutions to neutral-to-ground voltage problems are straightforward: correct faults to ground, repair wiring problems, add larger neutral wires, or add transformer isolation.
What are the likely impacts of power quality events on imaging systems?
Imaging systems represent a major advance in medical diagnostic and security operations. The technology, which includes computer tomography (CT scan), magnetic resonance imaging (MRI), ultrasound and X-ray scanning systems, also is highly sensitive to electrical interference. Power disturbances can affect imaging systems in several ways. High frequency impulses can degrade analog-to-digital conversion. Repetitive high-frequency disturbances can cause poor image quality in video or CRT displays. Power disturbances with sufficient energy can lock up the computer controls and damage hardware. Voltage fluctuations can activate low-voltage detection circuits and inhibit scanning operations. Because of the critical role of imaging devices in medicine and security, any deterioration of the system or compromise of its operation is unacceptable. Variables directly affecting the sensitivity of imaging systems include grounding, system design, operating speed, data links to other equipment, and the quantity of electronic equipment in the immediate area. Physical inspections of equipment subjected to power events can identify loose or broken connections, determine grounding continuity and physical integrity, identify excessive ground current, and pinpoint warm spots or warm circuit breakers. Modifications to the system may include the addition of environmental sensors for temperature and humidity, the addition of sensors for radio frequency interference, movement of the monitor to a new location, changing the threshold settings to increase or decrease sensitivity, or the addition of current transformers.
What are the basic elements of a power quality survey?
A power quality survey helps to identify and resolve power-related problems in equipment or facility systems. It represents a systematic, comprehensive approach to problem-solving and typically incorporates the following elements:
1. Planning for a survey to be successful, appropriate planning efforts must be performed. The two most important are determining the survey’s objectives and its scope of activities. Typical objectives include solving a particular equipment performance problem, identifying and correcting sources of interference in a facility, identifying the overall power quality for a facility, or establishing the suitability of available power before installing new equipment. The scope of the survey will be affected by factors ranging from the size of the facility and complexity of its electrical system to the number and length of equipment event logs, the degree of involvement of managerial levels, and the number of power monitors and their placement.
2. Survey Preparation
Once the design of the survey has been completed, preparation for the implementation will include the following:
• Data collection and documentation
• Assembly of required tools, such as a power monitor, circuit tester, multi-meter and infrared scanner
• Site inspection will include both visual and physical inspection
• Placement, connection and setup of power monitoring system
3. Data Analysis
Based on data gathered through the preparation phase, the following should be analyzed:
• Review physical inspection data
• Review site history and equipment event logs
• Plot power monitor event summaries
• Compare power events to equipment event logs
• Compare events to equipment performance specifications
• Extract key power monitor events
• Classify key power monitor events
• Confirm power monitor event correlation
• Identify cause of event
• Design and implement solution to event