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Numerous systems and types of equipment in a typical business facility are vulnerable to power quality disturbances. Among them:
- Computer Equipment
Unlike 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 Equipment
Increasingly 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 Control
Computer-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.
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 undervoltage 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 coltage 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 undervoltage 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 interruptionsoccur 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 traipping 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 panelboard or other location, which represents a safety hazard and diverts part of the return current flow through the grounding conductor.
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 nonsinusoidal 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, undervoltage circuit activation, motor failure and nuisance tripping. Nonsinusoidal 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.
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 Sag
A 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 recloser 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 Drives
Intermittent 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 Spikes
Rectifier 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 Lockups
In 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 rebonding 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.
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 failurewas 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 lockupson 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 errorsat 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.
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 Distortion
Repeated 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 Loops
At 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.
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.
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:
- Predictive Maintenance
By “seeing” the continuous performance of equipment and systems, cost-efficient maintenance procedures can be developed and implemented.
- Cost Management
Power 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 Management
Power 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 Investments
Planning 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 Control
Power 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.
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
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.
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.
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, loose 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.
Imaging systems represent a major advance in medical diagnostic and security operations. The technology, which includes computer tomography (CT scan), magnetic resonanace 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 hardward. 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.
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, multimeter 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