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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, undervoltage 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 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.
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 nonsinusoidal 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.
Panelboards 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, panelboards 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.
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.
Numerous systems and types of equipment in a typical business facility are vulnerable to power quality disturbances. Among them:
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:
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.
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:
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:
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 Signature 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 Signature System can be used for a range of other uses. Among them:
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:
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: