Dranetz Power Quality FAQ’s
The collection of Dranetz FAQ’s on power quality is a great information resource whether you have a question or just want to browse. Dranetz is proud to employ the best and brightest PQ professionals in the industry, and they actively enjoy authoring helpful and informative documents to help educate our customers. Please click on the questions below for the answers to the FAQ questions.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Unidirectional Impulse
- Oscillatory Impulse
- Repetitive Even
- Common and Normal Mode Event
- 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