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188 8-3 Contactor and relay troubleshooting chart (Continued)
189 8-3 Contactor and relay troubleshooting chart (Continued)
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CHAPTER
9 Troubleshooting Power Quality Problems
power quality problem is any change of voltage, current, or frequency that results in failure or reduced performance of end-user equipment In reallife electrical power systems, voltages and currents are generally not the pure 60-Hz sine waves shown in textbooks (Figure 9-1) Instead, the waveform is typically distorted by voltage transients, harmonics, and other phenomena (Figure 9-2) These waveforms can be displayed on the screens of power monitors and other instruments to diagnose power quality problems Power quality problems can be caused by many factors:
Voltage levels (steady state) and voltage stability (surges and sags) Current balance (phase loading) Harmonics Power factor Grounding Overheated terminals and connections Faulty or marginal circuit breakers 191
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9-1 Ideal sine waveform representing voltage or current
Monitoring
Recording monitors are typically installed to record power system characteristics over a period of time, such as 24 hours, or 7 days This long-term monitoring provides information on whether a power quality problem was caused by a one-time random event, or a repetitive recurring event Often, power quality
9-2 Sine waveform distorted by power quality problems 192
problems are not caused by a single event, but by a combination of factors (such as voltage drop, utility transients, harmonics, and improper neutral-to-ground connections) Power can be monitored at different locations in an electrical power system: At the load: Placing a monitor at the branch circuit supplying a motor or other piece of utilization equipment analyzes the power quality at the point of use At the distribution equipment: Placing a monitor on the feeder to a panelboard or motor control center (MCC) analyzes the power quality in an entire section of a building At the service: Placing a monitor at the incoming service conductors to a switchboard or other service equipment analyzes the power quality in an entire building (Figure 9-3) This is where capacitors are typically installed to improve power factor for the reason of avoiding utility penalty charges
Voltage Levels and Stability
Voltage Levels
Check voltage levels at the main panel terminals and each branch circuit Voltage at the panel should ideally be 120/208 or 277/480 V, three-phase, four-wire Voltage at receptacles or utilization equipment may be lower due to voltage drop on branch circuits, but should ideally be no less than 115/200 or 265/460 V 193
9-3 Service equipment: main distribution panel (Courtesy of Schneider Electric Company)
For safety, take voltage measurements on the load side of main or branch circuit breakers whenever possible This precaution helps protect the test instrument and operator from potential fault currents on feeders (Figure 9-4) Low voltage causes electric motors to run slower than their design speed, incandescent lights to burn dimmer, starting problems for fluorescent and HID lamps, and performance problems for electronic and data devices Overvoltage causes motors to run faster, shortens incandescent lamp life, and can damage sensitive electronic components
Volt
Volt
9-4 Safe voltage measurement technique at panel board 195
Most electrical and electronic equipment are designed to tolerate a range of 10 percent of rated voltage and still operate satisfactorily However, panelboard voltages in the range of 115/200 or 265/460 V will probably translate into unacceptably low voltages at receptacles or utilization equipment, due to additional voltage drop on the branch circuit conductors Common causes of low voltage at the panel are low tap settings at transformers, feeder conductors that are too long or too small, and loose connections The first condition results in lower supply voltage; the latter two result in higher impedance that increases voltage drop Voltage Stability Voltage sags can be caused by either loads on branch circuits, or elsewhere in the distribution system, including utility-generated sags and brownouts This is most easily analyzed using an instrument such as a power quality analyzer that measures both voltage and current simultaneously Take measurements at each branch circuit in the panelboard Voltage sag occurring simultaneously with a current surge usually indicates a problem downstream of the measurement point This would be a load-related disturbance on the branch circuit Voltage sag occurring simultaneously with a current sag usually indicates a problem upstream of the measurement point, originating elsewhere in the distribution system Typical source-related disturbances include large three-phase motors coming on line (starting) or sags in the utility network 196
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