Sections:
Definition |
Types |
Methods |
Equipment
Power Quality
p. 18-110
p 23-2
Definition. Power Quality is the characteristics of the delivered power. There are two categories of how Power Quality affects equipment:
1. Equipment Operating Problems range from a disruption in the programmed operation of computers or microprocessors-controlled equipment to the drop-out of motor starters.
2. Component Damage caused flashover due to excess electrical stress.
PQ Problems are classified into two general types (a) created by the switching of electrical loads or entire circuits; and (b) the interaction of electrical equipment and the electrical supply systems.
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Definition |
Types |
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Equipment
PQ Problem Types
1. Extended Outages result of a permanent electrical fault, usually from 30 cycles up to several days. Thus is is necessary to design the isolation of permanent faults and the resultant outages to the smallest area possible.
2. Momentary Interruptions are temporary total loss of voltage caused by the operation of automatic overcurrent protective devices.
3. Voltage Sags are voltage levels lower than nominal for a periof od 2s or less resulting from (a) the operation of large loads such as motors or electrical welders on the same circuit; (b) bus votlage collapse on the electric supply distribution circuit from motor starting and (3) electrical faults on circuits supplied from the same source.
4. Voltage Swell are faults which occur in one-phase of a three-phase, four-wire system usually lasting less then several seconds up to a minute.
5. Surges and Sytem Disturbances are temporary voltage increases caused by lightning or the switching of large loads.
6. Impulse and Noise. Impulse is the overvoltage conditions lasting for less than half a cycle, also known as spikes or transients. Noise is repetitive impulse superimposed on the power sine wave. THe sources of which are radio transmitters, flourescent lamps, battery chargers, computers, or losse electrical connections.
7. Harmonics basically are forms of electrical noise, the superposition of signals at multiples of the fundamental power frequency on the power sine wave.
7. Frequency Deviations are minute changes in the frequency rating of power supply.
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Methods to PQ
Power Quality problems can be minimized through the following:
A. Site Surveys Prior to Equipment Installation will show the need for power conditioning or other actions before installting sensitive electrical equipment. This involves Power Quality Monitoring which collects information on voltage, regulation, harmonic distortion levels, outages, momentary interruptions, and other aspects of steady-state and transient conditions; accurate and detailed logging of equipment operating problems and unscheduled downtime. Downtime logs should indicate which equipment, the date and time, and the duration of the problem.
B. Electrical System Design Complete eleimination of all Power Quality problems is impossible except in a controlled laboratory conditions. Thus, design of the electrical system is of prime importance:
(1) Power Delivery Circuits The probability of extended outages or momentary interruptions on a transmission and distribution circuit is roughly proportional to the number of total line miles on the circuits. Therefore, short, directly routed circuits, with a minimum number of taps, is likely to have a better performace than long, complicated lines. Dedicated Circuits are direct circuits without dispersed loads between the source and end load. Alternate Feeders and Automatic Transfer Switches are also, more often than not, used.
(2) Secondary Circuits The more feeders, the more reliable is the system. In some utilities, power factor penalties are imposed for users with low power factor, thus capacitors are added to circuits of inductive loads provide a leading reactive current, but capacitor banks can also cause surges and harmonics.
C. Investigative Techniques involve the following:
(1) Matching Problem to Events propositions that it is necessary to determine the root cause of the power quality problem to prevent recurrence, else the installation of power conditiong equipment might be necessary.
(2) Inspection of Building Wiring according to applicable codes should involve the following watchpoints: improper grounding, improprt wiring practices (splices).
(3) On-Going Test or Benchmarking involves initial tests to provide a benchmark for determining if conditions deteriorate overtime, plus the conduct of Predictive Tests, or regular internal tests to predict deterioration.
(4) Adjusting Monitoring Equipment Thresholds initial setting are as follows:
Parameter |
Range
Maximum Values |
1. Voltage Regulation |
+5 ±10%, - 15% |
2. Voltage Distribution |
Momentary, undervoltage |
-25 - 30%
duration: <5s
-100%
duration: 4 to 20ms |
Transient Voltage |
+150 - 200 %
duration: < 0.2 ms |
3. Harmonics |
3 - 5% linear |
4. Frequency Deviation |
60 Hz ±0.5 Hz to ±1.0 Hz |
5. Frequency Rate of Change |
1 Hz / second (slow) |
6. Noise |
No standard |
7. Three-phase Unbalance Voltage |
2.5 - 5.0 % |
8. Three-phase
Load Unbalanced |
5 - 20% maximum
at any one phase |
9. Power Factor |
0.80 to 0.90 |
10. Load Demand |
0.75 to 0.85 |
Sections:
Definition |
Types |
Methods |
Equipment
Power Conditioning Equipment
1. Transient Voltage Surge Suppressor is a simple spark gaps with breakdown voltages below the insulating level of the equipment.
2. Isolation Transformer
3. Magnetic Voltage Regualtor is a conventional transformer or autotransformer with automatic tap-changers. They saturate the magnetic field within the core and use feedback to control saturation level and resist sudden change in line voltage.
4. Motor-Generator Sets generated AC or DC at different frequencies. They also provide total isolation from the electric utility.
SCADA / Energy Management System
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