Origins of PQ Problems and Harmonics
Power disturbances can originate from many sources external and internal to a facility’s electrical power distribution system. External sources are
• Power system faults
• Lightning
• Switching
• Surges
• Accidents involving electric power lines and feeders
Examples of internal sources are
• Line and capacitor switching
• Motor starting or switching of large inductive loads
• Harmonic producing loads (linear and nonlinear (solid-state and electronic) loads)
The mechanisms involved in generating electrical disturbances often deter- mine whether occurrence of disturbances is random or repeatable, unpredictable, or easy to find. Untrained users often attribute power disturbances to the utility source. However, recent Electrical Power Research Institute (EPRI) studies indicated that most (up to 80%) electronic system malfunctions attrib- utable to power disturbances are the result of electrical wiring and grounding errors, or interactions of loads within the facility’s distribution system.
A brief description of some of the major sources of power disturbances follows:
Power system faults: Power system faults can cause a momentary voltage reduction to a complete loss of power lasting for a few cycles, seconds, minutes, hours, or days. Power system faults may be classified as temporary or permanent. Usually the temporary faults are confined to overhead distribution lines where a line may suffer a momentary fault which will open the circuit breaker. However, the circuit breaker will reclose immediately to restore the circuit. Permanent faults are confined usually to underground feeders. Due to their location, the detection and repair of these types of faults require a considerable amount of time. Also, the power system faults may result from power apparatus failure such as transformers, circuit breakers, etc. which require a longer time to repair or replace.
Lightning surges: Direct lightning strikes to the power system conductors cause overvoltages near their points of impact. Direct hits inject the total lightning surge into the system. As a result, current amplitudes can range from a few thousand amperes to a few hundred thousand amperes. The rapid change of current through the impedance of the conductors produces a high voltage drop, which causes secondary flashover to ground. This diverts current even in the absence of an intentional diverter. Lightning strikes also can activate lightning arrestors and/or surge arrestors. A flash- over of line insulators can trip a breaker, with reclosing delayed by several cycles, causing a power interruption. The power system also can be affected indirectly by lightning. These effects include overvoltages in conductors and ground potential rises in grounding grids or the earth.
Load switching and surges: Load switching forms a transient disturbance whenever a circuit containing capacitance and inductance, such as capacitors, starting motors, or switching feeders, is switched on or off. In these circuits, the currents and voltages do not reach their final value instantaneously. The sever- ity of such disturbances depends on the power level of the load being switched and on the available short-circuit current of the power system. Switching large loads on or off can produce long-duration voltage changes beyond the immediate transient response of the circuit. More complex switching can produce surge voltages reaching 10 times the normal circuit voltages, involving energy levels determined by the power rating of the elements being switched. Also, energizing loads, such as large motors, may cause voltage dip that can affect operation of microprocessor-based equipment.
Linear and nonlinear loads: The power system harmonic problem is an old problem and, in many instances in the past, we have been able to go around it and reduce its effects. The harmonic producing linear loads are devices such as transformers, generators, motors, electromagnetic ballasts, and saturated magnetic devices that have been around a long time. These are discussed in more detail in Section 12.4. The nonlinear loads are generally classified as those devices that are electronic and solid-state devices used in power conversion and control. It is clear that nonlinear loads draw nonsinusoidal currents from the power system, even if the power system has a perfect sinusoidal waveshape. These currents produce nonsinusoidal voltage drops in the system’s source impedance which distorts the sine wave produced by the power source. A typical nonlinear load is a direct current (DC) power supply with capacitor-input filter. They are used in most computers and draw current only at the peaks of the voltage sine wave. Nonlinear loads typically result in harmonic distortions (HDs) in the power system. These loads can be broadly classified into four categories as follows.
1. Power electronic devices: Power electronic devices are being employed in small appliances to huge converters on the transmission system. Typical applications of power electronics include switch-mode power supplies (SMPS), adjustable speed drives (ASDs), electronic ballasts, and the like.
2. Saturatable devices: Most saturatable devices are transformers which generate harmonics due to the nonlinearity of the transformer excitation. These harmonics are small unless the transformer is overexcited due to high voltage magnitudes.
3. Arcing devices: Arcing devices are used most commonly in fluorescent, high and low pressure sodium and mercury-vapor lamps. Other types of these devices include arc furnaces or arc welders.
4. Electrostatic discharges (ESDs): An ESD buildup results from a rub- bing action between two materials (solid or liquid) of different sur- face energy characteristics. This is due to an absence of a conductive path between the two materials. The ESD is quickly released when a conductive path (discharge arc) is established. Such discharges can be harmful to semiconductor devices in sensitive electronic equipment. Discharge voltages often range from 5 to 40 kV.
Grounding design and installation: Improperly grounded systems which have multiple ground points are common causes of PQ disturbances. Grounding systems which do not have sufficiently low ground impedance do not allow the proper amount of current flow necessary for the operation of the circuit protection devices, thereby compromising the safety of personnel and equipment. Such systems also cause failures in electronic equipment due to leakage currents. Leakage currents created by power line noise, coupled with high-ground impedances, cause voltage to develop on the ground conductor that can trigger a failure in electronic equipment. A ground system with multi- ground points can create multiground loops and impose stray currents on the logic chips of microprocessors. Therefore, it is essential that the design and installation of earth grounds and equipment grounds should be done carefully. Since the ground system also serves as an equal potential reference between peripherals, an improperly designed ground can affect microprocessor logic and inject unwanted signals. The logic circuitry of a microprocessor uses the ground system as a zero conductor. See Section 12.7 for a more detailed discus- sion on grounding design and installation.
Wiring design and installation:
Wiring design and installation problems can be classified as follows:
1. Problems involving the hot, neutral, and ground wires
2. Missing connections, improper connections, loose connections, open grounds, N–G shorts, two hot wires in an outlet, reversed polarity
3. Lack of an isolated ground (IG) receptacle when called for
Because microprocessors use the ground wire as the zero-voltage reference, stray currents imposed upon it can change information and damage microprocessor components. Additional power distribution problems can occur because many pieces of equipment typically are connected together through the building’s grounding system, including conduit or data cables. If the ground paths of individual pieces of equipment are not isolated from one another, currents carried on one can affect another’s operation. When a piece of equipment is plugged into a standard wall receptacle without an IG designation, its ground wire is immediately connected to every other piece of equipment in the building by means of building conduit. This is similar to the manner in which a large radio antenna picks up radio signals it was never meant to receive. Data cables are extremely sensitive to such cross talk.