The size of power quality problems must be deﬁned by estimating the cost of disruptions caused to distribution system equipment and the extent of its sensitivity to power quality. The equipment may be less sensitive to disturbances, allowing it to ride through the disturbances, that is, no need to apply a solution. Otherwise, a solution is needed, for example, by installing power quality devices that suppress or counteract the disturbances.
Solutions to power quality problems
The power quality devices are used to protect the electric equipment or eliminate the source of disturbances or mitigate the effect of disturbances. The devices that are commonly used for this application include the following:
- Surge protection devices (SPDs)
- Shielding (SH)
- Uninterruptible power supplies (UPSs)
- Voltage regulators (VRs)
- Series capacitors (SCs)
- Capacitor voltage transformers (CVTs)
- Wiring and grounding (W&G)
- Static var compensators (SVCs)
- Energy storage system (ESSs)
- Backup generators (BCKGs)
- Isolation transformers (ITRs)
Surge Protection Devices (SPDs)
The electric equipment in distribution systems may be exposed to internal or external surges. Internal surges are generated within a facility by the user’s equipment. They result from switching processes, for example, switching inductive or capacitive loads, fuse, or breaker opening in an inductive circuit.
External surges are generated outside a facility and brought into the facility by utility wires. They result from fuse operation, power system switching, and lightning.
SPDs protect the equipment against these surges by limiting the amount of undesired surge energy that reaches the equipment. The surge energy is diverted to a path rather than the equipment itself (neutral or earth).
The SPD is a nonlinear element acting as an isolating switch in normal conditions where its resistance is very high. When the voltage increases and reaches a certain value called “clamping voltage,” the SPD will change rapidly (in nanoseconds) from a very high resistance mode to a very low resistance mode. Then, the majority of surge energy is directed through SPD, and most of this energy is dissipated in its internal resistance.
Shielding is the use of a conducting and/or ferromagnetic barrier between a potentially disturbing noise source and circuitry. Shields are used to protecting cables (data and power) and electronic circuits. They may be in the form of metal barriers, enclosures, or wrappings around source circuits and receiving circuits.
Uninterruptible power supplies (UPSs)
The UPS is an alternative power source to supply power to the load during interruption or outage of the main power source (e.g., utility source). It includes a rectiﬁer circuit to convert AC input power into direct current (DC) power. The DC power charges a set of batteries to store energy and an inverter to convert the DC stored energy back onto AC power for the load. Six or twelve or twenty-four diode bridges can constitute the rectiﬁer circuit depending on the desired level of wave distortion. From the point of view of frequency stability as well as voltage stability, the inverter that constitutes the UPS generator has performance superior to that of the mains. It is designed to generate sinusoidal voltage even when supplying nonlinear loads, that is, dealing with highly distorted currents.
During normal operation, the utility supplies power to both the load directly bypassing the UPS unit and to the UPS to charge its batteries via the rectiﬁer circuit. In an emergency operation, for instance, an outage of a utility power source, the UPS supplies power to the load fast enough (a few milliseconds) to avoid any damage resulting from load interruption. This necessitates using an electronic transfer switch to change the power source to load.
UPSs are effective for microprocessor-based loads such as computer systems and PLCs where the loss of data is avoided. On the other hand, they have deﬁciencies where the transfer switch and rectiﬁer are exposed to line disturbances in normal operating conditions. In emergencies, the operation time of UPS is limited by the capacity of batteries.
The design of UPSs and, generally, ESS (energy storage system) depends on the required mode of operation. Three modes of operation are considered: Standby (off-line), on-line, and line-interactive. The standby mode of operation means that the ESS operates only during the interruption time, while it operates full time in the case of the online mode of operation. Line-interactive mode of operation includes both of these two modes.
Voltage regulators (VRs)
The function of the voltage regulator is to maintain the voltage at load within preset limits. During voltage sags, VRs increase the voltage to the desired level for sensitive loads, and, conversely, during overvoltages or swells, they decrease the voltage. Mostly, the usage of VRs is to mitigate the effect of sag events.
The common type used for regulating the voltage is a motor-driven variable-ratio autotransformer. A motor is used to change the location of a slider on transformer winding providing a change of transformer ratio to increase or decrease the voltage levels. The response time is slow, which may be inadequate for some loads and may not correct large short-term voltage variations.
Series capacitors (SCs)
Series capacitors are installed in transmission systems mainly in order to increase the power transfer capability and to reduce losses by optimizing load distribution between parallel transmission lines.
Series capacitors are also installed in distribution systems. Here, the main reason is to improve the voltage stability of the network. Series compensation of a network positively affects the voltage and reactive power balance. When the load current passes through the capacitor, the voltage drop over the capacitor varies in proportion to the current. The voltage drop is capacitive, e.g., it compensates for the inductive voltage drop, which also varies with the load current. The result is an automatic stabilizing effect on the voltage in the network.
Simultaneously, series capacitors generate reactive power, the power factor in the network is improved, whereby the line current and the line losses are reduced and the load capacity is increased. The generated reactive power varies proportionally to the square of the load current. Thus, the reactive power is automatically regulated.
Capacitor voltage transformers (CVTs)
A capacitor voltage transformer (CVT), also known as capacitor-coupled voltage transformer (CCVT), is a transformer used in power systems to step down extra-high voltage signals and provide a low voltage signal, for metering or operating a protective relay. Capacitive voltage transformers also ensure suitable electrical insulation between high voltage and low voltage measuring equipment and are available with accessories for PLC transmission and power quality measurement.
Wiring and grounding (W&G)
Nearly 80 % of the power quality problems are due to deficiencies in the wiring and more specifically to grounding, problems that are solved with relatively low cost if it is compared with the costs of the mitigation equipment. The grounding is the intentional linking of a conductor to earth, which is direct if it is carried out without interposing any impedance, otherwise, it is indirect. The primordial objective of that grounding is the protection of human life, equipment, and electric systems. The dimensioning of the power and protection wiring fulfilling the local and international specifications, not always avoid power quality problems, since in such standards their special requirements are not generally taken into account. The wiring mistakes are frequently so simple as caused by an unfastened connection, neutral of insufficient cross-section, incorrect grounding, or a damaged conductor. The priority in the design of the wiring is security and personal protection so that any necessary modification from the point of view of power quality should not put personnel at risk.
Static var compensators (SVCs)
Static var compensator (SVC) allows grid operators to gain accurate control of reactive network power, increase power transfer capability and improve the steady-state and dynamic stability of the grid. SVC controls transmission line voltage to compensate for reactive power balance by absorbing inductive reactive power when voltage is too high and generating capacitive reactive power when voltage is too low. Compared to the investment required for additional transmission networks, SVC provides customers with a flexible solution that has minimal infrastructure investment, low environmental impact, rapid implementation time, and improved return on investment.
In its simplest form, the SVC consists of a TCR in parallel with a bank of capacitors. From an operational point of view, the SVC behaves like a shunt-connected variable reactance, which either generates or absorbs reactive power in order to regulate the voltage magnitude at the point of connection to the AC network.
Energy storage systems (ESSs)
The increasing integration of distributed generation units into power grids has resulted from the degradation of the quality of supplied power in the form of deviations in current, voltage, and frequency from their standard values, leading to failures or miss operation of equipment. Energy storage systems (ESS) are being considered as a potential solution for this problem since they can regulate the power being injected into the grid, from the distributed generation units, making it more stable. ESS integration can improve power quality by providing constant power control, smoothing control, and energy shifting.
Backup generators (BCKGs)
In large industries and for long-duration interruptions, backup generation is essential to supply at least the critical loads. It is common to use a diesel-generator set with rating sufﬁcient for feeding these critical loads such as emergency lighting systems, electric lifts, industrial processes that cannot withstand long interruption, and hospitals.
Usually, the backup generator is used as a standby unit and connected to the distribution system of the industry at the main LV entrance. An automatic transfer switch (ATS) can be used to automatically transfer the power source from the utility incoming feeder to the backup generator in emergency cases. An electric interlock is provided between both sources to avoid parallel operation. Some of the outgoing feeders of uncritical loads may be disconnected to keep the power demand within the generator rating that is mostly less than the rating of the utility source.
In many cases, the main LV bus bar is sectionalized into two sections with a bus coupler to increase reliability. Each section is fed from a different utility source. The outgoing feeders of critical loads are preferred to be connected to one of these two sections that are supplied by utility sources in normal operation and backup generators in emergency operation.
Isolation transformers (ITRs)
They are generally composed of two separate windings with a magnetic shield between these windings to offer noise control. The noise can be transported to the electric device by electromagnetic coupling (EMC) in two basic ways: a differential mode noise and a common mode noise.
The ITR is connected between the power source and the electric device. Therefore, it carries the full load current, and thus must be suitably sized. The main beneﬁt offered by ITRs is the isolation between two circuits, by converting electric energy to magnetic energy and back to electric energy, thus acting as a new power source.
When the harmonic levels are too high, a harmonic ﬁlter solution is needed. Traditionally, passive ﬁlters, active ﬁlters (AFs), and recently hybrid ﬁlters have been used.
A passive ﬁlter consists of a series circuit of reactors and capacitors. Harmonic currents generated by, for example, a frequency converter are shunted by this circuit designed to have low impedance at a given frequency compared with the rest of the network.
Active filters are systems employing power electronics. They are installed either in series or in parallel with the nonlinear load to provide the harmonic currents required by the nonlinear load and thereby avoid distortion on the power system.
The two types of ﬁlters presented above can be combined in a single device, thus constituting a hybrid ﬁlter. This type of ﬁltering solution combines the advantages of the existing systems and provides a high-performance solution covering a wide power range.
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