Why shunt capacitor banks?
It’s quite simple, using shunt capacitor banks to supply the leading currents required by the load relieves the generator from supplying that part of the inductive current. Both distribution and transmission systems benefit due to the application of shunt capacitors include: reactive power support, voltage profile improvements, line and transformer loss reductions, release of power system capacity, savings due to increased energy loss.
Considerations in locating capacitors
Shunt capacitors provide reactive power locally, resulting in reduced maximum kVA demand, improved voltage profile, reduced line / feeder losses, and decreased payments for the energy. Maximum benefit can be obtained by installing the shunt capacitors at the load.
This is not always practical due to the size of the load, distribution of the load, and voltage level.
Let’s discuss now the most important locations where shunt capacitor banks are usually being installed.
- Pole-mounted capacitor banks
- Shunt capacitor banks at EHV levels
- Substation capacitor banks
- Metal-enclosed capacitor banks
- Distribution capacitor banks
- Fixed capacitor banks
- Switched capacitor banks
- Installation of capacitors on the transformer LV side
- Installation of capacitors on transformer HV side
- Mobile capacitor banks
These type of capacitors are probably the most visible and widely spotted by people. In the distribution systems, the power factor correction capacitors are usually installed on the poles. These installations are similar to the pole-mounted distribution transformers.
The interconnections are made using insulated power cables. Pole-mounted capacitor banks can be fixed units or switched units to meet the varying load conditions. The voltage rating can be 460 V–33 kV.
The size of the capacitor units can be 300–3,000 kVAR. A typical pole-mounted installation of a capacitor bank is shown in Figure 1.
In the case of capacitor banks, the following components are installed on a stable platform:
- Capacitor banks
- Vacuum or oil switches
- Controller to switch the capacitor units
- Control transformer
- Fuse units along with mounting
- Distribution class surge arrester
- Junction box
- Current limiter or harmonic filter reactor
Usually extra-high voltage (EHV) lines are used to transmit bulk power from remote generations to load centers. These long lines tend to produce significant voltage drops during peak loads. Therefore, shunt capacitors are used at the EHV substations to provide reactive power.
Sometimes these capacitor banks are switched as and when required. A typical high voltage harmonic filter bank is shown in Figure 2.
When large reactive power is to be delivered at medium or high voltages, then shunt capacitor banks are installed in substation locations. These open stack shunt capacitor units are installed for operating voltages 2.4–765 kV.
A typical substation type capacitor bank installation is shown in Figure 3. At high voltage levels, the shunt capacitor banks are used for reactive power support, voltage profile improvement, reduction in line, and transformer losses.
These shunt capacitor banks are also installed in select substations after careful load flow and stability analysis.
When the capacitor banks are installed in industrial or small substations in indoor settings, then metal-enclosed cabinet type construction is employed. Such units are compact and require less maintenance. A typical metal-enclosed capacitor bank is shown in Figure 4.
The life expectancy of these type of units is longer because they are not exposed to external environmental factors such as severe heat, cold, humidity, and dust.
Distribution capacitors are installed close to the load, on the poles, or at the substations. Although these capacitor units provide reactive power support to local load, they may not help reduce the feeder and transformer losses.
Low voltage capacitor units are cheaper than high voltage capacitor banks. Protecting distribution capacitor banks from all types of fault conditions is difficult.
Pad-mounted capacitor banks are usually available in ratings of 15-25kV.
Pad-mounted capacitor banks have broad applicability through out the industry, including non-utility facilities. The capacitors are installed to boost the voltage back within the operating tolerance of the system and, thereby, provide voltage stability.
Without capacitors, load circuits will operate at reduced voltage, motors will run slower and overheat, lights will not burn as bright, relays in process industries will drop out, etc., creating end-user system disturbances.
Capacitors extend the range of substations by allowing feeder circuits to have longer runs of cable. Extending the range of substations also means that capacitors serve to increase network capacity. For individual customer facilities, it may be necessary or desirable to provide improved voltage regulation atthe installation.
For this purpose, on-site pad-mounted capacitor banks near customer loads provide power factor correction.
Pad-mounted capacitor banks have three (3) major advantages:
- Voltage stability,
- Increased network capacity, and
- Power factor correction.
These all combine to provide cost savings through lower system losses. For application in the electric industry, individual capacitor units are rated in kvars (kilovars-amperes reactance) and are applied in banks called shunt- capacitor banks.
For underground distribution systems, capacitor banks are installed in pad-mounted enclosures as small, distributed installations that are connected to main-primer feeder circuits at a considerable distance from the substation. These distributed banks can be fixed on the circuit or switched on and off as dictated for system stability.
Pad-mounted capacitor banks have valued advantages for the underground distribution system:
- They extend the ability of the power supply system to support longer lines to the load.
- Growing systems into newer developments are more typically served underground and pad-mounted capacitor banks fit this growth segment.
- The enclosed components offer a more aesthetic appearance than exposed overhead components, making them well suited for utility, industrial, commercial, and institutional installations.
- The enclosure affords considerable protection from the environmental flora and fauna.
- Access to components is easier to achieve at ground level than on a pole.
- Component integration can be arranged in a fairly low-profile enclosure.
- Underground circuits are less prone to storm damage.
In distribution and certain industrial loads, the reactive power requirement to meet the required power factor is constant.
In such applications, fixed capacitor banks are used. Sometimes such fixed capacitor banks can be switched along with the load. If the load is constant for the 24-hour period, the capacitor banks can be on without the need for switching on and off.
Fixed MV capacitor banks are usually 50 to 4800 kvar capacity, insulation levels from 7.2 to 36 kV.
The most common configurations are parallel connection of three-phase units (internal star connection and internal fuses) and double star configuration with isolated neutral using single-phase capacitors. Other configurations are available.
In high voltage and feeder applications, the reactive power support is required during peak load conditions. Therefore the capacitor banks are switched on during the peak load and switched off during off-peak load.
The switching schemes keep the reactive power levels more or less constant, maintain the desired power factor, reduce overvoltage during light load conditions, and reduce losses at the transformers and feeders.
The switching controls are operated using one of the following signals:
- Voltage: since the voltage varies with load.
- Current: as the load is switched on.
- kVAR: as the kVAR demand increases, the capacitor banks can be switched on and vice versa.
- Power factor: as the power factor falls below a predetermined value, the capacitor banks can be switched on.
- Time: sometimes the capacitor banks can be switched on using a timer and switched off at the end of a factory shift.
In Figure 6(a), one capacitor bank is switched by a circuit breaker. Figure 6(b) shows one fixed capacitor and two automatically switched capacitor banks. The circuit breakers must have suitable short-circuit ratings to handle the energization and back-to-back switching requirements.
Table 1 – Selection of capacitors in binary order for power factor control
|Item||Bit 0||Bit 1||Bit 2||Remarks|
|1||0||0||0||All switches are open.|
|2||1||0||0||Switch 1 is closed.|
|3||0||1||0||Switch 2 is closed.|
|4||1||1||0||Switches 1 and 2 are closed.|
|5||0||0||1||Switch3 is closed.|
|6||1||0||1||Switches 1 and 3 are closed.|
|7||0||1||1||Switches 2 and 3 are closed.|
|8||1||1||1||All three switches are closed.|
Figure 6(c) shows the capacitor bank switching arrangement with one automatic and two non-automatic circuit breakers.
In certain applications with random variations in the reactive power requirements, the capacitors are to be switched in and out using a binary arrangement. Such a scheme is shown in Figure 6(d). The corresponding choice of capacitors is listed in Table 1 above.
This arrangement can be used to switch seven steps of capacitor banks using three capacitor banks and three circuit breakers.
The selection requires careful programming and is achievable using programmable controllers.
Figure 6(e) shows another scheme where one automatic circuit breaker can switch three capacitor banks equipped with fuses and non-automatic circuit breakers. The capacitor banks can be of equal size.
The capacitor bank is installed close to the load to provide reactive power locally. In a system in which a large number of small equipment are compensated, the reactive power demand may fluctuate, depending on the load.
Harmonics in the system should be checked to determine if the capacitor and the reactance of the power transformer are in series and create resonance. A typical scheme is shown in Figure 7.
This type of installation provides the same kind of reactive power compensation as a low voltage capacitor bank. The installation can be safe from overvoltage if it is switched on and off, depending on the reactive power requirement. One of the main advantages of high voltage capacitor installation is that the losses in the step-down transformer are reduced.
The cost of a high voltage capacitor scheme will be higher. As in the low voltage capacitor scheme, the possibility of over-compensation and resonance issues should be checked.
A typical scheme is shown in Figure 8. Sometimes it may be possible to correct the power factor at the individual load location. The relative advantages and disadvantages are presented in Table 2.
Table 2 – Power factor correction on the HV side versus at the load location
|On the Transformer Primary Side||Capacitor at the Load Location|
|Need one capacitor bank.||Need three capacitor banks.|
|One physical location.||Three physical locations.|
|Rack mounted outdoor or metal enclosed indoor.||Metal enclosed indoor or pole mounted.|
|Easy to maintain.||Multiple locations require more maintenance.|
|Can be designed as a tuned filter.||Filtering with transformer.|
|Controlled resonant point.||Multiple resonant points.|
|Stable system impedance from filter location.||System impedance sees resonance points in the impedance mode.|
|Relatively low cost due to one location.||Higher cost due to multiple locations.|
|May not be able to switch based on load changes.||Load changes can be handled.|
|Need to have one circuit breaker to handle capacitor switching.||Need to have circuit switches to handle capacitor switching.|
When there is need to apply shunt capacitors in distribution systems to relieve overloaded facilities until permanent changes are made, portable capacitor banks can be used. These banks are available in three-phase and single-phase units.
A typical mobile capacitor bank mounted on a truck is shown in Figure 9.
Mobile capacitor banks are unique and designed specifically to meet the requirements of the customer. Because mobile capacitor banks are often designed to be placed anywhere on the customer’s transmission or distribution system, the banks are designed to be fully self-contained.
Depending on customer requirements, mobile capacitor banks can be designed on a single- or multi-trailer platform.
10.1 Special considerations
When utilizing mobile capacitor bank solutions, it is important to consider national, state and local requirements for the transportation of electrical equipment. Special permitting and/or escort vehicles may be required for transportation of the mobile capacitor bank.
- Power System Capacitors by Ramasamy Natarajan
- Maintain required voltage with reactive power solutions by Eaton
- Pad-mounted capacitors 15-25 kVby Federal pacific