What is a Capacitor Bank : Types & Its Connections
- May 25, 2022
The active power which is generated from the power plant can be expressed in MW (Mega Watts) otherwise kW (KiloWatts). Even though the reactive power within alternating power system can be expressed in Mega VAR or Kilo VAR. The reactive power demand is mainly created from the inductive load which is connected to the electric system. So, these loads are usually electromagnetic circuits of transformers, motors, fluorescent lights, distribution networks, etc.
This reactive power must be compensated properly otherwise, the ratio of actual power utilized through the load, to the whole power of the electric system will become very less. Here whole power is the sum of reactive power & active power. This ratio is called the electrical pf (power factor). If the pf of an electric system is low then a load of ampere for the transmission, alternators, transformers which are connected to the system will become high for necessary active power. Thus the compensation of reactive power will become so essential. So this is usually done through a capacitor bank.
What is a Capacitor Bank?
Capacitor bank definition is when a combination of several capacitors are connected in series or parallel connection with the same rating then it is called a capacitor bank. Generally, an individual capacitor is used to store electrical energy. So once capacitors are increased within a bank then it will increase the energy capacity that is stored within a single device. The basic capacitor bank symbol or diagram is shown below.
In a substation, it is used to enhance the power factor & reactive power compensation. While installing a capacitor bank in a substation, some specifications need to consider. So capacitor bank specifications are voltage rating, temperature rating, KVAR rating, and basic instruction range.
Capacitor Bank Types
Generally, the unit of a capacitor bank is known as a capacitor unit. The manufacturing of these units can be done similarly to 1- phase unit. These units are mainly connected in the form of a star/delta connection to make a whole three-phase capacitor bank. At present most frequently available capacitor units are 1-phase type whereas 3-phase capacitor units are rarely manufactured. There are three types of capacitor banks which are discussed below.
- Internally Fused
- Externally Fused
- Fuse Less
The designing of an internally fused can be done within a particular arrangement. According to its rating, various elements are allied in series and parallel. The protection of each capacitor element can be done separately through a fuse unit. As the name suggests, the capacitor elements, as well as fuse units, are arranged within the same casing. In this type of bank, the size of every capacitor element is extremely small within ratings.
So if any of the capacitor elements are broken down, then there will be no effect within the act of the bank. This kind of capacitor bank can run suitably even one or above capacitor elements are broken.
The main advantages of an internally fused type are, it is very simple to install as well as maintenance is simple. The disadvantage of an internally fused is, once several capacitor elements are failed, then the whole bank needs to be changed. So there is no possibility for the replacement of a single unit.
In an externally fused type, the fuse unit for every capacitor unit is given externally. If any fault occurs in any capacitor unit then the fuse unit will be damaged. When the fuse unit detaches the defective capacitor unit, then this bank will maintain its service without any break.
In this type, the connection of capacitor units can be done in parallel for each phase of the bank. Once one unit fails, then there will be not a lot of effect on the whole bank’s performance. Whenever one capacitor unit is not there within a single phase, then the capacitance of that single phase will be less as compared to the other two phases.
This will affect high voltage in the remaining two phases of the bank. In this type, the identification of a faulty unit can be done through visual inspection once the fuse unit blows. The capacitor unit rating typically ranges from 50 KVAR – 40 KVAR. This is one of the main specifications.
The main disadvantage of this bank is, once any fuse unit fails, then an unbalance can be detected even all the units within the bank are well.
In a useless type, the connection of several fuse units can be done in series to make a capacitor string. These strings are connected in parallel to make a capacitor bank for each phase. After that, three similar phase banks are connected in the connection of star/delta to make a whole three-phase bank.
Through an arrangement of fusing in internal or external, the capacitor strings are not protected. So in this type of system, if any one of the string units fails because of the short circuit or fault, then there is no change within the flow of current throughout this string because there are several other capacitors allied in series connection through this path.
When the short circuit effect within the string unit is small, then the capacitor bank can be accumulated to extend the time before faulty unit replacement. So this is the main reason, why the fuse unit is not necessary to change the faulty unit from the system within the bank instantly once the unit turns defective.
Capacitor Bank Connections
The capacitor bank is connected in two ways like star and delta but most of the time, delta is used. So there is a bit of confusion about which connection is better for a bank. So here we are going to discuss these two connections along with benefits and drawbacks. The main application is power factor correction because, in a 3-phase system, a 3-phase capacitor bank is used for the power factor correction which may be connected in star or delta.
Capacitor Bank in Delta Connection
When these banks are used in delta connection then it is utilized for less to average voltage. The capacitor bank in delta connection can be utilized for high voltage however it is not achievable sometimes as in delta connection; the complete phase voltage is given across every capacitor while in star type connection, it is lesser as compared to applied phase voltage across the capacitor. So, 3 phase capacitor bank wiring diagram using two connections is discussed below.
So if we employ a delta connection at high voltage, then the capacitor’s voltage rating must be high. Consequently, manufacturing of high voltage capacitors is expensive & it is impossible sometimes.
The advantages of a capacitor bank in delta connection include the following.
When the capacitor generates Kilovolt-Ampere Reactive (KVAR) then that is proportional to the square of the voltage applied. So, if the voltage is higher, the KVAR is also more. So the capacitor in this connection will provide high KVAR compared to the bank connected in star connection because, in star type connection, the applied voltage is low compared to delta connection.
The capacitor bank in this connection can flow the harmonic current, thus it can decrease the effect of harmonic within an electrical system. When the bank is connected in delta connection, then it gives a balanced capacitance to every stage of the electrical system & keeps a balanced voltage.
If a capacitor cell within a single phase is not succeeding in the bank, then voltage beyond every phase remains the same, simply KVAR falls.
The disadvantages of a capacitor bank in delta connection include the following.
The main drawback of the capacitor bank in delta connection is, the pressure of voltage across every capacitor is maximum which decreases the capacitor’s life & it may not be utilized in the applications of high voltage.
Capacitor Bank in Star Connection
The star connection-based type is mainly used in the applications of medium to high voltage. In this type of connection, the voltage beyond every capacitor is smaller as compared to the voltage of the phase, so the pressure of voltage beyond the capacitors is less even in the applications of the maximum voltage. In the capacitor bank, there are 2 types of connections used like the following.
- Grounded Star Connection
- Ungrounded star Connection
Grounded Star Connection
In this type of connection, the unbiased point of the bank is stably earthed, which means the neutral should not be insulated toward the BIL level of the complete system. Thus, some price reductions can be realized with this connection. In addition, TRV (transient recovery voltage) may be less harsh within this connection. An error on the 1-phase of the bank will not affect the rise of voltage within the remaining legs of the bank. So a fault on one phase of the capacitor will not affect other phases.
Ungrounded Star Connection
In this kind of connection, the capacitor bank’s neutral point is not connected toward earthing. So this type of connection does not allow the supply of GND currents & zero series harmonic currents.
The advantages of capacitor bank in star connection include the following.
- It is a simple connection
- The voltage pressure across every capacitor is low, thus the capacitor’s life span is high.
The disadvantages of capacitor bank in star connection include the following.
- Star-connected type provides less KVAR than delta-connected type because the voltage across the capacitor is less.
- A star-connected type cannot circulate the harmonic current in an electrical system.
- The ungrounded star-connected type cannot maintain the balance voltage and cannot provide the balance capacitance.
- If a capacitor cell in one phase is failed, the unbalanced voltage occurs in the electrical system.
From the above two connections, the delta connection provides more benefits as compared to the star connection. For a capacitor bank, this connection is suitable so, most of the banks are connected in a delta connection.
Capacitor Bank Calculation
The main purpose of the capacitor bank calculator is to get the necessary kVAR for enhancing power factor (pf) from low range to high. For that, the required values are; current power factor, real power & the value of power factor to be enhanced over the system. So that we can calculate to get the value in kVAR.
If we want to measure the value in VAR or MVAR then the real power in MW/ W is necessary. For instance, if the value is used in kW then you can get the value in kVAR simply. Similarly, it works for MW & W. The required reactive power like Q(kVR) is equivalent to the real power like P(kW). The formula for the capacitor bank calculator is shown below.
- Required Reactive Power (kVAR) = P(kW) * tan (cos-1(PF1) – cos-1(PF2))
- Required Reactive Power (VAR) = P(W) * tan (cos-1(PF1) – cos-1(PF2))
- Required Reactive Power (MVAR) = P(MW) * tan (cos-1(PF1) – cos-1(PF2))
Why Capacitor Bank Testing is Important?
Capacitors banks within the power system provide accurate power factor (pf) correction. So pf correction unit includes different functioning settings based on the installation position. The different factors like time, moisture, change in temperature & harmonics will change the correction of power factor for capacitor banks.
If already connected capacitor banks are not tested properly in a specific time then they will turn incapable to utilize. The capacitor’s operation can weaken; reducing the power factor (pf) of your power system can cause power factor loss. During the testing, ANSI/ IEEE or standard is employed. There are three types of tests done like type tests/design tests, routine/production tests & pre-commissioning & field Tests.
The applications of capacitor banks include the following.
- Capacitor banks are mainly used to enhance the electrical supply quality & also to enhance the power systems efficiency.
- This is most frequently used for the correction of AC power supply in industries where electric motors and transformers are used.
- As this bank uses an inductive load, then they are vulnerable to power factor lags & phase shifts within the power supply, so it results in a system efficiency loss.
- When these are used in the system then the power lag can be solved at less cost for the organization by making some changes in the power grid
- These are used in radars, pulsed lasers, Marx generators, detonators, coilguns, fusion research, nuclear weapons, electromagnetic railguns, etc.
- Generally, capacitor banks decrease the phase difference among the current & voltage.
- The power factor (pf) can be maintained close to unity.
Thus, this is all about an overview of a capacitor bank and its working with the connection. Generally, these are used to power factor or pf correction and compensation of reactive power. As compared to the inductive type motors, capacitors include the reverse effect where it stops a maximum flow of current flow, so that this will reduce the power bill. Here is a question for you, what are the two connections used in capacitor bank wiring?