Capacitor Elements
A capacitor element is the fundamental building block of a capacitor unit, consisting of two metal electrodes separated by a dielectric medium. Multiple elements are connected in series or parallel to form the internal circuit of a capacitor unit. To match the system’s rated voltage, several capacitor units are connected in series. The total number of units in a capacitor bank is determined based on its required MVAR rating.
Each capacitor element contains dielectric materials such as polypropylene film or paper film, which are sandwiched between electrodes usually made of thin aluminum foil. Previously, thin metal sheets were used as conductive plates in capacitors. However, these have been replaced by aluminum foil due to its superior electrical and physical properties. As a result, tin sheets have become obsolete, and aluminum foil now dominates the market for this application. The capacitor element is constructed by stacking and folding alternating layers of aluminum foils and dielectric films, creating a compact and efficient capacitive element structure.
It is important to note that multiple dielectric sheets are typically placed between the aluminum foil layers in a capacitor element. Although a single sheet may be sufficient under ideal conditions, multiple sheets are used to minimize the risk of short circuits. Such faults can occur if conductive impurities or particles become trapped within the dielectric layers. Therefore, to enhance reliability and ensure better protection, it is standard practice to use more than one dielectric film.
Types of Capacitor Element
There are three main types of capacitor elements available in the market: internally fused, externally fused, and fuse less capacitor elements.
Internally Fused Capacitor Elements
Often, a capacitor element is equipped with a fuse connected in series with it. In some designs, a few capacitor elements are connected in parallel to form a small group, and each group is then connected in series with a fuse. When the fusing arrangement of an element is placed inside the casing of the capacitor unit, it is referred to as an internally fused capacitor element.
Externally Fused Capacitor Elements
Alternatively, a required number of capacitor elements may be connected in parallel to form a group capable of handling the current requirements of the unit. This entire group is then protected by a fuse connected in series with it. If this fuse is located outside the capacitor casing, the arrangement is referred to as an externally fused capacitor unit. The associated elements are called externally fused capacitor elements. In this case, the fuse protects the entire group and is externally accessible.
Fuse Less Capacitor Elements
The purpose of providing a fuse in capacitor elements is to isolate faulty components. If any capacitor element develops a short circuit due to dielectric failure, the associated fuse blows, thereby isolating the damaged element or group from the rest of the capacitor unit. This ensures that the function of the capacitor bank remains unaffected and that immediate maintenance is not required. Therefore, under normal operating conditions, each capacitor element or group is equipped with an individual fuse to maintain continuity of service and enhance reliability. However, this arrangement increases manufacturing complexity and cost. It requires additional space inside the unit to accommodate the fuses and adds to the overall design intricacy. To avoid these drawbacks some designs eliminate fuses altogether, leading to the development of fuse less capacitor elements.
In fuseless designs, all capacitor elements are connected in series to form a vertical column, with no fuses included in the circuit. The number of series-connected elements in each column is determined based on the voltage rating of the capacitor unit. To achieve the required current capacity, an appropriate number of such columns are connected in parallel. Each capacitor element has a defined voltage and current rating, and by combining these elements in suitable series-parallel configurations, the overall voltage and current ratings of the capacitor unit are established.
Internal Fuses
Internal fuses play an important role in protection of capacitor units. They are mainly used in the units of shunt capacitor banks. Internal fuses isolate faulty capacitor elements, ensuring continued operation of the capacitor unit within the bank. In general, one fuse is connected in series to each element in a capacitor unit. Sometimes, a single fuse is used for a small group of capacitor elements. This means each capacitor element, or each small group of elements within the casing of the capacitor unit, is equipped with a fuse. These capacitor elements, or small groups of elements with a fuse in series, are connected in parallel. These parallel-connected elements form groups, which are then connected in series to achieve the required voltage and capacitance ratings of the capacitor unit.
If a fault occurs in any capacitor element due to dielectric failure, the internal fuse associated with that element or with the group containing that element will blow to disconnect the faulty part from the unit. This prevents the failure of the entire capacitor unit or bank. In this way, internal fuses reduce the risk of catastrophic failures such as explosions or fires, enhancing the safety of the installation. Since an internal fuse isolates the faulty element from the unit, it ensures continuity of service with only a slight reduction in capacitance. Thus, internal fuses help minimize downtime and reduce maintenance requirements.
Internal fuses are installed inside the casing of the unit and are connected in series with each element or group of elements. Hence, a specific internal arrangement is required to accommodate the fuses within the unit. This increases the design complexity and requires additional space within the casing or container of the unit. However, this is justified by the inherent benefit of isolating faulty elements without interrupting overall service continuity.
Discharge Device
When a capacitor bank is switched off and isolated from the live system, the energy is still stored in each unit of the capacitor bank. It is essential to discharge this energy in view of safety. This is done by a discharge device connected across the terminals of the capacitor unit, inside the casing of the capacitor unit. This discharge device is essentially a resistor.
As per standard, a capacitor bank rated below 52 kV should be discharged to less than 50 volts within 5 minutes. This means the terminal voltage of the capacitor unit should come down to 50 volts or below within 5 minutes. If the capacitor unit is rated above 52 kV, the voltage should come down to 75 volts or less within 10 minutes after disconnecting the capacitor.
The value of the internal discharge device, i.e., the resistance of the discharge resistor of the capacitor unit, is calculated using the following formula: \[ R = \frac{t}{C \cdot \ln\left(\frac{\sqrt{2}U_N}{U_R}\right)} \]
R = (discharge resistance),
t = discharge time in seconds,
C = capacitance in farads,
\( U_N \) = rated voltage of the capacitor unit in volts, and
\( U_R \) = permissible residual voltage.
Residual voltage is the voltage that remains in the capacitor unit after it has been discharged for 5 or 10 minutes through the discharge resistor, as per the definition.
For example, if C = 10 microfarads, \( U_N \) = 10 kV, \( U_R \) = 50 volts, and t = 5 minutes or 300 seconds, the resistance of discharge device will be calculated to ensure the voltage drops to 50 volts within 5 minutes as follows.\[ R = \frac{300}{10\times 10^{- 6} \cdot \ln\left(\frac{10,000\sqrt{2}}{50}\right)} \approx 5.32 \, \text{M}\Omega\]