**INTRODUCTION TO CAPACITOR BANKS, ITS CHARACTERISTICS AND APPLICATIONS**

By: **Manuel Bolotinha**

Table of Contents

** DEFINITION OF AND CHARACTERISTICS OF CAPACITORS**

A **capacitor**, also known as “*condenser*” is an **electrical element** with **two electrical conductors** separated by an **insulator material **(*dielectric*), as shown in Figure 1.

Figure 1 – Simplified scheme of a capacitor

The **electric parameter** that defines a *capacitor* is the “**capacitance**” (symbol: **C**) and the *unit*, according to *International System of Units* (**SI**), is “**farad**” (symbol: **F**).

The most common used *dielectrics* are:

- Ceramics
- Plastic films
- Oxide layer on metal (Aluminum; Tantalum; Niobium)
- Mica, glass, paper, air and other similar natural materials
- Vacuum

**USE OF CAPACITORS AND CAPACITOR BANKS**

In *power electric systems* **capacitors** and **capacitors banks**, which must be in accordance with *IEC*^{[1]}* Standards 60143 and 60871* or *IEEE*^{[}^{2]}* Standard 824*, are used to:

*Compensate***reactive energy**(*power factor correction*) due to consumers (**MV and LV**) and the**inductive effect**of*long overhead lines and underground cables*(**MV and MV**).*Provide***voltage regulation**(**HV**^{[3]}).*Start*of**single phase squirrel cage motors**(**LV**).

A **shunt capacitor bank** (or simply *capacitor bank*) is a *set of capacitor units*, arranged in **parallel/series association** within a *steel enclosure*.

Usually **fuses** are used to *protect capacitor units *and they may be located **inside** the *capacitor unit*, on **each element**, or **outside** the *unit*.

*Capacitor banks* may be **star** or **delta** connected. *Delta-connected banks* are generally used only in **MV distribution networks** and in **LV installations**.

Figure 2 shows what was explained above.

Figure 2 – Schematic diagram of a capacitor bank

*Capacitors* may **retain a charge long after power is removed from a circuit**; *this charge* can **cause dangerous or even potentially fatal shocks** or *damage connected equipment*.

*Capacitors banks* may have **built-in discharge resistors** to **dissipate stored energy** to a **safe level** within a *few seconds after power is removed.*

*Capacitors banks* **shall be** *stored* with the **terminals shorted**, as *protection* from **potentially dangerous voltages** due to **dielectric absorption**^{[4]}.

*HV capacitor banks* are installed **outdoors**, **surrounded by a fence**, and *LV capacitor banks* are installed **indoors**, in **metallic enclosures** (*switchboards*).

In **MV installations** *capacitor banks* may be installed either **outdoors**, *surrounded by a fence* or in the **pole** of a *MV overhead line*, or **indoors**, in **metallic enclosures** (*switchgears*).

The fence must have a lock with a delayed opening to assure the time requested for the complete discharge of the capacitors.

Examples of capacitor banks are shown in figures 3 and 4.

Figure 3 – HV Capacitor bank

Figure 4 – LV Capacitor bank

**TRANSIENT DISTURBANCES AND HARMONICS**

During **electrical switching**** of capacitor banks**, **t****ransient disturbances** (*during a short time*) occur in power systems that *may damage key equipment*, **potentially having a great impact on system reliability**.

An *oscillation of the power system* and *electromagnetic pulses* (**EMP**) can be provoked by that sudden change of a circuit.

One of those *transient disturbances* is **overvoltage** (known as “*switching overvoltages*”) that influences the **required insulation level **of the network and of the equipments.

During the *switching of capacitor banks*, **high magnitude and high frequency transients** can occur.

The **impedance of a circuit** dictates the **current flow** in that circuit.

As the *supply impedance* is generally considered to be **inductive**, the *network impedance* **increases** with *frequency* while the *impedance of a capacitor* **decreases**. This encourages a greater proportion of the currents circulating at frequencies above the *fundamental supply frequency* (**50Hz or 60 Hz**) to be absorbed by the *capacitor*, and *all equipment associated with the capacitor*.

In certain circumstances such currents can **exceed** the value of the *fundamental* (**50Hz or 60 Hz**) *capacitor current*. These currents in turn cause **increased voltage** to be applied *across the dielectric of the capacitor*. The **harmonic voltage** due to each **harmonic current** added arithmetically to the fundamental voltage dictates the *voltage stress* to be sustained by the *capacitor dielectric* and for which the *capacitor* must be designed, to avoid **additional heating and higher dielectric stress**.

*Capacitors* may **catastrophically fail** when subjected to *voltages or currents* **beyond their rating**, or as they reach their **normal end of life**. *Dielectric or metal interconnection failures* may create **arcing** that *vaporizes the dielectric fluid*, resulting in that case **bulging, rupture, or even an explosion**.

**Capacitors units are intended to be operated at or below their rated voltage and frequency**.

*IEEE Std. 18-1992* and *Std 1036-1992* specifies the **standard ratings of the capacitors** designed for *shunt connection* to **ac systems** and also provide application guidelines. These standards stipulate that:

- Capacitor units should be capable of continuous operation up to
**110%**of*rated terminal rms*^{[5]}*voltage*and a*crest (peak) voltage*not exceeding**2 x √2**of*rated rms voltage*,**including harmonics**but**excluding transients**. The capacitor should also be able to carry**135%**of*nominal current*. *Capacitors units*should not give*less*than**100%**and*more*than**115%**of*rated reactive power*at*rated sinusoidal voltage and frequency*.*Capacitor units*should be suitable for continuous operation at*up to***135%**of*rated reactive power*caused by the combined effects of:

- Voltage in excess of the nameplate rating at fundamental frequency, but not over
**110%**of*rated rms voltage*. *Harmonic voltages*superimposed on the*fundamental frequency*:**reactive power***manufacturing tolerance of up to***115%**of*rated reactive power*.

If *harmonic* problems exist, they most often manifest themselves first at *shunt capacitor banks* in the form of **audible noise, blown fuses or capacitor unit failures**.

As *frequency* varies, so *reactance* varies and a point can be reached when the *capacitor reactance and the supply reactance are equal*. This point is known as the circuit **resonant frequency**.

Whenever *power factor correction* is applied to a distribution network, bringing together *capacitance and inductance*, there will always be a *frequency* at which the *capacitors are in parallel resonance with the supply*.

If this condition occurs at, or close to, one of the *harmonics* generated by any *solid state control equipment*, then **large harmonic currents** can *circulate between the supply network and the capacitor equipment*, limited only by the damping resistance in the circuit. Such *currents* will add to the **harmonic voltage disturbance** in the network causing an **increased voltage distortion**.

This result in an *unacceptably high voltage across the capacitor dielectric* coupled with an *excessive current through all the capacitor ancillary components*. The most common order of *harmonics* is **5th, 7th, 11th and 13th** but *resonance* *can occur at any frequency*.

*Capacitors* can be effectively applied in these types of environments by selecting compensation levels that do not tune the circuit or by the use of a **filter**.

** POWER FACTOR CORRECTION**

In electrical installations, namely in industry, exists several consumers, such as motors, that have an **important inductive load** that provokes a **phase shift** *between voltage and current*, as show on the figure below.

Figure 5 – Phase shift

**Phase shift** is represented as an *angle* (**Ф**) and *current* may be in **advance **or **delayed **of *voltage*.

If **capacitance** of the circuit *prevails* current is *advanced*; if *inductance prevails* current is *delayed*.

This *phase shift* provokes that “**useful power**” (*active power* – unit: **W or kW**) is **lower** than the **power supplied** (*apparent power* – unit: **VA or kVA**), the *difference* being the **reactive power** – unit: **VAr or kVAr**.

Representing those powers by *vectors*, the “**power triangle**” may be as shown on the figure below.

Figure 6 – Power triangle

The *cosine of Ф* (**cos Ф**) is the **power factor**, its value varies from **0 to 1** and the *relations between the powers* in the above triangle are:

**P = S x cos Φ**

**Q = S x sen Φ**

The consequences of a low power factor are:

- Higher currents in cables.
- Higher losses by Joule effect in the conductors.
- Higher voltage drops

In most countries *electricity distribution companies* **do not allow** *power factor* to be **lower** than a *defined value* (in *Europe* it must be **cos Ф ≥ 0.93 <> tg Ф ≤ 0.4**) and **impose penalties** to clients that do not comply with this requirement.

When correcting *power factor*, clients avoid those penalties and in addition the reduction of losses can provide the utility with the additional benefit of reducing the apparent substation demand during peak loading conditions and the released system capacity can then be used to deliver more real power from the existing system, resulting in a more efficiently run power system.

Also the cross-section of the conductors can be reduced as advantage by improving the power factor.

When installing capacitor banks it is necessary to:

- Calculate the bank size (
*the size of a capacitor bank is defined***kVAr**) - Determine the location for connection.
- Select a control method.

To calculate the size of the capacitor bank (Q), the following equation must be used: You may also convert the capacitor bank kVAr and Farads as well.

**Q [kVA _{r}] = P [kW] x (tan Ф_{1} – tan Ф_{2})**

**P** is the *active power* of the installation; **Ф _{1}** is the

*voltage and current phase shift of the installation*;

**Ф**is the

_{2}*desired voltage and current phase shift*.

To define the **location** of the *capacitor bank* it must be taken into account that three methods are used for power factor correction, which depends of the **location** of the *inductive loads* and their *requested reactive power*:

*Centralized correction*: one capacitor bank is installed near the main incoming switchboard (see Figure 7).*De-centralized correction*: capacitor banks are installed near distribution switchboards that supply energy to the main consumers responsible for the low power factor (see Figure 8).*Local correction*: capacitor banks are installed near individual consumers (see Figure 9).

Centralized Power Factor (P.f) correction

De-centralized P.f correction

Figure 9 – Local correction

For **MV installations** *capacitor banks* may be divided in **steps** and controlled by a **VAr relay** or an **electronic controller**, that *monitors and switches steps* or the *whole capacitor bank* based on **real time network conditions** what is shown in Figure 10, to avoid the *supply of reactive power to the network*, situation that is also **subjected to penalties** applied by the *electricity distribution companies.*

Figure 10 – VAr relay

^{[1]}** IEC**: International Electrotechnical Commission.

^{[2]}** IEEE**: Institute of Electrical and Electronic Engineers (USA).

^{[3]} **HV**: *High Voltage* (**V ≥ 60 kV**); **MV**: *Medium Voltage* (**1 kV < V < 60 kV**); **LV**: *Low Voltage* (**V ≤ 1 kV**).

^{[4] }**Dielectric absorption** is the phenomenon by which a *capacitor* that **has been charged for a long time** **does not discharges completely** *when submitted to a short discharge*.

^{[5]} **rms**: *root mean square*.

You may also read:

- How to Test a Capacitor? 6 Ways to Check a Capacitor
- Capacitors MCQs with Explanatory Answers
- What is the Role of Capacitor in AC and DC Circuit?

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