## Methods of voltage control in power system

Before learning the methods of voltage control in power system, we must first understand why do we need to control voltage. In power systems, voltage is supposed to be constant which is obviously not. So we have to control it in such a way that it remains constant. But why does the voltage need to be constant at all? Because, most of the devices, apparatus, electrical machines, consumer appliances etc. are all designed to work at a specified voltage. Wide variations of voltage may cause errors in operation, malfunctioning or performance deterioration. It is desirable that the consumers receive power at substantially constant voltage. In many countries, including India, the statutory limit of voltage variation is ±6% of the declared voltage at consumers' end.
Therefore, it is important to apply certain techniques, certain methods to control the power system voltage to keep it constant. Following are the methods of voltage control in power system.

## Methods of voltage control in power system

1. Using excitation control or voltage regulators at generating stations
2. By using tap changing transformers
3. Using induction regulators
4. By using shunt reactors
5. By using shunt capacitors
6. Using synchronous condensers

### Excitation control or voltage regulators at generating stations

Induced emf (E) of a synchronous generator (alternator) depends on the excitation current (field current). The terminal voltage of an alternator can be given as V = E - IZ. As the load current, and hence the armature current, increases, voltage drop in the armature also increases. The field current must be increased to compensate this voltage drop, such that the terminal voltage is at the target value. For this purpose, alternators are provided with excitation control or automatic voltage regulator systems. There are two main types of automatic voltage regulators (AVR):
1. Tirril regulator
2. Brown-Boveri regulator
An automatic voltage regulator detects the terminal voltage and compares it with the reference voltage. The difference between detected voltage and given reference voltage is called as the error voltage. The regulator then controls the excitation voltage of the alternator to cancel out the error voltage. Thus, an automatic voltage regulator controls the voltage by controlling the excitation.
Excitation control method is satisfactory only for short lines. For longer lines, the terminal voltage of alternator has to be varied widely for the voltage at far ends to remain constant. Obviously, this method is not feasible for longer lines.

### By using tap changing transformers

The voltage control in transmission and distribution systems is usually obtained by using tap changing transformers. In this method, the voltage in the line is adjusted by changing the secondary EMF of the transformer by varying the number of secondary turns. Secondary voltage of a transformer is directly proportional to the number of secondary turns. Thus, the secondary voltage can be adjusted by changing the turns ratio of the transformer. Secondary number of turns can be varied with the help of tappings provided on the winding. Basically, there are two types of tap changing transformers.

#### Voltage control using off-load tap changing transformers

In this method, the transformer is disconnected from the supply before changing the tap. Off load tap changing transformers are relatively cheaper. But the main drawback with them is that the power supply is interrupted while changing the tap.

#### Voltage control using on-load tap changing transformers

In modern power system, continuity of the supply is important. Therefore, on-load tap changing transformers are preferred to control the voltage.

### By using induction voltage regulators

An induction regulator is basically an electrical machine somewhat similar to an induction motor, except that the rotor is not allowed to rotate continuously. The rotor of induction regulator holds primary (excitation) winding which is connected across (parallel) the supply voltage. The stationary secondary winding is connected in series with the line which is to be regulated. From electrical point of view, it is immaterial whether primary winding is rotating or secondary winding is rotating. The magnitude of voltage in the secondary winding depends upon its position with respect to the primary winding. Thus, the secondary voltage can be adjusted by rotating the primary winding. Induction voltage regulators were used to control voltage of electrical network in earlier days, but they are now replaced by tap changing transformers.

### Voltage control by using shunt reactors

Shunt reactors are basically inductive elements that are provided at sending end and receiving end of long EHV and UHV transmission lines. When a transmission line is not loaded or lightly loaded, the line capacitance predominates and receiving end voltage becomes greater than the sending end voltage. This effect is known as Ferranti effect. In such situation, shunt reactors are switched in the line. Shunt reactors compensate the line capacitance and, hence, control the voltage.

### Voltage control by using shunt capacitors

Shunt capacitors are usually installed at the receiving end substations or near industrial loads. Most of the industrial loads draw inductive current and therefore the power factor is lagging (usually 0.3 to 0.6 lag). The line experiences IXL drop due to this lagging current. Switching in shunt capacitors compensate this inductive reactance, thereby, decreasing the IXL drop. Thus, shunt capacitors can be used to control the line voltage when the load is highly inductive.

### Voltage control by using synchronous condenser

A synchronous condenser is basically an over-excited synchronous motor running on no-load. Synchronous condensers are also called as synchronous phase modifiers. A synchronous condenser is located near the load end and can inject or absorb reactive power. And, thus, a synchronous phase modifier improves the voltage profile.

## What is a terminal block? | Significance and Types

How do we connect two wires? By stripping the insulation at the ends and twisting them together? Yes, it works. But, is it safe? We can apply insulation tape over the joint or use a wire connector. But what if there are a number wires that need to be joint/connected near each other? Or, what if multiple outgoing wires are to be connected to a single incoming wire? Then this method will neither be safe nor be convenient anymore. Here we use terminal blocks.

## What is a terminal block?

A terminal block (also called as connection terminal or terminal connector) is a modular block with an insulated frame that secures two or more wires together. It consists of a clamping component and a conducting strip. A typical simplest terminal block is as shown in the image below.
 Image Credit: Wikimedia Commons

The insulating body of a terminal block houses a current carrying element (a metal strip or terminal bar). It also provides a base for clamping element. The body has a mounting arrangement so that the block can be easily mounted on or unmounted from a PCB or a mounting rail. Most terminal blocks are usually modular and mounted on DIN rail. That allows us to increase the number of terminals according to the requirements. Terminal blocks keep connections much more secure and wires well organized.

## Types of terminal blocks

Electrical terminal blocks can be classified on the basis of structure, device type, termination options etc.
 Single level pass through terminal block

### Structure type

• Single level pass-through terminal blocks: These are simply used to connect two wires together, i.e. wire-to-wire connection. These are also called as single feed terminal blocks. Single level terminal blocks are of the most simple type having one input contact and one output contact.
• Dual level terminal blocks: These blocks have another level of connection terminal stacked on the first one. This arrangement is generally used to save space.
• Three level terminal blocks: Just like dual level blocks, these have an extra level at the top. An advantage of using multilevel blocks is that multiple connections can be made in the same block.
 Image credits: Connectwell.com

### Device type

 Image credits: Connectwell.com

#### - Ground terminal blocks

These blocks often look like a single level feed through terminals. The exception is that these blocks and the metal connection where the wire is terminated are grounded to the panel or DIN rail on which the block is mounted.

#### - Fused connection terminals

These are similar to the pass-through blocks with an exception of the metal connection strip is replaced with a fuse. Therefore, the wires will be connected through a fuse providing an added protection.

#### - Thermocouple terminal blocks

These are designed to accept thermocouple lead connections. Some thermocouple connectors essentially clamp the thermocouple leads together on both sides of the block, eliminating the metal connection strip inside the block. However, in some thermocouple blocks, the metal connection strip of the same metal as that of the wire may be present.

#### - I/O blocks and sensor blocks

I/O blocks are used to make a connection between a device and a controller. Whereas, sensor blocks handle three or four wire devices such as proximity sensors.

#### - Disconnect terminal blocks

These blocks allow wires to be easily disconnected just by lifting a lever or knife switch. They can be used for convenient disconnection and connection without removing the wires. They are also known as switch blocks.

### - Power Distribution blocks

These blocks are used in electrical power distribution. An electric power distribution terminal block is a convenient, economical and safer way to distribute power from a single input source to multiple outputs. One large wire is connected to the input terminal of the block and multiple output terminals are provided at the output. This way, wires are well arranged in a control panel giving it a neat, clean and professional look.
 Image source: ABB e-library

### Clamping options in terminal blocks

• Screw terminal: Screw clamp terminals are the most common type of connection method. The wire or conductor is simply pressed against the conductor strip in the block by tightening the screw. Screw terminals accommodate a very wide range of wire or conductor sizes.
• Spring clamp: These type of terminals use spring pressure to retain the wire clamped. Spring clamps are a newer alternative to screw clamps and are generally used for relatively small wires.
 Image credit:C J Cowie | Altech Corp.
• Push-in terminal blocks: Push-in terminals allow you to connect a wire simply by inserting it. Most push-in terminals require the use of a ferrule. A ferrule strengthens the end of the wire/conductor. However, some push-in terminal blocks allow to insert a solid conductor directly or a stranded conductor by inserting a screwdriver into the release hole.
• Insulation Displacement Connector (IDC): These connectors do not require us to strip the insulation for contact. We simply need to insert the wire without stripping the insulation, and the two sharp metal blades inside the terminal will cut through it to the conductor making proper contact.
• Barrier terminal block: These are used where vibration is an issue. A spade or ring terminal is attached to the wire and then inserted into a bolt and tightened with a nut on the terminal block. This prevents loosening of the wire due to vibrations.

## Radial, Parallel, Ring main and Interconnected Distribution Systems

An electric power distribution system can be classified according to its feeder connection schemes or topologies as follows -
• Parallel feeders distribution
• Ring main distribution system
• Interconnected distribution
There are few other variations of distribution feeder systems, but we'll stick to these four basic and commonly used systems.

[Also read: Classification of distribution systems according to number of phases and wires involved.]

This system is used only when substation or generating station is located at the center of the consumers. In this system, different feeders radiate from a substation or a generating station and feed the distributors at one end. Thus, the main characteristic of a radial distribution system is that the power flow is in only one direction. Single line diagram of a typical radial distribution system is as shown in the figure below. It is the simplest system and has the lowest initial cost.
 Image credit: Wikimedia commons
Although this system is simplest and least expensive, it is not highly reliable. A major drawback of a radial distribution system is, a fault in the feeder will result in supply failure to associated consumers as there won't be any alternative feeder to feed distributors.

## Parallel feeders distribution system

The above-mentioned disadvantage of a radial system can be minimized by introducing parallel feeders. The initial cost of this system is much more as the number of feeders is doubled. Such system may be used where reliability of the supply is important or for load sharing where the load is higher. (Reference: EEP - Distribution Feeder Systems)

## Ring main distribution system

A similar level of system reliability to that of the parallel feeders can be achieved by using ring distribution system. Here, each distribution transformer is fed with two feeders but in different paths. The feeders in this system form a loop which starts from the substation bus-bars, runs through the load area feeding distribution transformers and returns to the substation bus-bars. The following figure shows a typical single line diagram of a ring main distribution system.
Ring main distribution system is the most preferred due to its following advantages.

### Advantages of ring main distribution system

• There are fewer voltage fluctuations at consumer's terminal.
• The system is very reliable as each distribution transformer is fed with two feeders. That means, in the event of a fault in any section of the feeder, the continuity of the supply is ensured from the alternative path.

## Interconnected distribution system

When a ring main feeder is energized by two or more substations or generating stations, it is called as an interconnected distribution system. This system ensures reliability in an event of transmission failure. Also, any area fed from one generating stations during peak load hours can be fed from the other generating station or substation for meeting power requirements from increased load.

## Types of AC power distribution systems

As we all know, electrical power is almost exclusively generated, transmitted and distributed in it's AC form. A distribution system usually begins from a substation where the power is delivered by a transmission network. In some cases, the distribution system may start from a generating station itself, such as when consumers are located near the generating station. For larger areas or industrial areas, primary and secondary distribution may also be used.

## Types of AC power distribution systems

According to phases and wires involved, an AC distribution system can be classified as -
1. Single phase, 2-wire system
2. Single phase, 3-wire system
3. Two phase, 3-wire system
4. Two phase, 4-wire system
5. Three phase, 3-wire system
6. Three phase, 4-wire system

### Single phase, 2-wire distribution

This system may be used for very short distances. The following figure shows a single phase two wire system with - fig (a) one of the two wires earthed and fig. (b) mid-point of the phase winding is earthed.

### Single phase, 3-wire system

This system is identical in principle with 3-wire dc distribution system. The neutral wire is center-tapped from the secondary winding of the transformer and earthed. This system is also called as split-phase electricity distribution system. It is commonly used in North America for residential supply.

### Two phase, 3-wire system

In this system, the neutral wire is taken from the junction of two phase windings whose voltages are in quadrature with each other. The voltage between neutral wire and either of the outer phase wires is V. Whereas, the voltage between outer phase wires is √2V. As compared to a two-phase 4-wire system, this system suffers from voltage imbalance due to unsymmetrical voltage in the neutral.

### Two phase, 4-wire system

In this system, 4 wires are taken from two phase windings whose voltages are in quadrature with each other. Mid-point of both phase windings are connected together. If the voltage between the two wires of a same phase is V, then the voltage between two wires of different phase would be 0.707V.

### Three phase, 3-wire distribution system

Three phase systems are very widely used for AC power distribution. The three phases may be delta connected or star connected with star point usually grounded. The voltage between two phases or lines for delta connection is V, where V is the voltage across a phase winding. For star connection, the voltage between two phases is √3V.

### Three phase, 4-wire distribution system

This system uses star connected phase windings and the fourth wire or neutral wire is taken from the star point. If the voltage of each winding is V, then the line-to-line voltage (line voltage) is √3V and the line-to-neutral voltage (phase voltage) is V. This type of distribution system is widely used in India and many other countries. In these countries, standard phase voltage is 230 volts and line voltage is √3x230 = 400 volts. Single phase residential loads, single phase motors which run on 230 volts etc. are connected between any one phase and the neutral. Three phase loads like three-phase induction motors are put across all the three phases and the neutral.

## Classification on the basis of connection scheme

Distribution system can be classified according to its connection scheme or topology as follows -
2. Ring main system
3. Interconnected system

## DC Power Distribution Systems

At the end of 19th century, when Edison built the first electrical distribution networks, they were based on DC technology. However, with the invention of transformers, AC system proved to be much more superior to DC system at that time and AC systems were universally adopted for power generation, transmission as well as distribution.

## Why DC now?

Electrical power is almost exclusively generated, transmitted and distributed in AC form. However, for certain applications, DC supply is absolutely necessary. For example, variable speed machinery incorporating DC motors, critical areas where storage battery reserves are necessary. Following are some points that make us think about dc power distribution.
• Advancements in Power electronics have made it possible to transform DC voltage levels and conversion between AC and DC efficiently. It is now possible to replace existing AC distribution network with DC distribution network.
• Distribution generation from solar and wind energy is increasing rapidly and both of these sources are intrinsically DC.
• A large number of office and household appliances internally require low voltage DC. These appliances are fed with AC supply and then transformed to lower voltage and converted into DC by an internal circuitry.
• Harmonic issues, phase balancing problems, skin effect etc. are not present in DC systems.
• DC energy can be stored easily in batteries and fuel cells. Such backup batteries can be utilized easily in case of supply failure.

## Types of DC power distribution

Wherever DC power distribution is required, AC power from the transmission network can be rectified at a substation using converting equipment and then fed to the dc distribution system. AC consumers can also be connected to DC system using a DC to AC inverter. A low voltage DC distribution system is of two types.

### Unipolar DC distribution system (2-wire DC system)

As the name suggests, this system uses two conductors, one is positive conductor and the other one is negative conductor. The energy is transmitted at only one voltage level to all the consumers using this system. A typical unipolar dc power distribution system is as shown in the following figure.

### Bipolar DC distribution system (3-wire DC system)

This is basically a combination of two series connected unipolar DC systems. It consists of three conductors, two outer conductors (one is positive and the other is negative) and one middle conductor which acts as neutral. This system leaves following connection choices to a consumer -
1. between positive conductor and neutral
2. between negative conductor and neutral
3. between positive and negative conductor (double voltage)
4. positive to negative with neutral connected
The above figures of unipolar and bipolar dc distribution system suggest that, DC to DC converter or DC to AC inverter can be installed at the consumer's premises according to consumer's or load's requirement. Consumers can also be directly connected to the DC distributors if the distribution voltage level is similar as per their requirement.

## Types of DC distributors

DC distributors are usually classified on the basis of the way they are fed by the feeders. Following are the four types of DC distributors.
1. Distributor fed at one end
2. Distributor fed at both ends
3. Distributor fed at center
4. Ring distributor

### Distributor fed at one end

In this type, distributor is connected to the supply at one end and loads are tapped at different points along its length. The following figure shows the single line diagram of a distributor fed at one end. It worth to note that -
• The current in various sections of the distributor away from the feeding point goes on decreasing. From the above figure, the current in section DE is less than the current in section CD and likewise.
• The voltage also goes on decreasing away from the feeding point. In the above figure, voltage at point E will be minimum.
• In case of a fault in any section of the distributor, the whole distributor will have to be disconnected from the supply. Thus, continuity of supply is interrupted.

### Distributor fed at both ends

In this type, the distributor is connected to supply at both ends and voltages at feeding points may or may not be equal. The minimum voltage occurs at some load point which is shifted with the variation of load on different sections of the distributor.
• If a fault occurs at any feeding point, continuity of the supply is ensured from the other feeding point.
• If a fault occurs on any section of the distributor, continuity of the supply is ensured on both sides of the fault with respective feeding points.
• The conductor cross-section area required for a doubly fed distributor is much less than that required for a distributor fed at one end.

### Distributor fed at the center

As the name implies, the distributor is supplied at the center point. Voltage drop at the farthest ends is not as large as that would be in a distributor fed at one end.

### Ring main DC distributor

In this type, the distributor is in the form of a closed ring and fed at one point. This is equivalent to a straight distributor fed at both ends with equal voltages.