DISTRIBUTION SYSTEM EQUIPMENT

1  OUTDOOR CIRCUIT BREAKER (15 TO 230 KV)

            A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Types of circuit breaker

1. Low voltage circuit breakers

Low voltage (less than 1000 V_AC) types are common in domestic, commercial and industrial application, and include:

• MCB (Miniature Circuit Breaker)—rated current not more than 100 A, Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.

• MCCB (Molded Case Circuit Breaker)—rated current up to 2500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.

• Low voltage power circuit breakers can be mounted in multi-tiers in LV switchboards or switchgear cabinets.

2. Magnetic circuit breaker

3. Thermal magnetic circuit breaker

4. Common trip breakers

5. Medium-voltage circuit breakers

Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:

• Vacuum circuit breaker—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V,^[4] which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

• Air circuit breaker—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

SF₆ circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.

6. High-voltage circuit breakers

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

• Bulk oil
• Minimum oil
• Air blast
• Vacuum
• SF₆

2 GENERATORS AND MOTORS

               In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities. An electric motor converts electrical energy into mechanical energy. Most electric motors operate through interacting magnetic fields and conductors to generate force, although a few use electrostatic forces. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo. Many types of electric motors can be run as generators, and vice versa. For example a starter/generator for a gas turbine, or traction motors used on vehicles, often perform both tasks. Electric motors and generators are commonly referred to as electric machines.

Comparison of motor types

Comparison of motor types
Type	Advantages	Disadvantages	Typical Application	Typical Drive
AC polyphase induction squirrel-cage	Low cost, long life, high efficiency, large ratings available (to 1 MW or more), large number of standardized types	Starting inrush current can be high, speed control requires variable frequency source	Pumps, fans, blowers, conveyors, compressors	Poly-phase AC, variable frequency AC
Shaded-pole motor	Low cost Long life	Rotation slips from frequency Low starting torque Small ratings low efficiency	Fans, appliances, record players	Single phase AC
AC Induction (split-phase capacitor)	High power high starting torque	Rotation slips from frequency Starting switch required	Appliances Stationary Power Tools	Single phase AC
Universal motor	High starting torque, compact, high speed	Maintenance (brushes) lifespan Only small ratings economic	Drill, blender, vacuum cleaner, insulation blowers	Single phase AC or DC
AC Synchronous	Rotation in-sync with freq - hence no slip	More expensive	Industrial motors Clocks Audio turntables tape drives	Poly-phase AC

3 MEDIUM VOLTAGE SWITCHGEARS (5 TO 15 KV)

                 The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply.

4 CURRENT LIMITING REACTORS

                       

              Current Limiting Reactors reduce short circuit levels to meet the system needs and reduce stresses on busses, insulators, circuit breakers and other high voltage devices. They are connected between the neutral of a system  and earth for limiting the line to ground current under system fault conditions. They are also used as load sharing  reactors for balancing the current in parallel circuits.

5 TRANSFORMERS

         A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.A distribution transformer (also called a pole mount transformer, pylon transformer, or colloquially a pole pig) is a transformer mounted on a utility pole. If the distribution lines are located underground, distribution transformers are mounted on concrete pads and locked in steel cases; these are known as pad mount transformers. The transformer provides the final voltage transformation in the electric power distribution grid, stepping down the voltage used in the overhead distribution wires to the level used by the customer. Because of weight restrictions transformers for pole mounting are only built for primary voltages under 30kV. Distribution  transformers  are  generally  used  in  electrical  power  distribution  and  transmission systems.  This class of transformer has the highest power, or volt-ampere ratings, and the highest continuous voltage rating.    The  power  rating  is  normally  determined  by  the  type  of  cooling methods the transformer may use.   Some commonly-used methods of cooling are by using oil or some other heat-conducting material.  Ampere rating is increased in a distribution transformer by increasing the size of the primary and secondary windings; voltage ratings are increased by increasing the voltage rating of the insulation used in making the transformer. Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V).

            A delta-wye (Δ-Y) transformer is a transformer that converts three-phase electric power without a neutral wire into 3-phase power with a neutral wire. It can be a single three-phase transformer, or built from three independent single-phase units. The term Delta-Wye transformer is used in North America, and Delta-Star system in Europe.

Delta Wye Transformer

Delta-wye transformers are common in commercial, industrial, and high-density residential locations, to supply three three-phase distribution systems.

An example would be a distribution transformer with a delta primary, running on three 11kV phases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 400 V, with the domestic voltage of 230 available between each phase and an earthed neutral point.

6 LOAD BREAK SWITCHES

         

                In power applications, switches perform function to energize or no energized an electric load. On the high end of the scale are load-break switches and disconnecting switches in power systems at the highest voltages (several hundred thousand volts).

Load-break switches are required to maintain the capability of interrupting the load current. The load break switch in a circuit with several hundred thousand volts, designed to carry a large amount of current without overheating the open position, having enough insulation to isolate the circuit in closed position, and equipped with arc interrupters to interrupt the load current.

Load break switches of air break are of versatile switch gear for transformer control & protection. They can also be used for Motor feeder in conjunction with Vacuum contactors. They are highly useful for Ring feeders for isolation of faulty section either manually or through remote control if fitted with motor operation. Load break switch in conjunction with HRC fuse can tackle high fault current and offer very good protection against dead short circuit capacity up to 40KA.The fault clearance and isolation through this combination will be achieved in a few milli seconds provided a proper selection of LBS and fuse is done. HRC fuses are manufactured with silver strips/silver coated Cu. Strips wires surrounded by granular quartz. When short-circuit occurs the metal element melts and the molten metal melts the surrounding quartz, making the combination an insulating material, called fulgarite. There will be a number of sections in the element which will create similar insulating media in quick succession and thus the arc will be extinguished in a matter of few milliseconds, say 5 to 8 milli-seconds, i.e., less than 1/2 a cycle.

7 MEDIUM VOLTAGE STARTERS

   A small motor can be started by simply plugging it into an electrical receptacle or by using a switch or circuit breaker. A larger motor requires a specialized switching unit called a motor starter or motor contactor. When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. A motor soft starter connects the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.

Methods of Starting Three Phase Induction Motors

Direct Online Starting

                Direct online starting also known as across the line starting and full voltage starting, involves connecting each terminal of a three-phase induction motor to a separate line of a device. In this arrangement, the motor current is the same as the line current and the terminal voltage of the induction motor equals the line voltage.

Wye and Delta Connections

                   In a wye or star connection, the windings of the induction motor connect from the supply phases to the neutral. In a delta or mesh connection, the windings connect between the supply phases. A wye connection creates higher voltage to the windings of the three-phase induction motor than a delta connection. A starter with the ability to utilize both star and delta connections, also known as a wye start delta run connection, initializes the three-phase motor using a wye connection then transfers to a delta connection when the motor reaches a set speed.

Series Reactor Connection

           A reactor in series with the terminals of the motor decreases the terminal voltage of the induction motor, decreasing the initial current. The impedance decreases as the induction motor accelerates until a bypass method makes the motor run at full speed and full voltage.
Variable Frequency Drive

             A variable-frequency drive starts a three-phase induction motor at a frequency low enough to initialize a full-rated torque without an inrush of current. The low frequency increases the torque because it increases the impedance of the rotor circuit with slip frequency.

Methods of starting synchronous motor

Basically there are three methods that are used to start a synchronous motor:

• To reduce the speed of the rotating magnetic field of the stator to a low enough value that the rotor can easily accelerate and lock in with it during one half-cycle of the rotating magnetic field’s rotation. This is done by reducing the frequency of the applied electric power. This method is usually followed in the case of inverter-fed synchronous motor operating under variable speed drive applications.

• To use an external prime mover to accelerate the rotor of synchronous motor near to its synchronous speed and then supply the rotor as well as stator. Of course care should be taken to ensure that the direction of rotation of the rotor as well as that of the rotating magnetic field of the stator are the same. This method is usually followed in the laboratory- the synchronous machine is started as a generator and is then connected to the supply mains by following the synchronization or paralleling procedure. Then the power supply to the prime mover is disconnected so that the synchronous machine will continue to operate as a motor.

• To use damper windings or amortisseur windings if these are provided in the machine. The damper windings or amortisseur windings are provided in most of the large synchronous motors in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodically varying load.

8. LOW VOLTAGE SWITCHGEAR

                The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply. The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF₆ equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

9. CONDUCTORS

         The wires and cables over which electrical energy is transmitted are made of copper, aluminum, steel, or a combination of copper and steel or aluminum and steel. A conductor is a material that readily permits the flow of an   electric   current.   Materials,   other   than   those mentioned, that conduct. In the early days conductor used on transmission lines were usually Copper, but Aluminium Conductors have completely replaced Copper because of the much lower cost and lighter weight of Aluminium conductor compared with a Copper conductor of the same resistance. The fact that Aluminium conductor has a larger diameter than a Copper conductor of the same resistance is also an advantage. With a larger diameter the lines of electric flux originating on the conductor will be farther apart at the conductor surface for the same voltage. This means a lower voltage gradient at the conductor surface and less tendency to ionize the air around the conductor. Ionization produces the undesirable effect called corona.

The symbols identifying different types of Aluminium conductors are as follows:-
AAC     : All Aluminium conductors.
AAAC   : All Aluminium Alloy conductors
ACSR   : Aluminium conductors, Steel-Reinforced
ACAR   : Aluminium conductor, Alloy-Reinforced

Aluminium alloy conductors have higher tensile strength than the conductor of EC grade Aluminium or AAC; ACSR consists of a central core of steel strands surrounded by layers of Aluminium strands. ACAR has a central core of higher strength Aluminium Alloy surrounded by layer of Electrical-Conductor-Grade Aluminium. Recently AAAC are being used in some SEBs to overcome menace of pilferage of ACSR and AAC conductors, particularly lower voltage lines. AAAC cannot be re-cycled and it does not have any common use for other purposes, as that in case of pure Aluminium.
AAAC is made out of heat treated Aluminium-Magnesium-Silicon Alloy designed as 64401 T 81 covered under IS: 9997:1991 containing 0.6-0.9% Magnesium and 0.5-0.9% Silicon. Besides use of AAAC on lower voltage lines from the point of view of avoiding its pilferage, it is also better for use in coastal areas to avoid corrosion problem prevalent in Steel core of ACSR conductors.

10. RECTIFIERS

                              A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. The primary application of rectifiers is to derive DC power from an AC supply. Virtually all electronic devices require DC, so rectifiers find uses inside the power supplies of virtually all electronic equipment.

11. CAPACITORS

Shunt and series reactive compensation using capacitors has been a widely recognized and powerful method to combat the problems of voltage drops, power losses and voltage flicker in power distribution networks.

1.   Why Use Shunt Capacitor compensation in Distribution systems?

Fig. 1 represents an a.c. generator supplying a load through a line of series impedance (R+jX) ohms. Fig. 2(a) shows the phasor diagram when the line is delivering a complex power of (P+jQ) VA and Fig. 2 (b) shows the phasor diagram when the line is delivering a complex power of (P+jO) VA i.e. with the load fully compensated. A thorough examination of these phasor diagrams will reveal the following facts.

1. Current in the line, generator and intervening transformers, if any, is higher by a factor of  in the case of uncompensated load compared to compensated load. This results in a power loss, which is higher by a factor of () ² compared to the minimum power loss attainable in the system.
2. The loading on generator, transformers, line etc is decided by the current flow. The higher current flow in the case of uncompensated load necessitated by the reactive demand results in a tie up of capacity in this equipment by a factor of . i.e. compensating the load to UPF will release a capacity of (load VA rating X ) in all these equipment.
3. The sending-end voltage to be maintained for a specified receiving-end voltage is higher in the case of uncompensated load. The line has bad regulation with uncompensated load.
4. The sending-end power factor is less in the case of an uncompensated one. This due to the higher reactive absorption taking place in the line reactance. 
5. The excitation requirements on the generator are severe in the case of uncompensated load. Under this condition, the generator is required to maintain a higher terminal voltage with a greater current flowing in the armature at a lower lagging power factor compared to the situation with the same load fully compensated. It is entirely possible that the required excitation is much beyond the maximum excitation current capacity of the machine and  in that case further voltage drop at receiving-end will take place due to the inability of the generator to maintain the required sending-end voltage. It is also clear that the increased excitation requirement results in considerable increase in losses in the excitation system.

It is abundantly clear from the above that compensating a lagging load by using shunt capacitors will result in 

1. Lesser power loss everywhere up to the location of capacitor and hence a more efficient system 
2. Releasing of tied-up capacity in all the system equipments thereby enabling a postponement of the capital intensive capacity enhancement programmes to a later date.
3. Increased life of equipments due to optimum loading on them 
4. Lesser voltage drops in the system and better regulation 
5. Less strain on the excitation system of generators and lesser excitation losses. 
6. Increase in the ability of the generators to meet the system peak demand thanks to the released capacity and lesser power losses. 

Shunt capacitive compensation delivers maximum benefit when employed right across the load. And employing compensation in HT & LT distribution network is the closest one can get to the load in a power network. However, various considerations like ease of operation and control, economy achievable by lumping shunt compensation at EHV stations etc will tend to shift a portion of shunt compensation to EHV & HV substations. Power utilities in most countries employ about 60% capacitors on feeders, 30% capacitors on the substation buses and the remaining 10% on the transmission system. Application of capacitors on the LT side is not usually resorted to by the utilities. 

Just as a lagging system power factor is detrimental to the system on various counts, a leading system pf is also undesirable. It tends to result in over-voltages, higher losses, lesser capacity utilisation, and reduced stability margin in the generators. The reduced stability margin makes a leading power factor operation of the system much more undesirable than the lagging p.f operation.  This fact has to be given due to consideration in designing shunt compensation in view of changing reactive load levels in a power network.

Shunt compensation is successful in reducing voltage drop and power loss problems in the network under steady load conditions. But the voltage dips produced by DOL starting of large motors, motors driving sharply fluctuating or periodically varying loads, arc furnaces, welding units etc cannot be improved by shunt capacitors since it would require a rapidly varying compensation level. The voltage dips, especially in the case of a low short circuit capacity system can result in annoying lamp-flicker, dropping out of motor contactors due to U/V pick up, stalling of loaded motors etc and fixed or switched shunt capacitors are powerless against these voltage dips. But Thyristor controlled Static Var compensators with a fast response will be able to alleviate the voltage dip problem effectively.

2.   Why Use Series Capacitor Compensation in Distribution Systems? 

Shunt compensation essentially reduces the current flow everywhere upto the point where capacitors are located and all other advantages follow from this fact.But series compensation acts directly on the series reactance of the line. It reduces the transfer reactance between supply point and the load and thereby reduces the voltage drop. Series capacitor can be thought of as a voltage regulator, which adds a voltage proportional to the load current and there by improves the load voltage. 

Series compensation is employed in EHV lines to 1) improve the power transfer capability 2) improve voltage regulation 3) improve the load sharing between parallel lines. Economic factors along with the possible occurrence of sub-synchronous resonance in the system will decide the extent of compensation employed. 

Series capacitors, with their inherent ability to add a voltage proportional to load current, will be the ideal solution for handling the voltage dip problem brought about by motor starting, arc furnaces, welders etc. And, usually the application of series compensation in distribution system is limited to this due to the complex protection required for the capacitors and the consequent high cost. Also, some problems like self-excitation of motors during starting, ferroresonance, steady hunting of synchronous motors etc discourages wide spread use of series compensation in distribution systems.

12. STATIC VARIABLE SPEED DRIVE

Variable speed drives are equipment that is designed to regulate the torque output, or speed and rotational force, of an electric motor. The use of these drives significantly increase the efficiency of an electric motor by controlling the power fed into the machine based on the current demand or work required of the motor and are sometimes referred to as variable frequency drives. As a result, the motor is not driven at full capacity except when needed which can result in a significant increase in energy savings and a resulting prolonged life of the motor.

Types of Variable Speed Drives?

There are a number of types of variable speed or frequency drives in use throughout industry. Most of the drive types are designed to aid motors that use alternating current to achieve multiple speeds of operation.

AC Variable Speed Drive – Also known as an AC variable frequency drive (VFD). These drives control motors that use alternating current and are the most prevalent in industry.

DC Variable Speed Drive – These drives are designed to control a shunt would direct current (DC) motor which has separate shunt and armature circuits.

Pulse Width Modulated Frequency Drive (PWM) – The most complex of the variable speed drives, but results in the most efficiency through the use of transistors. These transistors are used to switch the DC at different frequencies which deliver voltage pulses to the motor.

Flux Vector Pulse Width Variable Speed Drives – These are one of the newer types of drives to be used in industry. The drive makes use of a control system that is similar to those used in a DC motor and have a microprocessor which is used to regulate motor operation.

Variable Voltage Input Speed Drive (VVI) – Also known as a VVI frequency drive. These drives are the most basic of the variable speed drives and make use of an output switching device which creates a new sine wave for the motors voltage through inputting a sequence of square waves at varying voltages.
Current Source Input Variable Speed Drive (CSI) – These drives are closely related to the VVI drives except that they force a square wave of current instead of voltage and require a large inverter in order to maintain a stable current.