Components of The Distribution System

In this section, we describe various components of the power distribution system, viz. substations, transformers, feeders, lines and metering arrangements.

Substation

A substation is the meeting point between the transmission grid and the distribution feeder system. This is where a fundamental change takes place within most T&D systems. The transmission and sub-transmission systems above the substation level usually form a network (about which you will study in the next section). But arranging a network configuration from the substation to the customer would simply be prohibitively expensive. Hence, most distribution systems are radial (also described in the next section), i.e., there is only one path through the other levels of the system.Typically, a substation consists of high and low voltage racks and buses for power flow, circuit breakers at the transmission and distribution level,metering equipment and the control house, where the relaying,measurement and control equipment is located. But the most important piece of equipment that gives the substation its capacity rating is the substation transformer. It converts the incoming power from transmission voltage levels to the lower primary voltage for distribution. Very often, a substation has more than one transformer.

Power Distribution Substations

Apart from the transformer, a substation has other equipment such as lightning arrestors, isolators, etc. You will learn about the substation equipment in detail in next unit. Here we give a brief introduction of the most critical component of a substation,
the transformer.

Transformer

A transformer is an electrical device that transfers power from one circuit to another without change in frequency. The purpose of a transformer is to convert one AC voltage to another AC voltage. A transformer comprises two or more coupled conducting coils (windings), which are wound on common laminated core of a magnetic material such as iron or iron-nickel alloy (Fig.). These are called primary and secondary windings.

The alternating current in the primary winding creates an alternating magnetic field in the core just as it would in an electromagnet. The secondary winding is wrapped around the same core. The changing magnetic flux (magnetic field per unit area per unit time) in the primary winding induces alternating current of the same frequency in the secondary winding. The voltage in the secondary winding is controlled by the ratio of the number of turns in the two windings.

If the primary and secondary windings have the same number of turns, the primary and secondary voltages will be the same. For step-down transformers, the secondary winding has lesser number of turns than the primary. For example, to step-down voltages from 240 V at the mains to 6 V, there needs to be 40 times more turns in the primary than in the secondary. In case of step-up transformers, the number of turns in the secondary winding is more than those in the primary winding.

The transformer is one of the simplest of electrical devices, yet transformer designs and materials continue to be improved every day.
For an ideal transformer, it is assumed that the entire magnetic flux linked with the primary winding is also linked to the secondary winding. However, in practice it is impossible to realize this condition. While a large portion of the flux called common or mutual magnetic flux links with both the coils, a small portion called the leakage flux links only with the primary winding. This leakage flux is responsible for the inductive reactance of a transformer.

Principle Underlying a Transformer

For an ideal transformer, it is assumed that the entire magnetic flux linked with the primary winding is also linked to the secondary winding. However, in practice it is impossible to realize this condition. While a large portion of the flux called common or mutual magnetic flux links with both the coils, a small portion called the leakage flux links only with the primary winding. This leakage flux is responsible for the inductive reactance of a transformer.

For an ideal transformer, it is assumed that the entire magnetic flux linked with the primary winding is also linked to the secondary winding. However, in practice it is impossible to realize this condition. While a large portion of the flux called common or mutual magnetic flux links with both the coils, a small portion called the leakage flux links only with the primary winding. This leakage flux is responsible for the inductive reactance of a transformer.

Types of Transformer

Transformers can be categorised based on the type of core used, type of cooling used, the method of mounting the transformer or the intended use for which it is designed. We shall deal with the first three categories of transformers in Unit 6. Here we give a brief introduction to the categorisation of transformers on the basis of their use as power transformers and distribution transformers. Power substations use power transformers while the distribution substations employ distribution transformers. While the underlying principle of operation is the same for both the transformers, they differ in their design since they are required to operate under different conditions at power and distribution substations.

Comparison of Power Transformers and Distribution Transformers

Feeders route the power from the substation throughout the service area.They are typically either overhead distribution lines mounted on wooden poles, or underground buried or ducted cable sets. Feeders operate at the primary distribution voltage in primary distribution system and secondary distribution voltage in the secondary distribution system.

Definition of a feeder

By definition, the feeder consists of all primary or secondary voltage level segments of distribution lines between two substations or between a substation and an open point (switch).The most common primary distribution voltages in use are 11 kV, 22 kV and 33 kV. The main feeder, which consists of three phases, may branch into several main routes.

The main branches end at open points where the feeder meets the ends of other feeders – points at which a normally open switch serves as an emergency tie between two feeders.Feeders are connected in a configuration, which depends on the type of network required in the distribution system. Three types of network are normally available in the electrical distribution system:

• radial;

• loop; and

• cross-loop network.

Since the radial feeder emanates from one point and ends at the other in the radial network, load transfer in the case of breakdown is not possible.Although a radial feeder can be loaded to its maximum capacity, in the case of breakdown, quite a large area may remain in dark until the fault is detected and repaired.

In loop arrangement, two feeders are connected to each other so that in the case of breakdown, the faulty section can be isolated and the rest of the portion can be switched on. In this type of system, the feeder is normally loaded to 70% of its capacity so that in the event of breakdown it can share the load of other feeders also.

A cross-loop network provides multiple paths and the flexibility further increases. In case of breakdown in any line, the faulty system can be isolated and supply can be resumed very quickly. In this type of network, feeders should normally be loaded to 70% of their current carrying capacity. This system is highly reliable, but very expensive.

In big cities, the concept of 33 kV ring main is very popular and two ring mains are laid: one outer and one inner. The outer ring main is laid using the panther conductor and the inner ring main is laid using the dog conductor.The use of these two types of ring mains provides excellent flexibility to the system and at the time of breakdown, supply can be immediately switched on from another 132 kV substation. While making any distribution planning for metros, the aspect of outer and inner 33 kV ringmains is extremely essential and should be included for providing uninterrupted supply.

Quantities

Meters are required to be installed at various points of the Distribution System including the substation equipment and the consumer end. They are required for correct recording of electrical quantities for operational and safety purposes as well as energy accounting. The meters installed at the interface points of generation-transmission and transmission-distribution are called interface meters. Meters installed at consumer premises by the utility are called consumer meters.

The Central Electricity Authority regulations on Installation and Operation of Meters provide for the type, standards, ownership, location, accuracy class,installation, operation, testing and maintenance, access, sealing, safety,meter reading and recording, meter failure or discrepancies, anti tampering features, quality assurance, calibration and periodical testing of meters,additional meters and adoption of new technologies in respect of the following meters for correct accounting, billing and audit of electricity:

Interface meter,

Consumer meter, and

Energy accounting and audit meter.

It is important to note that these regulations make the use of static meters mandatory for new consumers.