Indoor and Outdoor Substations | Electrical Engineering

In this article we will discuss about the indoor substations and outdoor substations.

Indoor Substations:

In these substations, the apparatus is installed within the substation building. Such substations are usually for a voltage up to 11,000 V but can be erected for 33,000 V and 66,000 volts when the surrounding atmosphere is contaminated with impurities such as metal corroding gases and fumes, conductive dust etc. The switchgear on supply or primary side will consist of oil circuit breakers only.

The high voltage supply is given to the primary of the transformer through circuit breaker. From the bus-bar various feeders emerge out. The panel for each feeder consists of an isolator switch and circuit breaker. In addition to isolator and circuit breaker, the panel is provided with the measuring instruments. As regards protection of feeders usually reverse power relay is employed. For the protection of oil filled transformers with conservator tanks Buchholz’s relay is most common.

The auxiliaries of the indoor type of substations are:

(i) Storage batteries,

(ii) Fire-fighting equipment such as water buckets, fire extinguishers, etc.

The batteries are used for the:

(i) Operation of the protective gear and switch operating solenoids, and

(ii) Emergency lighting in substations in case of failure of supply.

The several compartments in which the indoor substation is divided are control compartment, indicating and metering instruments and protective device compartment, circuit breaker and operating mechanism compartment, main bus-bar compartment and current transformer and cable sealing box compartment.

Indoor distribution and transformer substations as well as high voltage switchboards consist of a series of open and enclosed chambers or compartments. The main equipment of the given installation is arranged in these compartments. The chamber space, within which the equipment of any one main bus-bar connection is mounted, as a whole, is called as a cell, cubicle or compartment.

According to construction indoor distribution transformer substations and high voltage switchboards are further subdivided into the following categories:

1. Substations of the Integrally Built Type:

It is that in which the apparatus is installed on site. In such substations the cell structures are constructed of concrete or bricks.

2. Substations of the Composite Built-Up Type:

It is that type in which the assemblies and parts are factory or workshop prefabricated, but is assembled on site within a substation switchgear room. The compartments of such substations take form of metal cabinets or enclosures, each of which contains the equipment of one main connection cell. Within such cabinets or enclosures an oil minimum circuit breaker, a load-interrupter switch and one or more voltage transformers may be mounted.

3. Unit Type Factory Fabricated Substations and Metal Clad Switchboards:

These are built in electrical workshops and are shipped to the site of installations fully pre-assembled. After installations of substations and switchboards only connection to the incoming and outgoing power circuits are required to be made. Cubicles for unit type switchboards or substations take the form of fully enclosed metal clad cabinets.

Metal clad cubicles designed with withdrawable trucks and divided into several compartments are usually employed. The several compartments in which the cubicle is divided are control compartment, indicating and metering instrument and protective device compartment, circuit breaker and operating mechanism compartment, main bus-bar compartment and current transformers and cable sealing box compartment.

By partitioning of the cubicle space into compartments safe access to the apparatus is provided. The circuit breaker and its operating mechanism are mounted on the truck, which can be withdrawn from the cubicle. In withdrawable-truck unit-type cubicles the isolating device is of the plug-in-type.

When the truck is rolled out from the cubicle the holes in which the isolating devices enter for making contacts are automatically closed by metal shutters serving to isolate the live parts from possible casual contact. When the truck is rolled back into the cubicle, the shutters open automatically.

To prevent any possible opening or closing of the disconnecting devices when the circuit breaker is closed, these cubicles are designed with interlocks which prevent the truck from being rolled in or withdrawn when the circuit breaker is closed.

Outdoor Substations:

Outdoor substations are of two types namely:

1. Pole-mounted substations, and

2. Foundation-mounted substations.

1. Pole-Mounted Substations:

Such substations are erected for mounting distribution transformers of capacity up to 250 kVA. Such substations are cheapest, simple and smallest of substations. All the equipment is of outdoor type and mounted on the supporting structure of ht distribution line. Triple Pole Mechanically Operated (TPMO) switch is used for switching “on” and “off’ of ht transmission line. HT fuse unit is installed for protection of ht side.

To control ht side iron clad low tension switch of suitable capacity with fuses is installed. Lightning arresters are installed over the ht line to protect the transformer from the surges. Substation is earthed at two or more places.

Generally a transformer of capacity up to 125 kVA are mounted on double pole structure and for transformers of capacity above 125 kVA but not exceeding 250 kVA 4-pole structures with suitable platform is used. This type of pole-mounted substation is erected in very thickly populated location.

The maintenance cost of such substations is low and by using a large number of such substations in a town it is possible to lay the distributors, at a lower cost. But owing to increase in number of transformers, total kVA is increased, no load losses increase and the cost per kVA is thus more. Economy is the main consideration when a choice is made for such substations.

2. Foundation Mounted Substations:

These substations are built entirely in the open and in such substations all the equipment is assembled into one unit usually enclosed by a fence from the point of view of safety. Substations for primary and secondary transmission and for secondary distribution, (above 250 kVA) are foundation mounted outdoor type. Since equipment required for such substations is heavy, therefore, site selected for these substations must have a good access for heavy transport.

Again, owing to exposed bus-bars and other associated equipment the clearances and spacings are not only to be governed by the operating voltage but also from the considerations of the encroachment from outside. Low level types of substations are designed except when the space available is limited as these provide facility of easy inspection, cleaning and maintenance.

The switchgear consists of circuit breakers of suitable type on both the sides but with the increased reliability of the modern transformers, the practice is to dispense with the circuit breaker on the incoming side form the economic consideration. The isolating switches thus serve the purpose.

Advantages and Disadvantages of Outdoor Substations over Indoor Substations:

The outdoor substations have the following main advantages over indoor substations:

(i) All the equipment is within view and therefore fault location is easier.

(ii) The extension of the installation is easier, if required.

(iii) The time required in erection of such substations is lesser.

(iv) The smaller amount of building materials (steel-concrete) is required.

(v) The construction work required is comparatively smaller and cost of the switchgear installation is low.

(vi) There is practically no danger of a fault which appears at one point being carried over to another point in the installation because the apparatus of the adjoining connections can be spaced liberally, without any appreciable increase in costs.

(vii) Repairing work is easy.

The disadvantages of outdoor installations in comparison of indoor installations are:

(i) The various switching operations with the isolators, as well as supervision and maintenance of the apparatus are to be performed in the open air during all kinds of weather.

(ii) More space is required for the substation.

(iii) Protection devices are required to be installed for protection against lightning surges.

(iv) The length of control cables required is more.

(v) The influence of rapid fluctuation in ambient temperature and dust and dirt deposits upon the outdoor substation equipment makes it necessary to install apparatus specially designed for outdoor service and, therefore, more costly.

Notwithstanding the disadvantages, outdoor substations are very widely used in power systems.