Substation Layout

1- Types of Substations

Electrical power plants are generally constructed far from the centers of electrical loads due to the economic feasibility of establishing the plant. In order to transmit electrical energy over long distances, the voltage is raised to reduce the losses in the power lines, but when this energy is used, the voltage must be reduced again to adapt to the electrical loads. Transformer stations are generally divided into:

1. Transformer stations to raise the voltage of the generator in the generating stations through step up transformers , and the transformers are called machine transforms because of their connection to the generator. Most of these transformers contain two coils, where the transformer coil connected to the generator is connected in the form of a delta, and on the network side (the other coil) is a star (Star) and the tie point (Star point) is directly connected to the ground (Solidly earthed).In some cases (relatively few), the converter has three coils in order to obtain two different voltage values. Also, the impervious equivalence point of the transformer may be connected to the ground.

2. Network Transformer Substations

These stations contain transformers to reduce the voltage - but it does not yet reach the distribution voltage - and autotransformers may be used in these stations because of their economic feasibility, and these transformers may give one or two values of voltage in the secondary circuit according to the number of coils.

Electrical power grids usually contain another set of transformer stations to reduce the voltage again – but it does not yet reach the distribution voltage – for example, 220/500 kV transformers and 220/66 kV transformers, and the locations of these stations are selected according to the distribution of the electrical load centers.

C- Distribution Substations Distribution networks start with distribution transformer stations, which are stations to reduce the voltage for a second time (step-down transformer) and from the secondary circuit of the transformers, the power lines extend to the distribution transformers, from which the secondary feeder lines for the end user come out Electrical loads are divided in terms of the nature of consumption into industrial loads, commercial loads, residential loads , and special loads that require requirements that must be met in the source of electrical supply for them.

In terms of construction features, transformer stations are divided into:

1. Outdoor Substation

2. Indoor Substation

3. Underground Substation

4. Hanging stations, whether open or in a Pole mounting booth - Open or Kiosk

The type of station depends on the voltage conversion ratio, economic factors, the capacity and nature of the place, safety measures, and others. Figure 1 shows a diagram of a transformer station within a building.

Figure (1)

Mobile Substations

Despite the considerations of protection and backup, the collapse of the feeder buildings, transformers, or both may occur in the transformer station, which causes the power supply to be completely cut off for a period of time, in this case, a mobile transformer station can be used, all its devices are mounted on large tractors with a Large Tractor Trailer, and in general, the capacity of these stations does not exceed 40 MVA - this causes considerations of size and weight of the movement, and this station can be equipped within three to six hours. Shape No. (2) shows an example of this type of transformer station.

Figure (2): Portable Transformer Station

2- Civil and Electrical Works for Transformer Stations

Civil and Electrical works in Substations

2-1 Civil works include:

1. Buildings, whether residential or non-residential (offices, warehouses, repair workshops, control galaxies, etc.)

2. Railways and Suspended Cranes

Railway track and overhead cranes cable trenches

3. Cable Walkways

4. Roads and Trails

5. Fencing Cutting Devices Protection Fence Around Switch yard

2.2 Electrical Works

1. Choosing the arrangement of the distribution bars Bas Bar arrangement
2. Selection of Isolators
3. Selection of Instrument Transformers
4. Choosing Circuit Breakers
5. Lightning arrestors
6. Choosing Power Transformers
7. Protective relaying schemes
8. Choosing Voltage regulating equipment
9. Cable Selection
10. Choosing Earthing Systems
11. Illumination System Selection
12. Choosing Fire Protection System
13. Communication Systems Selection
14. Selection of Auxiliary Supply
15. Choosing Interlocks for Regulation

In general, the design of the transformer station should achieve flexibility , simplicity, low economic cost with high quality performance.

3- Electrical layout of substation

The electrical pattern of the substation shows the method of arranging the distribution rods and the relative places of the devices and connecting them with lines, and this diagram may also be called a key diagram . While in the civil diagram, the devices are illustrated while maintaining the distances and angle of inclination with an appropriate drawing scale. Figure (3) shows an example of a transformer station.

Figure (3): Transformer Station

4- Bus Bars

Distribution rods are one of the most important components of the stations, as they pass through high electrical power and any malfunction in them causes power interruption, so the quality of the electrical design must be taken into account. and the structural aspect of the installation to withstand the forces exposed to it.

4.1 Types of Distribution Rods

There are two main types:

1. Steel Rod Rigid bus

2. Strain bus

Rigid rods are usually used in the case of medium and low voltage, and they are made of aluminum or copper in the form of bars or tubes, and they are used for fixing and insulation, and in the case of high voltage, flexible rods made of stranded and reinforced aluminum wires are used or of copper, and they are installed on strain type insulators.

4-2 Bus conductor material

The choice of the material from which the rods are made depends on several factors, which are:

1. The value of the loss in the allowable voltage

2. Loss of electrical power

3. Passing Current Capacity

4. The value of the Qasr Stream

5. Metal corrosion

6. Dynamic wind load and static load of accumulated snow

In general, heat-treated aluminum, copper, and aluminum alloys are used, especially in the case of high-voltage and ultra-high voltage pipes.

Aluminum is distinguished from copper in that it is less in weight. It also requires less maintenance, and if it has a certain current capacity and a permissible temperature rise, the aluminum conductor section is about 33% larger than the copper conductor.

4.3 Common bus bar arrangement

4.3.1 Single bus bar

Figure 3 illustrates an example of this case.

4.3.2 Single bar with single bus bar divider with bus sectionalizer

The single rod is characterized by simplicity and low costs when implementing, and to increase the degree of reliability, it can be divided into parts – usually only two parts – as in Figure (4). Each part is connected to a feeder and on the other hand electrical loads are connected. To increase efficiency, it is preferable to use circuit breakers to separate the parts, rather than using an isolator breaker, as there can be a difference in voltage between the two parts, so that current passes through the connection.

Figure (4): Single rod with divider

4.3.3 Double bus bar

To increase the degree of reliability and with the possibility of increasing the cost, dual rods are used, and the operation is as follows:

1. Loads are distributed on the rails

2. The loads are divided into groups and each set is fed from the rod

3. Maintenance can be done for one rod without cutting off the current to the loads by transferring it to the other rod

Figure (5) shows how to connect the two rods to a bus coupler circuit breaker to transfer loads from one rod to another, since the separation knife cannot cut off the load current. When performing maintenance of the cutting devices, the feed must be disconnected. This system may call two rods with a single bus bar.

1. Double bus bar double circuit breaker

Figure (6) shows a diagram of this system which is considered to be high cost. Therefore, its use in transformer stations is relatively small and it is used in large transformer stations.

2. Double Rail and One and a Half Circuit Breaker Double bus bar one and half circuit breaker

Figure (7) shows a diagram of this system and shows the savings in the number of cutting devices, for every two circuits there is one spare breaker. This system requires complex protective devices and is therefore not commonly used.

Figure (5): Two-rail system

Figure (6): Dual Rails and Pieces

Figure (7): Two-rail, one-and-a-half cutting device

4.3.6 Main and transfer bus bar

The diagram shown in Figure (8) allows for maintenance of the cutting devices, but more isolators are used and care is required when moving from one rod to another.

4.3.7 Two rods with side separator

Figure (9) shows the diagram of this system, which is a mixture of a two-bar system, a basic rod system, and a conversion rod. Maintenance of cutting devices is allowed, but more separators are used. It is characterized by simplicity in operation. For maintenance, the feeder can be cut off briefly, for example during the period of light loads, during which the transfer is carried out while avoiding placing the separators on a single circuit.

4.3.8 Mesh or ring scheme

The diagram shown in Figure (10) shows the arrangement of the separator and cutting devices, which is characterized by the following:

1. Allows dual feed per circuit

2. Allows maintenance of cutting devices

3. Lower cost than a bi-rod system

It is recommended that the number of circuits fed by the system should not exceed 6 circuits, so it is not suitable in the case of networks in which expansion is underway.

Figure (8)

Figure (9): Two rods with side separator

Figure (10): Network or Loop Diagram

5 - Transformer Stations in Distribution Systems

Figure (11) shows the simplest picture of a distribution station, where the station is fed by one main distributor and shows the breaker device and the separation equipment, and the disconnect switch of the cutting device can be omitted in terms of electrical load, although it is preferable to place the disconnect switches before and after the cutter.

Figure (12) shows a transformer station containing two transformers, each transformer is connected to an independent feeder, and the transformer capacity is 75% of the station's capacity. This system is characterized by a relatively short yar value and the existing load disconnect switch connected to the distribution rod is closed only if one of the switches on the transformer's secondary coil is in the open position.

Figure (13) shows a transformer station with a higher economic cost and a higher degree of dependence. The station has cutting devices connected to the distribution rod as well as the terminal of the transformer secondary coil and the system works automatically to ensure continuous feeding.