Single-phase transformer banking system
Three-phase systems can be created through three single-phase transformer banking. This banking system is more eye-catching when traveling in rural areas. The Rural Electrification Board gives priority to the banking system for a three-phase supply.
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What is a Single Phase Transformer Banking System?
Single-phase transformer banking is the process of providing three phase supply by connecting three single phases 6.35 kV / 230-volt transformers.
How can this banking system be created?
Banking can be done in four ways with three single-phase transformers. E.g.
- Star Star Connection
- Delta Delta Connection
- Star Delta Connection
- Delta Star Connection
- Star Star Connection
For ease of understanding, the three terminals on the HV (High Voltage) side are marked as A, B, C, and the three terminals on the LV (Low Voltage) side are marked with a, b, c. The polarity is also denoted by A1, A2, B1, B2, a1, a2, b1, b2, c1, c2. The Star Star connection has two common connections on the HV and LV sides separately. Terminals A1, B1, and C1 on the HV side are common in the figure below.
Terminals a1, b1, c1 are common on the LV side. In this type of connection, the voltage on both sides is in the same phase. So this connection is also called Star Star 0 degree connection. If a2, b2, c2 terminals were common on the LV side then this connection would be called Star Star 180 degree connection. Because then the voltage on both sides would be in a 180-degree phase.
Delta Delta Connection
In this type of connection, A1 terminal B2, B1 terminal C2, and C1 terminal A2 are connected on the HV side. Similarly, on the LV side, the a1 terminal is connected to b2, the b1 terminal to c2, and the c1 terminal to a2. In this connection, the voltage on both sides is in phase. So it is called delta 0-degree connection. If the LV side connection was reversed, this connection would be a Delta Delta 180 degree connection.
Star Delta Connection
If you understand the above two types of connections, you can tell yourself how this connection will be. Since the connection is Star Delta, A1, B1, C1 terminal common connection is given on the HV side for Star connection. And for delta connection on the LV side a1 is connected to b2, b1 to c2, and c1 to a2.
In this connection, the voltage on the HV side is at a 30-degree lag. So this connection is called the Star Delta-30 degree connection. +30 degree connection is possible if the reverse connection is given on the LV side. This type of banking system is used by the rural electricity associations under the Bangladesh Rural Electrification Board.
Delta Star Connection
In this connection, the A1 terminal on the HV side is connected to B2, the B1 terminal to C2, the C1 terminal to A2. On the other hand, common connections are given at terminals a1, b1, c1 on the secondary side.
Now the question may arise in the minds of many, we can get a three-phase supply only by installing a three-phase transformer. Why bother with banking? In fact, there are some advantages of providing three phase supply by banking three transformers of a single phase.
Advantages-disadvantages of this system subject to a three-phase transformer
This system is easy to install and easily changeable. On the other hand, the installation of a single three-phase transformer is not easy. Maintenance costs are very low in the banking system. Because the market price of a single-phase transformer is much lower. But the maintenance cost of a single three-phase transformer is high.
Maintenance is easy as the three transformers are separate in the banking system. On the other hand, maintenance in three-phase transformers is not easy. However, the banking system needs a lot of space. But a three-phase transformer does not cost much space. The losses are relatively high as the single-phase transformer banking system has more cores.
The banking system requires six bushings on a Star Delta connection but three bushings on a single three-phase transformer are sufficient.