Why is an auxiliary Unit Transformer and a Station Transformer used?

What is the function of a station transformer and auxiliary transformer in a Power Plant and a Power Station?

If you work in a power plant or power grid station or are familiar with such kind of power generation or power transmission system, you must know the function of the station transformer or auxiliary transformer. I feel obliged to answer this question for those who are a little bit far from this kind of working field. 

Let it make it easier to understand the station transformer, assuming that you have some basic knowledge about how a power plant or power grid station, or substation works. Any electrical or mechanical device may go out of order at any time. In such a situation you cannot afford a plant shut down due to the failure of any device. 

Hence arrangements are made such that each and every device or facility has a backup. Station and auxiliary transformers are also a part of such an arrangement.

Station or auxiliary transformers are invariably used in all Grid Sub-Stations & Power plants. The KVA ratings of station transformers are normally 100 KVA, 33/0.4 kV. It is connected to the 33kV Main /Transfer Bus bar in the Grid substation with required protection & control.

Single Line Diagram of Auxiliary Unit Transformer 

Station transformers are transformers in which input is from the generator and output goes to the bus bar. And auxiliary transformers are those which are used for the auxiliary machines which are used inside the plant to run the plant.

The Station Transformer is used for 

In any power plant, station transformers will get the supply from the grid to provide supply to the auxiliaries which are common to all the units, all illumination of the Sub-Station yard & Control Room. Other loads such as ceiling fans and exhaust fans are also met from Station Transformer. 

The Station Transformer supply is used for Battery Charging too.

Routine works like cooling system using the external fan for transformers, air-conditioning of the control room, equipment includes Oil Filtration machine, Drilling machine, Grinder machine for sharpening Tools & Plants, etc.

The power supply, required for Civil work for the Mixture machine, Vibrator, etc. met from Station Transformer and any other temporary works that may come where power supply is required.

Unit Auxiliary Transformer is used for 

An auxiliary transformer is a smaller transformer than the main supply transformer in a piece of equipment that is commonly used to energize a master control relay which switches on the main supply.

The Unit Auxiliary Transformer is the same as other Power Transformers that provide power supply to the auxiliary equipment of a power plant or power generating station or power grid station or power grid substation during its normal operation. UAT is connected directly to the generator output or High voltage or Extra high voltage incoming bus by a tap-off of the isolated phase bus duct and thus becomes the cheapest source of power to the generating station.

UAT or Unit Auxiliary Transformer is generally a three-winding transformer i.e. one primary and two separate secondary windings. The primary winding of UAT is equal to the main generator voltage rating. The secondary windings can have the same or different voltages i.e. generally, 11KV and or 6.9KV as per plant layout.

Unit Auxiliary Transformer [UAT] will get the Supply from Generator Transformer which is charged only when the unit is running. If the unit is not running then UAT gets the supply from the station transformer.

These transformers are connected to the Generator Transformer bus. These transformers step down the voltage from 230kV or 400kV to 6.6kV (230/6.6kV or 400kV/6.6kV) and supply the power to the electrical auxiliaries present in the plant (motors, drives, lighting, and other plant loads as described above in station transformer).

Station service voltage transformer SSVT 

ABB developed the alternative auxiliary transformer called SSVT or Station Service Voltage Transformer, these captured the market in North America and remain an innovator in this field with new designs. 

SSVT inductive voltage transformers consist of a single-phase design intended for connection between phase and ground on 46 kV-362 kV HV grounded neutral networks to supply power to panels at low voltage or medium voltage.

The design allows for convenient sitting within the substation environment for simple mounting to single-phase supports. The SSVT is directly connected to the HV network and provides power to the panel within the substation, or to remote loads directly supplied for the HV service.

Capacity ratings 46kV to 362kV

ABB also introduced SSVM, meaning of SSVM is station service medium voltage transformer that a variation of the SSVT, the SSMV steps down the power from a high-voltage transmission line to a medium-voltage output. High-side voltage classes range from 46 to 138 kV with a 15 kV output and a maximum rating of 200 kVA. 

Applications of SSVT

1. Station service auxiliary power needs;
2. Railway applications for small power needs;
3. Remote site power for small industrial loads or mining sites;
4. Rural electrification for reducing the cost of first power.

Applications SSVM

1. Rural/remote area electrification
2. Oil and gas field power
3. Railroad electrification
4. Mining and construction power
5. Hospital and defense installations
6. Airports and transportation hubs
7. What is an Auxiliary Load

Auxiliary Load means, in relation to a generating CMU or a generating unit, the total amount of electricity used by that unit for purposes directly related to its operation (including fuel handling, fuel preparation, maintenance, and the pumping of water), whether or not that electricity is generated by the unit or used while the unit is generating electricity.

What is Auxiliary Power?

Auxiliary power is electric power provided by an alternate source and serves as a backup for the primary power source at the station's main bus or prescribed sub-bus. An offline unit provides electrical isolation between the primary power source and the critical technical load whereas an online unit does not.

You may know the details about the electrical transformer from the following articles:

 

1. Working Principle of Transformer;
2. Transformer Construction;
3. Core-type Transformers;
4. Shell-type Transformers;
5. Elementary Theory of an Ideal Transformer;
6. E.M.F. Equation of Transformer;
7. Voltage Transformation Ratio;
8. Transformer with losses but no Magnetic Leakage;
9. Transformer on No-load;
10. Transformer on Load;
11. Transformer with Winding Resistance but no Magnetic Leakage;
12. Equivalent Resistance;
13. Magnetic Leakage;
14. Transformer with Resistance and Leakage Reactance;
15. Simplified Diagram;
16. Total Approximate Voltage Drop in Transformer;
17. Exact Voltage Drop;
18. Equivalent Circuit Transformer Tests;
19. Open-circuit or No-load Test;
20. Separation of Core Losses;
21. Short-Circuit or Impedance Test;
22. Why Transformer Rating in KVA?;
23. Regulation of a Transformer;
24. Percentage Resistance, Reactance, and Impedance;
25. Kapp Regulation Diagram;
26. Sumpner or Back-to-back-Test;
    
27. The efficiency of a Transformer;
28. Condition for Maximum Efficiency;
29. Variation of Efficiency with Power Factor;
30. All-day Efficiency;
31. Auto-transformer;
32. Conversion of 2-Winding Transformer into Auto-transformer;
33. Parallel Operation of Single-phase Transformers;
34. Questions and Answers on Transformers;
35. Three-phase Transformers;
36. Three-phase Transformer Connections;
37. Star/Star or Y/Y Connection;
38. Delta-Delta or ∆/∆ Connection;
39. Wye/Delta or Y/ Connection;
40. Delta/Wye or ∆/Y Connection;
41. Open-Delta or V-V Connection;
42. Power Supplied by V-V Bank;
43. Scott Connection or T-T Connection;
44. Three-phase to Two-Phase Conversion and vice-versa;
45. Parallel Operation of 3-phase Transformers;
46. Instrument Transformers;
47. Current Transformers;
48. Potential or Voltage Transformers.