Transformer Open Circuit and Short Circuit Test

Open Circuit Or No Load Test and Short Circuit Or Impedance Test On Transformer can carry out economically without loading a Transformer.

Open Circuit Test And Short Circuit Test on Transformer are performed to find the parameters of the equivalent circuit of the transformer and losses of the transformer. These two tests are conducted to determine the core loss, copper loss, and equivalent circuit parameters of a transformer.

The performance of a transformer can be calculated on the basis of a Transformer  Equivalent Circuit which contains four main parameters:

1. the equivalent resistance R₀₁ as referred to as primary (or secondary R02); 
2. the equivalent leakage reactance X01 as referred to primary (or secondary X02);
3. the core-loss conductance G0 (or resistance R0); 
4. the magnetising susceptance B0 (or reactance X0). 

These constants or parameters can be easily determined by two tests:

1. open-circuit test and 
2. short-circuit test. 

These tests are very economical and convenient because they furnish the required information without actually loading the transformer. 

In fact, the testing of very large a.c. the machinery consists of running two tests similar to the open-circuit and short-circuit tests of a transformer.

Open Circuit Or No Load Test On Transformer

An open circuit test is performed in a transformer to find the resistance of the core and shell France we apply rated voltage in an open circuit test because to find the maximum admittance or resistance of the core and excitation branch.

The two components of no-load current can be given as,

I_μ = I₀sinΦ₀   and    I_w = I₀cosΦ₀.
cosΦ₀ (no-load power factor) = W / (V₁I₀). ... (W = wattmeter reading)

From this, shunt parameters of an equivalent circuit of the transformer (X₀ and R₀) can be calculated as

X₀ = V₁/I_μ  and  R₀ = V₁/I_w.

(These values are referring to the LV side of the transformer.)
Hence, it is seen that the open-circuit test gives core losses of the transformer and shunt parameters of the equivalent circuit.

Short Circuit Or Impedance Test On Transformer

we apply a short circuit test to find the series impedance of a transformer by adjusting the primary voltage until in secondary winding current is equal to the rated current.

Sense input voltage is low current flowing through the excitation branch is negligible and all voltage will drop in a transformer due to series element in a circuit.

The ammeter reading gives the primary equivalent of full load current (I_sc).

The voltage applied for full load current is very small as compared to the rated voltage. Hence, core loss due to small applied voltage can be neglected. Thus, the wattmeter reading can be taken as copper loss in the transformer.

Therefore, W = I_sc²R_eq....... (where R_eq is the equivalent resistance of the transformer)
 Z_eq = V_sc/I_sc.

Therefore, the equivalent reactance of the transformer can be calculated from the formula  Z_eq² = R_eq² + X_eq².

These values are referred to as the HV side of the transformer.
Hence, it is seen that the short circuit test gives copper losses of the transformer and approximate equivalent resistance and reactance of the transformer.

Transformer Open Circuit and Short Circuit Test Preparation.

To prepare for transformer open circuit and short circuit tests, you need to follow certain steps. These tests are conducted to determine the equivalent circuit parameters of a transformer. Here's a step-by-step guide to preparing for these tests:

Study the Theory: Familiarize yourself with the theoretical aspects of open circuit and short circuit tests. Understand the purpose of these tests, the equipment used, and the parameters that can be derived from the test results.

Review Transformer Equivalent Circuit: Gain a solid understanding of the equivalent circuit model of a transformer, which includes components like winding resistances, leakage reactances, and core losses.

Understand Test Equipment: Learn about the equipment required for these tests. For the open circuit test, you typically need a variable voltage source, wattmeter, and voltmeter. For the short circuit test, you'll require a variable voltage source, ammeter, and voltmeter.

Gather Test Setup: Make sure you have the necessary equipment and setup for conducting the tests. This may include connecting the transformer to the appropriate power sources and measuring devices.

Ensure Safety Precautions: Safety should always be a priority. Ensure that you're following proper electrical safety procedures, such as using insulated gloves and keeping a safe distance from live circuits.

Open Circuit Test: In the open circuit test, the secondary winding is left open while the primary winding is connected to a variable voltage source. Gradually increase the voltage and measure the primary voltage, primary current, and core losses using the wattmeter. Repeat this process for different voltage levels.

Short Circuit Test: In the short circuit test, the primary winding is short-circuited, and the secondary winding is connected to a variable voltage source. Gradually increase the voltage and measure the primary current, secondary voltage, and copper losses using the ammeter and voltmeter. Again, repeat this process for different voltage levels.

Record and Calculate Results: Record all the measured values from both tests. Use these values to calculate parameters such as winding resistances, leakage reactances, and core losses.

Compare and Analyze Results: Compare the calculated values with the manufacturer's specifications to ensure the transformer is functioning correctly. Analyze the results to identify any deviations or abnormalities.

Report and Documentation: Document your test procedure, measured values, and calculated parameters. Prepare a report summarizing the findings and any necessary recommendations or actions.

Remember, transformer testing can involve high voltages and currents, so it's crucial to have proper knowledge, training, and experience in working with electrical equipment. Always prioritize safety and seek professional assistance if needed.

What Test Equipment is Required for Transformer Open and Short Circuit Test?

The following test equipment is typically required for conducting transformer open circuit and short circuit tests:

Variable Voltage Source: A variable voltage source is needed for both the open circuit and short circuit tests. This source allows you to vary the voltage applied to the transformer during the tests. It can be a variable transformer or a variable autotransformer.

Wattmeter: A wattmeter is used in the open circuit test to measure the core losses of the transformer. It measures the power consumed by the primary winding when the secondary winding is left open.

Voltmeter: A voltmeter is used in both the open circuit and short circuit tests. It is used to measure the voltage across the primary winding of the transformer. In the open circuit test, it measures the primary voltage, while in the short circuit test, it measures the secondary voltage.

Ammeter: An ammeter is used in the short circuit test to measure the current flowing through the primary winding of the transformer when the secondary winding is shorted. It helps determine the copper losses of the transformer.

Safety Equipment: Safety equipment is essential for working with high voltages and currents. This may include insulated gloves, safety goggles, and other protective gear to ensure the safety of the operator.

It's important to note that the specific equipment requirements may vary depending on the size and rating of the transformer being tested. Additionally, some modern test equipment combines multiple functionalities, such as having built-in voltage and current measurement capabilities. Always refer to the manufacturer's instructions and safety guidelines when selecting and using the test equipment for transformer testing.

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.