Short Circuit and Open Circuit Test of Transformer

Short Circuit and Open Circuit Test of Transformer . Transformers are critical in electrical power systems, ensuring efficient voltage transformation for diverse applications. Testing their performance is essential for identifying losses, efficiency, and impedance. Among the various methods, the Short Circuit Test and Open Circuit Test of Transformer stand out as fundamental techniques. This guide explains these tests’ processes, purposes, and outcomes.

Understanding Transformer Testing

Transformer testing is a vital process to evaluate a transformer’s performance under different conditions. The key parameters assessed include losses, efficiency, and impedance. By simulating real-world conditions, these tests ensure optimal transformer operation and reliability.

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Basics of the Open Circuit Test

What is an Open Circuit Test?

An Open Circuit Test measures the core loss or no-load loss of a transformer. This test assesses the performance when the secondary winding is open, and only the primary winding is energized.

Importance and Objectives

Determines iron losses in the transformer core.
Identifies the magnetizing current required for efficient operation.
Helps calculate no-load parameters such as resistance and reactance.

Open Circuit Test (No-Load Test)

The open-circuit test (aka No-load test) is performed to determine the losses in a transformer such as core loss (iron loss), no-load current (I₀), and no-load equivalent circuit parameters (R₀ and X₀). This test is performed either on primary winding or secondary winding. But in most cases, this test is performed on low-voltage winding. Because it is difficult to obtain high voltage in laboratories and the current that passes through the high voltage winding is very small. So, it may be difficult to measure the accurate readings.

Therefore, the open circuit test is performed on low voltage winding. The experimental connection diagram of the open-circuit test on a single-phase transformer is shown in the figure below.

As shown in the above figure, the primary winding (low voltage winding) is supplied by rated voltage and frequency (commonly, a single phase supply from autotransformer). And the secondary winding is kept open. Now, a voltmeter V₀, an ammeter I₀, and a wattmeter W₀ are connected in the primary winding.

The secondary winding is kept open-circuited. Therefore, the current that passes through the secondary winding is zero. And the load is not connected. Hence, the current that passes through the primary winding is no-load current I0. The current that passes through the primary winding is measured by an ammeter that gives the value of no-load current.

The supply voltage given to the primary winding is rated voltage. So, the flux produced in the core of a transformer is normal. And this flux is the same for all loading conditions. The iron loss produced in the transformer depends on the supply voltage and frequency. In this test, we have given rated supply voltage and frequency. Hence, the iron loss or core loss produced in this test is the same for all loads.

The current that passes through the secondary winding is supplied to the iron loss and copper loss in the primary winding. No-load current passes through the primary winding that is very small (2 to 5 percent of full-load current). Therefore, we can neglect the copper loss. And the primary current is supplied for the core loss.

A wattmeter is connected to the primary winding that measures the supplied power. So, the wattmeter indicates the power loss occurred in the transformer core. In an open-circuit test, the reading of instruments is as below;

Ammeter: no-load Current I₀

Voltmeter: rated supply voltage V₁

Wattmeter: iron or core loss P_i

Observation Table

The observation table of an open circuit test is shown below.

Rated supply voltage V₁	No-load current I₀	Iron or core loss P_i
…..	…..	…..

Now, we can find the circuit parameter (R0 and X0) using the no-load current.

No-load power W₀ = V₁ I₀ Cos ϕ₀ = Iron loss

Working component of no-load current;

I_W = I₀ Cos ϕ₀

Magnetizing component of no-load current;

I_M = I₀ Sin ϕ₀

Now, from the working component and magnetizing component, we can find the no-load resistance and reactance as follows;

No-load Resistance;

Procedure for Conducting an Open Circuit Test

Test Setup:
- Connect a voltmeter, ammeter, and wattmeter to the primary winding.
- Leave the secondary winding open.
Steps:
- Apply rated voltage to the primary winding.
- Record the current, voltage, and power readings.
Key Measurements:
- Voltage: Measures the applied voltage to the primary winding.
- Current: Measures the no-load current.
- Power: Indicates core losses.

Analysis of Open Circuit Test Results

The Open Circuit Test primarily determines core losses (hysteresis and eddy current losses). Using the wattmeter readings, the no-load efficiency of the transformer can be calculated. These values are crucial for improving transformer design and operation.

Short Circuit Test (On-Load Test)

The short circuit test (aka On-load test) is performed on the high-voltage side and the low-voltage side is short-circuited. This test could be conducted on the low voltage side, but this test required hardly 5 to 7 percent of rated voltage. On the low voltage side, this voltage is quite small and has chances of a measurement error. Also, reduced voltage (5 to 7 percent) of the high voltage side is easily available in the laboratory. Therefore, it is convenient to perform short circuit tests on the high voltage side.

The schematic diagram of the short circuit test is shown in the figure below.

Usually, a low voltage winding is short-circuited using a thick wire. But in some cases, an ammeter is connected to measure the rated load current. An ammeter, a voltmeter, and a wattmeter are connected in the high voltage side as shown in the above figure. Here, we have considered primary winding as high voltage winding and secondary winding as low voltage winding.

The high voltage winding is supplied by the reduced input voltage from a variable supply source. The supply voltage gradually increases until full-load primary current flows through the primary winding. When full-load current passes through the primary winding, by transformer action, the current flows through the secondary winding are full-load secondary current.

So, the ammeter connected in the high voltage side measures the full-load primary current. The voltmeter measures the supplied voltage when full-load current flows through the primary winding. In this condition, the supplied voltage is hardly 5 to 10 percent of full load voltage. Due to low input voltage, the flux produced in the core is very low. And core loss is proportional to the square of flux. Hence, the core loss is very small that can be neglected.

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Also, the current that passes through the windings is a full-load current. So, a copper loss that occurs during a test is a normal full-load copper loss. And the wattmeter indicates the full-load copper loss. The secondary winding is short-circuited. So, the secondary voltage (output voltage) is zero. Therefore, the entire primary voltage is used to supply the voltage drop in total impedance referred to as the primary side.

The approximate equivalent circuit of the transformer under the short circuit test is shown in the figure below.

Observation Table:

V_SC Volts	I_SC Ampere	P_C Watt
…..	…..	…..

The reading of instruments in short-circuit test are as follows;

Ammeter: Full-load primary current (I_SC)
Voltmeter: Supplied voltage (V_SC)
Wattmeter: Full-load copper loss (P_C)

Full-load copper loss;

W_SC = I²_SC R₀₁

The equivalent resistance of transformer referred to primary;

equivalent-resistance-of-transformer-Primary

Equivalent impedance referred to primary;

equivalent-impedance-of-transformer-Primary

Equivalent reactance referred to primary;

equivalent-reactance-of-transformer-Primary

Power factor;

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Basics of the Short Circuit Test

What is a Short Circuit Test?

A Short Circuit Test evaluates the copper losses in the transformer windings. In this test, the secondary winding is short-circuited, and the primary winding is supplied with a reduced voltage.

Purpose and Significance

Determines full-load copper losses.
Calculates equivalent impedance and resistance of the windings.
Helps assess voltage regulation.

Procedure for Conducting a Short Circuit Test

Test Setup:
- Short-circuit the secondary winding.
- Connect measuring instruments to the primary winding.
Steps:
- Apply a small voltage to the primary winding.
- Gradually increase the voltage until the rated current flows in the primary winding.
- Record current, voltage, and power.
Safety Precautions:
- Ensure robust connections to prevent overheating.
- Use protective equipment to avoid accidents.

Analysis of Short Circuit Test Results

The wattmeter reading during this test indicates copper losses. Using these values, the transformer’s efficiency and impedance can be calculated. The results help in designing transformers with reduced operational losses.

Advantages of Performing Transformer Tests

Improved Efficiency:
- Identifies losses and enables corrective measures.
Enhanced Reliability:
- Detects potential weaknesses in design or manufacturing.
Cost Optimization:
- Prevents operational failures, reducing repair costs.
Performance Validation:
- Ensures the transformer meets design specifications.

Challenges and Limitations of Transformer Tests

Execution Complexity:
- Requires skilled personnel and specific instruments.
Safety Concerns:
- High voltage and current pose risks.
Equipment Constraints:
- Requires precise and calibrated measuring tools.

Applications of Short Circuit and Open Circuit Tests

These tests are widely used in industries such as:

Power Generation: Ensuring transformers deliver consistent performance.
Manufacturing: Validating transformer specifications during production.
Maintenance: Diagnosing faults and ensuring longevity.

Transformer Testing Standards and Guidelines

Adhering to industry standards, such as those from IEEE or IEC, ensures accuracy and safety during testing. Common guidelines include:

Using calibrated instruments.
Following safety protocols to prevent accidents.
Conducting tests in controlled environments.

Frequently Asked Questions (FAQs)

1. Why are Short Circuit and Open Circuit Tests important?

These tests are essential for evaluating transformer efficiency, losses, and impedance, ensuring optimal performance.

2. What losses are measured in the Open Circuit Test?

The Open Circuit Test measures core losses, including hysteresis and eddy current losses.

3. How does the Short Circuit Test determine copper losses?

The wattmeter reading during the Short Circuit Test reflects the power lost due to resistance in the transformer windings.