Transformer Operation and Testing

Transformer Operation and Testing Objectives

• The objective is to grasp transformers and their equivalent circuits for practical use.
• Another objective is to learn how to determine a transformer's polarity.
• It's important to understand transformer tests, including open and short circuit tests.
• There is also an aim to learn about voltage regulation and its significance.
• The main goal is to interpret the results derived from transformer tests.

Introduction to Transformers

• Electrical engineers need to understand the parts of an electricity pole.
• Transformers are a key components to understand.

Equivalent Circuit of a Transformer

• The equivalent circuit represents the transformer's electrical traits, including winding and core losses.
• It also represents the magnetizing current.

Types of Transformers

• Transformers come in step-down and step-up types.
• Step-down transformers reduce voltage.
• Step-up transformers increase voltage.

Winding Losses (Copper Loss)

• Copper losses occur in the primary(R1) and secondary (R2) windings.
• Leakage flux (X1 and X2) escapes from the core through one winding.

Core Loss

• Core losses include Eddy current and Hysteresis losses.
• Eddy current losses result from resistive heating of the core.
• Hysteresis losses are energy lost rearranging magnetic domains in the core.

Actual Transformer Components

• Transformers have primary bushings (H1 & H2).
• Secondary bushings (X1, X2, X3) and core.
• They also have grounding straps.

Actual Transformer Circuit Diagram

• Three-phase power lines (A, B, C) carry electricity to the transformer's primary windings from a power source.
• Primary terminals H1 and H2 connect the transformer to two phases, creating a magnetic field.
• This magnetic field induces voltage in the secondary windings through electromagnetic induction.

Secondary Side of Transformer

• The secondary side has a center-tapped winding with output points X1, X2, and X3.
• The center tap (X2) divides the secondary winding into two halves.
• The voltage between X1 and X3 is 240V, for appliances like air conditioners.
• Each winding half (X1 to X2, X3 to X2) delivers 120V for standard outlets.
• The center tap is neutral, grounded, creating a safe reference, stabilizing the 120V outputs.
• Single-phase transformers can be series or parallel.
• Transformers are AC devices without fixed polarity, needing comparative polarity markings when connected.

Polarity in Transformers

• Polarity refers to having two opposite points, determining current flow in electricity and magnets.
• In DC circuits, electrons flow neg to pos.
• In AC circuits, polarity switches causing current to change direction back and forth.
• Transformer polarity is either additive or subtractive.

Additive and Subtractive Polarity

• In additive polarity, X2 is left and X1 is right.
• In subtractive polarity, X1 is left and X2 is right.
• Transformer polarity depends on winding direction around the core.
• If primary and secondary coils are wound in opposite directions the polarity is additive.
• If they are wound in the same direction, it is subtractive.

AC Voltmeter Test for Polarity

• This test identifies transformer polarity before parallel connections.
• Incorrect polarity causes short circuits.

Additive and Subtractive Polarity Tests

• The secondary winding produces 24V from a 240V source because of the 10:1 ratio.

Additive Polarity Test

• X3 is connected to H1 using a jumper.
• The voltmeter measures 264V across H2 and X1.
• This additive polarity test shows the secondary voltage adds to the primary voltage

Subtractive Polarity Test

• X1 is connected to H1 using a jumper.
• The voltmeter measures 216V across H2 and X3.
• This subtractive polarity test shows the secondary voltage is 180° out of phase and opposes the primary voltage

Transformer Polarity Standards

• IEEE C57.12.00-2015 states single-phase transformers at 200 kVA or below and 8,660 volts or below must have additive polarity.
• All other single-phase transformers have subtractive polarity.
• Transformer polarity can be identified on the nameplate.
• The example transformer has a 15 kVA rating and a primary voltage of 7200V.
• It has additive polarity, meeting qualifications for an additive polarity transformer.

Transformer Testing

• Transformer testing checks if a transformer is working properly.
• Various tests measure performance and efficiency, guiding corrective actions.
• Transformer testing includes Type, Routine, Special, and Pre-Commission Tests.

Type Tests

• Type Tests confirm the basic design criteria.
• It includes winding resistance, transformer ratio, and vector group tests.
• The measurements include impedance voltage/short circuit impedance and load loss (short circuit test).
• There is also measurement of no-load loss and current (open circuit test).
• Insulation resistance and dielectric tests are conducted.
• Temperature rise tests and vacuum tests are also completed.

Routine Tests

• Routine Tests confirm performance of each unit in a production lot.
• These include winding resistance, transformer ratio, and vector group tests.
• The measurements include impedance voltage/short circuit impedance and load loss (short circuit test).
• There is also measurement of no-load loss and current (open circuit test).
• Insulation resistance and dielectric tests are conducted.
• There is further testing on on-load tap-changers and oil pressure.

Special Tests

• Special Tests are done under specific circumstances, not standard procedures.

Pre-Commissioning Tests

• Pre-Commissioning Tests ensure correct installation and good working condition.
• Two key tests covered are the Open Circuit Test and Short Circuit Test.

Open Circuit Test

• The Open Circuit Test finds shunt parameters of the transformer's equivalent circuit.
• The rated voltage is applied to one winding while keeping the other open.
• A voltmeter, wattmeter, and ammeter are connected to the LV side.
• A variac increases voltage until the voltmeter reads the rated LV voltage.
• Voltmeter, ammeter, and wattmeter readings are then recorded.

Open Circuit Test Computations

• Parameters like Pc can be computed using measured values.
• The no-load power factor (cos θ0) is computed using PC.
• Magnetizing and core loss are calculated using a vector diagram.
• Shunt parameters RC and Xm are computed from measured values.
• Calculations are often done on the LV side.
• If required, calculated values can be converted to the HV side.

Short Circuit Test

• The Short Circuit Test measures load loss and leakage impedance.
• A voltage is applied to the HV winding to circulate rated currents through both windings.
• LV winding is typically short-circuited for this test.
• The applied voltage is tuned to achieve full load current in both windings.

Short Circuit Test Computations

• Parameters like Req1, Xeq1, and Zeq1 are derived from measured power, voltage, and current.
• Transformer efficiency typically ranges from 95% to 99%.
• Large low-loss transformers can achieve efficiency as high as 99.7%.
• The efficiency is determined using losses measured via open-circuit and short-circuit tests.

Example Transformer Test Results

• A 50kVA, 250/500V, 50Hz single-phase transformer's test results are used to determine equivalent circuit parameters.

Voltage Regulation

• Voltage regulation measures percentage voltage rise at the transformer's terminals when the load is removed.
• It reflects the change in secondary voltage as the load varies, keeping primary voltage constant.
• Lower voltage regulation means better transformer performance.
• The general formula uses Vno load and Vfull load to calculate %VR.

Why Lower Voltage Regulation is Better

• Minimal voltage drop under load due to lower voltage regulation.
• Better efficiency because it indicates lower internal losses.
• Stable operation, so equipment works better with consistent voltage.

Importance of Voltage Regulation

• Voltage regulation stabilizes the voltage even with load changes.

Voltage Regulation Example

• Poor voltage regulation of 230V rated transformer reduces voltage to 190V under load.
• The voltage instability can cause device malfunction.

Learnings from Tests

• Open/short circuit tests determine transformer equivalent circuit parameters, efficiency, and voltage regulation.

Open Circuit Test Use

• Find core loss or constant loss at rated voltage and frequency.
• It helps to find no-load branch parameters.
• The test determines turns ratio.

Short Circuit Test Use

• Short Circuit Test can determine equivalent resistance and reactance.
• It can also find variable losses (copper loss).
• Using short circuit test data, voltage regulation can be determined.