How are open-circuit and short-circuit tests used to determine transformer parameters?
1 Answer
Open-circuit and short-circuit tests are two important tests conducted on transformers to determine their parameters, such as winding resistance, leakage reactance, and core losses. These tests are performed during the transformer's commissioning or routine maintenance to ensure its proper functioning and to gather critical data for transformer modeling and performance analysis.
Open-circuit test (also called No-load test):
In the open-circuit test, the secondary winding of the transformer is left open, and the primary winding is connected to a variable AC voltage source. The primary voltage is gradually increased until the rated current flows through the primary winding. During this test, the following parameters are measured:
a. No-load current (Io): The primary current when the secondary winding is open.
b. No-load voltage (Vo): The voltage applied to the primary winding.
c. No-load power (Po): The power consumed by the transformer when no load is connected to the secondary winding.
From these measurements, the following parameters can be determined:
Winding resistance (R): Calculated from the ratio of the no-load power (Po) to the square of the no-load current (Io) using the formula: R = Po / (Io^2)
No-load impedance (Z0): It is the equivalent impedance of the transformer at no-load and is calculated by dividing the no-load voltage (Vo) by the no-load current (Io) using the formula: Z0 = Vo / Io
Short-circuit test (also called Impedance test):
In the short-circuit test, the secondary winding of the transformer is shorted, and the primary winding is connected to a variable AC voltage source. The voltage is adjusted until the rated current flows through the short-circuited primary winding. The following parameters are measured:
a. Short-circuit current (Isc): The primary current when the secondary winding is short-circuited.
b. Short-circuit voltage (Vsc): The voltage applied to the primary winding to achieve the rated short-circuit current.
From these measurements, the following parameters can be determined:
Total impedance (Zt): The total impedance of the transformer, including both the winding impedance and the core losses. It is calculated by dividing the short-circuit voltage (Vsc) by the short-circuit current (Isc) using the formula: Zt = Vsc / Isc
Leakage impedance (Zl): The impedance of the transformer's leakage reactance, which represents the leakage flux in the windings. It is calculated by subtracting the winding resistance (R) from the total impedance (Zt) using the formula: Zl = √(Zt^2 - R^2)
Core losses (Pc): Core losses in the transformer can be calculated by subtracting the product of winding resistance (R) and the square of the short-circuit current (Isc) from the short-circuit power (Psc) using the formula: Pc = Psc - R * Isc^2
Once these parameters are determined, they are used for designing transformer models and to assess the transformer's performance under different load conditions.
Open-circuit test (also called No-load test):
In the open-circuit test, the secondary winding of the transformer is left open, and the primary winding is connected to a variable AC voltage source. The primary voltage is gradually increased until the rated current flows through the primary winding. During this test, the following parameters are measured:
a. No-load current (Io): The primary current when the secondary winding is open.
b. No-load voltage (Vo): The voltage applied to the primary winding.
c. No-load power (Po): The power consumed by the transformer when no load is connected to the secondary winding.
From these measurements, the following parameters can be determined:
Winding resistance (R): Calculated from the ratio of the no-load power (Po) to the square of the no-load current (Io) using the formula: R = Po / (Io^2)
No-load impedance (Z0): It is the equivalent impedance of the transformer at no-load and is calculated by dividing the no-load voltage (Vo) by the no-load current (Io) using the formula: Z0 = Vo / Io
Short-circuit test (also called Impedance test):
In the short-circuit test, the secondary winding of the transformer is shorted, and the primary winding is connected to a variable AC voltage source. The voltage is adjusted until the rated current flows through the short-circuited primary winding. The following parameters are measured:
a. Short-circuit current (Isc): The primary current when the secondary winding is short-circuited.
b. Short-circuit voltage (Vsc): The voltage applied to the primary winding to achieve the rated short-circuit current.
From these measurements, the following parameters can be determined:
Total impedance (Zt): The total impedance of the transformer, including both the winding impedance and the core losses. It is calculated by dividing the short-circuit voltage (Vsc) by the short-circuit current (Isc) using the formula: Zt = Vsc / Isc
Leakage impedance (Zl): The impedance of the transformer's leakage reactance, which represents the leakage flux in the windings. It is calculated by subtracting the winding resistance (R) from the total impedance (Zt) using the formula: Zl = √(Zt^2 - R^2)
Core losses (Pc): Core losses in the transformer can be calculated by subtracting the product of winding resistance (R) and the square of the short-circuit current (Isc) from the short-circuit power (Psc) using the formula: Pc = Psc - R * Isc^2
Once these parameters are determined, they are used for designing transformer models and to assess the transformer's performance under different load conditions.