I. Adverse Consequences of Secondary Short Circuit of Voltage Transformer
Adverse consequences: If the secondary side of the voltage transformer is short-circuited, a very high short-circuit current will be generated, which may damage the transformer and even affect the safe operation of the primary circuit.
Therefore, both the primary and secondary sides of the voltage transformer must be equipped with fuses for short-circuit protection (this is exactly the opposite of the article about current transformers that was written earlier).
II. Causes of the adverse consequences (working principle of voltage transformer)
To understand the causes of the adverse consequences, one needs to know the principle of the voltage transformer.
1. The essence of voltage transformers
The essence of a voltage transformer is: it is a step-down transformer that operates in an open-circuit state (this is a crucial point and should be remembered). As shown in Figure 2.
From the schematic diagram in the above figure, it can be seen that the voltage transformer is essentially a transformer. The primary winding has more turns and the secondary winding has fewer turns, so it is a step-down transformer.
What causes the manufacturing of a step-down transformer? This is determined by the function of the voltage transformer. Its function is to measure voltage. Since the voltage in the power system is relatively high and cannot be directly measured with a voltmeter, it is necessary to convert the high voltage into a low voltage. After the voltage is reduced, it is connected to the voltmeter (or the measured voltage value can be sent to other loads) for voltage measurement (as shown in Figure 2).
The function of a voltage transformer is to measure voltage. Therefore, its secondary side is usually connected to loads with high impedance (resistance), such as voltage meters. The ideal resistance of a voltage meter is infinitely large, which is equivalent to an open circuit. Thus, when it is operating normally, it is in an open-circuit state.
So, the essence of a voltage transformer is a step-down transformer that operates in an open-circuit state.
2. The reasons for the adverse consequences
Negative consequences: If the secondary side of the voltage transformer is short-circuited, a very high short-circuit current will be generated, which may damage the transformer and even affect the safe operation of the primary circuit.
Explanation: When a voltage transformer is operating normally, as shown in Figure 2, its secondary side is open-circuited, meaning the current on the secondary side is almost zero. After the secondary side of the voltage transformer is short-circuited, since the voltage transformer is a step-down transformer, the current on the secondary side will become extremely large. This is because for a step-down transformer, as the voltage decreases, the current increases (explained by the inverse relationship between current ratio and turns ratio or power balance: the power on the primary side and secondary side of the transformer is basically equal. When the voltage decreases, the current increases, which can also explain this issue).
If the secondary side of the voltage transformer cannot be short-circuited, then fuses must be installed on both the primary and secondary sides of the voltage transformer to provide short-circuit protection.
III. Precautions during Daily Use and Maintenance
If there is a short circuit on the secondary side, a very large short-circuit current will be generated, which may damage the transformer and even affect the safe operation of the primary circuit. Therefore, fuses must be installed on both the primary and secondary sides of the voltage transformer for short-circuit protection.
2. To prevent the high voltage on the primary side from leaking to the secondary side in case of insulation breakdown between the primary and secondary windings, the secondary side of the voltage transformer must be grounded; as shown in Figure 2, the principle diagram of the voltage transformer.
3. When connecting the voltage transformer, one should also pay attention to the polarity of its terminals.
GB1207-2006 “Electromagnetic Voltage Transformer” stipulates: For a single-phase voltage transformer, the primary and secondary winding terminals are marked as A, N and a, n, where A and a, N and n are respectively the “same-name terminals” or “same-polarity terminals”. For a three-phase voltage transformer, the primary winding terminals are marked as A, B, C, N, and the secondary winding terminals are marked as a, b, c, n. A and a, B and b, C and c, N and n are respectively the “same-name terminals” or “same-polarity terminals”. Here, N and n are the neutral points of the primary and secondary three-phase windings respectively. When connecting the voltage transformer, incorrect terminal polarity is also not allowed, and problems will arise if it does.
Post time: Oct-22-2025
