Ordinary current transformers convert large primary currents into smaller secondary currents through a certain transformation ratio for protection, measurement, and other purposes. Zero-sequence current transformers, on the other hand, are used to cut off the power supply when an electric shock or leakage fault occurs in the circuit. Although their names are similar, their functions are quite different. What are the specific differences and functions of these two types of current transformers? Using a through-type current transformer as a zero-sequence transformer is theoretically feasible, but its sensitivity differs significantly.
1. The function of a common current transformer
A standard current transformer can convert a large primary current into a smaller secondary current through a certain transformation ratio, for purposes such as protection and measurement. For example, a current transformer with a transformation ratio of 400/5 can convert an actual current of 400A into a current of 5A.
2. Working principle of zero-sequence current transformer
The basic principle of zero-sequence current protection is based on Kirchhoff's current law: the algebraic sum of the complex currents flowing into any node in a circuit is equal to zero. Under normal conditions of the line and electrical equipment, the vector sum of the phase currents is zero; therefore, the secondary winding of the zero-sequence current transformer has no signal output, and the actuator does not operate. When a ground fault occurs, the vector sum of the phase currents is not zero. The fault current induces magnetic flux in the toroidal core of the zero-sequence current transformer. The induced voltage on the secondary side of the zero-sequence current transformer causes the actuator to operate, driving the tripping device, switching the power supply network, and achieving the purpose of ground fault protection.
3. Function of zero-sequence current transformer
When an electric shock or leakage fault occurs in the circuit, the protection system will activate and cut off the power supply.
4. Operating conditions for zero-sequence current transformers
A current transformer can be installed on each of the three phase lines, or the three phase conductors can be passed through a single zero-sequence current transformer together. Alternatively, a single zero-sequence current transformer can be installed on the neutral line N to detect the vector sum of the three-phase currents.
In specific applications of zero-sequence current protection, a current transformer (CT) can be installed on each of the three-phase lines, or the three-phase conductors can be passed through a single zero-sequence CT together. Alternatively, a zero-sequence CT can be installed on the neutral line N. These CTs are used to detect the vector sum of the three-phase currents, i.e., the zero-sequence current Io, where IA + IB + IC = IO. When the three-phase loads connected to the line are completely balanced (without ground faults and without considering leakage currents from the lines or electrical equipment), IO = 0. When the three-phase loads connected to the line are unbalanced, IO = IN, and the zero-sequence current at this time is the unbalanced current IN. When a ground fault occurs in one phase, a single-phase ground fault current Id will inevitably be generated. The zero-sequence current IO detected at this time is IN + Id, which is the vector sum of the three-phase unbalanced current and the single-phase ground fault current.
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