Can ordinary current transformers be used as zero sequence current transformers?
Ordinary current transformers can convert a primary current with a larger value into a secondary current with a smaller value through a certain transformation ratio for protection, measurement and other purposes. The function of the zero sequence current transformer is to protect the circuit and cut off the power when an electric shock or leakage fault occurs in the circuit. The two are similar in name, but their functions are quite different. What are their differences and functions? The through-core current transformer is used as a zero-sequence transformer in principle, there is no problem, but there is a big difference in sensitivity.
1. The role of ordinary current transformers
Ordinary current transformers can convert a primary current with a larger value into a secondary current with a smaller value through a certain transformation ratio for protection, measurement and other purposes. 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 transformer
The basic principle of zero sequence current protection is based on Kirchhoff’s current law: the algebraic sum of the complex current flowing into any node in the circuit is equal to zero. When the circuit and electrical equipment are normal, the vector sum of each phase current is equal to 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 currents of each phase is not zero. The fault current causes magnetic flux to be generated 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 act and drive The tripping device switches the power supply network to achieve the purpose of ground fault protection.
3. Zero sequence transformer function
When an electric shock or leakage fault occurs in the circuit, the protection will act and the power supply will be cut off.
4. Zero-sequence transformer operating conditions
A current transformer can be installed on each of the three-phase lines, or the three-phase wires can pass through a zero-sequence current transformer together, or a zero-sequence current transformer can be installed on the neutral line N to use it to detect three-phase The vector sum of currents.
The specific application of zero-sequence current protection can install a current transformer (CT) on each of the three-phase lines, or let the three-phase wires pass through a zero-sequence CT together, or install a zero-sequence CT on the neutral line N, using These CTs are used to detect the vector sum of the three-phase currents, that is, the zero sequence current Io, IA+IB+IC=IO, when the three-phase load connected to the line is fully balanced (no ground fault, and the leakage current of the line and electrical equipment is not considered ), IO=0; when the three-phase load connected to the line is unbalanced, then IO=IN, the zero sequence current at this time is the unbalanced current IN; when a ground fault occurs in a phase, a single-phase grounding will inevitably occur The fault current Id, the zero sequence current detected at this time IO=IN+Id, is the vector sum of the three-phase unbalanced current and the single-phase ground current.