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Principle And Application Of Zero Sequence Current Transformer
Jan 25, 2018

Principle and application of zero sequence current transformer

In three-phase four wire circuit, the phasors of three phase currents are equal to zero. That is to say, if Ia+Ib+IC=0 is connected to a current transformer in three phases and four lines, the induced current is zero. When the electric shock or leakage fault in the circuit, a leakage current flowing through the loop, then through the three-phase current transformer phase and the phase ranging from zero, and the amount of: Ia+Ib+Ic=I (current transformer) so that the two secondary coil has an induced voltage, the voltage detection part of the electronic circuit, and reserve device compared predetermined action current values, such as greater than the current action, even if the sensitive relay action, acting on the actuator switch off. The mutual inductor attached here is called zero sequence current transformer. The phase amount of the three-phase current is not equal to zero, and the current produced is zero sequence current.


When the three-phase circuit is asymmetrical, the current can be decomposed into positive sequence, negative sequence and zero sequence current. The positive phase sequence of three-phase alternating current refers to normal (i.e. A, B, C three-phase space difference of 120 degrees, normal phase sequence), negative sequence refers to the three-phase sequence opposite and normal phase (phase difference is still 120 degrees, still balance), zero sequence (A, B, C refers to the current decomposition of the same size and phase three with the phasor. The zero sequence current transformer is set on the three core cable, and the zero sequence current is displayed outside the unbalanced three-phase (because of the same phase amount).

The zero sequence current transformer

Zero sequence current transformer is a line fault monitor, as only one core and two windings, when in use, will be the first three core cable through the transformer core fenestrae, two by wire connected to a dedicated relay, and then received by the relay output signal device or alarm system. Under normal conditions, the three-phase current in the primary loop is basically balanced, and its synthetic magnetic flux is near zero. There is no induced current in the two winding of the transformer. When a single phase grounding fault occurs in the primary circuit, an unbalanced current is generated in the primary circuit, which means zero sequence current. In the two winding, a tiny current is induced, causing the relay to act and signal. The current of the relay is very small (mA level). It is called the sensitivity of the two time current or zero sequence current transformer (it can also be expressed by a minimum action current), and it is the main operation index. The basic principle of zero sequence current transformer protection is based on Kirchhoff's current law: the algebraic sum of the complex currents of any node in the circuit is equal to zero, that is, sigma I=0, which uses zero sequence CT as the sampling element. In line with the electrical equipment under the condition of normal vector, the phase current and zero (zero sequence current protection assume novalued unbalanced current), therefore, the two winding of zero sequence CT (no output signal when the zero sequence current protection from unbalanced current), the actuator does not move. When the current vector grounding fault and fault current is not zero, the magnetic flux ring core zero sequence C.T in two secondary side induction voltage of zero sequence C.T the actuator movement, drives the tripping device, switching power supply network, to achieve the purpose of grounding fault protection.

The zero sequence current is different from the residual current. The zero sequence current is measured by Ia+Ib+Ic. When it is symmetrical load, it is equal to 0, but in reality, it can't be equal to 0. There will always be an unbalanced current. The residual current is measured by Ia +Ib+Ic+N, and at this time, the normal condition is equal to 0 regardless of the load. Therefore, the residual current will be more sensitive than zero sequence current. The working principle of low voltage leakage zero sequence current transformer is that if there is a current transformer in three phase four wire, the induction current is zero. When the electric shock or leakage fault in the circuit, a leakage current flowing through the loop, then through the three-phase current transformer phase and the phase ranging from zero, and the amount of: Ia+Ib+Ic=I (current transformer) so that the two secondary coil has an induced voltage, the voltage detection part of the electronic circuit, and reserve device compared predetermined action current values, such as greater than the current action, even if the sensitive relay action, acting on the actuator switch off. The mutual inductor attached here is called zero sequence current transformer. The phase amount of the three-phase current is not equal to zero, and the current produced is zero sequence current.

Zero sequence current transformer application in three-phase circuit are respectively provided with a current transformer (C.T), or let the three-phase conductor through a current transformer, also can install a current transformer in the neutral line N, and to detect the current vector using the three-phase current transformer, zero sequence the current Io, IA+IB + IC=IO, when the three-phase load balance on the line completely (no grounding fault, without considering the leakage current circuit and electrical equipment), IO=0; when the line three-phase load imbalance, IO=IN, zero sequence current for the time when the unbalance current of IN; one phase ground fault occurs, will produce a single-phase grounding fault current is Id, the zero sequence current detected at IO=IN+Id, is a vector and three-phase unbalanced current and single-phase grounding current

The scope of application of zero sequence current transformer

CXRDLJ series of zero sequence current transformer is cable type, the ABS engineering plastics shell, casting resin into sealing, good insulation, beautiful appearance, has the advantages of high sensitivity, good linearity, reliable operation, easy installation, its performance is better than the zero sequence current mutual inductor. A wide range of applications, not only adapt to the electromagnetic type relay protection, can adapt to the electronic and microcomputer protection device, the user can according to the operation mode of the system (neutral grounding, neutral grounding, high resistance grounding and low resistance grounding, Peterson coil grounding) zero sequence current transformer by selecting suitable.


Zero sequence current transformer is used more widely, not only adapt to the electromagnetic type relay protection, can adapt to the electronic and microcomputer protection device, the user can according to the operation mode of the system, effectively earthed neutral or neutral non effective grounding, the zero sequence current transformer corresponding.


Zero sequence current transformer is used in conjunction with relay protection devices and signals when generating zero sequence grounding current in power system, so that device elements can be moved to achieve protection or monitoring. The DH - LJ series zero sequence current transformer makes the cable type, uses the ABS engineering plastic shell, the resin is poured into full seal, the insulation performance is good, and the appearance is beautiful. It has the characteristics of high sensitivity, good linearity, reliable operation, convenient installation and so on. Zero sequence current mutual inductor is better than common, more widely used, not only adapt to the electromagnetic type relay protection, can adapt to the electronic and microcomputer protection device, the user can according to the operation mode of the system, effectively earthed neutral or neutral non effective grounding, the zero sequence current transformer corresponding. Principle: the basic principle of zero sequence current protection is based on Kirchhoff's law of current: the algebra of the complex current of any node in the circuit and equal to zero. When the line and electrical equipment are normal, the vector sum of each phase current is equal to zero. Therefore, the two side winding of zero sequence current transformer has no signal output, and the executive element doesn't act. When the current vector grounding fault and fault current is not zero, the flux ring cores of the zero sequence current transformer, two secondary side induction voltage of zero sequence current transformer so that the actuator action drives the tripping device, switching power supply network, to achieve the purpose of grounding fault protection.



It is possible to install a current transformer on three phase lines, or to connect three-phase conductors through a zero sequence current transformer, or to install a zero sequence current transformer on the neutral line N, and use it to detect three phase current vector sum.


Add: No.38 South Taoyuan Road, Yaozhuang Town,Jiashan County, Jiaxing City, Zhejiang Province, China
Contact person: Elva Zhuang&Sunny Ni
Tel:+86-573-84775555
Fax:+86-573-84776699
Phone:+86-13732570078
E-mail:
sales9@great-relay.com
       sunny.ni@great-relay.com