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# Working Principle of Zero Sequence Current Transformer

The basic principle of zero sequence current transformer protection is based on Kirchhoff's current law: the algebraic sum of complex current flowing into any node in the circuit is equal to zero, i.e. I = 0. It uses zero sequence CT as the sampling element. When the line and electrical equipment are normal, the vector sum of each phase current is equal to zero (for zero sequence current protection, it is assumed that unbalanced current is not considered).Therefore, the secondary winding of zero sequence CT has no signal output (avoiding unbalanced current during zero sequence current protection) and the actuator does not act. In case of ground fault, the vector sum of each phase current is not zero. The fault current generates magnetic flux in the annular iron core of zero sequence C.T, and the induced voltage at the secondary side of zero sequence C.T makes the actuator act, drives the tripping device and switches the power supply network, Achieve the purpose of grounding fault protection.Zero sequence current and residual current are different.

The zero sequence current measures Ia IB IC, which is equal to 0 when the load is symmetrical; But in reality, it can't be equal to 0. There will always be an unbalanced current. The residual current measures Ia IB IC n. at this time, regardless of the load, the normal condition is equal to 0.Working principle of low voltage leakage zero sequence current transformer

If a current transformer is connected in three-phase four wire, the induced current is zero. When an electric shock or leakage fault occurs in the circuit, there is leakage current flowing in the circuit. At this time, the phasor sum of the three-phase current passing through the transformer is unequal to zero, and the phasor sum is: Ia IB ic = I (leakage current), so there is an induced voltage in the secondary coil of the transformer. This voltage is applied to the electronic amplification circuit of the detection part, which is compared with the predetermined action current value of the device in the protection area, If it is greater than the action current, even if the sensitive relay acts, it will act on the tripping of the actuator. The transformer connected here is called zero sequence current transformer. The phasor sum of three-phase current is not equal to zero, and the generated current is zero sequence current.

The specific application of zero sequence current protection can install a current transformer (CT) on the three-phase line, or let the three-phase conductors pass through a zero sequence CT together, or install a zero sequence CT on the neutral line n, and use these C.T to detect the three-phase current vector sum, i.e. zero sequence Current IO, Ia IB ic = io. When the three-phase load connected to the line is completely balanced (there is no grounding fault, and the leakage current of lines and electrical equipment is not considered) , IO = 0; when the three-phase load connected to the line is unbalanced, IO = in, and the zero sequence current at this time is unbalanced current in; when a phase grounding fault occurs, a single-phase grounding fault current ID must be generated, and the detected zero sequence Current IO = in ID is the vector sum of three-phase unbalanced current and single-phase grounding current.

Action principle of zero sequence current transformerZero sequence current protection is generally suitable for TN grounding system.

When one phase grounding occurs, for TN-S system, ID loop impedance includes phase line impedance Z1, PE line impedance ZPE and contact impedance ZF, i.e. ZS = Z1 ZPE ZF; for TN-C system, ID loop impedance includes phase line impedance Z1, pen line impedance zpen and contact resistance ZF, i.e. ZS = Z1 zpen ZF; for TN-C-S system, ID loop impedance includes phase line impedance Z1, pen line impedance zpen, PE line impedance ZPE and contact resistance ZF , i.e. ZS = Z1 zpen ZPE ZF, the generated single-phase grounding fault current id = 220 / ZS is significantly greater than the three-phase unbalanced current without fault. As long as the setting is appropriate, the zero sequence current in case of grounding fault can be detected to cut off the fault circuit.

For it systems, generally, three-phase three wire distribution lines without neutral lines in industrial and mining enterprises that require high power supply reliability, do not need to cut off the power supply circuit immediately for single-phase grounding, but need to send insulation damage monitoring signals to maintain continuous power supply for a period of time.In case of single-phase grounding, the zero sequence current flowing through the fault line is the sum of the capacitance current of the whole system and non fault system, so it is easy to detect the grounding fault current. Therefore, the zero sequence current protection device can be used to monitor the first phase to ground grounding fault.

TT grounding system is often used in the three-phase four wire distribution system of lighting and power hybrid power supply for industrial, agricultural and civil buildings. It is often found that the three-phase unbalanced current is large. When one phase grounding occurs, the ID loop impedance includes phase line impedance Z1, PE line impedance ZPE, load side grounding resistance RA and power side grounding resistance Rb, contact impedance ZF, i.e. ZS = Z1 ZPE RA Rb ZF, and grounding fault current id = 22 0 / ZS, because RA RB Z1 ZPE ZF and RA RB values are generally large, it is obvious that the fault loop impedance of TT system is large, and the single fault current ID generated is far less than the unbalanced current, so it is difficult to detect the fault current, so it is not suitable for TT grounding system.

The installation of zero sequence CT for zero sequence current protection must comply with relevant process standards.For it grounding system, in case of single-phase grounding fault, the grounding current may flow back not only along the conductor surface of fault cable, but also along the conductor surface of non fault cable. Therefore, during installation, the cable head must be grounded through zero sequence CT, so as to ensure that the capacitive current of fault phase and non fault phase passes through the grounding point, that is, to prevent protection in case of external fault The device misoperates, which can ensure the reliable operation of the device in case of fault.For it grounding system, it is generally used to install zero sequence CT on neutral line n, and the zero sequence CT on low-voltage side busbar must be installed on the busbar between neutral line N and working grounding point (or repeated grounding).

For example, the zero sequence CT is installed on the n-wire busbar of the distribution panel. Since the metal shell of the distribution panel is generally directly connected with the grounding electrode, when the bus is short circuited to ground, the generated fault current ID will flow along the metal shell of the distribution panel grounding wire neutral point of the transformer without passing through the zero sequence CT, which can not achieve the required protection function, which is easy to be neglected during on-site construction.

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