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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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Several common detection wiring modes of transformer:The transformer winding deformation tester is mainly composed of main measuring unit and notebook computer, parallel three special measuring cables, measuring clips and grounding wires.The main measuring unit system and the test object are connected by 50W high-frequency coaxial cable. The frequency sweeping signal is injected into the test object through the signal clip (yellow) through the connecting cable through the output port (excitation output); The signal is obtained from the test object by the signal measuring clip (green) and transmitted to the (response input) via cable; The synchronous reference signal is obtained from the injection point of the tested object by signal measurement and transmitted to the input (reference input) through cable. The shell of the tested object and the shielding layer of the test cable must be reliably connected and grounded. Large transformers generally use the connection point between the outgoing line of the iron core grounding sleeve and the oil tank as the public grounding point, and the shell point of the transformer is grounded.Three phase yn shaped measuring wiring1. The Yellow clamp (input) is clamped on the o point of the transformer, the green clamp (measurement) is clamped on the a phase of the transformer, and the measurement of representative and three-phase yn type a phase.2. The Yellow clamp (input) is clamped on the o point of the transformer, the green clamp (measurement) is clamped on the B of the transformer, and represents the measurement of three-phase yn type B.3. The clamp (input) is clamped on the o point of the transformer, the green clamp (measurement) is clamped on the C of the transformer, and represents the measurement of three-phase yn type C.Three phase Y-shaped measuring wiring1. The Yellow clamp (input) is clamped on phase a of the transformer, the green clamp (measurement) is clamped on phase B of the transformer, and represents the measurement of three-phase Y-type ab.2. The color clamp (input) is clamped on phase B of the transformer, the green clamp (measurement) is clamped on phase C of the transformer, and represents the measurement of three-phase Y-type BC.3. The Yellow clamp (input) is clamped on phase C of the transformer, the green clamp (measurement) is clamped on phase a of the transformer, and represents the measurement of three-phase Y-type ca.Three phase delta measuring wiring1. The Yellow clamp (input) is clamped on phase a of the transformer, the green clamp (measurement) is clamped on phase B of the transformer, and represents the measurement of three-phase type ab.2. The Yellow clamp (input) is clamped on phase B of the transformer, the green clamp (measurement) is clamped on phase C of the transformer, and represents the measurement of three-phase BC.3. The Yellow clamp (input) is clamped on phase C of the transformer, the green clamp (measurement) is clamped on phase a of the transformer, and represents the measurement of three-phase ca.Single phase x, y, Z measuring wiring1. The Yellow clamp (input) is clamped on the X point of the transformer, the green clamp (measurement) is clamped on the a of the transformer, and represents the measurement of single-phase x, y and Z ax.2. The Yellow clamp (input) is clamped on the Y-point of the transformer, the green clamp (measurement) is clamped on the B-point of the transformer, and represents the measurement of single-phase x, y and Z-type by.3. The Yellow clamp (input) is clamped on the Z point of the transformer, the green clamp (measurement) is clamped on the C of the transformer, and represents the measurement of single-phase x, y and Z CZ.matters needing attention:The instrument shall be preheated for 15 minutes before measurement. If the temperature is low in winter, the preheating time shall be appropriately extended to ensure the normal measurement of the instrument.Pay attention to the grounding wire in strict accordance with the schematic diagram. In particular, the grounding clip of the response signal shall be connected with the grounding clip of the excitation signal through the connecting wire, and then the grounding wire of the excitation signal shall be grounded with the iron core to ensure the correct flow direction of the signal current.Author: Dingsheng power www.kv-kva.com reprint please indicate
The working principle of current transformer is similar to that of transformer. Its basic working principle is to work according to the electric magnetic electric conversion process. To a certain extent, it can be considered that current transformer is a special transformer. The function of making current transformer is mainly to measure the large current in the main circuit. During measurement, the large current in the main circuit is changed into small current according to the proportional relationship, and then sent to the meter to measure the current or power.Working process of current transformerCurrent transformer is actually a special converter that changes a large current into a small current. This converter can only be used in AC circuit and can measure the large current in AC circuit. The following is mainly about its measurement process. Friends familiar with the transformer structure know how the transformer works, so the current transformer is also similar to the double winding transformer in structure. Different from the transformer, the number of turns of the primary winding of the current transformer around the iron core is very small, generally only one to two turns, and the enamelled wire is very thick (mainly because the primary winding needs to be connected in high current), it is connected in series in the measured circuit when connected, and the wiring mode is shown in the figure belowThe number of turns of enamelled wire wound by the secondary of current transformer is many times more than that of the primary. Because the secondary is connected with the measuring instrument, the secondary is connected in series with the current coil of ammeter or watt hour meter to form a closed loop. The resistance value of these coils is very small, so the secondary can be similar to short circuit. We can deduce from the approximate short circuit of the secondary coil The voltage of the primary coil out of the transformer is almost zero. According to the electromagnetic principle, we can know that the primary input voltage determines the main magnetic flux. When the input side voltage is almost zero, the magnetic flux is zero and the excitation current is zero, so the total magnetic potential is zero. Finally, we will get such an expression: I1 = kxi2 (where I2 is the reading of the secondary connection ammeter of the current transformer).We can know from the expression I1 = kxi2 that the current transformer is equivalent to a step-up transformer. When the primary current is large, the current induced by the secondary is the current reduced by a certain proportion. In the formula, K is the reduced proportion, which we call the transformation ratio. For example, when k is 30 / 5, it means that the current flowing through the primary is 30a, and the current induced by the secondary side is 5A, The transformation ratio is 6. In other words, we reduce the current of the main circuit by 6 times through the current transformer, which can not only expand the measuring range of the instrument, but also ensure the safety of personnel and equipment.Precautions for use of current transformerThere are two points to pay attention to in the use of current transformer. One is that the shell and iron core of current transformer shall be reliably grounded for safety; the other is that its secondary cannot be disconnected when the current transformer is in use, otherwise it will induce high voltage and endanger personal safety.
Voltage transformer and current transformer have the same point. They all use the working principle of electricity magnetism electricity. As shown in Figure 1 below.Both voltage transformer and current transformer are components for measurement and protection in power supply system.Common voltage transformers are reduced to the rated voltage of the instrument to provide working voltage for the circuit; Its symbol is represented by TV; Its output voltage is connected in parallel with the instrument. See the figure below for its physical map.This configuration of high-voltage voltage transformer is mainly used for measurement and protection of voltage level of 10kV and above; The high-voltage winding of the voltage transformer is connected in parallel with the tested circuit, and the low-voltage winding is connected in parallel with the voltage coil of the measuring instrument. Due to the large internal impedance of the voltage coil, when the voltage transformer works, it is equivalent to a no-load transformer, so its secondary side cannot be short circuited, otherwise the winding coil will be burned.Its working principle is shown in the figure below.Current transformer provides rated 1 5A current with different ampere turn ratio for instruments and watt hour meters. It is represented by the symbol TA, and its physical diagram is shown in the figure below.Because the number of coils on the primary side of the current transformer is small and all are in series, its current depends on the line load current and has nothing to do with the secondary side load. Because the impedance of the current coil connected to the secondary side is very small, the current transformer is equivalent to a short-circuit transformer when it works normally. Therefore, the actual current value in the line is very large, so its secondary side cannot be open circuit during normal operation; Short circuit at secondary side of voltage transformer; One terminal on the secondary side of both of them is a protective grounding terminal, which must be protected and grounded according to the safety regulations during installation.The same point: they are all transformers. The primary side is connected with the measured circuit, and the secondary side is connected with instruments or control elements. They are generally used for the measurement and control of AC circuitsdifference:The primary side of the current transformer is connected in series in the measured circuit, and the voltage induced by the secondary side changes with the current of the primary side;The primary side of the voltage transformer is connected in parallel with the measured circuit, and the voltage induced by the secondary side changes with the terminal voltage of the primary sideThe key is the current transformer. The secondary side cannot open circuit, and the secondary side open circuit is equivalent to infinite impedance. According to the principle of mutual inductance, the resistance reflected to the primary side is infinite. When the current at the primary side is large, a large voltage drop will be generated at the primary side, which is very easy to burn the transformer. Therefore, when there is no measuring instrument at the secondary side of the current mutual inductor, the secondary side should also be short circuited with wires.In the same point, they all obtain monitoring data at the secondary side through electromagnetic induction. The difference is that the monitoring objects are different, one is voltage and the other is current.
Working principle of power transformerAs for the power transformer, it is a special transformer whose output and input share a set of coils. The step-up and step-down are realized with different taps. The tap voltage of less than the common coil is reduced. The tap voltage of more than the common coil increases. The working principle of power transformer is to use the principle of electromagnetic induction. The alternating current we use generates a magnetic field through coil n, and there is a coil m next to coil n. because the magnetic field generated by coil n is not constant, it changes accordingly according to the change of current. The change of magnetic field causes the corresponding current in coil m, and the frequency of coil m will change according to the different turns ratio of coil N and coil M. Of course, now many power transformers are becoming single coil (called autotransformer), and the principle is basically the same.Autotransformer is a transformer with only one winding. When it is used as a step-down transformer, some wire turns are extracted from the winding as the secondary winding; When used as a step-up transformer, the applied voltage is only applied to part of the wire turns of the winding. Generally, the part of the winding belonging to both primary and secondary is called common winding, and the rest of the autotransformer is called series winding. Compared with ordinary transformers, autotransformer with the same capacity has not only small size, but also high efficiency, and the larger the transformer capacity, the higher the voltage. This advantage is more prominent. Therefore, with the development of power system, the improvement of voltage level and the increase of transmission capacity, self coupled transformer is widely used because of its large capacity, low loss and low cost.Function of power transformerIn addition to the small volume of power transformer, there is no clear boundary between power transformer and electronic transformer. Generally, the power supply of 60Hz power network is very large, which may cover as large a capacity as half a continent. The power limitation of electronic devices is usually limited by the ability of rectification, amplification and other components of the system. Some of them belong to the power amplification, but compared with the power generation capacity of the power system, it still belongs to the scope of small power. Various electronic equipment are commonly used in transformers for the following reasons: providing various voltage levels to ensure the normal operation of the system; Provide electrical isolation of parts operating at different potentials in the system; Provide high impedance for AC current, but low impedance for DC; Maintain or modify the waveform and frequency response at different potentials.Loss of power transformerIn the power supply voltage regulator, when the current passes through the primary coil, a certain amount of heat will be generated. (the magnetic flux generated by the coil flows in the iron core, because the iron core itself is also a conductor, and the potential will be induced on the plane perpendicular to the magnetic line of force. This potential forms a closed loop on the broken surface of the iron core and generates current, like P a vortex, so it is called "eddy current". This "Eddy current" increases the loss of the transformer and heats the iron core of the transformer and increases the temperature rise of the power transformer.) more energy loss is generated in the "eddy current", which is the "iron loss" in the term. In addition, when a large number of copper wires are used in our power transformer, a large amount of heat will be generated when the current passes through, which is what we call "copper loss" The main heat sources of power transformer are "copper loss" and "iron loss" It is also these two phenomena that make the power transformer lose more power. Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss. Because the power transformer has iron loss and copper loss, its output power is always less than the input power. Therefore, we introduce an efficiency parameter to describe it, = Output power / input power.