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       sunny.ni@great-relay.com

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The Debugging And Technical Principle Of Current Transformer
Jun 25, 2018

The principle of voltage transformer and current transformer, voltage, connection mode of current transformer, voltage and current steady saturation and transition process, some problems encountered in the field debugging of voltage and current transformer and the debugging process are also expounded. The role of the transformer in the power system is to convert the first electrical quantity to the two quantity in proportion, and to provide the two equipment for use; to separate the primary system from the two system of the low voltage to ensure the safety of the two equipment and staff. A transformer and a current transformer. A voltage transformer is a two voltage that converts the one voltage of the system to a smaller number in proportion to a protective device and measuring device. The current transformer is a current transformer that converts the first current of the system to a two current in a magnetic field. It is provided to a protective device and a measurement. The quantity device is used. First, the principle of mutual inductor is added an AC potential in the primary side of the transformer. According to the law of electromagnetic induction, the flux of the alternating potential in the magnetic circuit is changed, and the known constant is known as the known constant of the 1=U1/4.44fN1 N1. 1, voltage transformer: voltage transformer is a very small internal resistance, and the two load is equivalent to the operation state of open circuit transformer.  When working, the flux density is close to the saturation value, and the magnetic flux density will decrease when the fault occurs. Working characteristic of voltage transformer: no short circuit operation. The reason is that, when the voltage transformer is short circuited, the two load impedance is zero, and a large short circuit current will be generated in the two winding, and the voltage transformer will burn out. In working, the magnetic flux density of voltage transformer should not be too high, so as to ensure larger excitation impedance and avoid excessive heat loss caused by core loss. 2. Current transformer: current transformer is a special form of transformer, equivalent to two short circuit transformer; the number of magnetized ampere turns in the primary winding of the current transformer is mostly offset by the degaussing ampere turn set up by the two winding of the short-circuit working state, only a very small part of the magnetizing ampere turn is left. To establish the magnetic flux in the iron core. The characteristics of current transformer are: the excitation current is small, the excitation impedance is very large, the two load impedance is very small, the two voltage is small, and the low part of the magnetization curve is working, thus ensuring the measurement accuracy. The two side of the current transformer can not open the road. The reason is: after the two side of the current transformer, the two current is zero and the two winding is disappearing, and the current in the primary winding is constant, and the number of the magnetic ampere number I1W1 is constant, and all of it is used to establish the magnetic flux in the core, so that the core of the current transformer is seriously saturated, so that the excitation impedance is greatly reduced and the excitation is excited.  The current transformer: 1) and the loss of the iron core increase, causing the current transformer to overheat and burn the winding insulation of the current transformer. 2) the magnetic force of the coercive excitation has a strong magnetic flux, which makes the core saturate seriously, and there is a great remanence in the iron core. Before the remanence is removed from the working current, the error will increase, and the high voltage at the two ends of the two winding will appear. F power frequency is also a known constant; in the case of ideal magnetic flux free, the secondary side will produce N2 a magnetic flux of 2, both 2= and 1; in the magnetic circuit, the alternating magnetic flux will produce alternating potential: U2=4.44 diameter 2F N2 f is a known constant two, in the power system, the connection mode of mutual inductor and the problem of saturated transition 1 Voltage transformer: 1), connection mode: (1), Y0/Y0 connection: three terminals of one winding, which can make the voltage transformer correctly reflect the relative voltage.  The three terminals of the two winding are grounded, the grounding mode has neutral grounding and the B connection; the voltage transformer is connected to the ground two times and has two functions: the two load of the access can obtain the relative voltage. B, when the insulation of the voltage transformer is damaged, prevent the high pressure from entering the low voltage (two times) side, and ensure the two equipment and personal safety safety grounding. (2), incomplete delta connection: this connection can not be grounded at once, and the two winding B is connected to each other; the relative system neutral point voltage and line voltage can only be obtained, and the relative voltage can not be obtained. The advantage is that the investment (3) and the three-phase five column connection can be saved: one round connection is connected to the Y0 and a group of auxiliary remainder. Group. The two winding is five two terminals of a, B, C, D and n. It can get line voltage, phase voltage and neutral point voltage of the system, and assist residual winding to get 3U0. 2) transition process of voltage transformer: voltage transformer is divided into electromagnetic voltage transformer and capacitive voltage transformer, but in 110kV and above voltage grade power grid system, capacitive voltage transformer is basically used, and the transition process of capacitive voltage transformer is far longer than that of electromagnetic voltage transformer. The oscillation process of voltage transformer will affect the correct action of relay protection. The transition process of capacitive voltage transformer: the two side voltage can not be changed when the system voltage is suddenly changed, and a stable state can be achieved through an oscillating transition process. The transition process depends on the combination of voltage divider and inductance and resistance in the whole circuit. The frequency of the transition process is much lower than that of the system, and the attenuation process is also longer.The additional oscillating components in the two circuit of a capacitive voltage transformer may affect the time and error of the relay protection, especially in the case of the low residual voltage of the bus bar near the end of the protective end, and the direction of the bus may be lost in the case of a bus fault. At the end of the protection range, the power frequency force of the distance measurement voltage U - IZ is zero, which remains the free oscillation component of the two times of the capacitive voltage transformer, which is the source of the transient transcendental error. In order to analyze the influence of the two transient voltage of the capacitor voltage transformer on the measurement accuracy of the ideal distance relay, a simple formula is given. The set impedance is Zm, the distance protection relay setting impedance is Zj, and the transient transcendental error generated by the two transient voltage is Zj. When the distance protection relay acts, the residual voltage of the bus bar is equal to Ue (Zj+ Zj) / (Zm+Zj+ Zj), and Ue is the rated voltage of the relay relay protection relay. In the case of ideal voltage transformer, the residual voltage of the bus when the distance relay acts is equal to (Ue * Zj) / (Zm+Zj). The difference between the two is derived from the two transient voltages of the capacitive voltage transformer, Ut={[(Zj+ Zj) / (Zm+Zj+ Zj)]-[Zj/ (Zm+Zj))] x Ue={(Zm x Zj) / [(Zm+Zj+]) x]). Assuming a metal short circuit at the outlet of a voltage transformer, when the two transient error voltage of the time is t, Ut (0), when the primary bus residual pressure is Ue (Zj+ Zj) / (Zm+Zj+ Zj), the two error voltages at the same time t (Zj+ Zj) / (Zm+Zj+ Zj+)] = [0] = [ The value of Zm/ (Zm+Zj+ Zj)] x Ut (0), and Ut (0) can be found in the technical data provided by the manufacturer. 2, current transformer: 1), wiring mode: (1), complete star connection: complete star connection is not suitable for small grounding current grid. (2), incomplete star connection: all current transformers are installed in the two phase of the same name when using incomplete star connection, which can guarantee the opportunity of 2/3 to remove the fault points and have the opportunity of 1/3 to be completely removed; but no selectivity, in order to overcome the shortcomings of the two phase incomplete star connection, two phase relay mode is adopted. Third relays are installed on the public line. (3) two phase current difference wiring: two phase current differential protection can not be used on the circuit with Y/ -11 transformer. *: the ratio of the current through the relay to the two current of the current transformer is different. 2) steady state saturation and transition process: (1) steady state saturation: current transformer saturation has different effects on different protection devices. In the actual operation of the power grid, more than one time due to the saturation voltage of the current transformer used, it is lower than the minimum voltage needed to start the over current relay (the starting current is large) under the actual two load, which causes the protection and rejection of the circuit when the circuit is short circuited. In addition, when the external short circuit occurs, the saturation voltage of the current transformer is too low, causing the bus differential protection device to malfunction. For some slower protective devices, a certain degree of saturation of the current transformer may also be irrelevant to its protection performance. The current transformer made in China has stipulated the current multiple of 10% error in the technical condition. Under the specified rated multiplier, the steady-state ratio error of the current transformer is not more than -10%. The load capacity of current transformer has a certain requirement. If it exceeds the carrying capacity of the current transformer, it will have a serious effect on the accuracy of the current transformer. The current transformer is used in the power grid of the switch field far away from the protection room. In order to ensure the carrying capacity of the current transformer, the resistance of the current transformer is greatly reduced. In order to ensure the precision of the current transformer, the resistance of the current circuit is reduced from the following aspects: A, reducing the contact resistance of the current circuit as much as possible. For example, reduce the number of crimping terminals, improve the contact resistance of the connection terminals, and tighten the current loop terminals. B, improve the efficiency of relay protection circuit, select a reasonable loop and pay attention to the necessary resistance matching. C, pay attention to proper selection of relay protection specifications, and select the appropriate current transformer. It is noted that the rated power of the current transformer refers to the value of the two current as the rated value. D, the current relay with low starting value is adopted to reduce the impedance of the current relay under the condition of large current. (2) the transition process of current transformer saturation: Generally speaking, it is simple to meet the requirements of the protective device for the steady saturation of the current transformer as long as it is paid attention to. But in the power grid with higher primary voltage, it is much more complicated to meet the requirement of fast protection when the transition process of current transformer is saturated. The reason is that the initial value of the fault current contains the DC component. However, in fact, there is always a process from the start of the fault to the beginning of the saturation of the current transformer. The saturation of the current transformer does not appear immediately from the start of the fault, but only after a certain time. In order to study the transition process of current transformer saturation, we use the core hysteresis loop of the current transformer to describe: before the flux density has not reached the maximum flux density Bmax, the required magnetic field intensity is zero, corresponding to the impedance equivalent circuit of the current transformer, and the equivalent Yu Lici impedance is infinitely large.When the flux density reaches Bmax, no matter how the magnetic field intensity H increases, the flux density remains unchanged, which corresponds to zero in the impedance equivalent circuit of the current transformer. The current transformer's core magnetic hysteresis loop is used to describe the output current of the two circuit of the current transformer: A, before the flux density reaches Bmax, the first current (after the conversion conversion) will be all faithfully transferred to the two circuit. B, when the flux density reaches Bmax, the primary current is completely not transferred to the two loop when the current continues to keep the original direction (and the saturation reaches the same half wave direction). Or, at this time, the primary current is all converted to the error current (Li Ci current). C, when a current is reduced to zero in the direction of saturation of the current transformer and starts to reverse phase, the flux density in the core of the current transformer is reduced by the saturation value and is converted to an ideal transformer. All the primary current (after conversion conversion) is transferred to the two circuit until once again. However, due to the existence of residual magnetism, even if the primary current value is less than the normal saturation level, it may saturate the current transformer. The remanence left by the fault in the near area often makes the next fault saturate, and the problem may be the most serious when it is reconnected to the fault. Because the switch disconnects the current when the current is zero, the remanence of the core is related to the impedance angle of the two circuit load. If the load of the two circuit is pure electrical inductance, when the current is zero, the two induction voltage is maximum, and the magnetic flux in the core of the current transformer is zero, and thus is not remanent; and when the two times are returned. The load is pure resistance. When the primary current is disconnected, the magnetic flux in the core of the current transformer is the largest, so the possible remanence is the largest. (3) the influence of the transition process of the current transformer saturation to the protection: A, in the line reclosing, when the switch appears reclosing in different periods, when the current is disconnected, the two circuit may appear the attenuation of the DC component; the DC component is directly transferred from one side to another, and its attenuation time constant is a single circuit single. The time constant of zero sequence network when phase reclosing is increased, and this time constant increases with the increase of line voltage. We should pay attention to the effect of whether the zero sequence current is used to distinguish whether the two sides are fully integrated. B, in UHV power grid, the DC component in the current may be decaying when the fault is quickly removed and the time constant of the primary loop is very large. If the DC and AC components in the current waveform are disconnected on the same side, the core is still in a highly saturated state when the fault is cut. After a current is disconnected, the flux in the core will rapidly decay to a corresponding remanent value, thus inducing a DC attenuation current in the two loop. The higher the saturation degree, the greater the difference between the saturation magnetic flux and the corresponding remanence, and the greater the two current of the induction, which may cause the delay of the relay to return to the check current in the two circuit of the current transformer. Three, some problems encountered in the process of debugging and debugging in the field: 1, field debugging: 1), voltage transformer: (1), checking the name brand parameters of the voltage transformer and the qualification certificate of the factory. (2) check the polarity of all windings. (3) check the ratio of all windings and their taps. (4) check the accuracy of voltage transformer under each capacity. (5) check the voltage two loop grounding point and grounding condition. A voltage transformer for a neutral grounding power network, such as a phase voltage or zero sequence voltage in a device, and can be supplied by two sets of voltage transformers (through a switching device), then the two and three windings of these two sets of voltage transformers are allowed to be grounded directly in one common place, and each set of voltages. The neutral point of the two winding of the transformer is grounded by the discharge device, and the voltage transformers for other use conditions are directly grounded in the neutral points of the two windings of each transformer. (6) check the reliability of the voltage transformer two times and three times winding in the terminal box and the wiring terminals of the respective terminals. (7) the voltage transformer that is grounded by the discharge device checks whether the installation of the discharge device is in conformity with the regulations and the discharge voltage of the discharge frequency of the discharge device. (8) check the fitting location of all fuse (automatic switch) in the two circuit of the voltage transformer and whether the fuse current is suitable (the release current of the automatic switch should be determined by the test), the quality is good, the selectivity is guaranteed, and the impedance of the automatic switch coil is suitable. (10) check the contact reliability of switch, knife switch and switchgear contact in series. (11) measure each phase DC resistance of the voltage circuit from the induction terminal to the voltage bus of the distribution screen, and calculate the voltage drop of the voltage transformer at the rated capacity, and the value is not more than 3% of the rated voltage. (12) check the insulation resistance of voltage transformer with 1000V shaking table. It includes the two winding of voltage transformer between the shell and the windings, all the two circuit to the ground and the core of the same cable. After the voltage of the voltage transformer is connected to the system, 1, measure the voltage of each two winding. 2. Measurement of interphase voltage. 3. Measurement of zero sequence voltage. For the voltage transformer of the small current grounding system, a suitable resistance load should be connected to the zero sequence voltage circuit before measuring the voltage transformer to avoid the phenomenon of ferromagnetic resonance.

4. Test the phase sequence. 5. Set the phase. 2), current transformer: (1) check the brand parameter of the current transformer and the factory certificate. (2) check the polarity of all windings. (3) check the ratio of all windings and their taps. (4) check the accuracy and level of each winding of the current transformer and the location of the internal installation. (5) check the DC resistance of the two winding of the current transformer (TAP). (6) test the polarity relationship between the current transformer windings, check the correctness of the polarity marking on the famous brand, check whether the connection mode of each secondary transformer of current transformer and its polarity relation with the design and whether it is correct or not. (7) when a current transformer is put into a large current, check the ratio of the working tap is correct or not. (the polarity and ratio test of transformer casing transformer is tested when the transformer is used for short circuit voltage test). (8) the excitation characteristic curve of the U2=f (I2) of the two winding working tap of the current transformer should be measured to the saturation part. The multi winding current transformer should be checked according to the U2=f (I2) curve to check the grade of each winding, so as to check whether the distribution of each winding is reasonable. (9) check the correctness of the wiring of the two circuit winding of the current transformer and the reliability of the screw crimping. (10) check the current two loop grounding and grounding conditions. Only one grounding point is allowed in the same current loop. (11) the voltage drop of the circuit is measured at the two terminal box of the current transformer and the voltage drop of the circuit is measured. The impedance of each phase and zero phase and phase of the current loop is calculated. The impedance of the current circuit is combined with the U2=f (I2) curve to check whether the U2=f (I2) curve meets the requirement of 10% error. After the current transformer is loaded, 1, measuring the current value of each phase and zero phase. 2, to measure the polarity and phase sequence of each phase current. 3. Set the phase.  Note: whether it is the voltage transformer and its voltage circuit, or current transformer and its current circuit; when all the work is completed, a 3 * 380V power supply should be added at one time to check the voltage transformer and its voltage circuit in the open circuit state, and check the current transformer and its current circuit in the short circuit state. 2, some problems encountered during the debugging process: 1), voltage transformers: (1), determination of the homopolar polarity of the voltage transformer: according to the principle of electromagnetic induction, a single DC power is added to the same winding between the one side of the same winding, and a digital milli ampere meter is set up between the two windings. To inspect the direction of the pointer deflection of the ammeter, the pointer is positive, and the end of the battery's positive pole is in the same polarity as the two winding end connected by the positive pole of the ammeter, and the back deviation turns the positive and negative of the ammeter to the end of the two winding, and repeats the above test. (2), voltage transformer voltage direction: the positive direction of the voltage in the primary winding is directed from the polar end of the primary windings to the non polar end, then the positive direction of the output of the two winding is also from the polar end of the two winding to the non polar end; it is opposite to the direction in the motor science, so it is called the polarity reduction. (3) the error and elimination of voltage transformer: the cause of the error caused by the voltage transformer is caused by the voltage drop produced by the excitation current on the primary winding and the load current on the one or two winding. The error includes the ratio error and the angle error. To reduce the error: 1, the leakage reactance of the one or two winding should be minimized. 2, the excitation current of the voltage transformer is reduced. 3, reduce the load current, that is, access to large loads. * remarks: the same voltage transformer has different Accuracy grades with the two loads. (4), unbalanced voltage: the generation of unbalanced voltage is caused by the following aspects: 1, the error of voltage transformer itself. 2. 3 times or 3 times harmonic voltage in one system. 3. The voltage caused by the different admittance of a system. The unbalanced voltage generally does not exceed the percentage of the maximum voltage of the filter terminal when the single-phase metal grounding fault occurs in the power grid. (5), zero sequence voltage: the farther the distance from the fault point, the lower the zero sequence voltage of the point; the zero sequence voltage of the fault point, the zero sequence voltage of the neutral point of the transformer, and the zero sequence voltage with the linear characteristic of the change of the distance from the fault point. 2), current transformer: (1), determination of the homopolar of current transformer: according to the principle of electromagnetic induction, in the same winding between the one side, a single DC power is added to the same winding between the one side, and a digital milli meter of ammeter is series between the two windings, and the direction of the deflection of the indicator of the ammeter is observed. On the other hand, the winding ends connected by the positive pole of the battery are in the same polarity as the two winding ends connected by the positive pole of the ammeter, while the reverse bias interchanges the positive and negative of the ammeter at the end of the two winding and repeats the above test. (2) current transformer error and its elimination method: current transformer error includes variable ratio error and angle error. To reduce the error, we must reduce the exciting current. The following methods are adopted: 1. Use high permeability material to make iron core and increase the load of iron core.

2. Increase the number of turns of the first winding. In order to reduce the error of the current transformer in the rated current range, we should try to reduce the increment of current and the general method to reduce the number of turns of the two windings artificially. In operation or inspection protection, the way to reduce the error is: 1. Limit the two load of the current transformer. 2, limit the primary current multiplier 1), and select a current transformer with a large current multiple allowable value. 2), choose the current transformer with large change. (3) the unbalanced current of the current transformer: the unbalanced current of the current transformer is due to: 1. The excitation characteristics of the current transformer are different. 2. There is remanence in the winding of the current transformer. (4), zero sequence current: zero sequence current is generated by the zero sequence voltage of the fault point. Its size and distribution are mainly determined by the zero sequence impedance of the line and neutral point grounding transformer in the power grid and the number and distribution of neutral point grounding transformer, but not directly related to the number and distribution of power supply in the power grid. The more the number of transformer grounding, the smaller the zero sequence impedance, the greater the zero sequence current. When the operation mode changes, if the number of transformer locations is constant, the magnitude of zero sequence current will not be affected.


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