Some problems of current transformer TA and its connection group in transformer differential protection

This paper describes the importance of several technical issues of current transformer (TA) and its connection group in transformer differential protection. The solutions adopted for these problems are given: using mathematical methods to solve the matching of TA connection group; additional stable zone judgment method for TA saturation; comprehensive current and voltage judgment method to identify TA secondary circuit disconnection or short circuit problems; strengthening the human-machine interface function of protection to avoid TA phase sequence, polarity and grounding problems.

Keywords: current transformer TA; TA connection group; TA saturation introduced

:adopt math method to solve the mismatch of TA connect group;the method of add-on stabilization area distinguish TA saturation; the integration

method of current and voltage distinguish the break and short of TA secondary circuit; The question of phase sequence and polarity and earth are avoided by stronger HMI. Key words: current transformer(TA);TA connect group;TA saturation;break and short 0 Introduction Power transformers are one of the main electrical equipment in power plants and substations, and are crucial to the safe and stable operation of power systems. In particular, large high-voltage and extra-high-voltage power transformers are expensive and have heavy operating responsibilities. Once a fault occurs and they are damaged, their maintenance is difficult and time-consuming, causing great economic losses. In addition, the sudden disconnection of the transformer after a fault will also cause disturbances to the power system. Therefore, the requirements for relay protection are very high. As one of the main protections for power transformers, transformer differential protection has always been valued by the majority of protection colleagues, and the research on its main protection principles has been quite fruitful. However, there is still room for further discussion on several issues of its current transformer (TA) and its connection group, such as: (1) ratio matching and phase correction of the TA connection group on each side of the transformer; (2) countermeasures when the TA is saturated; (3) countermeasures when the TA secondary circuit is disconnected or short-circuited; (4) TA phase sequence, polarity and grounding issues. If these issues are not handled properly, they will directly affect the reliable operation of the transformer differential protection and reduce the protection performance. In particular, the microcomputer-based transformer differential protection, which is now widely used, should handle and solve these problems better because it has more powerful software functions such as data processing, calculation, and logical judgment. In view of these problems and through long-term research, design, and application experience in transformer protection, this paper specifically discusses several issues of the current transformer TA and its connection group on each side of the transformer in the transformer differential protection. 1 Ratio matching and phase correction of the current transformer TA connection group Generally speaking, when current flows through the power transformer, the secondary current passing through the current transformer TA on each side of the transformer will not be completely balanced. This is related to the transformer ratio and wiring group and the ratio and wiring of the current transformer TA on each side of the transformer. Therefore, the following factors must be considered when designing the transformer differential protection system so that the currents on each side that have been reasonably matched can be compared. These factors are mainly: 1) the voltage level on each side of the transformer, including the tap situation; 2) the current transformer situation on each side of the transformer and its wiring method; 3) the current phase angle difference caused by the transformer Y-△ connection; 4) the neutral point grounding situation on the Y-connected winding side of the transformer; 5) whether there is a ground fault zero-sequence current power supply on the △ side of the transformer. Conventional transformer differential protection devices generally adopt a reasonable selection of the application wiring method of the current transformer TA to correct the phase difference, and use the internal device of the device to match the ratio or use a dedicated external auxiliary current transformer to match the ratio, so as to solve these problems. I will not go into details here. At present, microcomputer-based transformer differential protection devices generally use their own convenient calculation conditions, and use protection software to simply use mathematical methods to match the ratios of various transformers and their current transformers TA, as well as the phase difference formed by the wiring group of the protected transformer. There is no need for the internal device of the device to match the ratio or a dedicated external auxiliary current transformer to match the ratio. In general, the microcomputer-based transformer differential protection device can use the following mathematical expression to simulate the matching of the currents on each side of the transformer, and no longer requires the wiring method of the current transformer TA. Its usual programming coefficient matrix mathematical expression is as follows:
























Where: [IOUT] is the matrix of the currents Ia, Ib, and Ic on this side after matching; Kn is the transformation ratio balance coefficient on this side; [A] is the matrix of the phase balance coefficient on this side; [IIN] is the matrix of the currents IA, IB, and IC of the input device on this side;

if the zero-sequence current compensation method is adopted, the usual mathematical expression of the programming coefficient matrix is ​​as follows:


Where: [I0] is the matrix of zero-sequence current on this side; K0 is the zero-sequence ratio balance coefficient on this side.

For example, for the transformer connection as shown in Figure 1, if the current transformers TA on each side of the transformer main primary and secondary currents of the input microcomputer-type transformer differential protection device are set to be star-connected, and the same-name terminals are all outside the transformer, then the ratio matching and phase correction methods of the protection device current transformer TA connection group can be adopted in the following two ways:


Method 1: The transformation ratio matching and phase correction are performed in the conventional manner without zero-sequence compensation. The programming matrix equations of each side determined according to formula (1) are as follows:


Method 2 is the method with zero-sequence compensation. The programming matrix equations on each side are determined according to equations (2) and (1) as follows:


The main differences between them are: Method 1 conforms to the conventional usage and has rich application experience; Method 2 is more sensitive to ground faults on the primary side of the transformer than Method 1; Method 2 has no relevant synthesis for the current input to the current transformer on the primary side of the transformer, so the original characteristics of the excitation surge current generated by the transformer may be better preserved than Method 1; the disadvantage of Method 2 is that it needs to input the zero-sequence current on the ground side of the protection device, and the application experience is not rich enough.

Through the above analysis and experience in practical applications, the advantages of using pure mathematical methods to rely on software to achieve the ratio matching and phase correction of the current transformer TA connection group are: high reliability, flexible methods, no environmental impact, good economy, easy modification, etc. However, in the application, it is necessary to pay attention to the limited range of the selected ratio balance coefficient to avoid the ratio balance coefficient itself amplifying the protection sampling value and affecting the protection work.
2 Countermeasures when the current transformer TA is saturated

The current transformer TA of the conventional electromagnetic coupling method, due to the large fault current and (or) long system time constant and the residual magnetism of the current transformer TA itself, will have an extremely adverse effect on the transformer differential protection device. In particular, the transient saturation of the current transformer TA has a greater impact on the transformer differential protection that refers to the current on each side of the transformer. Corresponding identification methods should be adopted to distinguish whether the current transformer saturation is caused by a fault outside the transformer differential protection zone to avoid the transformer differential protection from malfunctioning.

At present, on the one hand, the selection of the current transformer TA has taken into account or paid attention to the transient saturation problem of the current transformer TA, such as the general design of the TPY-level current transformer on the high-voltage system or the high-voltage side of large-capacity power equipment, and the selection of the PR-level current transformer with a small air gap; on the other hand, the protection device itself is required to have a certain ability to resist the saturation of the current transformer, especially the ability to resist the transient saturation of the current transformer TA. The identification method used for the protection device is mainly to use the current characteristics after the current transformer is saturated, such as the current waveform identification method, the harmonic content identification method, the time difference identification method, etc. The following is a method for distinguishing the additional stable characteristic area of ​​the current transformer saturation selected in the transformer differential protection:

First, for the short-circuit fault occurring in the protected transformer area, the saturation of the current transformer TA caused by it is not easy to distinguish by the ratio of the differential current and the restraining current. This is because the measured values ​​of the differential current and the restraining current will be affected, and their ratio will immediately meet the protection action conditions. At this time, the action characteristics of the ratio differential protection are still valid, and the fault characteristics meet the action conditions of the ratio differential protection.

Secondly, for the fault occurring outside the protected transformer area, the current transformer saturation caused by the large through-short-circuit current it generates will generate a large false differential current, which is more serious when the saturation of the current transformer TA at each measuring point is different. If the operating point caused by the resulting value falls within the action characteristic area of ​​the ratio differential protection, and no measures are taken to stabilize the ratio differential protection, the ratio differential protection will malfunction. However, we know that the current transformer TA does not saturate at the beginning of the fault, but only after a period of time after the fault occurs, when the magnetic flux of its iron core reaches its saturation density. In this way, there will always be a period of time (not less than 1/4T-1/2T, T is the time of the power frequency cycle) from the beginning of the fault to the beginning of saturation of the current transformer TA, which can still linearly transform the current and will not be saturated immediately. Therefore, the differential current obtained by calculating the current on each side of the transformer according to Kirchhoff's current law is basically balanced in the initial short period of time, and only a small unbalanced current will be generated. Only after the current transformer TA is saturated will a larger differential current be generated, causing the transformer differential protection to malfunction.

In view of the above situation, the transformer differential protection can set an additional stable characteristic zone when the current transformer TA is saturated, which can distinguish between the internal and external fault conditions of this transformer zone. Its working characteristics are shown in Figure 2.


For the saturation of the current transformer TA caused by the fault outside the protected transformer area, it can be detected by the high initial restraining current (ITA) within the first 1/4T-1/2T time after the fault occurs. This restraining current will temporarily move the working point to the additional stable characteristic area. On the contrary, when the fault occurs in the transformer area, due to the large differential current, the working point caused by its ratio to the restraining current will immediately enter the action characteristic area of ​​the ratio differential protection. Therefore, the protection makes a decision within half a cycle by judging whether the working point caused by the measured current value is in the additional stable characteristic area. Once it is detected that the current transformer TA is saturated due to the external fault, the differential protection can be selected to automatically lock the ratio differential protection, and the ratio differential protection will be effectively locked for the set time (TTA) until the set time is reached. The judgment formula for checking the saturation of the current transformer TA caused by the fault outside the transformer area is as follows (7).


Where: ITA is the threshold value for checking TA saturation braking current; TTA is the TA saturation lockout time

During the period of ratio differential protection of power transformer TA saturation lockout caused by external fault, if a fault change occurs and a fault also occurs in the transformer protection zone, and the operating point caused by it stably works in the high-set action zone for two consecutive cycles, then the current transformer TA saturation lockout will be immediately released. In this way, the developing fault of the protected transformer can be reliably detected and quickly acted.
3 Countermeasures when the secondary circuit of the current transformer TA is disconnected or short-circuited

Historically, it is difficult for microcomputer-type transformer differential protection to distinguish the disconnection or short-circuit fault of the current transformer secondary circuit. The reason is that it is difficult to distinguish the various disconnections and short-circuit faults of the current transformer secondary circuit with complex wiring by simply judging the current itself. It is difficult to distinguish from various system abnormalities or faults. Therefore, many microcomputer-type transformer differential protections are only equipped with simple current transformer secondary circuit disconnection judgment elements. In view of this situation, a principle of judging whether the secondary circuit of the current transformer TA is disconnected or short-circuited by using the current and voltage is introduced. It is particularly suitable for the microcomputer-type transformer protection device with integrated main and backup mode.

The abnormal differential current alarm of the transformer differential protection and the judgment criteria for the disconnection or short-circuit of the secondary circuit of the current transformer TA are as follows:

1) Abnormal differential current alarm

When the effective value of the differential current of any phase is greater than the alarm threshold value, and the action condition is continuously met for more than 10 seconds, the protection device sends a differential current abnormal alarm signal, but does not lock the ratio differential protection. This function also has a comprehensive alarm function for the disconnection or short-circuit of the secondary circuit of the current transformer TA, abnormal sampling channel (device damage or characteristic change, etc.), abnormal external wiring circuit, etc.

2) Instantaneous current transformer TA disconnection or short circuit alarm

The ratio differential protection is opened when any of the following conditions are met after the protection is started:

① The voltage element of any phase on either side is suddenly started;

② The negative sequence voltage on either side is greater than the threshold value;

③ The current of any phase on either side after starting is greater than that before starting;

④ The maximum phase current after starting is greater than 1.2Ie.

If the above criteria for excluding system faults or disturbances are not met, and the working point of the differential current meets formula (8), then the protection is judged as a disconnection or short circuit fault of the current transformer TA secondary circuit, and it is not considered that a short circuit fault has occurred inside the transformer.


Where: Idset is the differential current threshold value for checking disconnection or short circuit; k is the ratio coefficient for checking disconnection or short circuit.

In the practical application of the above criteria, in order to meet the needs of different users, the criterion element can be designed to select only to send an alarm signal through the configuration word, or to send an alarm signal and lock the ratio differential protection, or to choose not to invest in this criterion element. When the alarm signal is sent and the ratio differential protection is locked, you can also choose to "permanently" lock the ratio differential protection or automatically release the lock ratio differential protection when the phase current increases by more than 1.2Ie.

Since the above criteria selects the comprehensive judgment of current and voltage, various disconnection or short circuit conditions of the current transformer secondary circuit can be well judged. Therefore, not only the judgment type range of the fault condition of the current transformer secondary circuit is comprehensively increased, but also the various disconnection or short circuit conditions of the current transformer secondary circuit are judged more accurately, more reliably and more comprehensively.

4 Phase sequence, polarity and grounding issues of current transformer TA connection

In accordance with relevant regulations, the phase sequence and polarity of the current transformer connection circuit of the input protection device must be strictly checked before the protection is put into operation to ensure the correct operation of the transformer differential protection. However, engineering practice reflects that due to various reasons, the wiring of the three-phase circuit of the current transformer on each side of the transformer is indeed wrong, resulting in incorrect phase sequence and polarity, causing the transformer differential protection to malfunction unnecessarily. If the protection device itself can intuitively display the phase angle and amplitude of the input current on each side of the transformer, it will be of great help to check the phase sequence and polarity of the current transformer connection on each side of the transformer differential protection, and it will provide another guarantee for the safe and stable operation of the transformer differential protection. Based on this consideration, the strong human-machine interface function of the microcomputer protection can be used to intuitively display the relative phase angle and amplitude of the current on each side of the transformer, display the amplitude of the differential current, etc., and observe the measurement of the input current. Therefore, when the transformer is put into operation with a light load, the on-site protection technicians can directly analyze and verify whether the TA circuit wiring of the current transformer on each side of the transformer is correct by observing the current and differential current measured by the transformer differential protection device. If the phase current circuit wiring on each side of the transformer connected to the transformer differential protection device is normal through observation and analysis and the obtained phasor diagram, and only the displayed differential current is abnormal, then there may be a problem with the setting value of the current on each side of the digital balance transformer of the protection device itself, thereby verifying whether the setting value of the current on each side of the digital balance transformer of the protection device is correct.

Another noteworthy issue in the secondary current circuit wiring of the transformer differential protection is the grounding point problem. The requirement that the secondary circuit of the instrument transformer must have a reliable grounding is clearly stipulated in the corresponding regulations at home and abroad. For example, in the "Technical Regulations for Relay Protection and Safety Automatic Devices" promulgated by the Ministry in 1983, there is the following provision: The secondary circuit of the current transformer should have a grounding point and be grounded through the terminal block near the distribution device. However, for protection devices with several groups of current transformers connected together, they should be grounded through the terminal block on the protection screen.

In engineering practice, it is reflected that the secondary circuit of the current transformer connected to the transformer differential protection device is grounded at multiple points, causing the transformer differential protection device to malfunction or abnormal. To solve this problem, on the one hand, it is necessary to strictly implement the relevant regulations for construction. On the other hand, when the transformer is put into operation and under load, the on-site protection technicians will analyze and solve it by observing the differential current measured and displayed by the transformer differential protection device. If the differential current measured and displayed by the transformer differential protection device is abnormal, after excluding the TA phase sequence wiring error and the setting value error of the current on each side of the digital balance transformer of the device itself, it is possible to check whether there is a multi-point grounding in the secondary circuit of the current transformer TA. In addition

, the ground network in the substation must also be safely and reliably formed into a complete equipotential surface ground network in accordance with the requirements of the relevant regulations. Both the ground network in the main control room and the ground network of the switch station must be reliably and safely interconnected, and the grounding point of the secondary equipment must also be safely and reliably connected to the ground network in accordance with the relevant regulations. In order to avoid the high voltage from burning the secondary cable and damaging the secondary protection control equipment or some unexpected things when a ground fault occurs in the switch station, which will affect the correct operation of the protection.

5 Conclusion

The above analysis discusses several problems of current transformers and their connection groups in transformer differential protection. These problems often have a great impact on the correct operation of transformer differential protection. If these problems cannot be solved well, it will directly affect the performance of transformer differential protection and even cause misoperation or refusal of transformer differential protection. In practical applications, abnormal operation of transformer differential protection caused by this also occurs from time to time.

The method introduced in this article has been well applied in actual devices. The RTDS digital simulation test, dynamic model test and actual field application have achieved satisfactory results and solved these problems well.

References

[1] Wang Weijian. Principle and Application of Relay Protection for Electrical Main Equipment. Beijing: China Electric Power Press, 1998.

[2] Wang Meiyi. Application of Relay Protection for Power Grid. Beijing: China Electric Power Press, 1998.

[3] Siemens Transformer Protection. 7UT512/513 Product Technical Manual.

[4] Li Hongren. Practical Relay Protection. Beijing: Machinery Industry Press, 2002.