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Main transformer differential protection several configuration methods analysis

First, the use of differential protection BCH-type differential relay constructed 

To achieve one of the main contradiction transformer differential protection need to be addressed is the use of various measures to avoid the effects of the current unbalanced, and there is a transformer inrush current differential protection Calculations need special consideration. At present, in order to reduce the impact of magnetizing inrush current on differential protection, a differential relay is widely used as a differential relay for a speed-saturated intermediate converter . 

(1) Differential protection (BCH-2 type) with reinforced speed-saturable intermediate converter:

          The setting of differential protection is based on the setting of avoiding maximum unbalanced current Differential protection of the device. The differential protection of this principle is effective in reducing the influence of the non-periodic components of the short-circuit current when external faults occur. However, it is not ideal to avoid magnetizing inrush. 

          BCH-2-type differential protection, is designed to solve the problem of inrush current design. At the heart of this differential protection is a saturated intermediate converter with shorted coils and a differential current relay. The existence of the short-circuit coil makes the operating current of the relay greatly increase when there is a non-periodic component current, thereby improving the performance of avoiding transient inrush current and transient unbalanced current in the external short circuit. 

          The use of BCH-2-type differential protection, pay attention to the determination of short-circuit coil turns, the more the number of turns, the better the performance of avoiding inrush, but the internal short-circuit, the relay action delay is long. For small and medium-sized transformers, the magnetizing inrush current multiples, the internal failure of non-periodic components decay fast, less protective action required, generally choose a larger number of turns, and for large transformers, the internal flow small multiples, non-periodic component attenuation Slow, but also requires protection of the fast, you should choose a smaller number of turns. The final choice of tap is appropriate, should be determined by the transformer drop test. At the same time, the sensitivity test should be carried out according to the shortest internal short circuit current. 

          If it does not meet the requirements, it should be selected with differential braking characteristics. The same principle with the BCH-2 type DCD-2-type differential relay consisting of differential protection

(2) Differential protection with brake characteristics (BCH-1 type): 

          Differential protection with braking characteristics is based on the principle of using the transformer through the current to produce a braking effect, so that when the current through the large braking effect generated, and the relay operating current also with the braking effect The size of the change. Thus, in the case of any external short-circuit current, the relay's operating current can be greater than the corresponding unbalanced current. Thus not only improve the sensitivity, but also will not malfunction. 

          The difference between the BCH-1 model and the BCH-2 model is that the latter has a short-circuit coil and the former has a brake coil. It should be emphasized here is the use of BCH-1-type differential protection, in order to improve the sensitivity of the protection device, according to the following principles to consider the brake coil access:

          1 for a single-sided power transformer, the brake coil connected Load side. 

          2 For a double-ended transformer with a double-sided power supply, the brake coil is connected to the large power supply side. 

          3 For a three-phase transformer with one-sided power supply, the brake coil should be connected to the maximum power-receiving side of the out-of-area short-circuit current. 

          4 For a three-turn transformer with a double-sided power supply, the brake coil is connected to the no-power side. 

          5 For a three-wire transformer with three-side power supply, the brake coil access point should be set by calculation, either on the side with the largest out-of-zone short-circuit current, on the large power side, or on the regulator side. 

          In short, the brake coil access principle is: external short-circuit, it should make the greatest braking effect, the protection does not mistakenly move, in the internal short-circuit, the braking effect should be minimal, the best protection sensitivity. 

          DCD-5-type differential protection and BCH-1-type differential protection the same principle. Generally for multi-winding transformers with load regulating transformers and multi-side power supplies, unbalanced current is larger when external faults occur, or when using BCH-2 type and DCD-2 type, the sensitivity can not be satisfied. 

(3) Multi-side brake differential protection (BCH-4 type): 

          BCH-4-type differential relay by the DL-11-type current relays and intermediate speed saturated converter composed of two parts. The former as an actuator, which has four windings and constitute some of the main relay differential performance, such as braking characteristics, to avoid DC inrush magnetizing inrush current characteristics and to eliminate the impact of unbalanced current performance and so on. 

          BCH-4 type differential relay is generally required in the following cases: 

          1 Differential protection of multi-winding three-winding or four-winding power transformer requiring multi-side braking;

          2 Multi-winding transformer differential protection with split winding; 

          3 Circuit breaker The number of more than three groups of transformer differential protection; 

          4 using BCH-1 type or DCD-5 type can not meet the sensitivity requirements. 

          Because BCH-4 differential relay brake winding can play a part of the action of the winding, so the internal fault sensitivity than the BCH-1 type, DCD-5 type is better. And BCH-4-type principle of the same DCD-4-type differential relay consisting of differential protection. 

Second secondary transformer differential protection harmonic restraint 

harmonic restraint inrush current relay using a larger second harmonic component, and the short-circuit current is almost no harmonic component 2 characterized in that, to distinguish magnetizing inrush And short circuit current.

          LCD series relay transformer differential protection, is composed of the ratio of the brake part, the differential part, the second harmonic brake part, the differential current breaking part and the polarization relay. Mainly LCD-4, LCD-5, LCD-5A-type differential protection relays. 

          The ratio of braking part is used to prevent external short circuit, due to the impact of unbalanced current caused by malfunction. The role of the second harmonic braking part is to prevent the magnetizing inrush current caused by no-load input of the transformer and cause the protection action. When a serious fault occurs inside the equipment, the short-circuit current is large, causing the current transformer to be seriously saturated. The secondary current may have a great amount of harmonic components and generate a great braking torque to cause the differential element to refuse. Set the differential breaking components, when the short-circuit current reaches 4 to 10 times the rated current, the breaking components action exit. 

Harmonic braking differential protection selection, the same should be noted that the brake coil access, the principle and BCH-1 type the same. The difference is the LCD-type differential relay 4DKB primary side brake coil, access has its own unique requirements. 

Second, the use of full-function main transformer differential protection device 

          Produced 992 two laps main transformer differential protection , 993 three laps main transformer differential protection device is mainly applied to 110KV, 66KV, 35KV and below the voltage level of the three sides Equipped with differential backup protection microcomputer differential protection products with differential instantaneous protection; proportional differential protection (with second harmonic brake, CT wire breakage); CT wire break alarm; Y / Y / △ compensation input; Electricity protection - Body light gas - Alarm; Body heavy gas - Tripped; Loaded light gas - Alarm; Gas loaded - Tripped; Pressure relief (cast - tripped; Trip and other protection features. Measurement of the amount of high-side current measurement: IAH, IBH, ICH; low-side current: IaM, IbM, IcM; differential current: DIa, DIb, DIc; At the same time with 485,232, Ethernet, a variety of communication interfaces, the main transformer differential protection is the best and choice. 

Third, the differential protection principle of differential waveform discontinuity angle 

          Differential protection principle of differential waveform discontinuity angle, there is a large inrush current is the use of the dead angle between the waveforms, but no difference between the two waveforms dead angle between the short-circuit current waveform, Differentiate whether it is inrush or short circuit, so as to avoid the influence of inrush current and utilize the braking characteristics to avoid the unbalanced current and form the differential protection. 

          Differential protection using the principle of BCD-type series and JCD Series relays. Commonly used BCD-23, BCD-32A, BCD-22A and JCD-2A, JCD-4A transformer differential protection. 

          It should be pointed out here is that the transformer intermittent magnetizing inrush current is generally 120 ° ~ 180 °. In order to avoid all types of transformer magnetizing inrush current, θb (closing angle) should take the lower limit of the intermittent angle of the magnetizing inrush current 120 °, if you can take * coefficient of 2, then θb = 60 °. 

          To sum up, relative to the differential relay with the speed-saturation characteristic, the differential protection reflecting the principle of the discontinuity angle and the second harmonic braking principle has unbalanced current that avoids external short-circuit due to the ratio braking characteristic Action time is fast (do not have to decay of the general non-periodic components, and then action), the minimum operating current is small (less than the transformer rated current) advantages. It is foreseeable that with the continuous development and improvement of rural power grids, some of the new principles of differential protection will also be more and more used by rural power grids.

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