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Abstract: The main transformer of the substation is differential protection. (1) Transformer differential protection composed of electromagnetic relay In the normal situation and external fault, the current Ij flowing into the differential relay under ideal conditions is 0, the protection device does not operate. . (2) Microcomputer type main transformer differential protection consists of the ratio difference of the second harmonic braking and the differential speed breaking.
(1) Transformer differential protection composed of electromagnetic relay In the normal case and external fault, the current Ij flowing into the differential relay under ideal conditions is 0, and the protection device does not operate. However, in fact, when the differential protection of the transformer is short-circuited at the near end (outside the protection zone), the voltage level of each side of the TA that constitutes the differential protection is different due to the large short-circuit current, and the ratio, capacity, and magnetic saturation characteristics are inconsistent, even if Compensation is performed by means of a balance coil, etc., and the ratio between the TAs on each side may not match, and the unbalanced current flowing into the differential relay may cause the differential protection to malfunction. At the same time, when the no-load transformer or external fault cut-off voltage is applied, once the core is saturated, a large amount of magnetizing inrush current appears accordingly. Since the magnetizing inrush current only exists in the primary winding, it flows into the differential relay after TA conversion, which also causes the differential protection to malfunction. The main transformer microcomputer differential protection can avoid the transformer differential protection misoperation caused by unbalanced current and magnetizing inrush current during external short circuit.
(2) Microcomputer type main transformer differential protection consists of the ratio difference of the second harmonic braking and the differential speed breaking. In addition to the differential protection that occurs frequently in the system, the TA disconnection locking device is designed when the differential circuit TA is disconnected or the contact is bad. The main transformers of Xiawei, Shishili, Quanzhuang and Guanzhuang Substation of our bureau adopt microcomputer differential protection, and the operation proves its reliability.
Ratio differential action characteristic equation:
Id>IqdIzd<Ie
Id-Iqd>k(Izd-Ie)Izd>Ie
Iqd-differential current starting value
Id-differential current action value, Id=|i1+i2|
Izd-brake current, Izd=0.5(|I1|+|I2|)
K-ratio braking coefficient, take 0.5
Ie-transformer rated current
That is: when Izd < Ie, the ratio difference does not have a braking effect.
When Izd>Ie, the ratio difference has a large braking effect.
Features: When the transformer is slightly faulty, for example, when the number of turns of the short circuit between the turns is small, the brake amount is not used, so that the protection has higher sensitivity when the transformer is slightly faulty. In the case of a more serious out-of-zone fault, there is a larger amount of braking to improve the reliability of the protection.
The main difference between the second harmonic braking is the fault current or the magnetizing inrush current. If it is determined as the magnetizing inrush current, the ratio differential action is blocked by the inrush current discriminating element, otherwise the ratio differential action. The second harmonic braking ratio is generally 0.12.
2 The role of differential quick disconnect:
Differential quick-break is to quickly trip open the circuit breaker on each side of the transformer and remove the fault point in the case of a more serious fault in the area. It is actually done by a differential overcurrent relay, which is no different from an electromagnetic differential quick-break. The fixed value of the differential quick-break is the larger of the unbalanced current caused by the excitation inrush current of the transformer and the through-fault fault in the maximum operating mode. The setting is generally taken from (4 to 14) Ie.
Calculation example:
The main transformer capacity of the substation is 2500kVA, the transformation ratio is 38.5±2×2.5%/11kV, the wiring group is Yd11, the high-voltage side TA ratio is 100/5, and the low-voltage side TA ratio is 200/5.
a. Determination of the basic side current:
Calculate the secondary rated current on each side according to the transformer capacity and the average voltage on each side.
b. Balance adjustment:
The balance adjustment in the LSA-P protection is based on the high pressure side.
Voltage side balance factor: 3.57/3.61=0.988, take 0.99.
