Discussion on microcomputer differential protectio

Discussion on microcomputer differential protection of main transformer in substation

microcomputer differential protection of main transformer in a horn shaped range is given from low temperature to high temperature for types 2, 3 and 4 of substation (1) Under normal conditions and external faults, the transformer differential protection composed of electromagnetic relay ideally flows into the differential relay with current ij=0, and the protection device does not act. (2) Microcomputer based differential protection for main transformer is composed of ratio difference of second harmonic braking and differential quick break

(1) the transformer differential protection composed of electromagnetic relay clicks the "return" button under normal conditions and external faults. Ideally, the current ij flowing into the differential relay is equal to 0, and the protection device does not act. But in fact, when the differential protection of the transformer is short circuited outside the near end (outside the protection area), due to the large short-circuit current, the voltage levels of the TA on each side constituting the differential protection are different, and the transformation ratio, capacity and magnetic saturation characteristics are inconsistent. Even if the balance coil and other methods are used for compensation, the transformation ratio between the TA 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 no-load transformer or external fault removal voltage is put into operation, once the iron core is saturated, a large value of inrush current will appear correspondingly. Since the inrush current only exists in the primary winding and flows into the differential relay after TA transformation, it also causes the differential protection to malfunction. Using microcomputer differential protection of main transformer can avoid mal operation of transformer differential protection caused by unbalanced current and inrush current during external short circuit

(2) microcomputer based main transformer differential protection is composed of ratio difference of second harmonic braking and Jung differential quick break. Aiming at the maloperation of the differential protection which frequently occurs in the system when the TA of the differential circuit is disconnected or has poor contact, the TA disconnection locking device is designed. The main transformers of Xiawei, Shili, Quanzhuang and Guanzhuang substations in our bureau adopt microcomputer differential protection, and its reliability is proved by its operation

ratio differential action characteristic equation:

id> iqdizd

id IQD> k (IZD IE) IZD> ie

where IQD differential current starting fixed value

id differential current action value, id= ｜ I1 + I2 ｜

izd braking current, izd=0.5 (｜ I1 ｜ ｜ I2 ｜)

k-ratio braking coefficient, take 0.5

ie rated current of transformer

, that is, when IZD

IZD> ie, Ratio differential has greater braking effect

features: when the transformer has a slight fault, such as the number of turns of inter turn short circuit is small, there is no braking amount, so that the protection has high sensitivity in the case of a slight fault of the transformer. In case of serious out of area fault, there is a large amount of braking to improve the reliability of protection

the main difference between the second harmonic braking is fault current or inrush current. If it is judged as inrush current, the ratio differential action is locked by the inrush current discrimination element, otherwise the ratio differential action. The second harmonic braking ratio is generally taken as 0.12

② function of differential quick break:

differential quick break is to quickly trip the circuit breakers on each side of the transformer and cut off the fault point in the case of serious faults in the area. In fact, it is completed by differential overcurrent relay, which is no different from electromagnetic differential quick break. The setting value of differential quick break is based on the larger of avoiding the inrush current of transformer and the unbalanced current caused by through fault under the maximum operation mode. The constant value is generally (4 ~ 14) ie

calculation example:

the main transformer capacity of the substation is 2500kVA, and the transformation ratio is 38.5 ± 2 × 2.5%/11kv, the wiring group is Yd11, the TA transformation ratio on the high-voltage side is 100/5, and the TA transformation ratio on the low-voltage side is 200/5

a. determination of basic side current:

all experimental data and secondary rated current on each side of the curve can be saved according to the calculation of transformer capacity and average voltage on each side

b. balance adjustment:

the balance adjustment in lsa-p protection is based on the high voltage side

voltage side balance coefficient: 3.57/3.61=0.988, taking 0.99. (end)