As a protective device, a differential relay is fed back by current transformers located at two different locations in the system. The differential relay compares the currents and if there is a difference, there is a fault in the protected area. These devices are often used to protect the coils of generators or transformers.
1. Introduction to differential relay
As a protective device, a differential relay is fed back by current transformers located at two different locations in the system. The differential relay adopts the swing-and-beat magnetic system.
2. Technical data of the differential relay
2.1 Rated current 5A, rated frequency 50Hz.
2.2 When there is no brake, the initial ampere-turn of the relay is AWo=60±4.
2.3 When used to protect a three-winding power transformer, its operating current can be set within the range of 3A to 12A (AWo=60). For the minimum setting value of the action, its maximum balance factor is close to 2.
When used to protect a two-winding power transformer, its operating current can be set within the range of 1.55A to 12A.
2.4 The braking coefficient Kz determined by the ratio of the operating current to the braking current can vary in a wide range. The braking characteristic AWp=f(AWz) in Figure 3 is its limit range, which is related to the operating current and the braking current. It is related to the phase angle between them, but no matter at any angle, it should not exceed the range of the curve.
2.5 The reliability factor (the ratio of the sinusoidal operating current of the relay when the primary operating current is equal to 5IDZ to the sinusoidal operating current when the primary operating current is equal to IDZ) is not less than 1.35.
2.6 The action time of the differential relay is not greater than 0.035s when the action current is 3 times.
2.7 The relay has a moving and closing contact. In a DC circuit with an inductive load (its time constant is not greater than 5×10-3s), and the voltage is not greater than 220V, and the current is not greater than 2A, the breaking capacity of the contact is 50W.
2.8 Under normal working conditions, when the current is 5A and the brake winding balance winding of the converter is turned on in all turns, the single-phase power consumption of the relay is not greater than 8.5VA.
2.9 The working, balancing and braking windings of the converter can pass a current of 10A for a long time.
2.10 Dielectric strength: The circuit of the relay should be able to withstand 2kV, 50Hz AC voltage for 1min without breakdown or flashover between the exposed non-charged metal parts.
3. Operation principle of differential relay
The basic principle of the differential relay is AC magnetic braking, the working winding is connected to the differential circuit for protection, and the braking winding is connected to the circulating current circuit. Its function is that under normal conditions or when a penetrating short circuit occurs, the secondary current of the current transformer or the entire short circuit current passes through the brake winding. According to the electromagnetic relationship shown in Figure 2, the corresponding braking flux φZ only circulates between the two side columns of the magnetizer. Its function is purely to saturate the iron core and reduce the magnetic permeability of the magnetic circuit, which is the AC magnetic braking effect.
Under normal circumstances, only a small unbalanced current passes through the working winding, and its effect has been eliminated. When a penetrating short circuit occurs, since the current multiple of the current transformer is already large and the errors vary, the value of the unbalanced current will increase significantly, and its effect cannot be eliminated, but the braking effect at this time is also very large. Large, the saturation degree of the magnetic conductor is very high, which greatly deteriorates the electromagnetic induction condition between the working winding and the secondary winding, thus constituting the braking characteristics of the differential relay.
When used to protect a three-winding power transformer, apply two balanced windings and connect them to the two arms of the circulating current circuit, so that the effect of unbalanced current in the three circulating current circuits can be eliminated. When used to protect two-winding power transformers, only one balanced winding needs to be applied. In the case of large unbalanced current, the balanced winding is connected to the circulating current circuit; when the unbalanced current is small, it can be connected to the differential circuit to expand the range of the setting value.