Internal short-circuit faults in generators mainly refer to phase-to-phase and turn-to-turn short-circuit faults in the stator windings. When a short-circuit fault occurs, it will generate a large inrush current, and the resulting powerful arc will burn out the stator winding insulation. It may also cause a large fire or even scrap the generator, with very serious consequences. Therefore, it is required to install generator longitudinal differential protection as the main protection for phase-to-phase and turn-to-turn short-circuit faults in the generator stator windings, which will trip the generator.
1. The function of longitudinal differential protection
The main protection against phase-to-phase short circuits in the generator stator windings and their leads is the protection against such short circuits .
2. Basic Principle Diagram of Longitudinal Differential Protection
By comparing the magnitude and phase of the current on both sides of the generator, it reflects phase-to-phase faults in the generator and its leads. The current transformers on both sides of the generator longitudinal differential protection system have the same transformation ratio and model.
Figure 1. Basic principle diagram of longitudinal differential.
Figure 2. Principles and current formulas during normal operation and external faults.
Figure 3 shows the principle and current formula during internal faults.
Working principle of ratio braking type longitudinal differential protection
The operating current of the ratio-controlled longitudinal differential protection is variable. It changes automatically with the change of short-circuit current, ensuring that it does not trip falsely when there is an external short-circuit fault, while also having high sensitivity to internal short-circuit faults.
Taking one phase of a generator as an example, the primary current flowing into the generator is defined as the positive direction. During normal operation and...
When a fault occurs outside the generator protection zone, the differential current flowing into the differential relay is zero, and the differential relay will not operate. When an internal generator fault occurs, the differential current flowing into the differential relay will be larger. When the differential current exceeds the set value, the differential relay will determine that an internal generator fault has occurred and will trip the generator.
1. Determine the fault location by comparing the phase and amplitude of the current at the generator terminals and the neutral point.
(1) When operating normally or under external fault, I1 and I2 are in the same direction and equal in magnitude, the differential current Id = I1 - I2 = 0; the braking current Iz = (11 + I2) / 2 - I.
(2) When there is a fault in the zone, I1 and I2 are in opposite directions, and the differential current Id = 11 - I2 = 0. The fault current is proportional to the sum of the absolute values of I1 and I2; the braking current IZ is proportional to the difference between the absolute values of I1 and I2.
2. In order to ensure that the device does not malfunction in the event of an external fault, a ratio braking differential element is adopted, so that the operating current changes with the braking current. The larger the external short-circuit current, the larger the operating current of the relay, ensuring that the relay can reliably brake in the event of an external fault.
Setting an appropriate braking coefficient ensures that faults outside the zone will not cause false tripping, while faults inside the zone will ensure reliable operation. As shown in Figure 1, a double-slope operating characteristic curve is used. Slope 1 is smaller to ensure good sensitivity during internal short circuits, while slope 2 is larger to account for the large through current generated by external faults, which will cause different saturation levels of the current transformers on both sides and generate a large differential current. Increasing the slope enhances the braking capability and prevents false tripping due to external short circuits.
II. Generator Longitudinal Differential Protection Principle
1. Generator longitudinal differential protection operation logic relationship
Because the generator neutral point is not directly grounded, when a phase-to-phase short-circuit fault occurs inside the generator, two or three phase differential relays will operate simultaneously. Based on this characteristic, corresponding considerations can be made in the protection logic design. When two or three phase differential relays operate, it can be determined that a short-circuit fault has occurred inside the generator; when only one phase differential relay operates, it is determined that the current transformer (CT) is disconnected. To handle short-circuit faults caused by one point being grounded within the zone and another point being grounded outside the zone, when one phase differential relay operates and there is a negative sequence voltage, it is also determined to be an internal short-circuit fault inside the generator. The characteristic of this operating logic is that a single-phase CT disconnection will not cause it to operate. Therefore, a dedicated CT disconnection interlocking mechanism can be omitted, and the protection is safe and reliable.
2. Principle of incomplete longitudinal differential protection for generators
Conventional longitudinal differential protection incorporates all phase currents *i* and *i* from the generator stator terminals and neutral point. When a phase-to-phase short circuit occurs in the stator windings, the two phase currents remain equal, and the protection will fail to operate. However, large generators typically have two or more parallel branches in each phase of the stator winding. If only a portion of the branch current *i* from the generator neutral point side is incorporated to form the longitudinal differential protection, and an appropriate current transformer (CT) ratio is selected, it can ensure no differential current during normal operation and external faults. However, differential currents will be generated during phase-to-phase and turn-to-turn short circuits, and when this current exceeds the set value, the fault can be cleared. This type of longitudinal differential protection is called incomplete longitudinal differential protection.
The number of neutral point current transformers (CTs) can be selected according to the following principles for incomplete longitudinal differential protection.
a/2 < N < (a/2) + 1
In the formula
N---Number of branches connected to longitudinal differential protection per phase on the neutral point side:
a----The total number of parallel branches for each phase of the generator.
Since the incomplete longitudinal differential protection of the generator only introduces a portion of the neutral point branch current, the following issues should be noted when applying it:
(1) The turns ratio of the generator terminal and the neutral point TA are no longer equal, so it is impossible to use the same type of TA. Therefore, the unbalanced current caused by the TA will increase.
(2) Increased error sources. In addition to the usual errors, incomplete longitudinal differential protection also has some special error sources, such as imbalances caused by slight differences in the parameters of each branch (air gap asymmetry, motor vibration, etc.).
(3) Setting values. Compared to the complete longitudinal differential protection of the generator. Due to the increased error of the incomplete longitudinal differential protection, the operating threshold and ratio braking coefficient of the longitudinal differential protection should be appropriately increased when setting.
(4) Sensitivity. The sensitivity of incomplete longitudinal differential protection is closely related to the location and number of current transformers (CTs) on the generator neutral point branch. Before applying incomplete longitudinal differential protection, necessary sensitivity analysis and calculation for generator internal short-circuit faults should be performed.
