Research on Stator Grounding Protection of New Digital Generator

Research on the grounding protection of the new digital generator stator Wan Hui! Wu Ji'an 2, Lu Yuping 1 1. Department of Electrical Engineering, Southeast University, Nanjing 210096, China; 2. Guodian Nanjing Automation Co., Ltd., Nanjing 210003, China) Protection. The protection can meet the requirements of the generator base grounding protection of different capacity and wiring mode, and the protection algorithm is not affected by the neutral point of the generator; at the same time, the grounding resistance criterion can be obtained by the protection algorithm based on the balance principle, the fundamental wave The zero-sequence voltage can be obtained from the voltage transformer TF) of the machine and neutral points. The value of the zero-sequence voltage of the fundamental wave should take into account the influence of the grounding of the high-voltage side of the main transformer.

Single-phase grounding of generator stator windings is the most common type of fault in generators and is a precursor to phase-to-phase or turn-to-turn short circuits. Therefore, the stator grounding protection is an important protection in the large unit protection 1, which is of great significance for preventing severe short-circuit faults in the stator windings.

In the previous research work, a new type of injected AC stator grounding protection based on the balance principle was proposed. It can improve the detection sensitivity of the stator grounding resistance under the grounding mode of the generator neutral point through the distribution transformer.

Based on the above principle of stator grounding protection, and considering the stator grounding protection requirements of different units, a new digital stator grounding protection scheme is proposed. The principle of the scheme is analyzed in detail, and the measurement method and value principle of protection criteria are given. .

1Proposed new grounding protection 1.1 Principle of protection based on the principle of balance The injection of AC power supply protection is based on the conventional injection of AC power grounding protection. The balance arm is set up to eliminate the neutral point of the generator through the low-resistance grounding of the distribution transformer. The sensitivity of the low-frequency injection type protection is not high, which is proposed to eliminate the influence of various imbalances and improve the detection sensitivity. For the generator to be grounded via the arc suppression coil, even if the conventional injection method is not at the neutral point, high sensitivity can be obtained without having to set the balance arm or not considering the protection at all.

Will be injected with AC power! The AC filter F, the balance arm F1, and the grounding resistance of the grounding transformer are considered to be injected into the AC power supply circuit.

As shown.

Before the generator is connected, the balance arm parameters are adjusted so that the AC filter branch 1 and the balance arm branch current 2 injected into the AC power supply circuit are equal, and the injected AC power supply circuit is in an equilibrium state. After the generator is connected, when there is no fault, due to the effect of the generator on the ground capacitance, there will be a small difference in the current between the filter branch and the balance arm branch; when a ground fault occurs, the two arm currents will appear obvious. Vector difference. Since the injected AC power supply circuit is in equilibrium before the generator is connected, the change in the current measurement amount reflects the change in the grounding impedance of the generator stator. And there is almost no 50Hz unbalance component in the measured current quantity, which improves the detection sensitivity of the protection to the low frequency measurement. The calculation of the grounding resistance will be more accurate, and 100* stator grounding protection can be obtained without setting another criterion. range.

1.2 New digital stator grounding protection 2 Stator grounding protection The fundamental zero-sequence voltage principle is the simplest and most reliable solution. No matter whether it is domestic or foreign, whether it is injected or not, or the neutral point of the generator is grounded. The mode and protection device are analog or digital. They all form the basis of the grounding protection of the stator with the fundamental zero-sequence voltage principle. Only when the protection dead zone near the neutral point of the generator is removed, other principles are used to form 100. * Action zone plan.

Based on the discussion of stator grounding protection based on the principle of balance, a new scheme for stator grounding protection is presented, as shown.

The ground protection logic diagram is based on the balanced principle of injected AC power grounding protection, which improves the sensitivity and accuracy of the grounding resistance criterion. The grounding resistance and zero-sequence voltage criteria use two sets of high and low settings. Considering that the grounding resistance is large when the calculated value of the grounding resistance is large, the calculation error may have an impact on the high-value grounding resistance criterion! "h, dz criterion, in addition to considering a certain reliability coefficient, the action value should also consider the influence of zero-sequence voltage on the fundamental zero-sequence voltage protection when the main transformer high-voltage side ground fault. For the fundamental zero-sequence voltage protection principle, When the generator stator single-phase grounding protection action time is less than the main transformer high-voltage side grounding protection time, if the protection cannot obtain the zero-sequence voltage of the main transformer high-voltage side 3=0), consider the action time of the fundamental zero-sequence voltage protection to avoid The action time of single-phase grounding on the high voltage side of the main transformer, so %0>0.5s, the value of 2 should take into account the influence of %0; otherwise, the zero-sequence voltage of the main transformer high-voltage side grounded 3"0,> lock fundamental wave Zero-sequence voltage is used as a criterion to prevent misoperation of the fundamental zero-sequence voltage protection; when the generator stator grounding protection action time is greater than the main-time high-voltage side grounding protection action time, it is not necessary to consider the base when the main transformer high-voltage side is grounded. The effect of the zero-sequence voltage of the wave on the zero-sequence voltage of the stator fundamental wave.

The fundamental zero-sequence voltage protection principle, 3"0n> and 3"0t use the protection algorithm of the balance principle to realize the protection criterion of the grounding resistance. Before the AC power supply circuit is connected to the generator, due to the role of the balance arm, /: =/2; After the AC power supply circuit is connected to the generator, due to the resistance of the generator stator to ground connected in parallel across the resistance of the distribution transformer, the /:>/2. dz criterion is detected after the injection of AC power or The door detects a ground fault.

2 Protection analysis 2.1 Protection The criteria for adapting different protection principles for different units are input or exited by the control words in the software.

For the stator windings of medium and small generator sets, there are many components that are electrically connected to the generator for the connection mode of the generator directly connected to the busbar. The capacitance of each external component is larger than that of the stator to the ground, so it flows from the external components. The capacitor current is also large, and the grounding current is also large when the ground fault occurs. Therefore, the conventional zero-sequence current principle 3/0>3/0,z is generally used as the main criterion for protection for the stator grounding protection of small units.

For the large-scale unit stator grounding protection, in order to ensure the safety of the Stator Core, it is required to protect the operating range and high sensitivity of 100; For the connection mode between the generator and the step-up transformer unit, the generator has no electrical connection with other components in the system. When the ground fault occurs, only the capacitance of the generator itself, the capacitance of the connected generator and the step-up transformer conductor, and the boost main The current caused by the low-voltage winding of the transformer and the capacitance of the high-voltage winding of the transformer and the transformer of the transformer is not very large. The complete set of two sets of grounding resistance and fundamental zero-sequence voltage and the fundamental zero-sequence current are used. 30>3/0, the grounding protection formed by the dz criterion.

In addition, in the case where the generator neutral point has no TV tap, the fundamental zero-sequence voltage uses only the zero-sequence voltage taken from the terminal TV. At this time, it is necessary to add a special terminal TV disconnection lock.

2.2 Protection scheme For large-scale generator sets with different neutral grounding modes, the neutral point is generally grounded by distribution transformer or arc suppression coil. Based on the balance principle, the AC power supply type grounding protection is applied. When the arc suppression coil is grounded, the balance arm is released from the injected AC power supply circuit, and the protection can be operated according to the conventional injection AC power supply mode. As shown.

The equivalent circuit diagram of the AC power supply stator grounding protection is used for the grounding resistance criterion, and the grounding protection algorithm based on the balance principle is adopted.

The generator is grounded through the distribution transformer, as shown in -). The grounding admittance of the stator is calculated by /i,/2: the grounding mode of the generator through the arc suppression coil, as shown in E), is obtained: if the above-mentioned derivation 1) and 2) are used, it can be seen that based on the principle of balance The grounding resistance algorithm is also applicable to the calculation of the grounding resistance of the generator through the arc-suppression coil grounding mode. The algorithm of the grounding resistance criterion is not affected by the neutral point of the generator.

2.3 Protection scheme The implementation of the measurement link and the principle of value injection into the AC power failure signal by the UE The grounding resistance R is calculated from the measured current. For the selection of the high and low fixed values, consider the low value of 3 k! The high value is 8 measured current injected into the AC power supply circuit, and the 2 arc suppression coil is not grounded, and is taken from the secondary side of the current transformer TA injected into the AC power supply circuit.

When single-phase grounding occurs at a certain point in the stator circuit of the generator, the stator circuit has zero-sequence voltage at each point. Therefore, the fundamental zero-sequence voltage of the protection action parameter can be taken from the generator neutral point single-phase grounding TV or arc suppression coil. The secondary voltage can also be taken from the voltage of the third winding of the third-phase TV of the machine, and the fundamental zero-sequence voltage of the generator through the neutral point distribution transformer is taken from the distribution transformer. Secondary voltage. For the zero-sequence voltage that forms a composite criterion with the grounding resistance, the action value of the 3U>Ui, dZ criterion only needs to be set to avoid the unbalanced voltage of the generator stator during normal operation. For the operating voltage protected by the conventional fundamental zero-sequence voltage principle, it is generally necessary to avoid the unbalanced voltage and the third harmonic voltage caused by the asymmetry of the three-phase capacitance of the generator stator during normal operation.

When a single-phase ground fault occurs outside the generator, it will also cause the generator neutral-to-ground voltage to change, which may result in protection malfunction. Therefore, it is also necessary to brake the fundamental zero-sequence voltage stator grounding protection of the generator with the fundamental zero-sequence voltage when the generator is externally grounded. Therefore, in addition to considering a certain reliability coefficient, the fundamental zero-sequence voltage criterion should consider the influence of the fundamental zero-sequence voltage on the zero-sequence fundamental voltage protection when the high-voltage side of the main transformer is grounded.

The zero-sequence current 3/0 can be taken from the secondary side of the TA at the outlet of the generator, the secondary side TA of the neutral distribution transformer or the secondary side TA of the arc suppression coil; the current is operated according to the conventional zero-sequence current principle. The zero-sequence unbalance current formed by the asymmetry of the parameters caused by the stator capacitance when the generator is normal and the external fault of the generator should be avoided.

3 Conclusion Based on the principle of low-frequency injection stator grounding protection and fundamental zero-sequence component protection based on the principle of balance, a new stator grounding protection criterion and implementation method are proposed. It has a 100% protection range and the advantages of reliability and sensitivity. The protection criterion consisting of two sets of high and low settings can meet the requirements of generator stator grounding protection with different capacity and wiring mode; the protection criterion is not affected by the neutral grounding mode of the generator.