CN1633000A - Longitudinal zero-voltage turn-to-turn protection of generators with fault-tolerant re-judgment negative-sequence directional blocking - Google Patents

Abstract

This invention discloses a method for discriminating turn-to-turn short circuit of generator by identifying longitudinal zero-order voltage and neutral point current and its end voltage including the following steps: a protection device samples the current voltage waveforms of an mutual inductor to get the instant value, evaluating the complex form of each electric volume by digital filtering algorithm plus Fourier algorithm, filtering the negative-order component of the neutral point current and voltage, protection is output when elements of the negative-order direction and longitudinal zero voltage operate at the same time, when the mutual inductor is disconnected, the negative-order elements are with drawn automatically and high demand longitudinal zero voltage protection is done.

Description

Longitudinal zero-voltage turn-to-turn protection of generators with fault-tolerant re-judgment negative-sequence directional blocking

technical field

The present invention relates to the field of power systems, and more particularly to a method for relay protection.

Background technique

Large-scale synchronous generator sets are the basic equipment for electric energy production and occupy a very important position in the power system. In recent years, the stand-alone capacity of generators has been continuously increasing. They are extremely expensive and have more complex structures. Once they are damaged due to faults, maintenance is difficult and takes a long time, causing huge direct and indirect losses to the national economy. In this way, higher requirements are put forward for the selectivity, rapidity, reliability and sensitivity of the unit protection device.

Regardless of whether it is a large turbogenerator or a large hydrogenerator, the proportion of the upper and lower layers of the stator winding in the same slot is in the same phase. Due to the water leakage of the stator water cooling coil, the dust and oil accumulation at the end of the stator winding, coupled with the long-term vibration, heat generation and strong electromagnetic force of the generator set, a short circuit between turns may occur in the stator slot or at the end Failure, operation statistics also show that there are instances of inter-turn short circuit, and there have also been vicious accidents in which the generator was severely burned out due to the failure to install the inter-turn protection of the stator winding. Therefore, large-scale generator stator winding turn-to-turn short circuit protection should be installed.

For generators with 4 to 6 current transformers (TA for short, the same below) that can be installed at the neutral point, main protections such as single element transverse difference, split phase transverse difference or incomplete longitudinal difference can be installed to protect the stator winding turns short circuit. But in practice, there are many generators at home and abroad that can only install 3 TAs in the neutral point. At this time, a special voltage transformer for inter-turn protection (referred to as special TV, the same below) can be installed at the generator end. The special TV is fully insulated, and the primary neutral point is connected to the neutral point of the generator through a high-voltage cable. When the inter-turn short circuit of the generator occurs, zero-sequence voltage will be generated at both ends of the open delta winding of the special TV; and the calculation of the symmetrical component method shows that no longitudinal voltage will be generated when an asymmetrical fault occurs on the high-voltage side of the main transformer, that is, the Y side. zero pressure. Utilizing this feature can constitute the generator turn-to-turn short-circuit protection of the longitudinal zero-sequence voltage.

However, the symmetrical component method is only an approximate model for generators, without considering the saturation phenomenon of the generator iron core and the asymmetry of the vertical and horizontal axes, and does not consider the mutual inductance between windings caused by the harmonic components in the air-gap magnetic field during faults , when used to analyze the transient process of faults, the analysis results are not very accurate, and cannot explain the phenomenon of non-fault phase voltage rise. The pure longitudinal zero-voltage turn-to-turn protection scheme has had many misoperation accidents. The reason is that when there is a fault outside the area, the open triangle of the special TV may generate a certain longitudinal zero-sequence unbalanced voltage.

In view of the misoperation of the simple longitudinal zero-voltage turn-to-turn protection scheme, a negative sequence directional element and a longitudinal zero-voltage element should be installed through the "AND gate" outlet. On this basis, the present invention proposes a fault-tolerant rejudgment type negative-sequence direction blocking longitudinal zero-voltage inter-turn protection to further improve the reliability of this type of generator inter-turn protection.

Contents of the invention

For a generator whose neutral point can only lead out 3 terminals and does not have a dedicated TV, in order to reliably protect the inter-turn fault of the stator winding, the present invention provides an voltage, to identify the short circuit between the turns of the generator, the method includes the following steps: the generator protection device samples the current and voltage waveform of the transformer to obtain the instantaneous value of the current and voltage; calculates the electrical quantities by digital filtering algorithm plus Fourier algorithm The complex number form; the neutral point current and the negative sequence component of the machine terminal voltage are filtered out through the filter sequence algorithm; when the negative sequence directional element and the longitudinal zero voltage element act at the same time, the outlet is protected; when the neutral point current transformer is disconnected or the machine terminal After the ordinary voltage transformer is disconnected, it automatically withdraws from the negative sequence directional element, and at the same time puts into high-value longitudinal zero-voltage protection. This method improves the reliability of the turn-to-turn protection of the longitudinal zero-voltage generator.

Description of drawings

Figure 1 shows the definition of the installation position and polarity of current transformers and voltage transformers;

Figure 2 shows the logic block diagram of the fault-tolerant rejudgment type negative-sequence directional blocking longitudinal zero-voltage turn-to-turn protection.

specific implementation plan

Please refer to Figure 1 for the definition of the installation position and polarity of TA and TV. Among them, TV1 is an ordinary TV at the machine end, and its primary neutral point is directly grounded. TV2 is a special TV between turns, and its primary neutral point is connected to the neutral point of the generator.

The action logic of the longitudinal zero-pressure element is shown in the second part of Figure 2, and its criterion is shown in formula (1):

3U ₀ >U _ZJ (1)

Among them, 3U ₀ is the fundamental wave component of the open triangle zero-sequence voltage of the TV dedicated to the terminal, and U _ZJ is the fixed value of the longitudinal zero-sequence voltage. When calculating 3U ₀ , zero-point filtering and Fourier algorithm should be used, so that the third harmonic filtering ratio can reach more than 100 times, and even in the case of system frequency deviation, a high third harmonic filtering ratio can still be guaranteed.

In order to prevent the misoperation of the longitudinal zero-sequence voltage element when the external fault and the secondary circuit of the voltage transformer are abnormal, a negative-sequence directional element is added as a blocking element. The voltage of the negative-sequence directional element is taken from the machine terminal TV1, and the current is taken from the neutral point TA. The most sensitive angle is generally taken from 75° to 85°. When a turn-to-turn fault occurs inside the generator, the fault source is inside the generator, the negative sequence power flows from the generator to the system, and the negative sequence directional element can act. When an external fault occurs, the source of the fault is outside the generator, the negative sequence power flows from the system to the generator, and the negative sequence directional element does not act. However, if the neutral point TA or TV1 has a secondary circuit disconnection, this criterion will lose its effect, and this criterion should be withdrawn.

The action logic of the negative sequence directional element is shown in the first part of Figure 2, and its criterion is shown in formula (2):

(I ₂ >I _2Q )∩(U ₂ >U _2Q )∩(P ₂ >0) (2)

In the formula: I ₂ is the negative-sequence current on the neutral point side, U ₂ is the negative-sequence voltage on the machine terminal side; I _2Q and U _2Q are the negative-sequence current and negative-sequence voltage thresholds respectively, which are fixed inside the device; P ₂ is the negative-sequence direction element.

As shown in the logic diagram of the second part of Figure 2, when the generator is not connected to the grid, since there is no negative sequence current, the negative sequence voltage is used as the blocking element at this time. When the primary circuit of TV2 dedicated to the generator is abnormal, the longitudinal zero-sequence voltage criterion is blocked. After both the longitudinal zero-pressure element and the negative sequence directional element operate, the protection exits through a short delay action.

As shown in the logic diagram of the third part of Figure 2, in order to prevent the negative sequence directional element from losing its function when the TV1 at the machine terminal or the neutral point TA is disconnected, the inter-turn protection may be blocked when an inter-turn fault occurs at this time, so it is automatically turned on A section of high fixed value (without negative sequence direction blocking), and exit through time delay.

The action logic of the longitudinal zero-voltage protection in the high-set value segment is shown in the third part of Figure 2, and its criterion is shown in formula (3):

3U ₀ >U _ZJH (3)

In the formula: U _ZJH is the fixed value of longitudinal zero-sequence voltage in the high fixed value section

See the fourth part of Figure 2 for the discriminant logic of the abnormality of the open triangle zero-pressure circuit. When the three line voltages of the special TV are greater than 90V, and the third harmonic component of the open triangle zero-sequence voltage is less than a fixed value, it will be judged that the special TV open triangle zero-voltage circuit is abnormal after a delay, and an alarm signal will be sent to notify the operating personnel in time to deal with it , to avoid the occurrence of protection refusal.

In summary, the present invention combines the longitudinal zero-sequence voltage criterion with the negative-sequence directional element, and considers the exit of the negative-sequence directional element when the neutral point TA or the ordinary TV at the machine end is disconnected, and improves the longitudinal zero-voltage type. Reliability and safety of generator turn-to-turn protection.

Claims (5)

1. A method for discriminating generator interturn short circuit by discriminating generator longitudinal zero-sequence voltage and neutral point current and machine terminal voltage, the method may further comprise the steps: The generator protection device samples the current and voltage waveform of the transformer to obtain the instantaneous value of the current and voltage; Calculate the complex number form of each electrical quantity through digital filtering algorithm plus Fourier algorithm; Filter out the neutral point current and the negative sequence component of the machine terminal voltage through the filter sequence algorithm; Protect the outlet when the negative sequence directional element and the longitudinal zero pressure element operate simultaneously; When the neutral point current transformer is disconnected or the common voltage transformer at the machine end is disconnected, it automatically exits the negative sequence directional element, and at the same time puts into high-value longitudinal zero-voltage protection. 2. The method as claimed in claim 1, in order to prevent the negative sequence directional element from losing its function when the terminal TV or the neutral point TA is disconnected, the turn-to-turn protection will be blocked when a turn-to-turn fault occurs at this time, and a section of high fixed voltage is automatically put into use for this purpose. value range, and it is exported after a delay, where the action criterion of the longitudinal zero-pressure element is shown in the following formula: 3U ₀ >U _2J Among them, 3U ₀ is the fundamental wave component of the open triangle zero-sequence voltage of the TV dedicated to the terminal, and U _ZJ is the fixed value of the longitudinal zero-sequence voltage. 3. The method according to claim 1, wherein the negative sequence directional element action criterion is as follows: (I ₂ ＞I _2Q )∩(U ₂ ＞U _2Q )∩(P ₂ ＞0) In the formula: I ₂ is the negative-sequence current on the neutral point side, U ₂ is the negative-sequence voltage on the machine terminal side; I _2Q and U _2Q are the negative-sequence current and negative-sequence voltage thresholds respectively, which are fixed inside the device; P ₂ is the negative-sequence direction element. 4. The method according to claim 1, when the generator is not connected to the grid, since there is no negative sequence current, the negative sequence voltage is used as the blocking element at this time. 5. When the three line voltages of the special TV are greater than 90V, and the third harmonic component of the open triangle zero-sequence voltage is less than a fixed value, after a delay, it is judged that the special TV open triangle zero-voltage circuit is abnormal, and an alarm signal is sent to notify the operator in time Handle it to avoid the occurrence of protection refusal.

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