Method for detection of upcoming pole slip

Abstract

A method for detecting an imminent pole slip of a synchronous generator electrically connected to a power supply network, whereby a signal characteristic of a power fault is detected and an imminent pole slip is determined via a predefinable value when a load angle of the synchronous generator increases, whereby the following steps are performed. Determination of a first load angle during operation without a power fault, determination of a generator frequency as a function of time when a power fault occurs, and precalculation of a second value of a load angle resulting from the power fault by adding the first value of the load angle to a load angle difference occurring during the power fault, whereby this load angle difference is caused by a deviation of a generator frequency relative to a power frequency.

Claims

1. A method for detecting an imminent pole slip of a synchronous generator electrically connected to a power supply network, wherein a signal characteristic of a power fault is detected and an imminent pole slip is determined based on a pre-defined value when a load angle of the synchronous generator increases, comprising the following steps: determining a first load angle during operation of the synchronous generator without a power fault, determining a synchronous generator frequency as a function of time when a power fault occurs, and precalculating a second value of a load angle resulting from the power fault by adding the first value of the load angle to a load angle difference occurring during the power fault, wherein this load angle difference is caused by a deviation of the synchronous generator frequency relative to a power frequency of a power grid, wherein the power fault comprises a power outage of the synchronous generator, wherein, on exceeding a second predefinable value for the second load angle resulting from the power fault, the synchronous generator is disconnected from the power supply network.

2. The method according to claim 1, wherein after detecting a signal characteristic of a power fault, the load angle difference is determined between a polar wheel voltage and a synchronous generator voltage, wherein the load angle difference is caused by a deviation of the synchronous generator frequency relative to the power frequency.

3. The method according to claim 1, wherein the first load angle in the operation without a power fault is determined by measuring a synchronous generator voltage, and a synchronous generator current.

4. The method according to claim 1, wherein, from the calculated second value of the load angle resulting from the power fault, a decision is derived as to whether the synchronous generator should remain connected to the power supply network.

5. The method according to claim 1, wherein, on exceeding a first predefinable value for the second load angle resulting from the power fault, at least one measure is taken to reduce the load angle.

6. A device for determining a load angle of a synchronous generator with a rotor electrically connected to a power supply network, comprising at least one measuring device used to determine a frequency of a synchronous generator voltage and/or a synchronous generator current of the synchronous generator, at least one rotary speed measuring device for determining a rotary speed, and at least one controller designed to perform the following steps: determine a first load angle during operation of the synchronous generator without a power fault, determine a synchronous generator frequency as a function of time when a power fault occurs, and precalculate a second value of a load angle resulting from the power fault by adding the first value of the load angle to a load angle difference occurring during the power fault, wherein this load angle difference is caused by a deviation of the synchronous generator frequency relative to a power frequency of a power grid, wherein the power fault comprises a power outage of the synchronous generator, wherein, on exceeding a second predefinable value for the second load angle resulting from the power fault, the synchronous generator is disconnected from the power supply network.

7. The device according to claim 6, wherein the device further consists of a mechanical power source that is mechanically coupled to the synchronous generator.

8. The device according to claim 7, wherein the mechanical power source is an internal combustion engine, in particular a stationary engine.

9. The device according to claim 7, wherein the mechanical power source is a wind power plant, a hydroelectric power plant or a gas turbine.

10. A controller, configured to: determine a first load angle during operation of a synchronous generator without a power fault, determine a synchronous generator frequency as a function of time when a power fault occurs, and precalculate a second value of a load angle resulting from the power fault by adding the first value of the load angle to a load angle difference occurring during the power fault, wherein this load angle difference is caused by a deviation of the synchronous generator frequency relative to a power frequency of a power grid, wherein the power fault comprises a power outage of the synchronous generator, wherein, on exceeding a second predefinable value for the second load angle resulting from the power fault, the synchronous generator is disconnected from a power supply network.

11. The controller of claim 10, wherein after detecting a signal characteristic of a power fault, the load angle difference is determined between a polar wheel voltage and a synchronous generator voltage, wherein the load angle difference is caused by a deviation of the synchronous generator frequency relative to the power frequency.

12. The controller of claim 10, wherein the first load angle is determined by measuring a synchronous generator voltage and a synchronous generator current.

13. The controller of claim 10, wherein, from the calculated second value of the load angle resulting from the power fault, a decision is derived as to whether the synchronous generator should remain connected to the power supply network.

14. The controller of claim 10, wherein, on exceeding a first predefinable value for the second load angle resulting from the power fault, at least one measure is taken to reduce the first load angle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail with reference to the figures. The figures show the following:

(2) FIG. 1 a representation of the load angle in the pointer model,

(3) FIG. 2 a schematic representation of a synchronous generator,

(4) FIG. 3 a schematic representation of a synchronous generator connected to a mechanical power source,

(5) FIG. 4 a diagram of the rotary speed and load angle difference after the occurrence of a power fault as a function of time, and

(6) FIG. 5 a flow diagram of an exemplary embodiment.

DETAILED DESCRIPTION

(7) FIG. 1 shows a representation of the load angle in the pointer model of a synchronous generator. The load angle ϑ has a span between the polar wheel voltage U.sub.Polarwheel and the generator voltage U.sub.G. The arrow at the load angle indicates the rotation direction. In the present case of an operation in parallel with a network, the generator voltage is equal to the power supply voltage. In generator operation of the synchronous generator, therefore, the polar wheel voltage U.sub.Polarwheel moves ahead of the generator voltage U.sub.G (i.e. also the power supply voltage). In normal generator operation, the load angle is generally between 20° and 30°. When a load angle of 180° is exceeded electrically, a pole slip occurs and the synchronous generator loses its synchronization with the power supply network.

(8) FIG. 2 shows a schematic representation of a synchronous generator 2 whose rotor (polar wheel) has two poles P.

(9) The phases 8 of the synchronous generator 2 are separably connected to a power supply network 1 through the evaluation unit 6 via a signal line 12 by means of a switching device 11. Via a further signal line 13, the evaluation unit 6 can perform interventions on a mechanical power source 7 (not shown). The evaluation unit 6 is designed in a control unit of the synchronous generator 2 and/or in a control unit of the mechanical power source 7.

(10) In addition, a rotary speed measuring device 5 for determining the rotary speed of the rotor 3 is shown. The measuring device 4 is used to determine a frequency of a generator voltage U.sub.G and/or a generator current I.sub.G of the synchronous generator 2. In addition, an evaluation unit 6, to which the signals of the measuring device 4 and the rotary speed measuring device 5 can be reported, is shown.

(11) FIG. 3 shows a further representation of a synchronous generator 2 which, in this representation, is connected to a mechanical power source 7 via the rotor 3. By way of example, the mechanical power source 7 is shown as an internal combustion engine 9 or a wind power plant 10.

(12) FIG. 4 shows a relationship of the rotary speed trend of a synchronous generator 2 with the load angle difference Δϑ on the occurrence of a power fault in the power supply network 1. The solid line represents the rotary speed development of the synchronous generator 2 as a function of time plotted on the X-axis. The corresponding Y-axis of the rotary speed in revolutions per minute is shown on the left-hand Y-axis of the diagram. It can be seen that the rotary speed of the synchronous generator 2 increases, starting from the nominal rotary speed, by the elimination of the electrical load in the power supply network 1.

(13) The dotted curve shows a load angle difference Δϑ in degrees, which increases due to the acceleration of the synchronous generator 2 after a power fault in the power supply network. The load angle difference Δϑ is understood to be the angle value that results from the deviation of the generator frequency f.sub.G relative to the power frequency f.sub.grid. Since the power frequency f.sub.grid can be regarded as constant and given, the load angle difference is essentially caused by the change in the generator frequency f.sub.G. The Y-axis-related load angle difference Δϑ is the right-hand Y-axis of the diagram. The calculation of the load angle difference Δϑ is illustrated by a numerical example: the generator speed prior to the power fault (starting speed) is 1500 rpm. Expressed as a frequency, this corresponds to a generator frequency f.sub.G of 25 Hz. At a time of 0.005 s (seconds) after the power fault, the speed has increased to 1,507 rpm. The speed difference divided by 60 times the time difference (0.005 s−0 s) times 360 times the number of pole pairs (the number of pole pairs is here 2) gives the current load angle difference at the time 0.005 s after the power fault, in this example 0.4 degrees. The calculation of the load angle difference is then continued by adding or integration, for example, until it exceeds a first predefinable value for the second load angle resulting from the power fault, ϑ.sub.fail, where ϑ.sub.fail=ϑ.sub.op+Δϑ.

(14) The increase in a load angle above a predefinable value can be interpreted as an imminent pole slip.

(15) FIG. 5 shows a flow diagram of the inventive method according to an exemplary embodiment. The routine which can be stored in a control device starts with the “start” circuit diagram and can, for example, be repeated every 10 ms (milliseconds). In a first step (“load angle measurement”), the load angle is measured during operation without a power fault of the synchronous generator 2. The load angle can be determined in known manner, for example, via a frequency of a generator voltage U.sub.G and a generator current I.sub.G of the synchronous generator 2.

(16) In the next step, “averaging the load angle”, the load angle is averaged over the last 500 ms (milliseconds).

(17) If a power fault does not occur, a new average value of the load angle is formed by the routine, into which the result of the last load angle measurement is fed. The average value of the load angle is thus continuously overwritten by this routine, such that the average value of the load angle always represents the average value of the load angle in the last 500 ms (milliseconds).

(18) If a power fault occurs, the last determined average value of the load angle of the operation without a power fault is stored as the initial value ϑ.sub.OP for the above-described integration. In other words, the last network-valid value is frozen.

(19) The routine ensures that the value used to determine the load angle ϑ.sub.fail in the event of a power fault corresponds to the load angle ϑ.sub.op of the operation without a power fault prior to the occurrence of the power fault.

(20) Thus, no artifacts caused by a power fault are included in the determination of the load angle ϑ.sub.op.

(21) As already explained above, the load angle difference Δϑ is then determined.

(22) The load angle difference Δϑ is, for example, determined such that an integral from the motor frequency (=generator frequency) is formed from a rotary speed measurement of the generator. For this purpose, the rotary speed development after the power fault is recorded based on a starting speed. Due to the acceleration of the generator, its speed increases. This is shown in the diagram of FIG. 4 together with the resulting values of the load angle difference Δϑ.

(23) Finally, the load angle on the occurrence of a power fault ϑ.sub.fail is calculated:
ϑ.sub.fail=ϑ.sub.op+Δϑ

(24) This gives us the information about the load angle on the occurrence of a power fault. From the load angle on the occurrence of a power fault, ϑ.sub.fail, we can then derive a decision as to whether the synchronous generator should remain connected to the power supply network.

(25) The detection of a power fault can, for example, consist of monitoring the generator frequency and interpreting a change in the generator frequency of greater than 0.1 Hz/10 ms as the occurrence of a power fault.

(26) If a change in the generator frequency is less than 0.1 Hz/10 ms and the speed of the power source 7 connected to the synchronous generator 2 is within a predefinable nominal speed +/−e.g. 10 rpm, then the report of a power fault is reset. This can be the case, for example, after the disappearance of a power fault.

(27) This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.