CH658557A5 - DEVICE FOR OPTIMIZING THE SUSPENSION BEHAVIOR OF THE CONTROL SIZE OF A VOLTAGE-CONTROLLED ELECTRICAL MACHINE. - Google Patents

Description

The invention relates to a device for optimizing the transient response of the controlled variable of a voltage-controlled electrical machine, in particular for the output voltage of a synchronous generator.

It is known that thyristor power converters with electronic control devices are used for the excitation and voltage control of large hydropower generators. Based on general experience in the control sector, without actually going into machine characteristics, a known system is used, namely a proportional, integrally connected voltage regulator for the generator terminals with a subordinate pole wheel current regulator.

In the first conventional excitation systems with a DC excitation machine, the voltage regulator was of little importance. On the one hand, the system was inherently sluggish due to the arrangement, especially variable resistors or switching contacts, on the other hand, the demands on the machines in terms of dynamics and network controls were not very high anyway. With the increasing meshing of the networks and the combined operation of several power plants, the requirements for the voltage regulating devices also increased. The requirements in normal operation are primarily voltage maintenance, parallel operation of several machines with each other and with the network with stable, adjustable reactive load distribution. When correctly dimensioned, these requirements are basically mastered by all modern excitation systems and voltage regulators.

Significant difficulties arise only in the case of unusual operating conditions and in the event of faults. Load shutdowns, which are particularly associated with hydropower generators with a strong increase in speed, fast compensation of strong reactive load surges during phase shifter operation, power transmission over long lines with load shutdowns at the end of the same, operation near the static stability limit under the influence of additional regulations are possible for the selection, design and dimensioning of the excitation system vital. In the event of short-circuits in the network, the main field of the generator must be supported by fast and strong surge excitation in order to enable continued operation after error advancements or to ensure overcurrent tripping.

All of these requirements can actually only be met by a proportional integrally connected voltage regulator, the proportional component being necessary for higher dynamics and the integral component serving for the accuracy in the static range. To further increase the dynamics, a magnet wheel current regulator can be added. At first glance, this arrangement, with the appropriate design, brings short settling times with little overshoot.

With this arrangement, however, a controller design can only be optimal for a single load case, e.g. only for idle. The demand for a short settling time is of minor importance, especially for idling, but an essential task of the voltage regulator is good dynamics in commercial operation.

The demands for good dynamics for idling and loading cannot be met at the same time. This has already been shown in practical examples. If the controller were to be made faster under load, it would have a tendency to become unstable when idling.

The basic idea of the conventional controller, the proportional-integral element with a subordinate pole wheel current controller, is correct, but it does have some major disadvantages.

As already mentioned, the selected circuitry of the operational amplifiers, which is usually only determined during the commissioning by tests, the transition function of the controller elements can only be optimal for a single load case, since the machine time constant and the amplification of the line change with the load.

Another disadvantage is that the integration time has to be quite long so that the integrator does not “run away” or only “runs away” a little with pole wheel ceiling tension. It can therefore not be used for dynamic considerations, but only serves for accuracy in the statonary case. However, this brings settling times of several seconds, especially in network operation, which should be avoided with electronic devices.

Another disadvantage is that with large load surges or when the generator is excited from zero to nominal voltage, the pole wheel ceiling voltage is present for a long time. Despite a large time constant of the integrator, it still migrates and causes the actual value to overshoot. It is therefore art circuits, such as Bridging of the integrator up to 70% nominal voltage or pole wheel current limits or the like is necessary.

If you still want to achieve good dynamics, you would have to select a high gain for the proportional part, which in turn tends to cause instability.

The object of the invention is therefore to create a device in which, regardless of the load on an electrical machine, in particular a synchronous gene5

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rators, the behavior of the overall control loop, i.e. the controller and the system, is always the same as far as possible and is dynamically optimal, especially when used. Another task is to shorten the settling time of the control loop compared to known control devices in the different load conditions.

The object of the invention is achieved by the device characterized in claim 1.

With the device according to the invention, it is possible for the first time to quickly adjust the controller, ie the current and voltage controller, to the route parameters with regard to its adaptation to the variable route.

Furthermore, no test signals are used, apart from the operational signals, so that normal operation is not disturbed.

The necessary connections for the operational amplifiers of the controllers can be calculated numerically during the project planning from the machine data or from the data of the model function and not only be determined by trials during commissioning.

In addition, the control behavior is neither unstable nor dynamically worse if the magnet wheel ceiling voltage is present for a long time or if one of the control loop elements appears to be saturated.

It is advantageous that the voltage regulator is a proportional integral operational amplifier and the current regulator is a proportional operational amplifier. As a result, the entire circuit for the self-adjusting or adaptive controller is so simple that it is sufficiently robust and can be manufactured at a reasonable price.

According to one embodiment of the invention, the model functions, the addition point and the feedback are simulated with appropriately connected operational amplifiers. This means that only a small amount of space is required, which means that the adaptive add-on can be accommodated right next to the controller or on the controller board. Furthermore, the additional costs for the adaptive part are very low.

Another advantage of the invention is that the model function, the addition point and the feedback can be simulated with a program in a digital computer. This achieves a very high level of accuracy and it is also easy to reproduce complicated model functions.

Based on the drawings, the invention will now be explained in more detail using an example.

1 shows an exemplary embodiment of the control loop according to the invention as a block diagram. 2 shows the transition function in the event of a setpoint step and simultaneous response of the non-linear limiters. FIG. 3 shows the 1st derivative of the transition function from FIG. 2 over time as a state variable and FIG. 4 shows the transition function in the phase plane.

The embodiment of the control loop according to the invention is thus in Fig. 1 with a voltage regulator (7), a current regulator (9), limiters (8, 10), a model function (15) with associated feedback (16) and a generator (4) with a downstream Actual value converter (14) shown.

In particular, the reference variable or the setpoint (2) and the actual value (3), which as the output signal of the actual value converter (14) is proportional to the output variable of the electrical machine (4) to be controlled, is applied to the subtraction point (1) a synchronous generator. The output variable (5) of the subtraction point (1) is the control difference, which is sent to a further subtraction point (6), the output variable of which, e.g. proportional integrally connected voltage regulator (7) is present. The voltage regulator (7) has in parallel a limiter (8) which is connected to the subtraction point (6). Furthermore, the series connection of the voltage regulator (7) and a e.g. proportional connected, current controller (9) a feedback consisting of the limiter (10) connected in parallel, the output signal of which is also applied to the subtraction point (6). An actuator (11) is connected downstream of the current regulator (9). This consists of a lattice control unit and a thyristor bridge, which controls the excitation voltage of the generator (4). The generator (4) is shown as a field transfer function (12) and a stator transfer function (13). This is followed by an actual value converter (14), which brings the output voltage of the generator (4) to electronics level.

In addition, the control loop also has an adaptive add-on, which consists of a model function (15) and a feedback (16). The model function (15) is connected in parallel 15 to the series connection of the current controller (9), actuator (11), generator (4) and actual value converter (14) and simulates the transfer function of this series connection. At an addition point (17) in the control loop, the output variable of the model function (15) is compared with the output variable 20 of the actual value converter (14) which is inverted to this. If a difference occurs, this is brought to the input of the current controller (9) via the feedback (16) and a summation point (18). This summation point (18) is arranged after the branch point of the model function (15), that is to say directly at the input of the current regulator (9).

A functional description of the control loop follows for further explanation. A distinction must be made between a response of the limiter, whereby individual transmission elements, in particular the voltage regulator or the actuator, change to the non-linear range and a working of the control loop in the linear range.

First, the mode of operation in the linear area will be briefly explained. The model function (15) is connected in parallel to the series circuit, consisting of a current controller (9), actuator (11), 35 generator (4) and actual value converter (14). This model function (15) is constructed in such a way that the output signal of the actual value converter (14) is equal in amount to the output signal of the model function (15) in a specific load case of the generator (4).

40 At this load, the model function (15) has the same transmission behavior as the series circuit connected in parallel. The two output signals, that of the model function (15) and that of the actual value converter (14), one of which is inverted, are compared at the addition point (17). 45 With the specific load on the generator (4), the deviation at the addition point (17) is zero and no additional signal reaches the addition point (18) via the feedback (16), directly at the input of the current controller (9).

For idling and for nominal load as well as for all other load cases that do not correspond to this preselected load, there is a deviation of the two output signals or a deviation in the transmission behavior, and thus an additional signal also reaches the addition point (18). Through this additional signal and thus through the entire 55 adaptive part, the regulation is significantly accelerated when a control deviation occurs and is approximately the same for all load cases.

Furthermore, the operation of the control loop when the limiters (8) and / or (10) respond will now be explained. 60 The analysis at the phase level is particularly suitable for describing the transition behavior of the control loop. It was therefore used to describe the function when the limiter was activated.

FIG. 2 shows the transition behavior of the controlled variable x 65 in the event of a setpoint step and a response of the non-linear limiters (8, 10). The controlled variable x is plotted on the ordinate, where xi is the new state to which the controlled variable is to settle, and the time t on the abscissa.

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3 shows the first derivative of the controlled variable x over time t. Ultimately, the first derivative of the controlled variable is plotted against the controlled variable x in FIG. 4.

Fig. 4 shows that when one of the non-linear limiters (8, 10) responds, i.e. when the maximum control limit of the voltage regulator (7), the current regulator (9) or the actuator (11), which is the power section, is reached, its maximum Control limit, due to the proportional gain can also be removed at its input, a switching line (19) is cut in the phase plane. The switching line (19) arises from the fact that the non-linear limiters (8, 10) work similarly to two Schmitt triggers and in such a way that when a certain voltage value is reached, in particular the positive or negative ceiling voltage of the voltage regulator (7), the Current controller (9) or the power section (11) one of the limiters (8, 10) suddenly takes a positive or negative output voltage. The two limiters (8, 10) are wired so that their output signal, which is present at the subtraction point (6), is always inverted compared to the signal (5) present at this point, which represents the control difference.

If, for the sake of clarity, the switching line (19) is thought of as two closely lying lines, the trajectories oscillate between these two lines,

and until the trajectory is reached which changes to the new state xi without another intersection with one of the switching straight lines (19). The target for the trajectories intersecting the two switching lines (19) results from s the size of the output voltage of the non-linear limiters (8, 10).

If you consider both switching lines (19) again as a switching line (19) as shown in Fig. 4, then the controlled variable x migrates when one of the non-linear Belo limiters (8, 10) along these switching lines (10) with the maximum possible increase in Control variable x and so far until the linear range can be reached again. The last trajectory (20) has a significantly lower overshoot than the first trajectory (21).

15 When the nonlinear limiter (8, 10) responds, the state of the controller is "frozen", so to speak, by the feedback corresponds to the step response Ax, which is reduced by the nonlinear range. From Fig. 2 it can be seen that the step response Ax can be expressed mathematically by the relationship Ax = Xi - xn. Here xN is the area in which one of the non-linear limiters (8, 10) responds.

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Claims (4)

658 557 2nd PATENT CLAIMS 1. Device for optimizing the transient response of the controlled variable of a voltage-controlled electrical machine, in particular for the output voltage of a synchronous generator, characterized in that a current regulator (9) has an actuator (11), a controlled system, which is the electrical machine (4), and this an actual value converter (14) is connected downstream, and that at the input of the current controller (9) there is an addition point (18) at which the output signal of a feedback (16) is present, and that at the input of this feedback (16) the output signal of a further addition point (17) is present, at which the output signal of a model function (15), the input of which is connected to a branch point in front of the addition point (18) at the input of the current controller (9), and the output signal of the actual value converter (14), which is inverted and at corresponds to a certain load case the output signal of the model function (15), but with any other load If there is a residual deviation at the output of the addition point (17), it can be added, and that a voltage regulator (7) is arranged in front of the branch point of the model function (15), and a limiter (8) is arranged antiparallel, the output of which is connected to a subtraction point (6 ) is connected to the input of the voltage regulator (7), and that a further limiter (10) is also guided from the output of the current regulator (9) to this subtraction point (6), and that another subtraction point (6) connected to the subtraction point (6) 1) is provided, to which the output signal of the actual value converter (14) and a signal which corresponds to the setpoint value (2) of the controlled variable arrive. 2. Device according to claim 1, characterized in that the voltage regulator (7) is a proportional integrally connected operational amplifier and the current regulator (9) is a proportional connected operational amplifier. 3. Device according to claim 1 or 2, characterized in that the model function (15), the addition point (17) and the feedback (16) are simulated with appropriately connected operational amplifiers. 5. Device according to claim 1 or 2, characterized in that the model function (15), the addition point (17) and the feedback (16) can be simulated with a program in a digital computer.