Patent Application
20080031024
The invention is now described in connection with preferred embodiments and with reference to the accompanying drawings, in which
The frequency converter shown in
According to the invention, the first control means 9 are configured to control all network control modules 3. Thus, every network converter module receives the same control if necessary. Similarly, in the embodiment of
According to the method of the invention, use of one or more network converter modules is prevented or allowed, whereby the network converter is reconfigured during the operation of the network converter. Preventing or allowing use means that the use of one or more network converter modules 3 is stopped. In such a case, the current passing through the network converter is not allowed to pass through the modules whose use is prevented. Correspondingly, all current passes through the network converter modules whose use is allowed.
In a manner similar to that in connection with the network converter modules 3, according to the embodiment disclosed in
According to a preferred embodiment of the invention, use of the network converter modules of the network converter is prevented or allowed using first switching means 5. These switching means are e.g. three-phase switches included, according to the invention, in every network converter module. The first control means 9 are configured to control these switches as necessary. Using the switching means 5 for allowing or preventing use of the network converter modules enables reliability of operation to be achieved since the modules 3 are then physically separated from a network 2. Consequently, the control means 9 may produce instructions to all network converter modules, irrespective of which modules are in operation.
Similarly, use of the inverter modules 4 of the inverter is prevented or allowed using second switching means 8. Preferably, these switching means are three-phase switches included in every inverter module. The second control means 10 are configured to control these switching means for connecting or disconnecting the inverter modules to/from the motor 1. Disconnecting an inverter module from a motor means that no current can pass through this particular module, irrespective of control.
According to an embodiment of the invention, use of a network converter module is prevented or allowed by the control means 9. The control means thus direct the semiconductor switches of a desired module into a state wherein no current passes through the particular module. In other words, modulation of a module which is to be excluded from the parallel coupling is stopped. The semiconductor switches may then be opened, for instance. Thus, the particular network converter module does not constitute a part of the current path of the frequency converter. It is to be noted, however, that if active use of the module is stopped by stopping modulation, parallel zero diodes of the power semiconductor components may, however, conduct current. This fact is to be taken into account e.g. by eliminating zero currents by an appropriate known method.
According to a preferred embodiment of the method of the invention, a network converter is reconfigured in response to a power signal or a reactive power signal. In such a case, in connection with an electric motor drive, for instance, a control system informs the control means 9, 10 about a change in power demand. When the power or reactive power demand increases, the number of network converter and inverter modules is naturally increased if the configuration at the time of increase in power does not suffice to transfer the increasing power or reactive power. Similarly, when power decreases, the number of modules may be reduced. It is to be noted that the active number of network converter and inverter modules is changed independently of one another if a modular inverter structure disclosed in
In addition to a power signal, the configuration may be implemented in response to a change in a torque instruction. A torque instruction is used in connection with both generator and motor drives as a control variable, which is typically obtained directly from higher control systems to be used by the control units. Other possible signals for indicating a change in power include actual values of current and velocity as well as the calculated power of a machine or a network. Furthermore, in connection with wind turbines, a higher level adjustment utilizes information on wind velocity. Wind velocity has a direct influence on the amount of power to be produced by a wind turbine drive. Consequently, wind velocity may also be used as a control signal for dimensioning the number of modules coupled in parallel.
The network converter may be configured during use also in response to failure of a certain module. The power semiconductor components used in modules often in themselves include circuits which detect component failure and which signal to an adjustment circuit about such failure. When a higher-level control system receives information about such failure, the control means responsible for controlling the particular module can be configured such that this is taken into account and the control of this particular module is stopped in all cases. Thus, the frequency converter according to the invention produces a redundant confirmation to the operation of the frequency converter.
According to the invention, every network converter module comprises a network filter 21, one being shown in the example of
According to a preferred embodiment of the invention, a network converter further comprises network-converter-module-specific second switching means 6 for separating the modules 3 from a direct voltage intermediate circuit 11 common to all network converter modules. The embodiment in
According to the invention, a network converter module further comprises a capacitor C for a direct voltage intermediate circuit. Such a capacitor is shown in
When the configurations of the inverter or the network converter are, according to the invention, changed during the operation of the frequency converter, the control means thereof should also be configured to conform with a given situation. In order to guarantee the operation of the device, the control means and the adjustment means should always know the module arrangement at any given time since adjustments are often made in the adjustment means as relative values. Relative values can be calculated when available rated current or power capacity is known. In addition, at least some higher-level control system has to know the number of modules that are in working order so as to limit the power if necessary.
In the device according to the invention, the reliability of operation can be further increased by implementing the control means such that there are several control means per network converter modules and inverter modules. In a typical case, one control card of the control means calculates the necessary instructions for controlling the power semiconductor components. These instructions are then copied to other control cards of the control means which, in turn, control their own modules. In order to increase redundancy, more than one control card of the control means may also be responsible for calculating the necessary controls. In such a case, each of the parallel modules itself determines its own control for the switch components to be controlled. This is further advantageous in terms of a reduction in disturbance currents since due to the separate controls, the parallel modules do not run synchronously, wherefore the disturbance currents produced by different modules do not equate with one another.
In the following, a simplified example will be shown of the operation of the invention, assuming that a frequency converter according to the invention is used in connection with a wind turbine. For the sake of simplicity, let us further assume that the frequency converter is formed by three network converter modules and three inverter modules. The rated power of each module is 500 kW, so that the rated power of the frequency converter is 1500 kW.
First, wind rotates a short-circuit generator connected to a wind turbine near its rated speed of rotation such that the torque of the generator can be adjusted to a value which gives a power of approximately 300 kW. The velocity of the wind is thus too low to reach the rated power of the generator. From an adjustment value of the torque, for example, information is obtained indicating that only one inverter module and only one network converter module are needed. In such a case, a higher-level control system gives the necessary information to the control means about putting one inverter module and one network converter module to use. Use of only one single module can be implemented as disclosed above, i.e. by opening the switching means of the modules or by controlling one single module only. At its simplest, modules are separated from use by detaching them on the AC side, e.g. the network converter modules from the network and the inverter modules from the load.
When the velocity and force of the wind increase, the torque of the machine can be adjusted higher, in which case the amount of electric power to be produced also increases. Furthermore, the adjustment value of the torque provides information about an increase in the electric power. The memory of a higher-level control system contains information indicating that the system includes three modules, one of which being in use. When the power exceeds the rated power of one module, i.e. 500 kW, two modules are to be coupled in parallel, in which case the current of the frequency converter is evenly divided between the modules. If two modules suffice to handle the power produced, no third module has to be put to use. When the wind gets stronger such that the torque of the machine can be further increased, and at the same time the power to be produced, a limit is reached where no two parallel modules will suffice. In the case of the example, when the torque exceeds a limit which corresponds to a power of 1000 kW, a third module should also be coupled in parallel in order to transfer power to the network. When the third module has also been put to use, the memory of a higher-level control system provides information indicating that the operation now takes place at the highest power level. In such a case, all modules of the frequency converter have been put to use, and the power can be increased all the way to the rated power of the frequency converter and, at the same time, to the rated power of the inverter.
For the sake of simplicity, it has been assumed in the above-disclosed example that both the inverter modules and the network converter modules are put to use and out of use simultaneously. In practice, the inverter and the network converter are configured independently, due to what has been disclosed above. Furthermore, no possible failures have bee taken into account in the example.
When wind and the power to be produced decrease, the operation is similar but instead of increasing the number of modules put to use, modules are put out of use. A higher-level control system knows how to control the control means to reduce the number of modules when the power to be produced is lower than the common power of the modules being used minus the power of one module.
In connection with adding and removing modules, the number of network filters and capacitors for an intermediate circuit changes according to the invention, thus enabling the network influences caused by the frequency converter to be optimized.
It is obvious to one skilled in the art that as technology advances, the basic idea of the invention can be implemented in many ways. The invention and its embodiments are thus not restricted to the above-described examples but may vary within the scope of the claims.
