fig1 shows an inventive inverter . it is formed by means of inverter elements or , as the case may be , power cabinets 1 to 5 . a further power cabinet n is shown with a dashed outline . that is intended to indicate that an expansion to higher numbers of power cabinets is possible . an already installed complete system can in particular be relatively easily expanded or scaled up if necessary by installing additional power cabinets . the structure of an element of such type is shown schematically with the aid of power cabinet 1 . power cabinet 1 has three input leads l 1 , l 2 , and l 3 . they constitute a three - phase current input of a three - phase power supply . the three input leads are applied to a voltage u of approximately 440 v or above ( medium voltage ). said input leads lead to a component 11 that has a power - line filter and a power - on control . component 11 includes a rectifier and switching elements ( capacitors , for example ) for continuous startup . component 11 is connected to a controller 12 for the switching elements provided in power cabinet 1 , in particular of a transformer 13 . electrical isolation from the line voltage is realized by means of transformer 13 which is dimensioned for 250 kva ( isolating transformer ). said transformer is a 250 - kva 50 / 60 - hz transformer having on its secondary side 5 three - phase systems or , as the case may be , cylinders 131 , 132 , 133 , 134 , and 135 having 5 × 3 windings . power cabinet 1 furthermore has a current / voltage transformer 14 and an output filter 15 . the speed of the voltage rise can be limited by means of output filter 15 to protect the output cable ( lc cable ). the requirements of such type of cable protection depend on the loads that arise and on the cable &# 39 ; s performance characteristics . finally , an internal air - conditioning system 16 is provided in power cabinet 1 . five corresponding power cabinets are provided . they are controlled by a central controller 20 . the individual power cabinets &# 39 ; air - conditioning systems ( element 16 for power cabinet 1 ) are fed by means of a central water / air heat cooler 21 . the five power cabinets are controlled by controller 20 with a time delay so that the output signals appear one after the other . that is illustrated with the aid of fig2 . top curves k 1 to k 5 correspond to the output signals of power cabinets 1 to 5 . said signals are sinusoidal pulses generated by means of an lc circuit . the individual oscillations are mutually offset by 10 μs and have ( as indicated in curve k 1 ) a period of 50 μs or , as the case may be , a frequency of 20 khz . these are then combined by means of another transformer into an overall signal g which is shown below in fig2 and has a frequency five times ( 100 khz ) that of the individual power cabinets &# 39 ; output signals . the power data can specifically be stated as follows : each inverter element 1 to 5 accepts a 100 - khz oscillator at a repetition rate of 20 khz . an inverter element &# 39 ; s output voltage will then be approximately 2 kv with use being made of a 250 - kva transformer . the result , therefore , is in each case a power unit or , as the case may be , converter unit for 200 kw and ( at most ) 20 khz . an overall system comprises , for example , 5 power units for 1 mw at a maximum frequency of 100 khz . the maximum power and frequency can then be scaled by using more or fewer power units , for example by using only a part of the five power cabinets for forming the overall output signal . electrical isolation from the line voltage will have been realized by the transformers in order to satisfy safety requirements here . the necessary input voltage of & gt ; 440 v can be easily realized ; a power unit &# 39 ; s output voltage is up to 2 kv . the overall unit thus consists of five identical power units with an output current of 200 kw and 20 khz that are connected in parallel to an output transformer 40 having 5 primary windings 401 , 402 , 403 , 404 , and 405 ( one primary winding 40 n is additionally indicated with respect to the system &# 39 ; s scalability ). it is , for example , a high - frequency transformer having a ferrite core . the 5 - fold or , as the case may be , n - fold primary windings of output transformer 40 illustrated in fig1 are not absolutely necessary ; hard parallel connection to one winding is also possible . that is shown in fig3 in which the signals of inverter elements 1 to 5 are fed into a primary winding 401 . the output frequency of the current can be increased or decreased in stages by up to 20 khz by adding power units or omitting them . the possible output power is likewise scalable up to 200 kw . each power unit generates up to 2 kw in the output voltage , thereby reducing the necessary currents and hence the lead cross - section and wiring requirements . an output transformer 50 is provided into which all the power units feed on the primary side . a superordinate controller 20 synchronizes and supplies the power units with the setpoint values . a liquid cooler 21 is also provided . circuit 50 enables the output signal to be employed for applications for , for example , inductive heating in the extraction of oil from bituminous sand . the converter can be tuned to the resonant frequency necessary therefor . another embodiment of the inventive subject matter is shown in fig4 . inverter units 1 , 2 , 3 , 4 , and 5 are here connected in series . here , too , it is possible to specify how many of the inverter units are used . inverter units that are not required can be switched off or , as the case may be , connected through so as to have no impact on the output signal . that is a solution offering additional outage protection insofar as most trouble involves a faulty inverter unit then behaving like a switched - off unit . the generating of middle - frequency currents in that high power category can thus be realized with little effort and expense . all that is required therefor are quite small power units rated for 200 kw and a maximum frequency of 20 khz . the maximum power frequency &# 39 ; s scalability is realized by using more or fewer power units . the overall system is for that reason less effort - intensive and more economical than conventional solutions . the invention has been presented within the scope of an exemplary embodiment intended as illustrative but not limiting . other variants based on the same principle and to be encompassed within the scope of protection will be immediately obvious to a person skilled in the relevant art . for example other embodiments of inverter elements can be used ; ways of realizing how superimposing of the individual output signals is effected other than by means of a transformer are also readily possible .