[0002] Inverters are used in frequency converters to generate a de- sired voltage or current to a load to be fed. A frequency converter typically re- ceives its supply voltage from an AC network. This voltage is rectified in a rec- tifier unit and further converted to alternating voltage in the inverter to control the load in a desired manner. A conventional device fed from a frequency con- verter is a motor, which may be controlled in a reliable manner with the fre- quency converter using various control and adjustment principles.

[0003] As the power demand of motors increases, it is not neces- sarily reasonable to increase the size and capacity of individual frequency con- verters correspondingly. It is therefore known to connect frequency converter inverter modules in parallel to obtain the required output power.

[0004] Parallel-connection of inverter modules is typically achieved by connecting the same output phase of each parallel-connected module to a common phase supply. In other words, the output phases of each inverter module are connected to the corresponding output phases of other inverter modules. The switching instructions of the frequency converter are generated in a unit common to all the inverter modules, which unit then copies the instruc- tions and sends them to each parallel-connected inverter module. Depending on the power needed, the number of parallel-connected modules may be any- thing from two upwards.

[0005] Inverter modules to be connected in parallel are typically fairly large and therefore placed in separate cabins. These cabins are cooled by blowing air into them to ensure proper operation and performance of the inverters. Today the temperatures of inverter output phases are measured for example directly from a power component of the output phase, or from the im- mediate vicinity thereof. The measured temperature is compared with a fixed temperature limit and thus, if the temperature rises above the limit, an overload possibly caused to the component is detected.

[0006] Prior art component temperature control thus relies on a sin- gle temperature measurement, but with this measurement only an overload of the component in question can be concluded with certainty. Other elements having an essential impact on the temperature of the switching component are properly functioning cooling and control of the switch component. Comparing the measured temperature with the fixed temperature limit does not, however, produce clear indication or diagnostic data of such disturbances that may oc- cur.

BRIEF DESCRIPTION OF THE INVENTION [0007] It is an object of the present invention to provide a method and an arrangement that allow the above drawbacks to be avoided and enable a more extensive fault diagnosis than before to be implemented by means of temperature measurements. This is achieved by a method of the invention characterized by what is stated in the characterizing part of independent claim 1 and by an arrangement of the invention characterized by what is stated in the characterizing part of independent claim 5. The preferred embodiments of the invention are disclosed in the dependent claims.

[0008] The invention is based on the idea that temperature control of parallel inverter modules is carried out by comparing the temperatures of the same output phases of parallel modules with one another.

[0009] The method of the invention provides distinct advantages in fault diagnostics, because in addition to failure indications based, up to pre- sent, almost solely on an overload current, failures associated with cooling and with the operation of the control of the power component are now indicated.

The failures relating cooling that the invention is capable of detecting include decreased circulation of cooling air, soiling of the cooling components in power semiconductors, and partial or complete breakdown of the cooling fan. In addi- tion, the method enables to detect failures in control electronics, i. e. when one semiconductor component is controlled differently than others, and in power semiconductors, i. e. when a power semiconductor is not conducting or does so in an obscure manner.

[0010] The arrangement of the invention provides a simple means for attaining the advantages of the invention.

BRIEF DESCRIPTION OF THE FIGURE [0011] In the following, the invention will be described in greater de- tail and with reference to the accompanying Figure, which is a schematic illus- tration of the connecting of parallel-connected inverter modules.

DETAILED DESCRIPTION OF THE INVENTION [0012] The Figure is a schematic example of parallel-connected three-phase inverter modules A, B, C. Although the Figure shows three in- verter modules, their number is not restricted thereto, but a required number of parallel modules may be used. Likewise, the number of phases may vary de- pending on the purpose of use.

[0013] Output phases Au, Av, Aw, Bu, Bv, Bw, Cu, Cv, Cw of in- verter modules A, B, C are interconnected to produce output u, v, w for the load. The connection has been implemented by connecting the same phases Au, Bu, Cu; Av, Bv, Cv; Aw, Bw, Cw of each module A, B, C together.

[0014] In accordance with the method of the invention, temperature TAu, TAv, TAw, Tabu, TBV, TBW, Tcu, Tcv, Tcw of each output phase of each inverter module is determined. This may be carried out in a conventional manner by direct temperature measurement. Many currently used power semiconductors have a built-in temperature measurement element that allows the temperature to be obtained directly from the semiconductor of the component or from the vicinity thereof. It is also possible to measure temperature from a cooling plate.

In addition, it is possible to determine the temperature of components indirectly by calculating it on the basis of the current passing through the component.

[0015] In accordance with the invention the temperatures of the output phases of each inverter module are compared with the temperatures of the same output phases of the other modules connected in parallel. With refer- ence to phase u, for example, temperature TAu of phase u of module A is thus compared with temperatures TBU, Tcu of phase u of modules B, C. This is im- plemented as a cross-comparison between all the modules and, in accordance with the invention, carried out in comparison means CM. As shown in the Fig- ure, each inverter module is connected with the comparison means by a signal connection At, Bt, Ct, respectively.

[0016] If the comparisons show that the output phase temperature of an inverter module differs from the temperatures of the same phase in other modules by more than allowed by a predetermined limit, an alarm signal as is generated. In the arrangement of the invention this alarm signal is generated by alarm signal means AM. It is apparent that means CM and AM may be im- plemented in a single circuitry, although in the Figure these means are sepa- rated by a signal line si.

[0017] Since all the substantially identical parallel inverter modules are controlled by substantially identical controls, the mutually corresponding phases of all modules are subject to substantially equal loads. Moreover, since the inverter modules correspond to each other structurally, ventilation operates in substantially the same way in connection with each module. According to the invention, the temperatures of the same phases are to be compared with each other, whereby decreased ventilation, for example, can be detected with the method of the invention.

[0018] The above-described cross-comparison of all temperatures of the same phase is fairly laborious if there is a significant number of parallel- connected modules, because a comparison between the temperatures of the phases of two modules alone does not necessarily provide a reliable result regarding a possible occurrence of failure or symptoms thereof. According to a preferred embodiment of the invention, an average of the temperatures of mu- tually corresponding output phases of parallel inverter modules is calculated, i. e. according to the example of the Figure, temperatures TAu, Tabu, Tcu deter- mined for phase u are used for calculating average Tu ave from the equation: <BR> <BR> <BR> <BR> <BR> <BR> TAu + TBu + TCu<BR> Tu_ave = .<BR> <P>3<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> [0019] When the average has been calculated, which according to the arrangement of the invention is carried out by means CM, each phase temperature is compared, in accordance with the embodiment, with the aver- age obtained and if the output phase temperature of one of the inverter mod- ules differs from the average of corresponding output phase temperatures more than allowed by a predetermined limit, an alarm signal is generated. In an embodiment of the arrangement the comparison is carried out by means CM configured for this purpose.

[0020] In this embodiment all phase temperatures are further com- pared with the calculated average of the phase temperatures. However, the method can be further simplified in accordance with the embodiment by select- ing the highest temperature among the temperatures of parallel modules in each phase and by comparing this maximum value with the calculated aver- age. Expressed mathematically, maximum temperature Tu max of phase u is in this embodiment selected using equation Tu max =max (TAuxTsusTcu) and it is compared with the average using equation Trait is a predetermined ratio.

[0021] If the result of the comparison carried out in means CM is greater than the ratio Tlimit, the alarm signal of the invention is generated by the means for generating an alarm signal in response to a signal of the means CM in signal line sl. The alarm signal as may contain all possible known preventive precautions that aim at reducing the risk of any major failures in the equipment.

Such precautions may include an alarm raised to the operator, interruption of the control of the inverter module in question, and possibly a controlled shut- down of the system as a whole.

[0022] The invention is described above, by way of example, with reference to a three-phase system having three parallel-connected inverter modules. However, it is evident that the number of both the modules and the switches may be varied according to need and implementation, the method and system of the invention still being applicable.

[0023] A person skilled in the art will find it apparent that the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples but they may vary within the scope of the claims.