Arrangement for monitoring a frequency converter

Abstract

An arrangement and method for monitoring a frequency converter, said frequency converter being a voltage-controlled PWM frequency converter provided with a control unit (12) and having an uncontrolled or a mains-frequency controlled mains bridge (10) connectable to an alternating-current source (UL1, UL2, UL3), a direct-voltage intermediate circuit and a controlled load bridge (11) for producing a variable-frequency multi-phase output voltage (UU, UV, UW), which direct-voltage intermediate circuit of the frequency converter is provided with a small-capacitance direct-voltage capacitor, the mains bridge (10) being connected to the load bridge (11) directly without a large-capacitance direct-voltage capacitor functioning as an intermediate energy storage, and which arrangement comprises a measuring unit for measuring the direct voltage of the intermediate voltage circuit. In the arrangement, the measuring unit (MEAS123) measures at least one quantity of the intermediate-circuit direct voltage, and the control unit determines on the basis of the aforesaid quantity at least one input voltage value and/or a failure situation of the frequency converter or its load circuit.

Description

The present invention relates to a method and arrangement for monitoring a frequency converter. In particular, the invention concerns a method and arrangement for monitoring a frequency converter provided with a small capacitor in the intermediate voltage circuit and controlled by pulse-width modulation (PWM), said method and arrangement being based on measurement of the intermediate circuit voltage.

FIG. 1 presents a three-phase voltage-controlled PWM-frequency converter having a mains bridge (rectifier) 10 for rectifying the three-phase alternating voltage of a supply network, consisting of phase voltages U_L1, U_L2, U_L3, to produce an intermediate-circuit direct voltage UDC and a load bridge (inverter) 11 for inverting the intermediate-circuit direct voltage to produce a three-phase alternating voltage of variable frequency having phase voltages U_U, U_V, U_W when power is flowing from the network to the load. The frequency converter may feed e.g. a three-phase alternating-current motor. The load bridge 11 is a full-wave bridge with a control unit 12 controlling the phase switches of each phase by pulse-width modulation. ‘Phase switch’ refers to the switch formed by the semiconductor switches of the upper and lower branches of a phase together, with a diode connected in inverse-parallel with each semiconductor power switch. The mains bridge 10 is an uncontrolled full-wave bridge, having a diode bridge consisting of diodes. The intermediate circuit of the frequency converter is provided with a DC capacitor, and an inductor LAC is connected between the mains supply and the diode bridge of the frequency converter.

FIG. 2 presents voltage measurements and measuring units generally used for them in a frequency converter:

• • MEAS1=input voltage measurement, which is needed when monitoring e.g. mains symmetry, phase absence, voltage brownouts or interruptions etc.
  • MEAS2=measurement of intermediate-circuit direct voltage, normally included in all PWM-frequency converters because this information is essential e.g. for purposes of motor control,
  • MEAS3=measurement of the voltage of a semiconductor power switch, e.g. IGBT, which is needed for short-circuit protection (in a short-circuit situation, the voltage of one of the semiconductor power switches is too high even if the switch is conducting).

A PWM frequency converter provided with a small intermediate circuit capacitor is described e.g. in specification U.S. Pat. No. B2-6,801,441. In the frequency converter of this specification, the mains bridge 10 is connected to the load bridge 12 (FIG. 1) without a high-capacitance direct-voltage capacitor unit serving as an intermediate energy storage. The direct-voltage intermediate circuit of such a PWM-frequency converter is implemented using only a low-capacitance capacitor CDC for limiting the voltage spikes caused in switching situations by the energy stored in leakage inductances.

The object of the present invention is to overcome the drawbacks of prior art and to create a monitoring method and arrangement wherein all the information produced by the measurements indicated in FIG. 2 is obtained by merely measuring the intermediate-circuit direct voltage. Such a measurement is needed in the frequency converter in any case for motor control purposes, and thus the monitoring of input voltage and short-circuit situations according to the present invention requires no additional electronics or expenses.

The features of the method, measuring arrangement and use of the invention are presented in detail in the claims below.

In the following, the invention will be described in detail with reference to an example and the attached drawings, wherein

FIG. 1 presents a voltage-controlled PWM-frequency converter,

FIG. 2 illustrates voltage measurements on a frequency converter,

FIGS. 3 and 4 illustrate the formation of the intermediate-circuit voltage,

FIG. 5 represents the intermediate-circuit voltage in a frequency converter provided with a small intermediate-circuit capacitor, and

FIG. 6 illustrates the measurement of frequency converter voltages according to the invention.

FIG. 3 represents the formation of the intermediate-circuit voltage from the input voltages via rectification when the intermediate-circuit capacitor is small. The intermediate-circuit voltage follows the highest input main voltage.

FIG. 4 represents the direct voltages of intermediate circuits provided with differently sized capacitors:

Udc1 corresponds to the voltage of an intermediate circuit provided with a very small capacitor not serving as an energy storage. The voltage follows the highest input main voltage. The present invention concerns a frequency converter in which the intermediate-circuit voltage has a curve form substantially like this curve

Udc2 represents a theoretical, perfectly smoothed voltage of an intermediate circuit provided with an infinitely large capacitor

Udc3 represents intermediate-circuit voltage in the case of traditional capacitor rating, wherein the capacitor serves as a significant energy storage. The voltage is nearly completely smoothed, but fluctuates slightly with the rectified supply network voltage. The voltage level is substantially lower than the maximum value of the supply main voltage.

FIG. 5 represents the intermediate-circuit voltage when the intermediate-circuit capacitor is small and the supply voltage asymmetric.

FIG. 6 presents a measuring arrangement according to the invention in a PWM-frequency converter provided with a small intermediate-circuit capacitor as presented in FIG. 1, in which arrangement the intermediate-circuit voltage is measured by a measuring unit MEAS123. In the invention, a measuring arrangement is used wherein all the information produced by the measurements shown in FIG. 2 is obtained by measuring the intermediate-circuit direct voltage alone.

In the arrangement of the invention, the measuring unit MEAS123 measures the intermediate-circuit direct voltage UDC. Based on this measurement data, the control unit 12 determines the voltage level, frequency and asymmetry of the mains supply voltage as well as the presence of a motor-circuit short circuit situation. For the observation of supply-network phenomena of relatively low frequency, it may be advantageous to have the measurement signal filtered by a low-pass filter that filters out high-frequency voltage components, e.g. above 1000 Hz, from the measurement signal.

In detail, the determinations based on the intermediate-circuit measurement data are performed as follows:

In the frequency converter described above, the intermediate-circuit voltage UDC follows the highest main voltage of the supply network. Therefore, by indicating the instants of occurrence of the peak values of UDC (which occur consecutively depending on the mains frequency (50 or 60 Hz) at intervals of about 2.7 . . . 3.3 ms) as well as their voltage values at the peaks, it is possible to determine the following:

1. From the time interval between peaks, it is possible to calculate the mains frequency, because in the three-phase system there occur 6 peaks during one cycle

2. From the voltage value at the peaks, it is possible to calculate the root mean square value (Urms) of the mains supply voltage

3. If the voltage values at the peaks differ in magnitude and the same inequality is repeated regularly in periods of three peaks, then this is a sign of supply voltage asymmetry. The magnitude of asymmetry can be calculated directly from the largest differential voltage ΔUdc between voltage peaks (FIG. 5)

4. Since the intermediate-circuit capacitor is very small and the circuit has on the supply side an AC inductor, which represents a large impedance for fast phenomena, the intermediate-circuit voltage collapses very rapidly in a short circuit situation occurring in the motor circuit. Therefore, this situation can be indicated by means of a voltage comparator, which is a fast and cheap solution. A short circuit may occur either in the motor circuit (cable) or it may be caused by a failure of a semiconductor power switch of the inverter. Especially low-power frequency converters may lack a separate AC inductor for reasons of cost, but this does not necessarily prevent the function described in this paragraph from being implemented, because in a short-circuit situation the intermediate-circuit voltage still collapses due to the short-circuit impedance of the supply network.

Let it be noted that, in a traditional frequency converter provided with a large intermediate-circuit capacitor, the above-described determinations are practically impossible because the intermediate-circuit voltage in a normal operating situation is almost completely smoothed (U_dc3, FIG. 4) and, due to the large energy content of the capacitor, does not collapse rapidly in a short-circuit situation.

It is obvious to the person skilled in the art that different embodiments of the invention are not exclusively limited to the example described above, but that they may be varied within the scope of the claims presented below. Thus, from the point of view of functionality and sphere of protection of the invention, for example the mains bridge 10 may just as well be mains-frequency controlled according to patent specification F1108819, and instead of the inductor LAC it is equally possible to use a DC-inductor placed between the mains bridge 10 and the intermediate-circuit capacitor CDC.

Claims

1. A method for monitoring a frequency converter, said frequency converter being a voltage-controlled PWM frequency converter having an uncontrolled or a mains-frequency controlled mains bridge (10) connectable to an alternating-current source (UL1, UL2, UL3), a direct-voltage intermediate circuit and a controlled load bridge (11) for producing a variable-frequency multi-phase output voltage (UU, UV, UW), which direct-voltage intermediate circuit of the frequency converter is provided with a small-capacitance direct-voltage capacitor, the mains bridge (10) being connected to the load bridge (11) directly without a large-capacitance direct-voltage capacitor functioning as an intermediate energy storage, and in which method the direct voltage of the intermediate voltage circuit is measured, characterized in that the method comprises measuring at least one quantity of the intermediate-circuit direct voltage, and determining on the basis of the aforesaid quantity at least one input voltage value and/or a failure situation of the frequency converter or its load circuit.

2. A method according to claim 1, characterized in that the said quantity is the voltage value of the direct-voltage peaks, on the basis of which is determined the voltage of the alternating-current mains supply network.

3. A method according to claim 1, characterized in that the said quantity is the time between peaks of the direct voltage, on the basis of which is determined the frequency of the alternating-current mains supply network.

4. A method according to claim 1, characterized in that the said quantity is the voltage value of the peaks of the direct voltage, on the basis of which is determined the asymmetry of the alternating-current mains supply network.

5. A method according to claim 1, characterized in that the said quantity is the direct-voltage level and the failure situation is a short-circuit situation, which is detected from the direct voltage level falling rapidly down.

6. An arrangement for magnetic reluctance a frequency converter, said frequency converter being a voltage-controlled PWM frequency converter provided with a control unit (12) and having an uncontrolled or a mains-frequency controlled mains bridge (10) connectable to an alternating-current source (UL1, UL2, UL3), a direct-voltage intermediate circuit and a controlled load bridge (11) for producing a variable-frequency multi-phase output voltage (UU, UV, UW), which direct-voltage intermediate circuit of the frequency converter is provided with a small-capacitance direct-voltage capacitor, the mains bridge (10) being connected to the load bridge (11) directly without a large-capacitance direct-voltage capacitor functioning as an intermediate energy storage, and which arrangement comprises a measuring unit for measuring the direct voltage of the intermediate voltage circuit, characterized in that in the arrangement the measuring unit (MEAS123) measures at least one quantity of the intermediate-circuit direct voltage, and the control unit determines on the basis of the aforesaid quantity at least one input voltage value and/or a failure situation of the frequency converter or its load circuit.

7. An arrangement according to claim 6, characterized in that the said quantity is the voltage value of the direct-voltage peaks, on the basis of which is determined the voltage of the alternating-current mains supply network.

8. An arrangement according to claim 6, characterized in that the said quantity is the time between peaks of the direct voltage, on the basis of which is determined the frequency of the alternating-current mains supply network.

9. An arrangement according to claim 6, characterized in that the said quantity is the voltage value of the peaks of the direct voltage, on the basis of which is determined the asymmetry of the alternating-current mains supply network.

10. An arrangement according to claim 6, characterized in that the said quantity is the direct-voltage level and the failure situation is a short-circuit situation, which is detected from the direct voltage level falling rapidly down.

11. Use of direct-voltage measurement of the intermediate voltage circuit of a frequency converter, said frequency converter being a voltage-controlled PWM frequency converter having an uncontrolled or a mains-frequency controlled mains bridge (10) connectable to an alternating-current source (UL1, UL2, UL3), a direct-voltage intermediate circuit and a controlled load bridge (11) for producing a variable-frequency multi-phase output voltage (UU, UV, UW), which direct-voltage intermediate circuit of the frequency converter is provided with a small-capacitance direct-voltage capacitor, the mains bridge (10) being connected to the load bridge (11) directly without a large-capacitance direct-voltage capacitor functioning as an intermediate energy storage, to determine at least one supply voltage value and/or a failure situation of the frequency converter or its load circuit.