FI116649B - Pulse width modulation mains inverter units synchronizing method for motor feeding device, involves measuring circulating current between inverter units and synchronizing modulators based on measurement data - Google Patents

Abstract

The method involves measuring a circulating current between inverter units. Phase currents are measured simultaneously and sum of the phase currents is formed. Instantaneous value of circulating current is detected by multiplying the sum of the phase currents by a polarity of a triangular wave. Successive instantaneous values are summed. Modulators in the units are synchronized to each other, based on the measurement data. Independent claims are also included for the following: (A) an inverter apparatus comprising parallel-connected inverter units (B) a method for synchronizing parallel-connected frequency converters (C) a converter apparatus comprising frequency converters re-injecting braking energy into a supply network.

Description

116649

SYNCHRONIZATION OF PARALLEL CONVERTERS

The present invention relates to a method for synchronizing parallel PWM frequency converters which are capable of feeding back the braking energy from the load into a so-called supply network. with network switches. The invention also relates to electric drive consisting of parallel-connected network-braking PVVM frequency converters. The invention relates specifically to a parallel circuit arrangement in which the input of the frequency converters is common and the outputs are galvanically coupled, for example, at the terminals of the motor 10 or at the common star point of the separate windings.

The basic unit of a frequency converter operating on PVVM principle is an inverter, which produces a three-phase alternating voltage from the DC voltage of the intermediate circuit whose frequency and amplitude can be adjusted. The mains-braking frequency inverter may consist of, for example, two physically identical inverter units with DC connections connected to a common DC intermediate circuit and a second (INU)

1) AC connections to the supply network via a mains filter (LFU) (see figure 1) and another (INU 2) AC connections to the motor.

Mains inverters are used between the mains and the DC bus when the system requires network braking capability and / or the mains harmonics must be very small. The network inverter consists of an inverter unit (INU) and a network filter (LFU). The inverter unit used in the network inverter rectifiers may be mechanically similar to the motor supply, except for the software. As a mains AC converter, the inverter unit forms a DC bus 25 from the DC voltage to the AC side of the supply network. The voltage pattern formed by the PWM principle is known to cause current harmonics, which are typically limited by a filter, which is usually a · ·> · 'LCL type, see Figure 1. In the power supply application, a sufficient power supply is provided - 30 sit.

,,; i * For high power, multiple inverter units or complete inverters must be connected in parallel. An example of parallel connection is shown in Fig. 2 which shows two parallel connected network braking drives ;;; FC1 and FC2, which are connected from the supply side to the same three-phase alternating current network U1 through the filter units LFU1 and LFU2 and which are coupled ,,; i * From side to side at the common star point of the individual windings of the motor.

·; ·: The problem with such an arrangement is that the upper control system common to the parallel units normally only takes care that the basic waves of the voltages generated by the 2 116649 inverter units are equal in magnitude and phase but do not depend on their actual inverter phase alternators. In this case, it may happen that the switches of the same phase in the parallel-connected INUs do not simultaneously rotate in the same direction, but may be temporarily in opposite positions, for example as shown in FIG. The figure shows the situation where the same phase of the network-side switches invertase riyksiköissä have a different position, but the engine side of inverter switches are in the same position. As can be seen in the example of the figure, in the situation 10, the voltage of the second drive inverter Udc2 causes a circulating current (discharges), which is mainly limited by the inductances of the filter units and the motor cables. Circulation current causes additional strain eg. main circuit power switches.

One known method for preventing this problem is to use frequency-varying discrete transformers, whereby the supply circuits are galvanically separated from one another and thus no circulating current can be generated. However, the solution is in many respects a bad one; it takes up a lot of space, is very expensive, etc.

Another known method of overcoming the problem is to use a common PWM modulator whose output of phase switch control signals is distributed to all parallel inverter units. However, this cannot be done without the special 20 lace.

U.S. Patent No. 5,257,180 discloses a control system having a separate synchronization circuit for synchronizing parallel-connected inverters, which provide common synchronization signals. Ko. the apparatus further includes detectors for detecting current components traveling between the inverters, and a control circuit for controlling the output voltages of the inverters, which may reduce the inverter; circulating currents between terrains.

The object of the present invention is to eliminate the disadvantages of the prior art. ; points, and provide a simpler solution to the synchronization of parallel inverter assemblies for network braking electric drive with at least two parallel inverter units on both the network and motor sides.

In the present invention, it is utilized that the inverter unit, normally includes a constant phase-specific current measurement (iu, iv, iw and ir, is, ΐτ il! In Figure 1). In accordance with the present invention, the inverter-specific modulators are synchronized to each other based on the current measurement data without any additional electronics. Thus, standard units can be used for parallel connections, which "* * is a great advantage over known methods.

3, 116649

In the method according to the invention, all inverter units included in the line inverter use the same switching frequency fKi, which is different from the switching frequency fa used in all motor-supplying inverter units. The modulating pulse patterns of both the line braking units and the motor supplying units are synchronized by measuring the modulation frequency rotation current characteristic of both switching frequencies caused by the phase difference between the zero components of the three-phase voltage pattern formed by the phase switches.

Detailed features of the solution according to the invention are set forth in the appended claims.

In the following, the invention will be explained in more detail by way of example with reference to the accompanying drawings in which

Figure 1 shows a frequency converter with a network inverter,

Figure 2 shows the rin-coupling of two network-braking drive inverters,

Fig. 3 shows the phase switches of the parallel-connected network-braking frequency inverters and the circulating current between them, and Fig. 4 shows the formation of a circulating current.

Fig. 1 shows a three-phase network braking PWM drive with a network bridge (INU1) for rectifying a three-phase alternating voltage of a supply network with phase voltages Uu, Uv, Uw to a direct voltage UDc of a DC link. The network bridge has oh-25 semiconductor switches V1-V6 leaving each branch of the bridge, which consist of e.g.

! IGBTs and the resistive diodes connected to them. A mains bridge is connected to the alternating current network ► ► ♦ ;; via the LCF filter unit LFU. The connection also allows the negative intermediate circuit current to flow in the direction of the feeder network. The semiconductor switches of the network bridge are controlled, for example, by a known modulator based on the blue-triangle comparison. The frequency converter of Fig. 1 also includes a load bridge (INU2), which forms a three-phase, [·,, output voltage Ur, Us, Ut, the DC voltage of the DC link, whose amplitude and frequency can be adjusted as desired !! '. way.

Figure 2 shows two parallel inverters FC1 and FC2 connected to a common three-phase alternating current network UL through filter units LFU1 and LFU2 and a common double-winding motor via chokes L1 and L2. According to the invention, the phase switches of each of the grid inverters (INU11 and INU21) of each network are controlled by their own sine-triangle modulators, in which the reference frequencies (= switching frequencies) of the triangle waves in both parallel inverter units are set (fKi). According to the invention, the phase switches of the inverter units 5 (INU12 and INU22) of each load bridge are also controlled by their own sine-triangle modulators, in which the reference frequencies of the triangle waves in the two parallel inverter units are set equal (½). According to the invention, the switching frequencies fKi and fx2 are set differently, whereby their frequency circulating currents can be detected and thereby synchronized both the modulation pulse patterns of the network bridges and the modulation pulse patterns of the load bridges to attenuate the circulating currents.

The modulation pulse patterns are synchronized with each other by measuring the modulating frequency circulating current caused by the phase difference between the zero components of the three-phase voltage pattern formed by the phase switches. This is illustrated in Figure 15 4, which illustrates modulation by the known sine-triangle comparison principle and the resulting virtual star point voltage, called the zero component of the voltage. The zero voltage of the inverter unit INU 11 represents the zero voltage generated by the modulation according to the figure and the zero voltage of the inverter unit INU 21 provided by the modulation of the second parallel inverter unit. As can be seen from the figure, the frequency and phasing of the zero voltage are linked to the triangular wave. In the example of the figure, the triangular waves of the modulators are in phase shift a, which phase shift is shown to be the same size between the zero: · sort voltages.

; To measure the circulating current, the first step is to measure three. 25 sum current, which differs from zero just by the amount of the circulating current (in a normal circular, non-current three-phase system, the sum current is 0). The total current can be measured, for example, by taking simultaneous samples of the phase currents in a triangular wave; lon tip, whereby the normal waveform is eliminated.

'···' The direction of the circulating current is expressed by multiplying the measurement result by the polarity of the triangle wave (positive vertex = 1, negative vertex = -1).

The instantaneous value of the circulating current measured is added to the previous measurement results

I * I

sum, resulting in the so-called circulating current sum. If the total current exceeds. . ·. of the maximum value given, momentarily select a higher switching frequency (= triangle frequency) following the reference for one half of the modulation period, ·· *.

Conversely, if the sum of the circulating currents falls below the negative maximum value, a switching frequency one step lower than the reference frequency is used for the next half of the modulation period. By this procedure, the phase difference of the triangular wave to the coil of the INU of the adjacent INU is reduced. While changing the switching frequency, the circulating current amount is reset.

The total current measurement shows the circulating current component caused by both mains bridges and load bridges. However, due to unequal switching-5 hairs, in the long run, the bridge current sum only shows the switching frequency (fKi) current of the network bridges, as does the bridge bridge current sum only the switching frequency (f «2) of the bridge.

The switching frequency can be changed proportional to the circulating current 10 in other ways, e.g., as a P1-controlled phase-locked loop whereby the frequency of the triangle is adjusted proportional to the modulated frequency circulating current. The above summing method has the advantage that at the same time the summing provides filtered measurement noise and that the instantaneous interference or offset in the sum signal cannot be amplified.

15 All triangular waves can be used in all parallel devices to synchronize the same algorithm, or alternatively one, it may be omitted without however impairing the functionality of the method.

It will be apparent to one skilled in the art that the various embodiments of the invention are not limited to the above example, but may vary within the scope of the following claims.

Claims (10)

116649 1. A method for synchronizing parallel-connected network-braking frequency converters (FC1, FC2) having inverter-unit-specific modulators in both network and load bridges, characterized in that the method comprises: ) la. A method according to claim 1, characterized in that the switching frequency (ίκι) of the network bridges is set to the same but different from the switching frequency (fo) of the load bridges. Method according to claims 1 and 2, characterized in that for measuring the circulating current in the inverter units, | measuring the phase currents simultaneously and generating the sum of the phase currents, expressing the instantaneous value of the switching frequency current by multiplying the sum of the phase currents by 20 polarity of the triangle wave or the corresponding modulation signal, and generating the sum of successive instantaneous value results The method of claims 1-3; : characterized by the fact that in order to prevent the circulating current in parallel-connected drives *. The triangular waves of the modulators, or the corresponding modulation signals, are synchronized, ', by the following operations which are performed on all inverter units: generating a circulating current sum, *; increasing the switching frequency by a fixed step, for example, for one half of the modulation period if the circulating current sum 30 exceeds the set positive limit value while zeroing the circulating current:: decreasing the switching frequency by a fixed step, for example value while zeroing the circulating current amount Method according to claims 1 to 3, characterized in that the synchronization v: of the triangular waves or the like modulation signals is performed by adjusting their frequency in proportion to the switching frequency indicated by the circulating current. 116649 The method of any one of claims 1 to 5, used in all but one of the inverter units of the network bridges and / or load bridges of the parallel inverters. 7. An apparatus consisting of parallel-connected network-braking frequency converters (FC1, 5 FC2) having inverter-specific modulators for both network and load bridges, characterized in that the apparatus: measures the circulating current between the drives and synchronizes the modulators. Apparatus according to claim 7, characterized in that the switching frequency (fKi) of the network bridges is set to one another but different from the switching frequency (f «2) of the load bridges. Apparatus according to claims 7 and 8, characterized in that, in order to prevent the circulating current between the parallel inverters 15, the triangular waves or corresponding modulation signals of their inverter unit-specific modulators are synchronized such that the sum of the circulating current is increased, the fixed frequency is increased for example, for one half of the modulation period if the sum of the circulating current 20 exceeds the set positive limit while zeroing the amount of circulating current is reduced by a fixed step, for example one half of modulation period if the sum of the circulating current is below the set negative threshold Apparatus according to any one of claims 7 to 9, used in all but one of the inverter units of the network bridges and / or load bridges of the parallel inverters. * # »A * a> '·> a a» a a> i a 8 116649