GB2131210A - Regulating power and phase of inverters operable in parallel - Google Patents

Abstract

Regulating means for inverters (WRI, WRII, WRIII) supplying a load bus bar, comprises first circuits for phase regulation to the same position (not shown), and second circuits (I, II, III) for power control of the inverters. Each second circuit includes a ring modulator as a power detector (DI, DII, DIII), connected to the secondary winding of a voltage transformer receiving the inverter output voltage (uI, uII, uIII). The centre point of the secondary winding is connected through the secondary winding of a current transformer responsive to the inverter current (iI, iII, iIII), and through a series resistor-capacitor member (R, C). The connection between the resistor and capacitor provides a voltage proportional to the active inverter load. The detector output is connectible through a resistor (RI, RII, RIII) to a bus bar (S), which conducts a mean value of the active powers of the inverters as target value. Each resistor is connected across the inputs of a differential amplifier (VI, VII, VIII), by the output of which the active power of each inverter is controlled. <IMAGE>

Description

SPECIFICATION Regulating means for power control of inverters operable in parallel The present invention relates to regulating means for power control of a plurality of inverters operable in parallel on a single load bus bar.

In regulating means for current supply installations with at least two inverters operated in parallel, groups with phase position regulating circuits and power control circuits are independent one of the other in the sense that these must be effective without mutual influencing.

Regulating means with only phase position regulating circuits are known from, for example, DE-AS 22 21152. These operate with phase position comparators, wherein the actual phase position of each one of the inverters to be connected in parallel here is compared with a "mean" phase position of the remaining inverters connected in parallel, which position is represented as target phase position on a bus bar for the phase positions of these inverters.

Associated with the known regulating means is equipment for monitoring of the phase positions of the inverters connected in parallel with a detection of phase deviations and a separate monitoring bus bar.

Also known is equipment forthe power control of inverters connected in parallel with the phase position regulating circuits necessary for the phase position regulation and with control and/or regulating means for parallel operation each time with equal loading of the individual inverters. Employed as control means for this purpose are, for example, current divider chokes (DE-OS 2421125) or a current divider transformer with multi-limb core (DE-AS 28 43 886), which are arranged in the power current circuits of the inverters. Due to their weight and volume, these control means are relatively expensive, particularly for three-phase inverters.

Known as regulating means for achieving equal driving of inverters operating in parallel in power operation are power regulators (DE-AS 22 31 152) with measuring transformers and the like, mostly expensive regulating circuit components.

Finally, regulating means for power control of inverters connected in parallel is known from DE-OS 29 11 342, which regulating means is equipped for active and reactive power regulation of the individual inverters with a system having a plurality of interdependent regulating circuits.

The delivered active power of an alternating current generator is generally represented by the product P = U.l.cos, wherein the effective values of a sinusoidal generator voltage and of a sinusoidal generator current are designated by U and I and the phase displacement of the generator current relative to the generator voltage is designated by . The active power P is usually detected with the aid of multipliers which operate on the current division method or on the time division method. Components with active elements are used for this purpose and these methods are likewise expensive in terms of circuit and equalisation requirements.

It would thus be desirable to provide regulating means in which the number of the regulating circuits for power control is restricted to the necessary number, which is equal to the number of inverters connected in parallel, and in which these regulating circuits contain mainly passive components, whereby power control can be performed even with different loading of the individual inverters.

According to the present invention there is provided regulating means for power control of a plurality of inverters operable in parallel on a single bus bar, the regulating means comprising a corresponding plurality of regulators each associated with a respective one of the inverters and each comprising phase ppsition regulating circuit means for regulation of the phase position of the output voltage of the respective inverter to correspond with the output voltage phase position of the or each other inverter, and power control circuit means comprising a power detector, which comprises a ring modulator coupled to an output low-pass filter and which is responsive to the output voltage and load current of the respective inverterto provide an output guide voltage proportional to the active power of that inerter, and comparison means for comparing the guide voltage with a target guide voltage derived from the other inverter or inverters and for providing an output signal to control the active power of the respective inverter in dependence on the comparison result, the modulator of each power control circuit means comprising a voltage transformer having its primary winding connected to receive said inverter output voltage in single phase and its secondary winding connected at each end thereof by way of a respective one of two equally poled diodes to a respective one of two equal value resistors of a voltage divider, and a current transformer having its primary winding connected to receive said inverter load current and its secondary winding connected at one end to a centre tap of the voltage transformer secondary winding and at its other end to the voltage divider by way of the low-pass filter, the low-pass filter comprising a serially connected resistor and capacitor arranged with the capacitor connected to a centre tap of the voltage divider and with an output for the guide voltage at the junction of the resistor and capacitor, and the comparison means of each power control circuit means comprising resistance means selectably connected between the output of the low-pass filter of the associated power detector and a control bus bar common to all of the regulators, and a differential amplifier having input means connected in parallel with said resistance means.

In a preferred embodiment, the regulating means comprises a group of first and a group of second regulating circuits which are independent one from the other, each inverter being associated with a first regulating circuit with a phase comparator for regulation of the output voltage of the associated inverterto an equal phase position for the inverters operating in parallel, and also associated with a second regulating circuit with a target-actual value comparator and means for the detection of the actual value of the delivered power of the associated inverter for the power control.Each second regulat ing circuit comprises a half-wave ring modulator as a power detector with two equally poled diodes which are connected togetherthrough a voltage divider with two equal resistors, which series circuit is connected to the secondary winding of a voltage converter, to which the output voltage of the associated inverter is fed at the primary side in single phase.The modulator also as a series circuit, which connects the centre tap of the secondary winding of the voltage transformer to the centre tap of the voltage divider, and which comprises the secondary winding of a current transformer, to which the load current of the inverter is fed at the primary side, and a series resistor-capacitor member, which serves as a low-pass filter and the capacitor of which is connected with the centre tap of the voltage divider and forms an output from which a guide voltage proportional to the active power is derivable as actual value of the active power delivered by the inverter.The output is connected through a resistor and a switching cintact, which is closed when the inverter is disposed in parallel compound, with a control bus bar as target value rail for the corresponding guide voltages of the other inverters, the resistor being connected in parallel with the input of a differential amplifier, through the output of which the active power of the inverter is controlled.

In order that the ring modulator diodes of the power detectors may have fault-free switching behaviour so that the temporal course of the measure- ment voltage proportional to load current will be free of obliterations, the amplitude ratio of half the secondary voltage of the voltage transformer to the secondary voltage of the current transformer may be greater than or at least several times higher than the maximum secondary voltage of the current transformer occurring in operation according to whether the secondary voltage of the voltage transformer is rectangular or sinusoidal.

For the monitoring of the power control and in order, in the event of an overload due to balancing currents or unequal load division, to effect protective switching-off of the or each inverter concerned; each second regulating circuit of the regulating means may be executed twice, namely onceforthe power control and once for monitoring of load current, and may comprise a detector detecting the actual value ofthe load current, a monitoring bus barforthe load currents of the inverters operated in parallel and a signal path, coming from the output of the differential amplifier, of the control magnitude for the current monitoring, which path includes a threshold value stage with a set threshold value effecting, on being exceeded, switching-off of the inverter.

For some applications of the power control of inverters operated in parallel, a high regulating speed is required. For this purpose, each second regulating circuit may contain a power detector executed as a full-wave ring modulator in centre tap connection. In such a modulator, two half-wave series circuits each with two diodes and two resistors can be connected in parallel opposition to the secondary winding of the voltage transformer.

Expediently, the resistance values of the burdens of the current transformers associated with the inverters are dimensioned to be inversely proportional to the load shares for an intended division of the total load of the inverters with different load shares.

There may thus be provided regulating means by which, also for several inverters which are differently loaded, the total loading of the same in parallel operation may be apportioned overthese according to the loads of the individual inverters so that the common bus bar voltage remains the same or is practically unchanged on each switching for parallel operation as well as thereafter.

Embodiments of the present invention will now be more particularly described with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a power detector with a half-wave ring modulator circuit, in regulating means embodying the invention; Figure 2 are diagrams illustrating aspects of the operation of the power detector of Figure 1; Figure 3a is a circuit diagram of part of regulating means embodying the invention, showing circuits for power control of inverters operated in parallel; and Figure 3b is a circuit diagram of a modified regulating means embodying the invention, showing current circuits for monitoring of the power control of the inverters.

Referring now to the drawings, there is shown in Figure 1 a power detector which is used in a respective regulating circuit of regulating means for the power control of inverters operated in parallel, the detector having the function of detection of the active power delivered by an associated one of the inverters. The power detector comprises a series circuit with two equally poled semiconductor diodes d and d' and two equal resitors rand r', which form an ohmic voltage divider, through which the two diodes are connected together. The series circuit is connected to the secondary winding of a voltage transformer SpW, to which an alternating voltage u is fed at the primary side.A centre tap M of the secondary winding of the transformer SpW is connected to the centre tap M' of the voltage divider through a series circuit comprising a secondary winding, which is connected in parallel circuit with a burden B, of a current transformer StW and a series resistor-capacitor member R and C. An alternating current i is fed to the primary winding of the current transformer from an alternating current generator, from which the alternating voltage u is fed to the primary winding of SpW. The circuit according to Figure 1, apart from the resistor-capacitor member, is in principle a ring modulator which by the series connection of equally poled diodes d and d' is only unilaterally conducting current and thus can be designated as half-wave ring modulator.

By means of this circuit and as shown in Figure 2, an alternating voltage u' proportional to the alternating current i (diagram b) is superimposed by the diodes d and d' on half the seconary voltage 1/2 u' of the voltage transformer, which is proportional to the alternating voltage u (diagram a), during each half period, during which the secondary voltage u' is poled in the passage direction for the diodes d and d'. Because the centre taps M and M' have equally high potentials in the conducting half periods, there thus exists at the resistor-capacitor member alone the current-proportional voltage u'j, the temporal course of which is illustrated in the diagram c of Figure 2, in each second half period of u.The resistor-capacitor member, the centre point of which forms the output A of the detector, acts as a low-pass filter for the voltage u'j so that the temporal halfperiod mean value of u'j at the capacitor C is represented as an output voltage and a guide voltage U, which is proportional to the product of I and powerfactorcos .

Voltages and currents are not represented true to scale in the diagrams of Figure 2.

In use of the power detector for detection of the active power delivered by the associated inverter, the output voltage u of the inverter is fed to the primary winding of the voltage transformer and the load current i is fed to the primary winding of the current transformer. On the assumption that the output voltage of the inverter is kept constant, the guide voltage obtained at the output of the detector can be regarded as proportional to the active power delivered by the associated inverter, according to the relationship U. P = K.l.cosB wherein K stands for the constant output voltage of the inverter with the magnitude dimension of a voltage.

The detector inclusive of the transformers consists of only passive components and can be produced without undue cost. These detectors are employed advantageously as power detectors for the actual value detection of the active power as well as for the detection of the power flow direction in regulating circuits for power control.

Illustrated in Figure 3a is a part of regulating means for three inverters WRI to WRIII operating in parallel on a common load bus bar, namely the part with second regulating circuits I to Ill, in which the power detectors are designated by Dl to DIII. Each of the detectors receives the output voltage u, at one input and the load current iv at another input (v = I, II, III). Derived from the power detectors at their outputs are guide voltages Uv, which are each formed at the respective low pass filter of the detector, namely at the capacitor C.Each guide voltage is the scalar product of the vectors of the output voltage and the load current of respective inverter, which have a phase difference pr from each other determined by the load thereof, and is proportional to the active power delivered by the inverter according to P = K.l.cosm, wherein K signifies a constant and I the effective value of the load current of the inverter.

The tap A from the capacitor C of each low-pass filter is connected through a respective resistor Rv and a respective mechanical switching member Sch with a bus bar S, which is common to all the second regulating circuits and on which a certain value U' is predetermined, which represents the mean value of the guide voltages Uv conducted from the inverters operating in parallel through like detectors and connections to the bus bar. The terminals of each resistor Rv are connected with the input of a respective differential amplifier Vv having the function ofthetarget-actual value comparator.On deviation of the guide voltage Uv from the comparison magnitude U' predetermined at the bus bar S, the difference magnitude Uy - U' fed to the input of the amplifier V arises at the resistor Rand a regulation deviation signal s", which is employed as setting magnitude for the driving of the inverter concerned, arises at the output of the amplifier.On a desired switching of an inverter WRV into the parallel compound ofinverters, the associated switching contact Schv is closed, whereupon if there is a power deviation of that inverter, a regulation process is effected, whereby a power adaptation of the switched-in inverter is controlled through the setting magnitude in the sense that balancing currents and unequal load currents iv between the inverters are regulated out.

In Figure 3b, current circuits I to Ill of Figure 3a are executed in like manner for the monitoring of the active power control, with detectors detecting the load currents iv of the inverters WR, measurement current transformers DL, resistors RL, switching contacts Schiz and a monitoring bus bar SLforthe load currents iv (v= I, II, Ill). Connected to the output of each amplifier VL is a signal path for the current deviation of a load current, which path leads to a threshold value transmitter GL in which a threshold value of the load current is set. This is chosen so that it will be exceeded by the load current in the case of a great current deviation of the associated inverter.

In this case, switching-off of the inverter is effected through a signal from the threshold value transmitter. By this extra circuit associated with each inverter, the equipment according to Figure 3b is able to provide constant monitoring of the power control of inverters operated in parallel.

Claims (7)

1. Regulating means for power control of a plurality of inverters operable in parallel on a single bus bar, the regulating means comprising a corresponding plurality of regulators each associated with a respective one of the inverters and each comprising phase position regulating circuit means for regulation of the phase position of the output voltage of the respective inverterto correspond with the output voltage phase position of the or each other inverter, and power control circuit means comprising a power detector, which comprises a ring modulator coupled to an output low-pass filter and which is responsive to the output voltage and load current of the respective inverter to provide an output guide voltage proportional to the active power of that inverter, and comparison means for comparing the guide voltage with a target guide voltage derived from the other inverter or inverters and for providing an output signal to control the active power of the respective inverter in dependence on the comparison result, the modulator of each power control circuit means comprising a voltage transformer having its primary winding connected to receive said inverter output voltage in single phase and its secondary winding connected at each end thereof by way of a respective one of two equally poled diodes to a respective one of two equal value resistors of a voltage divider, and a current transformer having its primary winding connected to receive said inverter load current and its secondary winding connected at one end to a centre tap of the voltage transformer secondary winding and at its other end to the voltage transformer secondary winding and at its other end to the voltage divider by way of the low-pass filter, the low-pass filter comprising a serially connected resistor and capacitor arranged with the capacitor connected to a centre tap of the voltage divider and with an output for the guide voltage at the junction of the resistor and capacitor, and the comparison means of each power control circuit means comprising resistance means selectably connected between the output of the low-pass filter of the associated power detector and a control bus bar common to all of the regulators, and a differential amplifier having input means connected in parallel with said resistance means.

2. Regulating means as claimed in claim 1, wherein the voltage and current transformers of each power control circuit means are so arranged that, in use, the amplitude ratio of half the secondary voltage of the voltage transformer to the secondary voltage of the current transformer is greater than the maximum secondary voltage of the current transfor mer by a factor dependent on the shape of the waveform of the secondary voltage of the voltage transformer.

3. Regulating means as claimed in either claim 1 or claim 2, the power control circuit means of each regulator further comprising a load current detector arranged to provide an output current proportional to the actual load current of the respective inverter, load current comparison means for comparing the detector output current with a target load current derived from the other inverter or inverters and for providing an output signal having a value indicative of the magnitude of any difference between the compared currents, and a threshold value stage for supplying a signal to switch off the associated inverter when said output signal value exceeds a predetermined threshold value, the load current comparison means comprising respective resistance means selectably connected between output means of the load current detector and a monitoring bus bar common to all of the regulators, and a differential amplifier connected at its inputs in parallel with that resistance means and at its output with the threshold value stage.

4. Regulating means as claimed in any one of the preceding claims, wherein the modulatorofeach power control circuit means is arranged to function as a full-wave ring modulator.

5. Regulating means as claimed in any one of the preceding claims, wherein the inverters are operable with respectively different shares of the total operating load thereof and the value of the burden of the current transformer of each regulator is inversely proportional to the load share of the associated one of the inverters.

6. Regulating means substantially as hereinbefore described with reference to Figures 1, 2 and 3a of the accompanying drawings.

7. Regulating means substantially as hereinbefore described with reference to Figures 1,2 and 3b of the accompanying drawings.