DC/AC POWER CONVERTING APPARATUS INCLUDING DC COMPONENT REMOVER

Abstract: In a DC/AC power converting apparatus (100), a DC component is removed from an AC output. The DC/AC power converting apparatus (100) includes: a DC source (1) for supplying DC power; a DC/AC inverter circuit (2) for inverting the DC power into AC power; a transformer (3) for transforming the AC power so as to obtain desirable AC power; a detector (5) for detecting an output signal derived from the DC/AC inverter circuit (2) to produce an output detection signal (i D ); a calculation circuit (6) for performing a control calculation so as to follow the output detection signal (i D ) to a reference signal (i R ), whereby a first output instruction signal (V I ) is derived; and, a DC component remover (50) for removing a DC component from the first output instruction signal (V I ) to derive a second output instruction signal (V I-DC ), whereby the inverting operation of the DC/AC inverter circuit (2) is controlled based upon the second output instruction signal (V I-DC ) so as to prevent DC magnetization of the transformer (3).

Description: Background of the Invention Field of the Invention The present invention generally relates to a DC-to-AC power converting apparatus. More particularly, the present invention is directed to a DC/AC power converting apparatus including either a DC power source, or a DC energy storage element; a DC/AC power inverting circuit; a control circuit for controlling either an output voltage, or an output current of the DC/AC inverter circuit; and also a DC component removing circuit. Description of the Related Art The DC/AC power converting apparatus set forth in the preamble is known in this field from, for instance, "A SEMICONDUCTOR POWER CONVERTING CIRCUIT" issued on March 31, 1987 from Ohm-sha Co., Ltd. in Japan, Figure 6.3.53 on page 150. In Fig. 1, there is shown a conventional DC/AC power converting apparatus. A DC power supply 1 is connected to supply DC power to a DC/AC inverter circuit 2. The DC/AC inverter circuit 2 is arranged by, for instance, a switching transistor, IGBT (insulated-gate bipolar transistor), gate turn-off thyristor (GTO) and the like. AC power produced from this DC/AC inverter circuit 2 is supplied via a transformer 3 to a load, or other power supply systems. An output current (otherwise an output voltage, similarly) of the DC/AC inverter circuit 2 is detected by a current detector 5 such as a current transformer, and supplied as a detected current signal "i D " to a calculation circuit 6. A reference current signal "i R " is also supplied to this calculation circuit 6. In the calculation circuit 6, the detected current signal "i D " is compared with the reference current signal "i R ", and a controlling calculation, e.g., a proportional integration is carried out in such a manner that a difference between the detected current signal "i D " and the reference current signal "i R " becomes a small value. As a result, an output instruction signal "V I is produced from this calculation circuit 6 and then supplied to a pulse converting circuit 8. The output instruction signal "V I " is converted into a gate signal "S g " in the pulse converting circuit 8. Then, the gate signal "S g " is supplied to the DC/AC inverter circuit 2 so that the AC output from the DC/AC inverter circuit 2 is followed to the above-described reference current value "i R ". More specifically, the inverting operation of the DC/AC inverter circuit 2 is controlled based upon this gate signal "S g " in the PWM (pulse width modulation) form. The above-described current detector 5, and calculation circuit 6 will constitute part of a current, or voltage feedback loop. If a DC component is produced due to a change in offset values in the feedback loop circuit, the AC output of the DC/AC inverter circuit 2 contains DC components. Since the output terminal of this DC/AC inverter circuit 2 is connected to the transformer 3 made of an iron core, the DC components contained in the AC output therefrom may cause DC magnetization of the transformer 3. For instance, the current transfer 5 has the offset value. Moreover, even if the offset adjustment would be effected at the beginning, a drift may occur in the offset value, due to the aging offset. As a consequence, a power loss in the transformer 3 is increased, and furthermore noises are produced therein. In the specific conditions, the transformer 3 is saturated so that no power can be supplied to the load 4, or an excessive current output from the DC/AC inverter circuit 2 flows through the output circuit thereof. Accordingly, the performance of the DC/AC inverter circuit 2 may be considerably deteriorated and the reliabilities of the overall DC/AC power converting apparatus may be lowered. Summary of the Invention The present invention has been made in an attempt to solve the above-described problems, and therefore, has an object to provide a highly safe and reliable DC/AC power converting apparatus capable of removing the DC components from the AC output of the DC/AC inverter circuit. To achieve the above-described object, a DC/AC power converting apparatus according to the present invention comprises: means (1) for supplying DC (direct current) power; DC-to-AC inverting means (2) for inverting the DC power into AC (alternating current) power; transformer means (3) for transforming the AC power so as to obtain desirable AC power; means (5) for detecting an output signal derived from the DC/AC inverting means (2) to produce an output detection signal (i D ); calculating means (6) for performing a control calculation for the DC/AC inverting means (2) so as to enable the output detection signal (i D ) to be followed to a reference signal (i R ), thereby deriving a first output instruction signal (V I ); characterized in further comprising: DC component removing means (50) for removing a DC component from the first output instruction signal (V I ) to derive a second output instruction signal (V I-DC ), whereby the inverting operation of the DC/AC inverting means (2) is controlled based upon the second output instruction signal (V I-DC ) so as to prevent DC magnetization of the transformer means (3). Brief Description of the Drawings For a better understanding of the present invention, reference is made to the following descriptions in conjunction with the accompanying drawings; in which: Fig. 1 is a schematic block diagram of a conventional DC/AC power converting apparatus; Fig. 2 is a schematic block diagram of a DC/AC power converting apparatus according to a preferred embodiment of the present invention; Fig. 3 is a circuit diagram of an internal circuit of the DC component remover 50 shown in Fig. 2; and, Fig. 4 represents waveforms of the output instruction signals. Detailed Description of the Preferred Embodiments OVERALL ARRANGEMENT OF DC/AC POWER CONVERTING APPARATUS In Fig. 2, there is shown an overall circuit arrangement of a DC/AC power converting apparatus 100 according to a preferred embodiment of the invention. It should be noted that the same reference numerals shown in Fig. 1 will be employed for indicating those in the following figures. As shown in Fig. 2, the DC/AC power converting apparatus 100 according to a preferred embodiment includes the DC source 1 such as a battery and a smoothing capacitor of an active filter. The DC source 1 is connected to the DC/AC inverter circuit 2 constructed of, for instance, a GTO (gate turn-off thyristor) and an IGBT (insulated-gate bipolar transistor). The output of the DC/AC inverter circuit 2 is connected to the primary winding (not shown in detail) of the transformer 3 made of an iron core. The secondary winding (not shown in detail) of the transformer 3 is connected to the load 4 (e.g., 10 KVA to 1 MVA). The current detector 5 such as a current transformer capable of detecting not only an AC current but also a DC current is electromagnetically coupled to the power line between the DC/AC inverter circuit 2 and the primary winding of the transformer 3, so as to detect a current detection signal "i D ". This current detection signal "i D " is supplied to an adder 6A of the calculation circuit 6. From the reference source 10, the reference current signal "i R " is derived and supplied to this adder 6A. A difference between the current detection signal "i D " and reference current signal "i R " is produced from the adder 6A and then supplied to an integrator 6B. The integrator 6B in the calculation circuit 6 outputs the first output instruction signal "V I ". The first output instruction signal "V I is supplied to a DC component remover 50, whereby the DC component contained in the first output instruction signal "V I " can be removed (will be discussed later). A second, or desirable output instruction signal "V I-DC " from which the DC component has been removed is supplied to the pulse converting circuit 7 so as to finally obtain the gate signal for controlling the control gates of the switching elements employed in the DC/AC inverter circuit 2. Thus, these switching elements are controlled in the PWM (pulse width modulation) mode. DC COMPONENT REMOVER 50 As a major feature of the preferred embodiment, the DC component remover 50 is newly employed in the current feedback path of the DC/AC power converting apparatus 100. Fig. 3 shows an internal circuit of the DC component remover 50. The DC component remover 50 is mainly constructed of a low-pass filter 50A, an inverter 50B, and an adder 50C. A cut-off frequency of this low-pass filter 50A is selected to be considerably lower than the frequency (e.g., 50 to 60 Hz) of the AC output from the DC/AC inverter circuit 2, for instance, lower than 2 Hz. As a result, if the first output instruction signal "V I " contains the DC component, only this DC component can sufficiently pass through this low-pass filter 50A. Then, the resultant DC component signal is firstly inverted by the inverter 50B and thereafter supplied to one input of the adder 50C. To the other input of this adder 50C, the above-described first output instruction signal "V I " is supplied. Accordingly, the adder 50C can output the second (desirable) output instruction signal "V I-DC " from which the DC component has been completely removed. In other words, the first output instruction signal "V I " containing the DC component is subtracted from the filtered DC component in the adder 50C, so that the desired second output instruction signal "V I-DC " can be finally obtained therefrom. For a better understanding of this DC component removing operation, a description will now be made with reference to waveforms shown in Fig. 4. Fig. 4A represents both the first output instruction signal "V I " and the DC component contained therein. This DC component is indicated by a dot line and is gradually increased in accordance with the lapse of time. Fig. 4B represents the desired second output instruction signal "V I-DC " from which the DC component has been completely removed. With the above-described circuit arrangement, in accordance with the DC/AC power converting apparatus 100, the AC output derived from the the DC/AC inverter circuit 2 contains no longer DC component. As a consequence, since the transformer 3 made of the iron core is no longer DC-magnetized by this DC component, the reliabilities and safety performance of the DC/AC power converting apparatus 100 can be improved. MODIFICATIONS In the DC component remover 50 shown in Fig. 3, the low-pass filter was constructed of an active element. Alternatively, a passive element such as a capacitor and a resistor may be employed. Furthermore, the first output instruction signal output from the calculation circuit 6 may be integrated so as to derive the DC component therefrom. Also, in the DC/AC power converting apparatus 100, the current transformer was employed to detect the current flowing through the output line of the DC/AC inverting circuit 2. Alternatively, a potential transformer may be utilized. While has been described above, in the DC/AC power converting apparatus according to the invention, since the DC component appearing in the voltage feedback loop circuit, e.g., the calculation circuit can be completely removed by the DC component remover, the AC output from the DC/AC inverter circuit contains no DC component. As a consequence, the transformer is not DC-magnetized by the DC component contained in the AC output, resulting in the higher reliabilities of the DC/AC power converting apparatus. Also, in accordance with the present invention, the DC component contained in the control signal (i.e., first output instruction signal) for the DC/AC inverting operation can be completely removed, so that the correct DC component removing operation can be accomplished, as compared with such a case that the DC component is removed from the feedback signal, i.e., the current detection signal. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A power converting apparatus (100) comprising: means (1) for supplying DC (direct current) power; DC-to-AC inverting means (2) for inverting the DC power into AC (alternating current) power; transformer means (3) for transforming the AC power so as to obtain desirable AC power; means (5) for detecting an output signal derived from the DC/AC inverting means (2) to produce an output detection signal (i D ); calculating means (6) for performing a control calculation for the DC/AC inverting means (2) so as to enable the output detection signal (i D ) to be followed to a reference signal (i R ), thereby deriving a first output instruction signal (V I ); characterized in further comprising: DC component removing means (50) for removing a DC component from the first output instruction signal (V I ) to derive a second output instruction signal (V I-DC ), whereby the inverting operation of the DC/AC inverting means (2) is controlled based upon the second output instruction signal (V I-DC ) so as to prevent DC magnetization of the transformer means (3). A power converting apparatus (100) as claimed in Claim 1, further comprising: means (7) for converting the second output instruction signal (V I-DC ) output from the DC component removing means (50) into a pulsatory gate signal (S g ) so as to control the inverting operation of the DC/AC inverting means (2). A power converting apparatus (100) as claimed in Claim 1, wherein said DC component removing means (50) includes: a low-pass filter (50A) for filtering said first output instruction signal (V I ) to obtain the DC component contained therein; an inverter (50B) for inverting the DC component derived from the low-pass filter (50A) to output an inverted DC component; and, an adder (50C) for adding the inverted DC component to said first output instruction signal (V I ) so as to obtain the second output instruction signal (V I-DC ). A power converting apparatus (100) as claimed in Claim 3, wherein a cutoff frequency of said low-pass filter (50A) is selected to be considerably lower than a frequency of said AC power derived from the DC/AC inverting apparatus (2). A power converting apparatus (100) as claimed in Claim 4, wherein said cutoff frequency of the low-pass filter (50A) is approximately 2 Hz, whereas said frequency of the AC power is from 50 to 60 Hz. A power converting apparatus (100) as claimed in Claim 1, wherein said DC component removing means (50) includes an integrator circuit and a subtracting circuit. A power converting apparatus (100) as claimed in Claim 1, wherein said detecting means (5) is a current transformer. A power converting apparatus (100) as claimed in Claim 1, wherein said detecting means (5) is a potential transformer.

Inventors: YASUDA, NOBUYUKI
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H02M7
IPC codes: H02M1, H02M7