Power conversion device

Details Power conversion device Power conversion device

2002-09-12

2004-05-25

Berhane, Adolf (Department: 2838)

Electric power conversion systems

Current conversion

Including an a.c.-d.c.-a.c. converter

C323S207000

Reexamination Certificate

active

06741482

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application claims benefit of priority to Japanese Application No. JP 2001-279981 filed Sep. 14, 2001 and JP 2001-329405 filed Oct. 26, 2001, the entire content of which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an economic, but high-efficiency power conversion device consisting of a combination of power diode rectifiers and voltage-type self-commutated power converters and/or a combination of power diode rectifiers and multi-level output self-commutated power converters.
2. Description of the Related Art
For electric railway power generation systems, a system is often adopted in which three-phase AC power is converted to DC power by power diode rectifiers in a three-phase bridge connection. This system has the advantages of excellent ability to withstand overloading and that the converter cost can be kept low. However, there was a problem that when regenerative braking was applied to the vehicle, the power involved in this could not be regenerated on the AC power source side, resulting in repeated absence of regeneration. Another drawback was load current dependence, resulting in considerable fluctuations of the DC generated voltage depending on the load.
FIG.
1
A and
FIG. 1B
illustrate the circuit layout of a prior art PWM converter (pulse width modulation control converter) capable of power regeneration. In PWM converter CNV, the AC terminals are connected with terminals R, S, T of a three-phase AC power source SUP through an AC reactor Ls and the DC terminals are connected with the DC terminals of a DC smoothing capacitor Cd and three-phase output VVVF (variable voltage variable frequency) converter INV. The AC terminal of converter INV is connected with an AC motor M. The PWM converter CNV comprises six arms i.e. six rectifying high-speed diodes D
1
to D
6
connected in the form of a three-phase bridge and self-turn-off elements S
1
to S
6
consisting of a switching element for a regenerating inverter connected in anti-parallel with these diodes. Diodes D
1
to D
3
and self-turn-off elements S
1
to S
3
are arranged on the positive side and diodes D
4
to D
6
and self-turn-off elements S
4
to S
6
are arranged on the negative side. Inverter INV also has the same as circuit layout as converter CNV, so detailed description thereof is here omitted.
PWM converter CNV is equipped with a control device comprising comparators C
1
, C
3
, voltage control compensator Gv(S), multiplier ML, current control compensators Gi(S) and pulse width modulation control circuit PWMC. Comparator C
1
and voltage control compensator Gv(S) are common to each phase but multiplier ML, comparator C
3
, current control compensators Gi(S) and pulse width modulation control circuit PWMC are provided for each phase. Only the internal circuit layout of the R phase is described in detail herein, but the layout of the S phase and T phase control circuits is the same. Gate signals g
1
, g
4
for the self-turn-off elements S
1
, S
4
of the R phase are output from the R phase control circuit; gate signals g
2
, g
5
for the self-turn-off elements S
2
, S
5
of the S phase are output from the S phase control circuit; and gate signals g
3
, g
6
for the self-turn-off elements S
3
, S
6
of the T phase are output from the T phase control circuit.
PWM converter CNV uses the control circuit constructed as above to control the input currents Ir, Is, It such that the DC voltage Vd applied to the DC smoothing capacitor Cd matches the voltage instruction value Vd*. In more detail, the deviation between the voltage instruction value Vd* and the voltage detection value Vd is obtained by comparator C
1
and amplified by voltage control compensator Gv(S) and is taken as the amplitude instruction value Ism of the input current. Multiplier ML multiplies the amplitude instruction value Ism of the input current with a unit sine wave sin &ohgr;t synchronized with the voltage of the R phase and this product is taken as the current instruction value Ir* of the R phase. Comparator C
3
compares this R phase current instruction value Ir* with the R phase current detection value Ir and the deviation is subjected to inverse amplification by current control compensator Gi(S). Normally proportional amplification is employed, the gain being Gi(S)=−Ki, where Ki is the constant of proportionality (proportional constant).
The voltage instruction value er*=−Ki X(Ir*−Ir), which is the output of current control compensator Gi(S) is input to PWM control circuit PWMC to create the gate signals g
1
, g
4
of self-turn-off elements S
1
and S
4
of the R phase of converter CNV. PWM converter circuit PWMC compares the voltage instruction value er* and the carrier signal X (for example a 1 kHz triangular wave) and, when er*>X, turns element S
1
ON (S
4
is OFF) and when er*<X turns element S
4
ON (S
1
is OFF). As a result, as the R phase voltage VR of the converter there is generated a voltage proportional to the voltage instruction value er*.
Regarding the input current Ir of the R phase, when Ir*>Ir, the voltage instruction value er* has a negative value and Ir is increased. Contrariwise, when Ir*<Ir, the voltage instruction value er* has a positive value and Ir is decreased. In this way, control is performed such that Ir*=Ir. Control is performed in the same way in respect of currents Is and It of the S phase and T phase.
The voltage Vd applied to DC smoothing capacitor Cd is controlled as follows. Specifically, if Vd*>Vd, the amplitude instruction value Ism of the input current is increased. The current instruction value of each phase is in phase with the power source voltage so active power Ps proportional to the current Ism is supplied from the AC power source SUP to the DC smoothing capacitor Cd. As a result, the voltage Vd rises and is controlled such that Vd*=Vd. Contrariwise, if Vd*<Vd, the amplitude instruction value Ism of the input current has a negative value and power Ps is regenerated on the AC power source side. Consequently, the accumulated energy of DC smoothing capacitor Cd is reduced, lowering the voltage Vd and thereby achieving control such that Vd*=Vd.
The VVVF (variable voltage variable frequency) inverter INV and AC motor M are loads whose voltage source is the DC smoothing capacitor Cd; thus during power running operation (motoring operation) they act in a direction such as to consume the accumulated energy of capacitor Cd and to lower voltage Vd. Also, during regeneration operation, this regenerated energy is returned to smoothing capacitor Cd, so they act in a direction such as to raise voltage Vd. Since, as described above, control is performed by the PWM converter CNV such that the DC voltage Vd is constant, matching active power is automatically supplied from the AC power source during power running operation and, during regenerative operation, active power matching the regenerated energy is regenerated on the AC power source side.
Thus, with the conventional PWM converter, the DC voltage Vd can be stabilized and power regeneration achieved, enabling the problem of absence of regeneration in an electric railway DC power-generation system to be solved.
However, a PWM converter has the drawback of considerable switching loss of the switching elements, due to switching being performed at high frequency. Also, the switching elements need to have the ability to interrupt the maximum value of the AC input current, constituting the interruption current. There was therefore the problem that they had to be designed so as to be capable of withstanding interruption current even in the case of overloading for a short time (for example 300% of the rated current); the power converter therefore had to be of large size, resulting in an uneconomic system.
Thus, as described above, although self-commutated converters (called PWM converters) using pulse width modulation control were available as power converters capable of powe

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