JP7193248B2 - power converter - Google Patents

Description

TECHNICAL FIELD Embodiments of the present invention relate to power converters.

2. Description of the Related Art Attempts have been made to increase voltage in power converters that convert DC power into AC power or convert AC power into DC power. A neutral point clamp type power converter is one of the types of power converters that can handle high voltages and can output phase voltages of three levels.

In a neutral-point clamp type power converter, a capacitor divides the DC voltage equally between the high-potential side and the low-potential side, and the neutral point maintains the high-potential side DC voltage and the low-potential side DC voltage equally. Potential control is required.

Various neutral point potential control methods have been proposed, but since it is necessary to detect the current flowing through the converter for neutral point potential control, when there is no load or light load on the AC side, It is difficult to stably execute neutral point potential control. Therefore, the neutral point potential control may be stabilized by forcibly supplying an AC current to the AC side to flow the current through the converter. However, there is a concern that the alternating current supplied to the alternating current side may affect an alternating current circuit such as a power system.

JP-A-11-113263

Embodiments provide a power converter capable of stably controlling the neutral point potential.

The power conversion device according to the embodiment is connected between a neutral point clamp type converter having a full bridge configuration provided for each phase of an AC circuit and the AC circuit and the converter, and the conversion a transformer that transforms the AC voltage of the converter and supplies it to the AC circuit; and a control device that controls the operation of the converter. The transformer is delta connected on the side of the AC circuit. The controller controls the converter to actively provide a zero sequence current that circulates in the delta connection of the transformer to provide neutral point potential control .

In this embodiment, since a full-bridge converter is provided for each phase, a transformer capable of delta-connecting the primary side can be provided, and the zero-phase current can be circulated in the delta-connection. Therefore, the neutral potential can be controlled stably without affecting the AC circuit side.

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram which illustrates the power converter device which concerns on embodiment. 2 is a block diagram illustrating a part of the power converter of FIG. 1; FIG. 2 is a block diagram illustrating a part of the power converter of FIG. 1; FIG. 2 is a block diagram illustrating a part of the power converter of FIG. 1; FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Note that the drawings are schematic or conceptual, and the relationship between the thickness and width of each portion, the size ratio between portions, and the like are not necessarily the same as the actual ones. Also, even when the same parts are shown, the dimensions and ratios may be different depending on the drawing.
In addition, in the present specification and each figure, the same reference numerals are given to the same elements as those described above with respect to the previous figures, and detailed description thereof will be omitted as appropriate.

FIG. 1 is a block diagram illustrating a power conversion device according to an embodiment.
As shown in FIG. 1 , the power converter 10 of this embodiment includes a power converter 20 and a control device 50 . The power converter 10 is connected to an AC circuit via AC terminals 21u to 21w. For example, the AC circuit may include a three-phase or single-phase 50 Hz or 60 Hz AC power supply 1 and a transformer 2 . The AC circuit may include AC loads such as induction motors, AC transmission lines, and the like. The power converter 10 is connected to the DC circuit 3 via DC terminals 21p and 21n. DC circuit 3 includes, for example, a DC transmission line, a storage battery, a solar battery, and the like. The DC circuit 3 may include a DC power supply or the like generated by another power converter.

The power converter 10 converts an AC voltage into a DC voltage, or converts a DC voltage into an AC voltage. The power electronics device 10 may perform such bidirectional power conversion, or may perform unidirectional power conversion.

Power converter 20 includes AC terminals 21u to 21w and DC terminals 21p and 21n. Between the AC terminals 21u-21w and the DC terminals 21p, 21n, converters 23u-23w are connected corresponding to the phases of the AC circuit. Transformers 22u to 22w are connected between the AC terminals 21u to 21w and the converters 23u to 23w, respectively. The transformers 22u-22w are delta-connected on the side of the AC terminals 21u-21w. In this specification, regarding the transformers 22u to 22w, the windings connected to the AC circuit are called primary windings, and the windings connected to the converters 23u to 23w are called secondary windings. . The primary winding side is also simply referred to as the primary side, and the secondary winding side is simply referred to as the secondary side.

More specifically, a transformer 22u and a converter 23u are provided corresponding to the U phase of the AC circuit, a transformer 22v and a converter 23v are provided corresponding to the V phase, and a transformer 22v and a converter 23v are provided corresponding to the W phase. A transformer 22w and a converter 23w are provided.

The converter 23u includes AC terminals 24ua and 24ub, DC terminals 24up and 24un, and a neutral point terminal 24uo. AC terminals 24ua and 24ub are connected to the secondary winding of transformer 22u. DC terminals 24up and 24un are connected to DC terminals 21p and 21n of power converter 20 . A capacitor 25p is connected between the DC terminal 24up and the neutral point terminal 24uo. A capacitor 25n is connected between the neutral point terminal 24uo and the DC terminal 24un.

Converter 23v is configured in the same manner as converter 23u described above, and is connected to the secondary winding of transformer 22v by AC terminals 24va and 24vb, and to the DC voltage of power converter 20 by DC terminals 24vp and 24vn. It is connected to terminals 21p and 21n.

The converter 23w is also configured in the same manner as the converters 23u and 23v described above. are connected to the DC terminals 21p and 21n of the .

The DC sides of all converters 23u-23w are connected in parallel. That is, DC terminals 24up, 24vp, and 24wp are connected to DC terminal 21p of power converter 20 . DC terminals 24un, 24vn, and 24wn are connected to DC terminal 21n of power converter 20 . Neutral terminals 24uo, 24vo, and 24wo are connected to each other.

The voltage between the DC terminal 21p and the neutral point O is VPO, and the voltage between the neutral point O and the DC terminal 21n is VON. The potential of DC terminal 21p is set to a value higher than the potential of DC terminal 21n. The potential of the DC terminal 21p is positive when viewed from the potential of the neutral point O, and the potential of the DC terminal 21n is negative when viewed from the neutral point O.

Control device 50 inputs DC voltages VPO and VON, phase currents iu to iw, and voltages vu to vw corresponding to AC phases, and generates a drive signal vgu for driving switching elements constituting converters 23u to 23w. ˜vgw are generated and supplied to each converter 23u˜23w. As will be described in detail later, the control device 50 generates various command values based on the input DC voltages VPO and VON, the phase currents iu to iw, and the AC voltages vu to vw. Drive signals vgu to vgw are generated based on the command values obtained.

FIG. 2 is a block diagram illustrating a portion of the power converter of FIG. 1;
FIG. 2 shows a simplified circuit configuration of converter 23u. In this example, a diode-clamped neutral point clamp circuit is shown.

As shown in FIG. 2, converter 23u includes switching elements 31-38 and diodes 39-42. The switching element 31 is, for example, an IGBT (Insulated Gate Bipolar Transistor), and a diode is connected in anti-parallel.

The switching elements 31 and 32 are connected in series at a connection node n1. The switching elements 33 and 34 are connected in series at a connection node n2. The switching elements 35 and 36 are connected in series at a connection node n3. The switching elements 37 and 38 are connected in series at a connection node n4. Series-connected circuits of switching elements 31 and 32, switching elements 33 and 34, switching elements 35 and 36, and switching elements 37 and 38 are called arms.

The arm of switching elements 31 and 32 is connected in series with the arm of switching elements 33 and 34 . A connection node of the arm formed by the switching elements 31 and 32 and the arm formed by the switching elements 33 and 34 is connected to the AC terminal 24ua.

Series-connected diodes 39 and 40 are connected between the connection nodes n1 and n2. A connection node of the series connection circuit of the diodes 39 and 40 is connected to the neutral point terminal 24uo.

The arm of switching elements 35 and 36 is connected in series with the arm of switching elements 37 and 38 . A connection node of the arm formed by the switching elements 35 and 36 and the arm formed by the switching elements 37 and 38 is connected to the AC terminal 24ub.

Diodes 41 and 42 connected in series are connected between the connection nodes n3 and n4. A connection node of the series connection circuit of the diodes 41 and 42 is connected to the neutral point terminal 24uo.

A series circuit of the switching elements 31 to 34 and a series circuit of the switching elements 35 to 38 are connected in parallel between the DC terminals 24up and 24un. An arm formed by switching elements 31 and 32, an arm formed by switching elements 33 and 34, an arm formed by switching elements 35 and 36, and an arm formed by switching elements 37 and 38 form a full bridge circuit having AC terminals 24ua and 24ub and DC terminals 24up and 24un. is. This full bridge circuit is clamped to the potential of the neutral point terminal 24uo (neutral point potential) by diodes 39-42.

The respective potentials Vu(A) and Vu(B) of the AC terminals 24ua and 24ub with respect to the neutral point potential are potentials controlled for each converter for neutral point potential control, which will be described in detail later.

The neutral point clamp type converter generates a zero-phase voltage command value based on the deviation of the positive and negative DC voltages VPO and VON with respect to the neutral point voltage to control the neutral point potential. In the power converter 10 of this embodiment, transformers 22u to 22w are connected between the converters 23u to 23w and the AC circuit. The primary sides of the transformers 22u to 22w are delta-connected.

Even if there is no load or a light load on the AC circuit side, the power conversion device 10 generates a zero-phase voltage according to the command value. phase current flows. By applying a zero-phase current to the primary sides of the transformers 22u to 22w, a current flows through each of the converters 23u to 23w, so that the converters can stably continue their operation. In addition, since the zero-phase current flowing on the primary side of the transformers 22u to 22w circulates in the delta connection, it is possible to prevent the current from flowing out to the AC circuit side of the electric power system or the like.

In the power conversion device 10 of the present embodiment, a voltage command value for controlling the neutral point potential is generated for each AC phase separately from the generation of the zero-phase voltage command value.
FIG. 3 is a block diagram illustrating a portion of the power converter of FIG. 1;
FIG. 3 shows a configuration example for controlling the neutral point potential for the U-phase converter 23u. The control device 50 has the same configuration for the V-phase and W-phase converters 23v and 23w, and detailed description thereof will be omitted.

As shown in FIG. 3, the control device 50 includes an adder/subtractor 51, coefficient multipliers 52 and 59, a PI controller 53, multipliers 54 and 57, encoders 55 and 56, and adders 58 and 60. ,including. The control device 50 uses an adder/subtractor 51 , a coefficient multiplier 52 and a PI controller 53 to generate a control output (neutral point potential control output) for neutral point potential control. An adder/subtractor 51 and a coefficient multiplier 52 calculate the deviation of the DC voltages VPO and VON, and a PI controller 53 generates a neutral point potential control output.

Control device 50 sets the direction of neutral point potential control by multiplier 54 and encoders 55 and 56 . The encoders 55 and 56 output "+1" when the sign of the input signal is positive, and output "-1" when the sign of the input signal is negative. The output from the converter 23u or the current iu input to the converter 23u and the output of the encoder 55 are input to the multiplier 54 . The U-phase voltage command value vu* is input to the encoder 55 . Encoder 56 determines the direction of controlling the neutral point potential, taking into consideration the signs of line current iu of converter 23u and U-phase voltage command value vu*.

The multiplier 57 gives a sign to the neutral point potential control output v0 and adds it to the voltage command value vu*. Positive side voltage command value Vu(A)* is generated by being added to the output of multiplier 57 by adder 58 . Negative voltage command value Vu(B)* is generated by being inverted by coefficient multiplier 59 and then added to the output of the multiplier by adder 60 .

In the above description, the circuit configuration of the neutral point clamp system is the case of the diode clamp system. be able to. For example, some switching elements and diodes of the converter may be replaced with bidirectional switches, and the configuration of the converter may be a T-type neutral point clamp system or the like.

In the above example, the AC circuit has three phases, but the number of phases of the AC circuit is not limited to this, and may be four or more.

Effects of the power converter 10 of the present embodiment will be described.
The power converter 10 of the present embodiment includes a full bridge circuit corresponding to each phase of an AC circuit, and a transformer delta-connected between the AC circuit and the full bridge circuit. Even if the AC circuit is in a no-load or light-load state, the control device 50 generates a zero-phase voltage command value and causes a zero-phase current to flow through the primary side of the transformer, thereby continuing the current of the converter. can flow. Therefore, the power conversion device 10 can stably perform a neutral point potential control output even when there is no load or a light load.

Since the primary side of the transformer is delta-connected, the zero-phase current output by the converter circulates in the delta-connection and does not flow out to the AC circuit side. Therefore, the power conversion device 10 can stably perform neutral point potential control without affecting the AC circuit side.

The AC circuit is not limited to an AC power source such as a power system, and may be an AC load such as an electric motor. In the power converter 10 of the present embodiment, when the AC circuit is an AC load, the neutral point potential can be stably controlled even when power supply to the AC load needs to be cut off. Therefore, the operation can be continued without stopping the power converter 10 .

It is known that the response of neutral point potential control changes according to the DC current output from the converter or the DC current input to the converter. A response of the neutral point potential control is represented by the following formula (1).

Response of neutral point potential control ∝ Amount of change in DC neutral point current
= ± (v0 × converter current) / DC voltage between PN (1)
where v0 is the neutral point potential control output (FIG. 3) and the converter currents are iu to iw.

As shown in equation (1), the amount of change in the DC neutral point potential is proportional to the magnitude of the currents iu to iw of the converters 23u to 23w corresponding to the respective phases. Therefore, the control response of the neutral point potential can also be improved by setting the current to be applied to each phase to circulate the zero-phase current on the primary side of the transformer to an appropriate value. That is, in the power conversion device 10 of the present embodiment, by executing the neutral point potential control corresponding to each phase, the neutral point potential control is stably performed and the response of the neutral point potential control is made faster. be able to.

(Modification)
In the above-described embodiment, the neutral point potential control is stably performed by constantly flowing the zero-phase current to the primary side of the transformer, including when the AC circuit side is under no load or light load. On the other hand, constant circulating zero-phase current causes a constant loss in the transformer and the converter, and noise and the like are generated due to the operation of the converter. Therefore, in this modification, when a preset condition is satisfied, the power conversion device 10 causes the zero-phase current to flow through the primary side of the transformer, and when there is no need for stabilization by the zero-phase current, prevents the zero-phase current from flowing.

FIG. 4 is a block diagram illustrating a portion of the power converter of FIG. 1;
FIG. 4 shows a zero-phase current conducting circuit 70. As shown in FIG. Zero-phase current conducting circuit 70 is provided in control device 50, for example.

As shown in FIG. 4, the zero-phase current conducting circuit 70 includes comparators 71 and 75, an adder/subtractor 72, a coefficient multiplier 73, an arithmetic unit 74, AND circuits 76 and 80, an off-delay circuit 77, An inverter circuit 78 and a one-shot circuit 79 are included. When the AC current of the converter falls below a predetermined drop level and the deviation of the DC voltages VPO and VON with reference to the neutral point potential exceeds a predetermined unbalance level, the zero-phase current conducting circuit 70 A zero-phase current command value is supplied to the controller so as to energize the primary side of the transformer with zero-phase current.

A case will be described below in which it is detected that any one of the currents iu to iw of the converters 23u to 23w of each phase has fallen below a predetermined drop level, but the current detection conditions are limited to this. Any suitable condition may be used. For example, it may be detected that the average value of the effective values of the currents iu to iw has fallen below a predetermined drop level, or other appropriate conditions may be used.

When the deviation of the DC voltages VPO and VON exceeds a predetermined imbalance level, a zero-phase current command value is supplied so that the zero-phase current for a certain period of time is supplied. When the imbalance between the DC voltages VPO and VON is eliminated, the zero-phase current is cut off.

A comparator 71 receives a preset converter current drop level iT1. Comparator 71 compares current iu of transducer 23u with transducer current drop level iT1. Comparator 71 supplies an H level signal to AND circuit 80 when current iu falls below converter current drop level iT1.

The DC voltages VPO and VON are input to an adder/subtractor 72 and a coefficient multiplier 73, and the deviation (arithmetic mean of voltage from neutral point potential) is calculated. The output of coefficient multiplier 73 is input to calculator 74 that calculates the absolute value, and is compared with DC voltage unbalance level VUB by comparator 75 . When the absolute value of the deviation of the DC voltages VPO and VON is equal to or higher than the DC voltage unbalance level VUB, the AND circuit 76 can be supplied with a signal of H level.

The output of the off-delay circuit 77 connected to the output of the AND circuit 76 is inverted by the inverter circuit 78 and supplied to the other input of the AND circuit 76 . The off-delay circuit 77 has a preset off-delay time. Even if the output of the AND circuit 76 becomes L level (OFF), the OFF delay circuit 77 becomes L level after the set OFF delay time elapses. That is, the AND circuit 76 enables the AND circuit 76 to output H level when the deviation of the DC voltages VPO and VON instantaneously exceeds the unbalance level VUB.

One-shot circuit 79 is connected to the output of AND circuit 76 . The one-shot circuit 79 supplies an H-level one-shot signal having a preset pulse width to the AND circuit when an H-level signal is input.

The operation of the zero-phase current conducting circuit 70 will be described.
The zero-phase current energization circuit 70 becomes operable when the alternating current output by the converter or input to the converter drops below the converter current drop level iT1.

The pulse width set by the one-shot circuit 79 when the deviation of the DC voltages VPO and VON of the power converter 20 becomes equal to or greater than the DC voltage unbalance level VUB while the zero-phase current conducting circuit 70 is operable. A zero-sequence current for a period determined by is supplied to the primary side of the transformer.

In this modified example, it is detected that the current flowing through the converter has decreased, and the zero-phase current is forced to flow through the transformer. Potential control can be performed.

In addition, in this modification, in addition to the decrease in the current of the converter, when the imbalance of the deviation of the DC voltages VPO and VON is large, the zero-phase current flows, so the neutral point potential control should be actively performed. It can only be in the state If the imbalance between the DC voltages VPO and VON is large, the voltage applied to the switching elements in the converter will be excessive and the switching elements may be damaged. Therefore, by appropriately setting the DC voltage unbalance level VUV, in the modified example, accidents such as damage to the switching elements can be prevented without reducing the conversion efficiency due to frequent zero-phase current flow. .

As described above, in this modification, when the predetermined condition is not satisfied, the zero-phase current does not flow through the transformer, so the efficiency of the transformer does not decrease.

Since the zero-phase current is passed when neutral point potential control is required, there is no need to pass the zero-phase current when other factors or when the DC voltage is not unbalanced.

Thus, in this modification, the zero-phase current supply to the transformer is started or stopped according to the current status on the AC side and the imbalance status on the DC side. can operate under optimal operating conditions.

According to the embodiments described above, it is possible to realize a power converter capable of accurately calculating the sign of the zero-phase voltage and outputting the command value of the zero-phase voltage.

Although several embodiments of the invention have been described above, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, replacements, and modifications can be made without departing from the scope of the invention. These embodiments and modifications thereof are included within the scope and spirit of the invention, and are included within the scope of the invention described in the claims and equivalents thereof. Moreover, each of the above-described embodiments can be implemented in combination with each other.

1 AC power supply, 2 transformer, 3 DC circuit, 10 power converter, 20 power converter, 22u ~ 22w transformer, 23u ~ 23w converter, 25p, 25n capacitor, 31 ~ 38 switching element, 39 ~ 42 diode, 50 control device, 51 adder/subtractor, 52, 59 coefficient unit, 53 PI controller, 54, 57 multiplier, 55, 56 encoder, 58, 60 adder, 70 zero-phase current conducting circuit, 71, 75 comparator, 72 adder/subtracter, 73 coefficient unit, 74 computing unit, 76, 80 AND circuit, 77 off-delay circuit, 78 inverting circuit, 79 one-shot circuit

Claims (5)

a full-bridge neutral-point-clamped converter provided for each phase of the AC circuit;
a transformer connected between the AC circuit and the converter for transforming the AC voltage of the converter and supplying the AC voltage to the AC circuit;
a controller for controlling the operation of the converter;
with
the transformer is delta-connected on the side of the alternating current circuit;
The controller controls the converter to actively supply a zero-phase current that circulates in the delta connection of the transformer to perform neutral point potential control . The converter is
a neutral point;
a first DC terminal having a higher potential than the neutral point;
a second DC terminal having a potential lower than the neutral point;
including
The control device is
Based on the deviation between the first DC voltage between the first DC terminal and the neutral point and the second DC voltage between the neutral point and the second DC terminal, producing a neutral point potential control output for controlling the potential;
2. The method of claim 1, wherein a first DC voltage command value for said first DC voltage and a second DC voltage command value for said second DC voltage are generated based on the direction of AC line current flowing through said converter. power converter. The controller forcibly sets a zero-phase current command value when the AC line current drops below a predetermined threshold current, and a zero-phase voltage command value based on the zero-phase current command value. 3. The power converter according to claim 2, which generates 4. The power converter according to claim 3, wherein said zero-phase current command value is forcibly set when a deviation between said first DC voltage and said second DC voltage is equal to or greater than a predetermined threshold voltage. The power converter according to any one of claims 1 to 4, wherein a neutral point clamping system of said converter is either a diode clamping system or a T-type clamping system.

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