# US 10,374,522 B2 · Three-level circuit and control method for balancing neutral point voltage of the same

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- Number: US 10,374,522 B2
- Document ID: US10374522B2
- Status: Granted
- Kind: B2
- Application number: 16/183,715
- Filed: 2018-11-07
- Published: 2019-06-13
- Granted: 2019-08-06
- Earliest claimed: 2017-12-08
- Assignees: Delta Electronics (Shanghai) Co., Ltd.
- Applicants: Delta Electronics (Shanghai) Co., Ltd.
- Inventors: Hong Liu; Hao Peng; Cheng Lu
- Web page: https://patentgrep.com/patent/US10374522B2

## CPC

- H02M 7/487: Neutral point clamped inverters
- H02M 7/4833: {Capacitor voltage balancing}
- H02M 7/53873: {with digital control}
- H02M 7/5395: by pulse-width modulation
- H02M: APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF (transformers H01F; dynamo-electric converters H02K47/00; controlling transformers, reactors or choke coils, control or regulation of electric motors, generators or dynamo-electric converters H02P)

## Abstract

The present invention relates to a control method for balancing a neutral point voltage of a three-level circuit, comprising obtaining the neutral point voltage through sampling, adjusting a duty ratio of a zero level in one phase to maintain balance of the neutral point voltage when an absolute value of the neutral point voltage is larger than a threshold value, and distributing duty ratios of positive and negative levels in the phase, such that an output voltage of switch bridge arm in the phase includes positive, negative and zero levels in at least one switch period. The present invention can be carried out in a three-phase four-wire system and a three-phase three-wire system. In the three-phase three-wire system, it may also be combined with zero sequence voltage injection to form a mixed modulation method, thereby acquiring more effective neutral point balance control performance, and reducing switch loss.

## Claims

### Claim 1

A control method for balancing neutral point voltage of a neutral point clamped three-level circuit, wherein the three-level circuit is for converting a DC voltage into a three-phase AC voltage, wherein the three-level circuit comprises a capacitance bridge arm including upper and lower capacitors connected in series and three switch bridge arms electrically connected to the capacitance bridge arm and being a, b and c phase switch bridge arms, respectively, each of the a, b and c phase switch bridge arms includes at least four switch devices to output a positive level, a zero level, or a negative level, wherein the method comprises:
step 11, obtaining a first neutral point voltage to represent a voltage difference between the upper and lower capacitors;
step 12, acquiring a first value range of a duty ratio of a zero level of x phase switch bridge arm through a modulating reference voltage of the x phase switch bridge arm and the DC voltage, wherein x represents a, b and c phase switch bridge arms included in the three-level circuit, and presetting a first set value of the duty ratio of the zero level of the x phase switch bridge arm within the first value range;
step 13, when an absolute value of the first neutral point voltage is larger than a first threshold value, adjusting the first set value to be within the first value range, distributing a second set value for a duty ratio of the positive level of the x phase switch bridge arm and a third set value for a duty ratio of the negative level of the x phase switch bridge arm on the basis of the adjusted first set value, and modulating the switch devices of the x phase switch bridge arm according to the adjusted first, second and third set values, such that the absolute value of the first neutral point voltage is smaller than the first threshold value.

### Claim 2 (depends on 1)

The method according to claim 1, wherein the step 12 further includes:
step 121, obtaining the maximum value dxomax of the first value range of the duty ratio of the zero level of the x phase switch bridge arm through
d
xomax
=
1
-

u
xref

0.5
⁢
u
d
,
wherein uxref is the modulating reference voltage of the x phase switch bridge arm, and ud is the DC voltage; presetting the first set value of the duty ratio of the zero level of the x phase switch bridge arm within the first value range (0,dxomax].

### Claim 3 (depends on 2)

The method according to claim 2, wherein the step 12 further includes:
step 122, presetting the first set value as the maximum value dxomax of the first value range of the x phase switch bridge arm.

### Claim 4 (depends on 1)

The method according to claim 1, wherein the three-level circuit outputs the three-phase AC voltage to a load, and the step 13 further includes:
step 131, on the basis of the first neutral point voltage, when it is determined that a voltage of the upper capacitor is larger than a voltage of the lower capacitor: decreasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when a current of the x phase switch bridge arm flows from the three-level circuit to the load, and increasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when the current of the x phase switch bridge arm flows from the load to the three-level circuit; or
on the basis of the first neutral point voltage, when it is determined that the voltage of the upper capacitor is smaller than the voltage of the lower capacitor: increasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when the current of the x phase switch bridge arm flows from the three-level circuit to the load, and decreasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when the current of the x phase switch bridge arm flows from the load to the three-level circuit.

### Claim 5 (depends on 1)

The method according to claim 1, wherein the step 13 further includes:
step 132, modulating the first set value dxo a through the following formula:
d
xo
=
{
1
-

u
xref

0.5
⁢
u
d
-
k
p
⁢
u
o
⁢
sign
⁡
(
i
x
)
,
(
u
o
⁢
⁢
and
⁢
⁢
i
x
⁢
⁢
have
⁢
⁢
the
⁢
⁢
same
sign
,
and
⁢
⁢

u
o

>
u
th
)
,
1
-

u
xref

0.5
⁢
u
d
,
(
others
)
wherein kp is a proportioncontrol parameter, uo is the first neutral point voltage, ix is an output current of the x phase switch bridge arm, uth is the first threshold value, uxref is the modulating reference voltage of the x phase switch bridge arm, and ud is the DC voltage.

### Claim 6 (depends on 1)

The method according to claim 1, wherein the step 13 further includes:
step 133, distributing the second set value dxp for the duty ratio of the positive level of the x phase switch bridge arm and the third set value dxn for the duty ratio of the negative level of the x phase switch bridge arm as:
d
xp
=
(
0.5
+
u
xref
u
d
)
-
0.5
⁢
d
xo
d
xn
=
(
0.5
-
u
xref
u
d
)
-
0.5
⁢
d
xo
,
wherein uxref is the modulating reference voltage of the x phase switch bridge arm, dxo is the first set value, and ud is the DC voltage.

### Claim 7 (depends on 1)

The method according to claim 1, wherein the first, second and third set values of the x phase switch bridge arm do not equal to zero in at least one switch period, such that an output voltage of the x phase switch bridge arm includes the positive level, the negative level and the zero level in the at least one switch period.

### Claim 8 (depends on 1)

The method according to claim 1, wherein output voltages of at least two of the a, b and c phase switch bridge arms of the three-level circuit include the positive level, the negative level and the zero level in at least one switch period.

### Claim 9 (depends on 1)

The method according to claim 1, wherein when the three-level circuit is applied to a three-phase three wire electrical system, the method further comprises:
step 14, when the absolute value of the first neutral point voltage is smaller than a second threshold value, injecting a zero sequence voltage, wherein the second threshold value is smaller than or equals to the first threshold value.

### Claim 10 (depends on 1)

A three-level circuit for converting a DC voltage into a three-phase AC voltage, wherein the three-level circuit comprises a capacitance bridge arm including upper and lower capacitors connected in series and three switch bridge arms electrically connected the capacitance bridge arm and being a, b and c phase switch bridge arms, respectively, wherein each of the a, b and c phase switch bridge arms includes at least four switch devices to output a positive level, a zero level, or a negative level, wherein the three-level circuit adopts the method according to claim 1.

### Claim 11 (depends on 10)

The three-level circuit according to claim 10, wherein the capacitance bridge arm includes a first DC input end, a neutral point level end and a second DC input end, wherein a first end of the upper capacitor is electrically connected to the first DC input end, a second end of the upper capacitor is electrically connected to the neutral point level end, a first end of the lower capacitor is electrically connected to the neutral point level end, and a second end of the lower capacitor is electrically connected to the second DC input end;
any one of the a, b and c phase switch bridge arms includes a first switch device, a second switch device, a third switch device, a fourth switch device, a first diode and a second diode; a first end of the first switch device is electrically connected to the first end of the upper capacitor, a second end of the first switch device is electrically connected to a first end of the second switch device, a second end of the second switch device is electrically connected to a first end of the third switch device, a second end of the third switch device is electrically connected to a first end of the fourth switch device, a second end of the fourth switch device is electrically connected to the second end of the lower capacitor; an anode of the first diode and a cathode of the second diode are electrically connected to the neutral point level end, a cathode of the first diode is electrically connected to the second end of the first switch device, and an anode of the second diode is electrically connected to the second end of the third switch device.

### Claim 12 (depends on 10)

The three-level circuit according to claim 10, wherein the capacitance bridge arm includes a first DC input end, a neutral point level end and a second DC input end, wherein a first end of the upper capacitor is electrically connected to the first DC input end, a second end of the upper capacitor is electrically connected to the neutral point level end, a first end of the lower capacitor is electrically connected to the neutral point level end, and a second end of the lower capacitor is electrically connected to the second DC input end;
any one of the a, b and c phase switch bridge arms includes a first switch device, a second switch device, a third switch device and a fourth switch device; a first end of the first switch device is electrically connected to the first end of the upper capacitor, a second end of the first switch device is electrically connected to a first end of the fourth switch device and a second end of the third switch device, a first end of the second switch device is electrically connected to a first end of the third switch device, a second end of the second switch device is electrically connected to the neutral point level end, and a second end of the fourth switch device is electrically connected to the second end of the lower capacitor.

### Claim 13 (depends on 10)

The three-level circuit according to claim 10, wherein the capacitance bridge arm includes a first DC input end, a neutral point level end and a second DC input end, wherein a first end of the upper capacitor is electrically connected to the first DC input end, a second end of the upper capacitor is electrically connected to the neutral point level end, a first end of the lower capacitor is electrically connected to the neutral point level end, and a second end of the lower capacitor is electrically connected to the second DC input end;
any one of the a, b and c phase switch bridge arms includes a first switch device, a second switch device, a third switch device, a fourth switch device, a fifth switch device and a sixth switch device; a first end of the first switch device is electrically connected to the first end of the upper capacitor, a second end of the first switch device is electrically connected to a first end of the second switch device and a first end of the fifth switch device, a second end of the second switch device is electrically connected to a first end of the third switch device, a second end of the third switch device is electrically connected to a first end of the fourth switch device and a second end of the sixth switch device, a second end of the fourth switch device is electrically connected to the second end of the lower capacitor, and a second end of the fifth switch device is electrically connected to a first end of the sixth switch device and the neutral point level end.

### Claim 14 (depends on 10)

The three-level circuit according to claim 10, wherein the three-level circuit is applied to a three-phase four wire electrical system.

### Claim 15 (depends on 10)

The three-level circuit according to claim 10, wherein the three-level circuit further comprises a fourth switch bridge arm electrically connected to the capacitance bridge arm and having the same topology as the a, b and c phase switch bridge arms.

### Claim 16

A control method for balancing neutral point voltage of a neutral point clamped three-level circuit, wherein the three-level circuit is for converting a DC voltage into a three-phase AC voltage and comprises a capacitance bridge arm including upper and lower capacitors connected in series, and three switch bridge arms electrically connected the capacitance bridge arm and being a, b and c phase switch bridge arms, respectively, each of which includes at least four switch devices to output a positive level, a zero level, or a negative level, comprising:
step 21, obtaining a first neutral point voltage to represent a voltage difference between the upper and lower capacitors;
step 22, adjusting a first set value of a duty ratio of the zero level of the x phase switch bridge arm, such that in a same switch period, when modulating reference voltages of the a, b and c phase switch bridge arms are different, duty ratios of the zero levels of the a, b and c phase switch bridge arms are different, wherein x represents the a, b and c phase switch bridge arms included in the three-level circuit;
step 23, distributing a second set value for a duty ratio of the positive level of the x phase switch bridge arm and a third set value for a duty ratio of the negative level of the x phase switch bridge arm on the basis of the first set value, and modulating the switch devices of the x phase switch bridge arm according to the first, second and third set values, such that an output voltage of the x phase switch bridge arm includes positive, negative and zero levels in at least one switch period, and an absolute value of the first neutral point voltage is smaller than a first threshold value.

### Claim 17 (depends on 16)

The method according to claim 16, wherein the step 22 further includes:
step 221, obtaining the maximum value dxomax within an adjustable range of the duty ratio of the zero level of the x phase switch bridge arm through
d
xomax
=
1
-

u
xref

0.5
⁢
u
d
,
wherein uxref is the modulating reference voltage of the x phase switch bridge arm, and ud is the DC voltage; presetting the first set value of the duty ratio of the zero level of the x phase switch bridge arm within a value range (0,dxomax]; and the adjusted first set value remains within the value range.

### Claim 18 (depends on 17)

The method according to claim 17, wherein presetting the first set value of the duty ratio of the zero level of the x phase switch bridge arm within the value range in step 221 includes:
step 222, presetting the first set value of the x phase switch bridge arm as the maximum value dxomax of the value range.

### Claim 19 (depends on 18)

A three-level circuit for converting a DC voltage into a three-phase AC voltage, wherein the three-level circuit comprises a capacitance bridge arm including upper and lower capacitors connected in series, and three switch bridge arms electrically connected the capacitance bridge arm and being a, b and c phase switch bridge arms, respectively, each of which includes at least four switch devices to output a positive level, a zero level, or a negative level, wherein the three-level circuit uses the method according to claim 18.

### Claim 20 (depends on 19)

The three-level circuit according to claim 19, wherein the capacitance bridge arm includes a first DC input end, a neutral point level end and a second DC input end, wherein a first end of the upper capacitor is electrically connected to the first DC input end, a second end of the upper capacitor is electrically connected to the neutral point level end, a first end of the lower capacitor is electrically connected to the neutral point level end, and a second end of the lower capacitor is electrically connected to the second DC input end;
any one of the a, b and c phase switch bridge arms includes a first switch device, a second switch device, a third switch device, a fourth switch device, a first diode and a second diode; a first end of the first switch device is electrically connected to the first end of the upper capacitor, a second end of the first switch device is electrically connected to a first end of the second switch device, a second end of the second switch device is electrically connected to a first end of the third switch device, a second end of the third switch device is electrically connected to a first end of the fourth switch device, a second end of the fourth switch device is electrically connected to the second end of the lower capacitor; an anode of the first diode and a cathode of the second diode are electrically connected to the neutral point level end, a cathode of the first diode is electrically connected to the second end of the first switch device, and an anode of the second diode is electrically connected to the second end of the third switch device.

### Claim 21 (depends on 19)

The three-level circuit according to claim 19, wherein the capacitance bridge arm includes a first DC input end, a neutral point level end and a second DC input end, wherein a first end of the upper capacitor is electrically connected to the first DC input end, a second end of the upper capacitor is electrically connected to the neutral point level end, a first end of the lower capacitor is electrically connected to the neutral point level end, and a second end of the lower capacitor is electrically connected to the second DC input end;
any one of the a, b and c phase switch bridge arms includes a first switch device, a second switch device, a third switch device and a fourth switch device; a first end of the first switch device is electrically connected to the first end of the upper capacitor, a second end of the first switch device is electrically connected to a first end of the fourth switch device and a second end of the third switch device, a first end of the second switch device is electrically connected to a first end of the third switch device, a second end of the second switch device is electrically connected to the neutral point level end, and a second end of the fourth switch device is electrically connected to the second end of the lower capacitor.

### Claim 22 (depends on 19)

The three-level circuit according to claim 19, wherein the capacitance bridge arm includes a first DC input end, a neutral point level end and a second DC input end, wherein a first end of the upper capacitor is electrically connected to the first DC input end, a second end of the upper capacitor is electrically connected to the neutral point level end, a first end of the lower capacitor is electrically connected to the neutral point level end, and a second end of the lower capacitor is electrically connected to the second DC input end;
any one of the a, b and c phase switch bridge arms includes a first switch device, a second switch device, a third switch device, a fourth switch device, a fifth switch device and a sixth switch device; a first end of the first switch device is electrically connected to the first end of the upper capacitor, a second end of the first switch device is electrically connected to a first end of the second switch device and a first end of the fifth switch device, a second end of the second switch device is electrically connected to a first end of the third switch device, a second end of the third switch device is electrically connected to a first end of the fourth switch device and a second end of the sixth switch device, a second end of the fourth switch device is electrically connected to the second end of the lower capacitor, and a second end of the fifth switch device is electrically connected to a first end of the sixth switch device and the neutral point level end.

### Claim 23 (depends on 19)

The three-level circuit according to claim 19, wherein the three-level circuit is applied to a three-phase four wire electrical system.

### Claim 24 (depends on 19)

The three-level circuit according to claim 19, wherein the three-level circuit further comprises a fourth switch bridge arm electrically connected to the capacitance bridge arm and having the same topology as the a, b and c phase switch bridge arms.

### Claim 25 (depends on 16)

The method according to claim 16, wherein the three-level circuit outputs the three-phase AC voltage to a load, and the step 22 further includes:
step 223, on the basis of the first neutral point voltage, when it is determined that a voltage of the upper capacitor is larger than a voltage of the lower capacitor: decreasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when a current of the x phase switch bridge arm flows from the three-level circuit to the load, and increasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when the current of the x phase switch bridge arm flows from the load to the three-level circuit; or
on the basis of the first neutral point voltage, when it is determined that the voltage of the upper capacitor is smaller than the voltage of the lower capacitor: increasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when the current of the x phase switch bridge arm flows from the three-level circuit to the load, and decreasing the first set value of the duty ratio of the zero level of the x phase switch bridge arm when the current of the x phase switch bridge arm flows from the load to the three-level circuit.

### Claim 26 (depends on 16)

The method according to claim 16, wherein distributing the second set value for a duty ratio of the positive level of the x phase switch bridge arm and the third set value for a duty ratio of the negative level of the x phase switch bridge arm on the basis of the first set value in the step 23 includes:
step 231, distributing the second set value dxp for a duty ratio of the positive level of the x phase switch bridge arm and the third set value dxn for a duty ratio of the negative level of the x phase switch bridge arm:
d
xp
=
(
0.5
+
u
xref
u
d
)
-
0.5
⁢
d
xo
d
xn
=
(
0.5
-
u
xref
u
d
)
-
0.5
⁢
d
xo
,
wherein uxref is the modulating reference voltage of the x phase switch bridge arm, dxo is the first set value, and ud is the DC voltage.

### Claim 27 (depends on 16)

The method according to claim 16, wherein output voltages of at least two of the a, b and c phase switch bridge arms of the three-level circuit include the positive level, the negative level and the zero level in at least one switch period.

### Claim 28 (depends on 16)

The method according to claim 16, wherein when the three-level circuit is applied to a three-phase three wire electrical system, the method further comprises:
step 24, when the absolute value of the first neutral point voltage is smaller than a second threshold value, injecting a zero sequence voltage, wherein the second threshold value is smaller than the first threshold value.