DESCRIPTION
Title of Invention:
UNINTERRUPTIBLE POWER SUPPLY AND CONTROL METHOD THEREOF
Technical Field
[OOO 11
The present invention relates to an UPS (Uninterruptible Power Supply) having a threelevel
converter.
Background Art
[0002]
A three-level converter can output three levels of DC-link voltages. A three-level
converter comprises capacitors at an upper arm and a lower arm of a DC portion respectively
(positive side capacitor and negative side capacitor). If a voltage between both ends of each
capacitor becomes unequal, there arise inconveniences such that, for example, a harmonic
voltage is included in an output voltage. Therefore, each of voltages between both ends is
controlled to be equal.
[0003]
However, due to the operating principle of three-level converter or factors such as
characteristic variations of switching devices, each of voltages between both ends becomes
unequal in some cases. This is referred to as neutral point voltage fluctuation. Neutral point
voltage fluctuation can be suppressed by controlling a neutral point current. A neutral point
current flows when the three-level converter transmits and receives electric power.
[0004]
Patent Literature 1 indicated below describes a method that can appropriately suppress
the neutral point voltage fluctuation in both cases of motoring and regeneration.
Citation List
Patent Literature
[OOOS]
Patent Literature 1 : JP Patent Publication (Kokai) No. 2006-1 91 743 A
Summary of Invention
Technical Problem
[0006]
In the technique described in the stated Patent Literature 1, if an input current and an
output current become smaller, the electric power transmitted and received by the three-level
converter also becomes smaller. As a result, the neutral point current also becomes smaller,
and it could be supposed that the effect of suppressing the neutral point voltage current is
reduced along with it.
[0007]
For example, some of UPSs employing a three-level converter convert alternating
current power provided from a grid into direct current by connecting a direct current portion of
a converter and an inverter configured as the three-level converter, and further convert the
direct current into alternating current. Such UPSs provide electric power from a storage
battery to a load if the electric power supply from the grid stops. If the load is light at this
time, the electric power transmitted and received by the three-level converter becomes small,
and a problem similar to the above-described one could be arisen.
[OOOS]
The present invention has been made to solve the problem stated above, and it is an
objective of the present invention to suppress a neutral point voltage fluctuation in a UPS
using a three-level converter even if an input current and an output current are small.
Solution to Problem
[0009]
An uninterruptible power supply according to the present invention, if a neutral point
unbalance occurs during a period in which electric power is not provided from a grid, suspends
an operation of a converter to disconnect from the grid, and resumes the converter operation
after initiating an electric power supply from a storage battery to an inverter.
Advantageous Effects of Invention
[OO 1 01
With an uninterruptible power supply according to the present invention, a neutral point
voltage fluctuation can be effectively suppressed even if an input current and an output current
are small.
Brief Description of Drawings
[OOl 11
[Figure 11 Figure 1 is a diagram showing a circuit configuration of an UPS 200 according to
an embodiment 1.
[Figure 21 Figure 2 is a circuit diagram of a converter 202.
[Figure 31 Figure 3 is a block diagram describing an operation in which a controller 210
controls the converter 202.
[Figure 41 Figure 4 is a block diagram describing a detail of a pulse output determination unit
5 16.
[Figure 51 Figure 5 is a block diagram describing a detail of a switch 524.
[Figure 61 Figure 6 is a circuit diagram showing another configuration example of the
converter 202.
[Figure 71 Figure 7 is a circuit diagram showing yet another configuration example of the
converter 202.
Description of Embodiments
[OO 1 21

Figure 1 is a diagram showing a circuit configuration of an UPS 200 according to an
embodiment 1 of the present invention. For the sake of convenience of description, a multiphase
alternating circuit is shown with a one-line wiring diagram. The UPS 200 comprises a
converter 202, an inverter 204, a controller 210, and a DC circuit 214. Other detailed
configurations will be described later. At the side of the converter 202, the UPS 200 is
connected with a grid 100 through a breaker 300 and an input transformer 102. At the side of
the inverter 204, the UPS 200 is connected with a load 206 through a breaker 302.
[00 1 31
The converter 202 and the inverter 204 are connected with each other through the DC
circuit 214. The DC circuit 214 includes three terminals of P, M, and N. A positive side
capacitor 220 (positive side capacitor) is connected between the terminals P and M. A
negative side capacitor 222 (negative side capacitor) is connected between the terminals M
and N. A storage battery 212 is connected between the terminals P and N through a buckboost
chopper 208 and a breaker 304. The buck-boost chopper 208 includes a function for
controlling a voltage between the terminals P and N to be constant.
[00 1 41
In a case where the grid 100 operates normally, the electric power consumed by the
load 206 is provided from the grid 100. The converter 202, by controlling a converter current
Icnv, controls an amount of current for charging the positive side capacitor 220 and the
negative side capacitor 222, controls the voltage between the terminals P and N to be constant,
and balances the voltage between the terminals P and M (positive side voltage) with the
voltage between the terminals M and N (negative side voltage).
[00 1 51
The inverter 204 supplies electric power to the load 206 connected with the alternating
current side utilizing an electric charge stored in the positive side capacitor 220 and the
negative side capacitor 222. At this time, the inverter 204 outputs an AC voltage
synchronized with the grid 100.
[OO 1 61
The controller 210 outputs pulse signals PC, Pi, and P3 driving semiconductor
switching devices included in the converter 202, the inverter 204, and the buck-boost chopper
208, respectively.
[00 171
If the electric power supply from the grid 100 stops due to electric power outage or the
like, the controller 210 stops the operation of the converter 202 (operation of switching
devices), and provides electric power to the load 206 using the electric power stored in the
storage battery 212. The controller 210, after initiating the electric power supply from the
storage battery 212, opens the breaker 300. The breaker 302 is used when disconnecting the
UPS 200 from the load 206 in maintenances of the UPS 200.
[00 1 81
Figure 2 is a circuit diagram of the converter 202. The main circuit of the converter
202 is configured as a three-level power converter. In Figure 2, the three-level power
converter is configured by applying reverse-blocking semiconductor switches as the switch S2
and S3. However, other configurations can be employed.
[OO 1 91
The converter 202 is configured using, for example, semiconductor switching devices
(GTO, IGBT, MOS, Sic, and the like). The semiconductor switching devices S - 1U to S - 4U,
S-IV to S-4V, and S-1 W to S-4W perform switching operations according to the pulse signal
PC from the controller 2 10.
[0020]
The pulse signal PC includes signals of "0" and "1". If the pulse signal PC is 0, the
switch is in an OFF state and no current flows. If the pulse signal PC is 1, the switch is in an
ON state and current flows. It is the same for the pulse signals Pi and P3. This switching
operation allows the converter 202 to output a pulse-like quasi AC voltage from a three-level
DC-link voltage. When converting AC power into DC power, the conversion is performed
by similar switching operations. This enables electric power conversion between AC and DC
mutually.
roo2 11
An AC filter circuit for attenuating harmonic currents and harmonic voltages
configured by a filter reactor 400 and a filter capacitor 402 is disposed at the AC output
terminal of the converter 202.
[0022]
The inverter 204 is configured as a three-level power converter including switching
devices in a fashion similar to the converter 202.
[0023]
Next, wirings and devices for controlling the converter 202 and the inverter 204 will be
described using Figure 1 and Figure 2.
[0024]
A voltage sensor 230 detects a three-phase voltage at the grid side of the breaker 300,
and outputs the detection result to the controller 210 as a voltage detection signal Vcnv (grid
voltage). A current sensor 240 detects a three-phase current at the converter 202 side of the
breaker 300, and outputs the detection result to the controller 210 as a current detection signal
Icnv (grid current). A voltage sensor 232 detects an output voltage of the inverter 204, and
outputs the detection result to the controller 21 0 as a detection signal Vinv.
[0025]
A voltage Vdc-P between both ends of the positive side capacitor 220 and a voltage
Vdc - N between both ends of the negative side capacitor 222 are detected by appropriate
voltage sensors and the detection result is inputted to the controller 2 10.
[0026]

The principle for suppressing a neutral point voltage fluctuation at electric power
outage will be described using Figure 2. As an example, it is assumed that all of switching
devices in the converter 202 are in an OFF state and the DC-link voltage Vdc-P > Vdc - N.
[0027]
If the switching devices S-1U and S-3V are turned ON, the charge stored in the
positive side capacitor 220 moves in the order of : the positive side capacitor 220 +- the
terminal P of the DC circuit 214 -+ the switching device S - 1U -+ the phase U of the filter
reactor 400 -, the phase U of the filter capacitor 402 --+ the phase V of the filter capacitor 402
-, the phase V of the filter reactor 400 -, the switching device S - 3V -, the terminal M of the
DC circuit 214 +- the positive side capacitor 220. In this process, resistive components of
circuits and ON resistance of switching devices cause losses, and the charge of the positive
side capacitor 220 is consumed to decrease the DC-link voltage Vdc-P. As a result, the
difference between the DC-link voltages Vdc-P and Vdc-N can be reduced.
[0028]
When the DC-link voltage Vdc-P < Vdc-N, if the switching devices S2W and S-4V
are turned ON, the charge stored in the negative side capacitor 222 moves in the order of: the
negative side capacitor 222 + the terminal M of the DC circuit 214 -+ the switching device
S-2W -, the phase W of the filter reactor 400 --, the phase W of the filter capacitor 402 +
the phase V of the filter capacitor 402 --, the phase V of the filter reactor 400 -, the switching
device S - 4V -, the terminal N of the DC circuit 214 -+ the negative side capacitor 222. In
this process, resistive components of circuits and ON resistance of switching devices cause
losses, and the charge of the negative side capacitor 222 is consumed to decrease the DC-link
voltage Vdc-N. As a result, the difference between the DC-link voltages Vdc-P and Vdc-N
can be reduced.
[0029]

Figure 3 is a block diagram describing an operation in which the controller 2 10 controls
the converter 202. The controller 210 switches, according to the state of the grid 100, a
three-phase alternating current voltage command outputted to the converter 202. A case
where the grid 100 is operating normally will be described first, and then a case where the
power supply fiom the grid 100 is stopped will be described next.
[0030]
An outage determination unit 522 determines, using the grid voltage Vcnv, whether the
grid 100 is in a power outage state. Specifically, if sufficient electric power cannot be
supplied to the load 206 using only the electric power supplied from the grid 100, the grid 100
is determined to be in an outage state. The determination result SGl is outputted to the
switch 524.
1003 11
The converter 200 inputs, to a DC-link voltage control calculator 504, the grid voltage
Vcnv, the grid current Icnv, the positive side capacitor voltage Vdc-P, and the negative side
capacitor voltage Vdc-N. The DC-link voltage control calculator 504 calculates three-phase
alternating voltage commands Vun, Vvn, and Vwn so that sum of voltages between both ends
of capacitors 220 and 222, Vdc-P + Vdc-N, matches with a DC-link voltage command
VdcREF, and so that difference between voltages between both ends, Vdc-P - Vdc-N,
becomes 0. This allows suppressing current including harmonic distortions to flow into the
grid 100.
[0032]
The voltage commands Vun, Vvn, and Vwn, which are outputs from the DC-link
voltage control calculator 504, are inputted to a pulse calculator PWMl (518). The pulse
calculator PWMl (518) calculates, using the inputted three-phase alternating voltage
commands, a pulse Pcl of pulse width modulation, and outputs it to the switch 524.
COO331
If the grid 100 goes outage, the voltages Vdc-P and Vdc-N between both ends are
inputted into a subtractor 502. The output of the subtractor 502 is inputted to an outage
voltage command calculator 5 12 and an absolute value calculator ABS 506.
[0034]
The absolute value calculator ABS 506 outputs an absolute value of an input value to
the outage control determination unit 520. When the grid 100 is in a power outage, the
outage control determination unit 520 outputs SG2=1 to a pulse output determination unit 5 16
if the input signal exceeds a predetermined threshold and outputs SG2=0 if the input signal is
at or below the predetermined threshold.
LO03 51
The outage voltage command calculator 512 calculates, using the above-described
principle, three-phase voltage commands Vun', Vvn', and Vwn' so that difference between the
voltages Vdc-P and Vdc-N between both ends becomes small. The result is outputted to a
pulse calculator P WM2 (5 1 4).
[0036]
The pulse calculator PWM2 (514) calculates, using the inputted three-phase alternating
voltage commands, a pulse Pcc2 of pulse width modulation, and outputs it to the pulse output
determination unit 5 16. The pulse output determination unit 5 16 determines whether the
pulse Pcc2 is outputted to the switch 524.
COO371
Figure 4 is a block diagram describing a detail of the pulse output determination unit
5 16. The pulse output determination unit 5 16 determines, according to an output signal SG2
of the outage control determination unit 520, whether the pulse Pcc2 outputted from the pulse
calculator PWM2 (514) is outputted to the switch 524. If SG2=1, the Pcc2 is selected and it
is outputted to the switch 524 as Pc2. If SG2=0,O is selected and a zero pulse is outputted to
the switch 524 as Pc2. By outputting the zero pulse, it is possible to prevent losses caused
along with switching operations of the converter 202.
[003 8 J
Figure 5 is a block diagram describing a detail of the switch 524. The switch 524
selects, according to the signal SG1 outputted from the outage determination block 522, either
one of Pcl or Pc2, and outputs the selected one as a pulse PC.
[0039]

As described above, the UPS 200 according to the embodiment 1, if the power supply
from the grid 100 is stopped, operates the converter 202 so that the positive side capacitor 220
and the negative side capacitor 222 are discharged to balance the neutral point voltage. This
allows effectively controlling the neutral point voltage even if the electric power transmitted
and received by the converter 202 is small.
[0040]

Figure 6 is a circuit diagram showing another configuration example of the converter
202. In Figure 6, semiconductor switches are reversely connected with the terminal M of the
DC circuit 214. The same effect is achieved by employing a configuration as Figure 6
instead of using reverse-blocking semiconductor switches described in Figure 2.
[004 1 ]
Figure 7 is a circuit diagram showing yet another configuration example of the
converter 202. The same effect is achieved by employing a configuration as Figure 7 instead
of using reverse-blocking semiconductor switches described in Figure 2.
[0042]
The present invention is not limited to the above-described embodiments and various
modifications are included. The above-described embodiments describe the present
invention in detail for the sake of better understanding, and the present invention is not limited
to ones including all described configurations.
[0043]
A part or all of above-described configurations, functions, processing units, processing
means, and the like can be implemented with hardware by designing it with such as integrated
circuits. The above-described configurations, knctions, and the like also can be
implemented with software by interpreting and executing programs implementing those
functions using processors. The information implementing those functions such as programs,
tables, files, and the like can be stored in storage devices such as memories, hard disks, SSDs
(Solid State Drive), or storage media such as IC cards, SD cards, or DVDs.
Reference Signs List
[0044]
100: grid
200: UPS (Uninterruptible Power Supply)
202: converter
204: inverter
206: load
208: buck-boost chopper
2 10: controller
2 12: storage battery
2 14: DC circuit
I 222: negative side capacitor
230,232: voltage sensor
300,302,304: breaker
400: filter reactor
402: filter capacitor
404u, 404v, 404w: current sensor

CLAIMS
[Claim 11
An uninterruptible power supply, comprising:
a converter that converts an alternating current power into a direct current power
having a predetermined voltage;
a breaker that connects the converter with a grid;
an inverter that converts the direct current power outputted by the converter into an
alternating current power;
a filter circuit that connects an alternating current output terminal of the converter with
the grid;
a direct current circuit that connects between the converter and the inverter;
a storage battery connected with the direct current circuit; and
a controller that controls an operation of the converter,
wherein
the converter includes a switching device, and is configured as a three-level converter
that can selectively output three electric voltages of a positive side voltage, a neutral point
voltage, and a negative side voltage,
and wherein
if the controller detects that a difference between the positive side voltage and the
neutral point voltage does not balance with a difference between the negative side voltage and
the neutral point voltage,
the controller suspends the operation of the converter, opens the breaker, and resumes
the operation of the converter after initiating a power supply from the storage battery to the
inverter.
[Claim 21
The uninterruptible power supply according to claim 1, wherein
the direct current circuit comprises:
a positive side capacitor to which a voltage caused by the positive side voltage and the
negative side voltage is applied; and
a negative side capacitor to which a voltage caused by the negative side voltage and the
neutral point voltage is applied,
and wherein
i
the controller balhces the difference between the positive side voltage and the neutral
point voltage with the difference between the negative side voltage and the neutral point
voltage by resuming the operation of the converter to discharge an electric charge stored in the
positive side capacitor or in the negative side capacitor.
[Claim 31
The uninterruptible power supply according to claim 2, wherein
the controller, when resuming the operation of the converter, discharges the charge
stored in the positive side capacitor or in the negative side capacitor by operating the switching
device so that a discharge current flows in a path from the positive side capacitor or from the
negative side capacitor to the filter circuit.
[Claim 41
The uninterruptible power supply according to claim 2, wherein
the uninterruptible power supply further comprises a voltage detector that detects a
voltage between both ends of the positive side capacitor and a voltage between both ends of
the negative side capacitor,
and wherein
the controller balances the difference between the positive side voltage and the neutral
point voltage with the difference between the negative side voltage and the neutral point
voltage by discharging one of the positive side capacitor and the negative side capacitor whose
voltage between both ends is larger.
[Claim 51
A method for controlling an uninterruptible power supply,
the uninterruptible power supply comprising:
a converter that includes a switching device, is configured as a three-level converter
that can selectively output three electric voltages of a positive side voltage, a neutral point
voltage, and a negative side voltage, and converts an alternating current power into a direct
current power having a predetermined voltage;
a breaker that connects the converter with a grid;
an inverter that converts the direct current power outputted by the converter into
alternating current power;
a filter circuit that connects an alternating current output terminal of the converter with
the grid;
a direct current circuit that connects between the converter and the inverter;
a storage battery connected with the direct current circuit; and
a controller that controls an operation of the converter,
the method comprising:
a step of detecting that a difference between the positive side voltage and the neutral
point voltage does not balance with a difference between the negative side voltage and the
neutral point voltage; and
a step of suspending the operation of the converter, opening the breaker, and resuming
the operation of the converter after initiating a power supply from the storage battery to the
inverter.
[Claim 61
An uninterruptible power supply, substantially as herein described with reference to
accompanying drawings and examples.
[Claim 71
A method for controlling an uninterruptible power supply, substanti s herein
described with reference to accompanying drawings and examples.