KR102639213B1 - Power system stability improvement device - Google Patents

Abstract

The present invention relates to a Modular Multilevel Converter (MMC) submodule of STATCOM (Static Synchronous Compensator), a power system stabilization device. The MMC submodule includes a battery that absorbs or supplies active power and generates reactive or active power. Immediate compensation can improve the stability of the power system.

Description

Power system stability improvement device

The present invention relates to a device for improving power system stability.

To improve power system stability, a reactive power compensation device is generally installed. Reactive power compensation devices include SVC (Static Var Compensator), TCSC (Thyristor Controlled Series Capacitor), and STATCOM (Static Synchronous Compensator), and each has different strengths and weaknesses.

Reactive power is power that actually does nothing and does not consume heat. Reactive power cannot be used in practice because it only travels back and forth between the power source and the electrical device and does not generate energy.

If reactive power consumption increases, the voltage may become too low during the transmission process, resulting in a power outage or power interruption. Therefore, it is necessary to properly compensate for reactive power to prevent the above situation from occurring.

For this purpose, the transmission system uses the Flexible Alternating Current Transmission System (FACTS).

A flexible transmission system includes a series compensator that is connected in series to the system and a parallel compensator that is connected in parallel, and a series-parallel compensator that combines the strengths and weaknesses of the two devices.

Series compensation devices include compensation devices such as TCSC (Thyristor-Controlled Series Compensation).

Parallel compensators include a parallel reactor using a mechanical switch, a static var compensator (SVC) using a thyristor that replaces a mechanical switch with a parallel capacitor and a power semiconductor element to enable transient characteristics and linear control, and There is a Static Synchronous Compensator (STATCOM) that uses an Insulated Gate Bipolar Transistor (IGBT) device.

Among these, the STATCOM system can attach the devices that make up the system, such as valves using IGBTs, cooling systems, and controllers, to each of multiple STATCOM banks, feed them into the system, and supply or absorb reactive power.

These improve the stability of the power system by compensating for reactive power, but because they cannot supply active power, devices are needed to compensate for the frequency or supplement active power when the frequency of the system changes in certain situations such as accidents in load-dense systems. need.

Similarly, ESS (Energy Storage System) prevents a decrease in the supply of active power to loads due to system accidents through independent control of active and reactive power and simultaneously enables the supply of appropriate reactive power. In addition, it helps solve the problem of system volatility by expanding the connection points of new and renewable energy. However, in large-capacity transmission networks, it is difficult to connect directly to the high-voltage system due to issues such as the price and capacity of ESS.

The above-mentioned background technology is technical information that the inventor possessed for deriving the present invention or acquired in the process of deriving the present invention, and cannot necessarily be said to be known art disclosed to the general public before filing the application for the present invention.

The present invention was created to solve the above-mentioned problem, and includes a battery that absorbs or supplies active power to the sub-module of the MMC (Modular Multilevel Converter) STATCOM, an existing power system stabilization device, The purpose is to provide an MMC submodule that can improve the stability of the power system by providing high-speed compensation of reactive power and immediate provision of active power using the proposed device.

As a means to solve the above-described problem, the present invention has embodiments with the following features.

The present invention relates to a Modular Multilevel Converter (MMC) submodule of a Static Synchronous Compensator (STATCOM), which is a power system stabilization device, wherein the MMC submodule includes a battery that absorbs or supplies active power; It is characterized by being able to improve the stability of the power system by immediately compensating for reactive power or active power, including.

The MMC sub-module includes a first converter that compensates for reactive power; and a second converter that compensates for active power; It is characterized by including.

The first converter includes a first power semiconductor switch and a second power semiconductor switch connected in series with each other in the same direction, and a first power semiconductor switch and a second power semiconductor switch connected in parallel throughout the series. Characterized by including a capacitor.

The semiconductor switch for power includes a semiconductor switch and a diode connected in anti-parallel to the semiconductor switch.

The second converter is characterized by including a third power semiconductor switch, an inductor, a diode, a capacitor, and a battery.

The second converter includes a switch controller that controls on/off of the third power semiconductor switch; It includes, and the voltage applied to the battery is characterized in that it is determined by the on period (D) of the third power semiconductor switch.

The voltage (V_bat) applied to the battery is, It is characterized by being.

The inductor accumulates energy while the third power semiconductor switch is on, and transfers the accumulated energy to the battery through a diode while the third power semiconductor switch is off. It is characterized by delivering.

The diode is turned off during the on period of the third power semiconductor switch, and is on during the off period of the third power semiconductor switch, thereby accumulating in the inductor. It is characterized in that the energy is transmitted to the battery.

The present invention has the effect of improving the stability of the power system by enabling high-speed compensation of reactive power possessed by existing STATCOMs and immediate provision of active power using the proposed device.

1 is a diagram showing the basic structure of MMC STATCOM
Figure 2 is a diagram showing that the MMC STATCOM submodule is configured as a half-bridge.
Figure 3 is a diagram showing that the MMC STATCOM submodule is configured as a full-bridge
4 is a diagram of an MMC submodule according to an embodiment of the present invention.
Figure 5 is a diagram for explaining the circuit operation of the MMC sub-module in the section where the third switch is on.
Figure 6 is a diagram for explaining the circuit operation of the MMC sub-module when the third switch is off.
Figure 7 is a diagram showing the waveforms of inductor current and voltage according to the on/off section of the third switch.
Figure 8 is a graph showing the transfer ratio (Gv) of the output voltage to the input voltage of the second converter according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Figure 1 is a diagram showing the basic structure of MMC STATCOM.

The topology of the power conversion device that makes up STATCOM is basically adopting a multi-level converter (MMC), which has advantages such as reducing voltage stress due to an increase in the number of levels, reducing the switching frequency of power semiconductor devices, and reducing generated harmonics. Because there is

In MMC (Modular Multilevel Converter) STATCOM (Static Synchronous Compensator), the three phases R, S, and T can be connected through Y-connection or D-connection. In general, because the current limit of power electronic devices is limited, a method of relatively increasing voltage and lowering current using D-connection compared to the same capacity is commonly used.

The core hardware element of MMC-based STATCOM is the valve. The valve is made up of several sub-modules connected in series, and the number of sub-modules is determined according to the voltage rating of the system.

As shown in Figure 1, all submodules 50 of each bridge arm of the MMC valve 10 are grouped into N groups, where N is at least 1, and all submodules 50 includes faulty submodules and normal submodules. Each submodule in each group may or may not have the same number. The number of normal submodules in each group changes dynamically, and each group operates corresponding to one MMC valve 10-based controller. Each MMC valve 10 based controller operates separately, and the upper controller provides the number of input submodules to the MMC valve based controller.

The controller of MMC STATCOM can be configured at multiple levels, including the top controller, upper controller, and lower controller.

The highest level controller monitors system information, controls reactive power, stops the system in the event of an emergency, and is responsible for overall control.

The upper controller performs control signals for the use of sub-modules and control to resolve direct current voltage imbalance that may occur when multiple sub-modules are connected in series. In addition, it also acts as a communication intermediary between the lower-level controller and the top-level controller.

The lower-level controller controls the amount of current and reactive power required by the upper-level controller by applying control signals received from the upper-level controller to the sub-module.

Generally, the submodule 50 is configured as a half-bridge structure or a full-bridge structure.

FIG. 2 is a diagram showing a submodule of MMC STATCOM configured as a half-bridge, and FIG. 3 is a diagram showing a submodule of MMC STATCOM configured as a full-bridge.

The submodule 50, which is composed of a half-bridge, is composed of two switches that can be turned on and off, including anti-parallel diodes, and a submodule capacitor. The submodule composed of a half bridge generates '0' and 'Vsm' unipolar voltages.

The sub-module 50, which is composed of a full-bridge, is composed of four switches with anti-parallel diodes and a capacitor to generate positive voltages of '0', 'Vsm', and '-Vsm'.

As shown in Figures 2 and 3, the half-bridge structure has fewer switch elements than the full-bridge structure, which is advantageous in terms of system loss and economics, and reduces the amount of current. It has the advantage of being advantageous from a control perspective as the balancing algorithm of the capacitor voltage of the sub-module can be easily implemented depending on the direction.

However, the half-bridge structure system has the disadvantage of being vulnerable to DC faults. Specifically, the half-bridge structure system consists of bypass thyristors and arm reactors in series and parallel to block and reduce fault current, but this cannot be a reliable measure against short-circuit faults in the DC terminal. . Generally, a half-bridge structure system uses a DC current breaker connected to the DC power transmission line as a way to suppress overcurrent caused by a short-circuit accident at the DC terminal of the DC power transmission line. However, current DC current circuit breakers have the problem that the short-circuit current increases for several milliseconds (msec) and the manufacturing cost is also high.

The full bridge structure has the advantage of increasing the degree of control freedom, but has the disadvantage that the number of semiconductor elements is greater than that of the half bridge structure system and the system loss is large during normal operation of the system.

Figure 4 is a diagram of an MMC submodule according to an embodiment of the present invention.

The MMC sub-module provided by the present invention includes a first converter 100 that compensates for reactive power and a second converter 200 that compensates for active power.

The first converter 100 is composed of a half bridge, and includes a first power semiconductor switch 110 and a second power semiconductor switch 120 connected in series in the same direction, and a first power semiconductor switch 120 connected in series. It includes a capacitor 130 connected in parallel across the semiconductor switch and the second power semiconductor switch.

The semiconductor switches 110 and 120 for power include semiconductor switches 111 and 121 and diodes 112 and 122 connected in anti-parallel to the semiconductor switches.

The second converter 200 includes a third power semiconductor switch 210, an inductor 220, a diode 230, a capacitor 240, a battery 250, and the third power semiconductor switch. It includes a switch controller 260 that controls on/off of.

The second converter 200 uses the capacitor voltage of the first converter 100 as the input voltage V_in, and the voltage V_bat applied to the battery 250 as the output voltage.

The voltage V_bat applied to the battery 250 is expressed in Equation 1 below.

That is, the voltage V_bat applied to the battery 250 is determined by the on period (D) of the third power semiconductor switch 210.

When the voltage V_bat applied to the battery 250 is lower than the voltage charged in the battery, the active power stored in the battery is supplied to the system through the first converter.

When the voltage V_bat applied to the battery 250 is higher than the voltage charged in the battery, the battery 250 absorbs the active power of the system through the first converter.

Ultimately, whether the battery 250 operates in a mode that supplies active power to the system or operates in a mode that absorbs active power from the system depends on the on section (D) of the third power semiconductor switch 210. is determined, and this control is performed by the switch controller 260, which controls the on/off of the third power semiconductor switch.

The switch controller 260 always discharges and recharges excess and insufficient energy of the connected system, and turns on/off the third power semiconductor switch when it receives information such as a change in frequency or assumed failure from a connected monitoring element. Controlled to improve system transient, frequency, and voltage stability through energy charging and discharging of the battery 250.

The inductor 220 accumulates energy while the third power semiconductor switch 210 is on, and the accumulated energy is stored during the third power semiconductor switch 210 is off. Energy is transferred to the battery 250 through the diode 230.

The diode 230 is turned off during the on section of the third power semiconductor switch 210, and is turned on during the off section of the third power semiconductor switch 210. (on) and transfers the energy accumulated in the inductor 220 to the battery 250.

The capacitor 240 functions to reduce ripple of the voltage V_bat applied to the battery 250.

The battery 250 includes a battery pack, and the battery pack consists of battery cells (not shown) in series and/or parallel, and the battery cells include high voltage batteries such as nickel metal batteries, lithium ion batteries, and lithium polymer batteries. It could be a battery. In general, high voltage batteries refer to high voltages of 100V or higher. However, it is not limited to this, and low-voltage batteries are also possible.

Figure 5 is a diagram for explaining the circuit operation of the MMC sub-module in the section where the third switch is on.

The section in which the third switch is turned on is D*T.

At this time, the voltage VS applied to the third switch (S) is VS = 0[v].

The voltage VD of the diode (D) becomes VD = Vi + Vo, which corresponds to the reverse voltage of the diode, so the diode (D) is turned off.

As a result, all of the current is flowing through the third switch flows into the inductor, the inductor current iL = is, and the inductor accumulates energy.

And the inductor voltage VL becomes VL = Vi.

Figure 6 is a diagram for explaining the circuit operation of the MMC sub-module when the third switch is off.

The section in which the third switch is off is (1-D)*T.

At this time, the voltage VS applied to the third switch (S) is VS = Vi + Vo.

The diode D is turned on, and the voltage VD of the diode D becomes VD = 0[v].

The energy accumulated in the inductor (L) is transferred to the battery through the diode (D), and the current flowing through the diode becomes iD = iL.

And the inductor voltage VL becomes VL = -Vo.

Figure 7 is a diagram showing waveforms of inductor current and voltage according to the on/off section of the third switch.

During the switching period T of the third switch, the average voltage of the inductor voltage VL is 0 [v].

During the third switch-on period, VL = Vi, and during the off period, VL = -Vo.

The average voltage of VL is

VL = Vi*D*T = (-Vo)*(1-D)*T, and this average voltage value is 0[v],

VL = Vi*D*T = (-Vo)*(1-D)*T = 0,

To summarize Vo,

It becomes.

Here, Vo is the voltage V_bat applied to the battery.

Figure 8 is a graph of the transfer ratio (Gv) of the output voltage to the input voltage of the second converter according to an embodiment of the present invention.

By organizing, to obtain the transfer ratio Gv of the output voltage to the input voltage,

It becomes.

Depending on the value of the on section D of the third switch, the Gv value varies,

If D=0.5, Gv=1, that is, the output voltage is equal to the input voltage, and charging and discharging of the battery does not occur.

If D < 0.5, Gv < 1, that is, the output voltage becomes smaller compared to the input voltage, and charging of the battery occurs. In other words, it absorbs active power from the system.

If D > 0.5, Gv > 1, that is, the output voltage increases compared to the input voltage, and battery discharge occurs. In other words, it supplies active power to the system.

The embodiments described above should be understood in all respects as illustrative and not restrictive. The scope of the present invention is indicated by the claims described below rather than the detailed description above, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the claims of the present invention. .

10: MMC valve
50: MMC submodule
100: first converter
110: First power semiconductor switch
120: Second power semiconductor switch
130: capacitor
200: second converter
210: Third power semiconductor switch
220: inductor
230: diode
240: capacitor
250: battery
260: switch controller

Claims (9)

In the MMC (Modular Multilevel Converter) submodule of STATCOM (Static Synchronous Compensator), a power system stabilization device,
The MMC submodule is,
A first converter that compensates for reactive power; and
It includes a second converter that compensates for active power,
The second converter includes a third power semiconductor switch, an inductor, a diode, a capacitor, a battery, and a switch controller that controls on/off of the third power semiconductor switch,
The switch controller,
When the voltage applied to the battery through the first converter is lower than the voltage charged in the battery, controlling the third power semiconductor switch to supply active power stored in the battery to the system,
When the voltage applied to the battery through the first converter is higher than the voltage charged in the battery, the MMC submodule controls the third semiconductor switch for power so that the battery absorbs the active power of the system.
delete According to paragraph 1,
The first converter,
A first power semiconductor switch and a second power semiconductor switch connected in series with each other in the same direction, and a capacitor connected in parallel across the first power semiconductor switch and the second power semiconductor switch connected in series. MMC submodule.
According to paragraph 3,
The semiconductor switch for power is an MMC submodule, characterized in that it includes a semiconductor switch and a diode connected in anti-parallel to the semiconductor switch. delete According to paragraph 1,
MMC sub-module, characterized in that the voltage applied to the battery is determined by the on period (D) of the third power semiconductor switch.
According to clause 6,
The voltage (V_bat) applied to the battery is, MMC submodule characterized in that.
Here, V_in is the capacitor voltage of the first converter.
In clause 7,
The inductor is,
The third power semiconductor switch accumulates energy during the on period,
The MMC sub-module is characterized in that the accumulated energy is transferred to the battery through a diode while the third semiconductor switch for power is off. According to clause 8,
The diode is,
The third power semiconductor switch is turned off during the on period,
The MMC sub-module, wherein the third power semiconductor switch is turned on during an off period and transfers energy accumulated in the inductor to the battery.

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