KR20200142330A - The control system and method of mmc converter - Google Patents

Abstract

According to an embodiment of the present invention, provided is a modular multilevel converter (MMC) control system which comprises: an MMC converter including one gaseous phase module divided into an upper arm and a lower arm, wherein each arm has a structure in which a plurality of sub-modules are connected in series; and a control unit selecting the number of insertion indexes of the inserted sub-module from measured values of the MMC converter, and generating a predicted insertion index evaluated at the next sampling moment by using the number of the inserted insertion indexes and a circulating current of the MMC converter. According to an embodiment of the present invention, by reducing the number of control options, the sub-module can be effectively controlled without reducing the speed even if the number of sub-modules increases.

Description

Modular multilevel converter control system {THE CONTROL SYSTEM AND METHOD OF MMC CONVERTER}

The embodiment relates to a modular multilevel converter control system for improving the control performance of the modular multilevel converter.

In recent years, continuous technology development for grid-connected systems is being made. Such grid-connected systems include not only inverters, but also high voltage direct current (HVDC) systems, static synchronous compensator (STATCOM) systems, and power conditioning systems (PCS).

The HVDC and STATCOM systems may be composed of a modular multilevel converter (MMC: Modular Multilevel Converter, hereinafter referred to as an MMC converter). This MMC converter converts the input voltage and becomes a current path for power transmission.

To this end, a plurality of sub-modules are connected in series to each other in the MMC converter, and the direct current is converted into alternating current through switching of a semiconductor switch included in the sub-module to be output.

Control elements of the conventional MMC converter include output voltage magnitude control, capacitor control, and circulating current control. However, in the case of a large MMC converter with a large number of submodules, there are a problem that the number of cases for simultaneously controlling them is large and control is complicated .

An object of the embodiment is to provide a modular multilevel converter control system for improving control performance.

The modular multilevel converter control system of the embodiment includes one gas phase phase module divided into an upper arm and a lower arm, and each arm includes an MMC converter having a structure in which a plurality of sub modules are connected in series, and the MMC And a control unit for selecting the number of insertion indexes of the inserted sub-module from the measured values of the converter, and generating a predicted insertion index evaluated at the next sampling moment by using the number of inserted insertion indexes and the circulating current of the MMC converter. I can.

The control unit includes a future prediction model unit that measures a future output current and a future circulating current from a current in an upper arm of the MMC converter, a current in a lower arm, and a capacitor voltage of an individual sub-module of the arm, and the future prediction model unit A cost function application unit that calculates a cost function using the value measured from and outputs the number of insertion indexes of the submodule that minimizes the cost function, the number of insertion indexes of the submodule, and the MMC converter It may include a prediction insertion index generator for generating a prediction insertion index of the sub-module evaluated at the next sampling moment by using a cyclic current.

The cost function application unit may be determined by the following equation.

[Equation]

The λ2 may be 0.05.

The cost function of the output current can be determined by the following equation.

[Equation]

The cost function of the circulating current can be determined by the following equation.

[Equation]

The control unit may determine the number of insertion indexes to be inserted using a switching state in which the cost function is minimized.

In order to predict the insertion index index, the number of sub-modules (Sj) in the ON state, the measured circulating current (i _circ (k)), and the reference circulating current (i * _circ (k)) may be used.

The control unit reads the number of insertion indexes of the inserted sub-module, determines the number of sub-module indexes to be inserted using the direction of the circulating current, and determines which sub-module to be connected using the voltage and output voltage of the capacitor. In order to do this, it may further include a voltage classification algorithm unit that generates the switching signal Sxj.

The output voltage level of the MMC converter may be set to 2N+1.

In implementation, by reducing the number of control options, even if the number of sub-modules increases, there is an effect of being able to control effectively without reducing the speed.

In addition, the embodiment has an effect of effectively maintaining a balance between a sinusoidal output current, a suppressed circulating current, and a capacitor voltage of a sub-module even if the number of control options is reduced.

Further, in implementation, by determining the weight of the cost function application unit, there is an effect that the optimum performance of the THD value of the output current and the rms value of the circulating current can be generated.

In addition, in implementation, by setting the output voltage level to 2N+1, there is an effect that the control performance can be further improved.

1 is a block diagram showing the structure of an MMC converter.
FIG. 2 shows a modular multilevel converter control system of an embodiment for controlling the MMC converter of FIG. 1.
3 is a graph showing a simulation waveform in a normal operating state.
4 is a graph showing an experimental waveform in a normal operating state.

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

1 is a block diagram showing the structure of an MMC converter, FIG. 2 is a modular multilevel converter control system of an embodiment for controlling the MMC converter of FIG. 1, and FIG. 3 is a graph showing a simulation waveform in a normal operating state, and FIG. 4 is a graph showing the experimental waveform in a normal operating state.

Referring to Figure 1, the MMC converter 100 is composed of one or more phase modules (phase module, 110), each phase module 110 is connected to each terminal 130 for connection to the AC system Each terminal may be divided into an upper arm 110a and a lower arm 110b. Each of the arms 110a and 110b is composed of N submodules 120 connected in series with each other. Here, the AC system may be a three-phase AC power system.

In the sub-module 120, two semiconductor switches S1 and S2 are connected in series, and a capacitor 121, which is an energy storage unit, may be connected in parallel to the semiconductor switch. The capacitor 121 is a component that stores the DC voltage stored in the sub-module 120, and the two semiconductor switches S1 and S2 are devices that switch the flow of current, for example, IGBT, FET, transistor, etc. Can include.

The semiconductor switches S1 and S2 may be controlled to be turned on/off by a control signal of the switching control unit of the MMC converter 100.

Referring to FIG. 2, the controller 200 may be connected to the MMC converter 100 to control the MMC converter 100. The controller 200 generates a predicted insertion index using the number of insertion indices of the sub-module 120 and circulating current, and determines which sub-module 120 to access. That is, the control unit 200 determines how many submodules 120 of the MMC converter 100 should be turned on using the number of submodules 120 currently turned on and the circulating current, and which submodule 120 is You decide whether to turn it on or not. From this, the control system of the embodiment can more effectively control the MMC converter 110 by reducing the number of control options.

The control unit 200 may include a future prediction model unit 210, a cost function application unit 220, a prediction insertion index generation unit 230, and a voltage classification algorithm unit 240.

The future prediction model unit 210 is a future output current (i _oj (k+1)) from the current (i _uj ) in the upper arm (110a), the current (i _lj ) in the lower arm (110b), and the future circulating current (i _circj (k+1)) can be predicted. The cost function application unit 220 may calculate the cost function J according to a value measured from the future prediction model unit 210. The control unit 200 may determine the number of insertion indexes of the sub-module 120 to be inserted using a switching state in which the cost function J is minimized.

The cost function (J) may be determined by Equation 1 below.

[Equation 1]

Here, J1 is the cost function of the output current, J2 is the cost function of the circulating current, and λ1 and λ2 are arbitrary weights. λ2 may be 0.05. It was confirmed through experiment that when λ2 is 0.05, the optimum performance of the THD value of the output current and the rms value of the circulating current occurs.

Hereinafter, the process of calculating the cost function J of the output current and the process of calculating the cost function of the circulating current will be described in detail in FIG. 1.

<Cost function of output current>

As shown in FIG. 1, the AC output current i _oj of phase -j can be expressed by Equation 2 below.

[Equation 2]

Here, i _uj and i _lj are the current of the upper arm and the current of the lower arm, respectively.

By Equation 2, the output current can be expressed as Equation 3.

[Equation 3]

Here, v _uj and v _lj are the voltages of the upper arm and the lower arm, respectively, and v _com is the voltage of the common mode of the MMC converter.

As the controller MPC operates in the discrete time domain, the mathematical model of the output current can be expressed by Equation 4 using the Euler approximation method.

[Equation 4]

i _oj (k+1) is the k+1 th predicted output current, i _oj (k) is the k th output current, and Tsp is the sample period.

As a result, the cost function J1 of the output current can be expressed by Equation 5 below.

[Equation 5]

Here, i* _oj (k+1) is the k+1th future reference output current, and i _oj (k+1) is the k+1th future predicted output current.

<Cost function of circulating current>

Cyclic current control is to suppress the circulating current generated by unbalanced voltages at the upper and lower arms of the phase leg.

Equations describing the model of the circulating current i _circj can be represented by Equations 6 and 7.

[Equation 6]

[Equation 7]

If the Euler approximation method for the derivative of the circulating current is used in a manner similar to the equation for obtaining the output current, the circulating current can be expressed by Equation 8.

[Equation 8]

According to Equation 8, the cost function of the circulating current may be defined as Equation 9.

[Equation 9]

Here, i* _circj (k+1) is the future reference circulating current value of the k+1 th, and i _circj (k+1) is the future predicted circulating current value of the k+1 th.

Accordingly, the final cost function may be defined by Equation 1 by Equation 5 and Equation 9. At this time, the cost function of the predicted energy balance control of the arm can be added to the final cost function. However, since the weight of the cost function of the predicted energy balance control of the arm is set to 0, in this embodiment, the cost is calculated using the output current and the circulating current. You can calculate the function.

The cost function of the cost function application unit 220 controls the correction of the square wave form, the magnitude of the output current, and suppression of the circulating current inside the converter. The controlled values are generated by the voltage classification algorithm unit 240, which will be described later, to generate a switching signal Sj to balance the voltage of the capacitor 121 of the sub-module 120.

Using the number of insertion indices of the sub-module 120 and the circulating current of the MMC converter 100, a predicted insertion index of the sub-module evaluated at the next sampling moment may be generated.

The predictive insertion index generation unit 230 includes the number of sub-modules in the ON state (Sj), the measured circulating current (i _circ (k)), and the reference circulating current (i) to predict the insertion index index evaluated at the next sampling moment. * _circ (k)) can be used.

Using Equations 7 and 8, the circulating current of the MMC converter 100 can be induced by the voltage of the upper arm 110a and the lower arm 110b, and under the condition of the capacitor voltage of the balanced MMC converter 100 The dark voltage may be determined by the following equation.

[Equation 10]

[Equation 11]

[Equation 12]

Here, Vcuavg is the average value of the capacitor voltage of the upper arm 110a, and Vclavg is the average value of the capacitor voltage of the lower arm 110b.

When Equations [10], [11] and [12] are used, Equation 8 can be expressed as Equation 13 as follows.

[Equation 13]

Here, Sj represents the on-state sub-modules in one phase. Sj can be expressed as Equation [14].

[Equation 14]

The output voltage of the MMC converter 100 may be 0 to 2N+1 (N is the number of sub-modules). Here, if the output voltage of the MMC converter 100 is used, a lot can be divided. In this case, the number of cases increases, but in the embodiment, since the control elements are considerably reduced, the performance can be improved compared to the conventional one. Here, the level lj of the output voltage may be determined by the following equation.

[Equation 15]

The voltage classification algorithm unit 240 serves to balance voltages of all capacitors of the MMC converter 100.

The voltage classification algorithm unit 240 reads the number of insertion indices of the inserted sub-module 120 and determines the number of indexes of the sub-module 120 to be inserted using the direction of the circulating current, and the voltage and output of the capacitor A switching signal Sxj may be generated to determine which sub-module 120 is to be connected using the voltage.

The voltage classification algorithm 240 determines which sub-module is to be connected or to be bypassed. Then, the switching signal Sj may be generated to be sent to the MMC converter applied at the sampling moment k.

3 shows a simulation was implemented using the PSIM software using Table 1 below.

[Table 1]

Referring to FIG. 3, assuming that the total simulation time is 0.06 seconds, a simulation result in a steady state state is shown. The A phase output current is shown in CH1. You can check that the output current tracks the threshold of THD=0.54%. In addition, when CH2 is N=3, it monitors the 7 level output voltage waveform changing from -Vdc/2 to Vdc/2.

It can be seen that the circulating current at this time is minimized and the capacitor voltage of the sub-module of the MMC converter is balanced.

In FIG. 4, CH1 represents the output current, CH2 represents the output voltage, CH3 represents the circulating current, and CH4 represents the voltages of the capacitors of the submodules.

4, the output voltage is 7 levels and the output current shows a correct sine wave. When the waveform of FIG. 4 shows the capacitor voltages of sub-module 1 and sub-module 3, it can be seen that they fluctuate at about 33.3V and are maintained in balance. In addition, it can be seen that the circulating current is suppressed.

As described above, the control system of the MMC converter has the effect of minimizing circulating current while balancing the capacitors.

Features, structures, effects, and the like described in the embodiments above are included in at least one embodiment of the present invention, and are not necessarily limited to only one embodiment. Further, the features, structures, effects, etc. illustrated in each embodiment may be implemented by combining or modifying other embodiments by a person having ordinary knowledge in the field to which the embodiments belong. Accordingly, contents related to such combinations and modifications should be interpreted as being included in the scope of the present invention.

100: MMC converter
200: control unit
210: future prediction model unit
220: cost function application unit
230: prediction insertion index generation unit

Claims (10)

An MMC converter including one gas phase phase module divided into an upper arm and a lower arm, each arm having a structure in which a plurality of sub modules are connected in series; And
A control unit for selecting the number of insertion indices of the inserted sub-module from the measured values of the MMC converter, and generating a predicted insertion index evaluated at the next sampling moment by using the number of inserted insertion indices and the circulating current of the MMC converter. Modular multilevel converter control system including. The method of claim 1,
The control unit
A future prediction model unit for measuring a future output current and a future circulating current from the current in the upper arm of the MMC converter, the current in the lower arm, and the capacitor voltage of each sub-module of the arm;
A cost function application unit that calculates a cost function using a value measured from the future prediction model unit and outputs the number of insertion indices of the submodule minimizing the cost function; And
A modular multilevel converter control system comprising a prediction insertion index generator for generating a prediction insertion index of the sub-module evaluated at a next sampling moment by using the number of insertion indices of the sub-module and a circulating current of the MMC converter. The method of claim 2,
The cost function application unit is a modular multi-level converter control system determined by the following equation.
[Equation]

Here, J1 is the cost function of the output current, J2 is the cost function of the circulating current, and λ1 and λ2 are arbitrary weights. The method of claim 3,
The λ2 is 0.05, the modular multilevel converter control system. The method of claim 3,
The cost function of the output current is determined by the following equation.
[Equation]

Here, i*oj represents the reference output current in the j-phase, and ioj represents the predicted output current in the j-phase. The method of claim 3,
The cost function of the circulating current is determined by the following equation.
[Equation]

Here, i*circ,j represents the reference circulating current in the j-phase, and icirc,j represents the predicted circulating current in the j-phase. The method of claim 1,
The control unit is a modular multilevel converter control system that determines the number of insertion indexes to be inserted using a switching state in which a cost function is minimized. The method of claim 2,
Modular multilevel converter control system using the number of on-state sub-modules (Sj), measured circulating current (i _circ (k)), and reference circulating current (i* _circ (k)) to predict the insertion index index . The method of claim 1,
The control unit reads the number of insertion indexes of the inserted sub-module, determines the number of sub-module indexes to be inserted using the direction of the circulating current, and determines which sub-module to be connected using the voltage and output voltage of the capacitor. Modular multilevel converter control system further comprising a voltage classification algorithm for generating a switching signal (Sxj) to do. The method according to any one of claims 1 to 9,
The output voltage level of the MMC converter is 2N+1 modular multilevel converter control system. (N is the number of submodules)

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