KR102457332B1 - Energy storage system photovoltaic inverter for low voltage ride through - Google Patents

Abstract

The present invention relates to an energy storage system for maintaining a low-voltage grid connection and, more specifically, to an energy storage system for maintaining a low-voltage grid connection for contributing to the recovery of a grid voltage by supplying reactive current within a set voltage range when the grid voltage drops instantaneously in a grid-connected energy storage system, thereby improving the reliability of the grid voltage and preventing the energy storage system from being disconnected from a grid.The energy storage system for maintaining a low-voltage grid connection according to the present invention comprises: an energy storage device that charges and discharges electrical energy; a PCS that transmits the electrical energy transferred from the grid to the energy storage device for charging or discharges the electrical energy stored in the energy storage device to supply the electrical energy to the grid; and an LVRT control device that controls to supply reactive current to the grid when a low voltage occurs in the grid.The LVRT control device controls the PCS to determine and supply reactive power to be supplied to the grid through the PCS when it is determined that the grid voltage detected in the grid is low voltage.

Description

Energy storage system for maintaining low-voltage grid connection

The present invention relates to an energy storage system for maintaining a low-voltage grid connection, and more particularly, when the grid voltage momentarily drops in a grid-connected energy storage system, a reactive current is supplied within a set voltage range to restore the grid voltage. By contributing to the system, it relates to an energy storage system for maintaining a low voltage grid connection that improves the reliability of the grid voltage and prevents the energy storage system from being separated from the grid.

Resources such as wind and solar energy are unreliable and not predictable resources because they change with environmental conditions. Therefore, an energy storage system (ESS) is used to preserve energy generated by such a renewable energy resource in a battery system.

The energy storage system is connected to the grid, receives electric energy from the grid, and charges it. When the grid requires power, the energy storage system supplies the charged electric energy to the grid to maximize the efficiency of electric energy while improving the quality of the electric energy of the grid. perform the function

1 shows a schematic configuration of a general energy storage system, a battery for charging electric energy, an inverter (PCS, power conditioner) for charging the electric energy of the system to the battery or supplying the electric energy charged in the battery to the system system), and a filter that removes high frequencies.

Such an energy storage system is configured to include a power conditioner system (PCS), a battery and battery management system (BMS), an energy management system (EMS), etc. As a system that can be supplied when necessary, it is widely used for peak reduction, frequency adjustment, new and renewable linkage, and emergency generator replacement.

On the other hand, low-voltage continuous power generation means that when an accident occurs in the grid and the grid voltage drops by 10% or more, the converter is put into the grid until the grid voltage is restored to a normal voltage, and a predetermined time (e.g., 3 seconds), which means that the requirements are divided according to the voltage reduction rate and the accident time, and it supports reactive current in a specific voltage range to keep it contributing to the system voltage recovery.

Such LVRT has been applied only to grid-connected wind power inverters, but research is being conducted to apply it to various power facilities as demand for large-capacity energy storage devices is recently supplied and increased.

Accordingly, as a technology capable of performing a functional test of a power conversion device for an energy storage system (ESS), a test apparatus for performance evaluation of a multipurpose power conversion device and a test method thereof are disclosed in Korean Patent Publication No. 10-1852711. .

The technology includes an emergency PCS performance test performing unit that performs a performance test including a LVRT: Low Voltage Ride Through (LVRT) evaluation for a power converter for emergency power.

However, the above technology evaluates the compatibility of grid connection with respect to the power converter, but is not actually configured to be applied to an energy storage system to supply reactive current from the energy storage system when a low voltage of the grid is generated to perform continuous power generation.

Therefore, in a system in which the energy storage system and the system are connected and operated, a technology capable of maintaining the power of the system in case of an instantaneous low voltage accident of the system is required.

Registered Patent Publication No. 10-1852711 (April 20, 2018)

The present invention was created to solve the problems of the prior art, and the problem to be solved in the present invention is to reduce the peak current to within the allowable range of the protection relay during a low voltage accident or voltage recovery of the grid voltage to an energy storage device It is to provide an energy storage system for maintaining the low voltage grid connection that allows the PCS of the system to maintain the input state.

In addition, it is an object of the present invention to provide an energy storage system for maintaining the low voltage grid connection that can improve the reliability of the grid voltage by accurately and quickly detecting the grid voltage and supplying reactive current in the event of a grid low voltage accident.

In order to solve the above problems, the energy storage system for maintaining the low-voltage grid connection according to the present invention is an energy storage device for charging and discharging electrical energy, and transferring the electrical energy transferred from the system to the energy storage device for charging or the energy A PCS for discharging electric energy charged in a storage device and supplying it to the system, and an LVRT control device for controlling to supply a reactive current to the system when a low voltage of the system occurs based on the system voltage of the system, When it is determined that the system voltage detected in the system is low, the LVRT control device determines and supplies the reactive power to be supplied to the system through the PCS, and controls the PCS.

Here, the LVRT control device includes a voltage detection unit for detecting the system voltage; an instantaneous peak value detection unit configured to detect an instantaneous peak value of the grid current when the system voltage detected by the voltage detection unit is determined to be a low voltage and when the low voltage state is restored to a normal state; and LVRT control to synchronize the phase with the grid voltage detected by the voltage detector, and to supply reactive power to the grid within a limited current harmonic range to maintain connection with the grid in a low voltage condition in the event of a low voltage accident of the grid including the device.

In addition, the voltage detector includes a low-pass filter for removing harmonic components; a advance compensator for compensating for a delay phase of a frequency from which harmonics are removed in the low-pass filter; a comparator for comparing an output frequency output from the advance compensator with a normal voltage frequency; and an RMS converter that converts a root mean square (RMS) of a voltage frequency corresponding to the difference value into voltage information based on the difference between the normal voltage frequency and the output frequency compared by the comparator and outputs the converted voltage information.

In addition, the instantaneous peak value detection unit detects and stores in advance the peak current value generated in the current phase of 90° and 270° and the peak current value generated in the other current phases, and stores the current at the moment when a low voltage accident occurs. The magnitude of the peak current in the case of a voltage accident is detected by comparing the peak current values detected in the phases of 90° and 270° with the stored comparison data.

In addition, the LVRT control device may include: a current controller for controlling an output current based on a phase and an instantaneous peak value detected by the instantaneous peak value detecting unit when the system voltage detected by the voltage detecting unit is determined to be a low voltage; a phase synchronizer for synchronizing an output phase output from the PCS to a phase of a grid voltage detected by the voltage detector; And it includes a reactive current controller for controlling the output of the reactive current while maintaining the linkage with the system in the low voltage condition in the case of the system accident.

According to the present invention, since the peak current of the energy storage device can be reduced when the system voltage is abnormal, the stability of the grid voltage can be secured through the LVRT control.

In addition, it is possible to prevent an operation error of the PCS due to the reduction of the peak current, and by quickly and accurately detecting the system voltage and guaranteeing a spare time for the LVRT control operation, there is an advantage in that accurate LVRT control can be derived.

In addition, since the reactive current can be effectively supplied when an abnormality occurs in the grid voltage, but the phase of the reactive power supplied to the phase of the supplied grid voltage can be synchronized and supplied, it is possible to solve the problem that the energy storage device is separated from the grid in the event of a low voltage accident. have.

1 is a block diagram of a conventional energy storage system;
2 is a schematic configuration diagram of an energy storage system for maintaining low voltage grid connection according to the present invention;
3 is a block diagram of an LVRT control device applied to an energy storage system for maintaining low-voltage grid connection according to the present invention;
4 is a configuration diagram of a voltage detection unit applied to an energy storage system for maintaining low voltage grid connection according to the present invention;
5 is a graph (a) and error rate (b) showing the frequency for the voltage passing the low-pass filter, the effective voltage of the system, and the voltage for which the phase delay is compensated in the energy storage system for maintaining the low-voltage grid connection according to the present invention; ,
6 is a diagram for detecting the instantaneous peak value applied to the instantaneous peak value detector in the energy storage system for maintaining the low voltage grid connection according to the present invention;
7 is a current control block diagram of a current controller applied to an energy storage system for maintaining low voltage grid connection according to the present invention;
8 is a structural diagram of a phase synchronizer applied to an energy storage system for maintaining low voltage grid connection according to the present invention;
9 is a phase small-signal model for a phase synchronizer applied to an energy storage system for maintaining low-voltage grid connection according to the present invention;
10 shows a control block diagram of SRF using SOGI-QSG in a phase synchronizer applied to an energy storage system for maintaining low voltage grid connection according to the present invention.

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

The present invention improves the reliability of the grid voltage and improves the reliability of the grid voltage by contributing to the recovery of the grid voltage by supplying a reactive current within a set voltage range when the grid voltage momentarily drops in the grid-connected energy storage system. It relates to an energy storage system for maintaining a low-voltage grid connection that prevents separation.

2 is a configuration diagram of an energy storage system for maintaining low-voltage grid connection according to the present invention.

2, the energy storage system for maintaining the low-voltage grid connection according to the present invention includes an energy storage device (10, battery), a PCS (20), a grid (30, Grid) and an LVRT control device (40) is composed by

The energy storage device 10 performs a function of charging electrical energy, and converts and stores chemical energy into electricity.

The energy storage device 10 may be configured in a modularized form in which a plurality of battery cells are managed by a Battery Management System (BMS).

The BMS monitors the voltage, current, temperature, etc. of the battery to maintain and manage the battery in an optimal state, prevent overcharging and overdischarging during charging and discharging of the battery, and equalizing the voltage between battery cells. It plays a role in improving efficiency and battery life.

In addition, the EMS (Energy Management System) shown in the figure manages the entire energy of the energy storage system (ESS), and serves as a control tower for the PMS. In other words, the EMS manages the supply and demand of the power system and operates the energy system.

Although not shown in the drawings, the PMS (Power Management System) serves to monitor and control the states of the battery and the PCS of the energy storage device.

PCS (Power Conversion System) is a function of charging the electric energy supplied from the system 30 to the energy storage device 10 , or discharging the electric energy charged in the energy storage device 10 and supplying it to the system 30 . carry out

On the other hand, when the voltage of the grid is lowered, that is, when a power accident occurs, the grid voltage is quickly increased in order to support LVRT (Low Voltage Ride Through) by supplying the electric energy charged in the energy storage device 10 to the grid. It must be detected to ensure the stability of the system 30 in response to the system low voltage.

For example, various protection devices such as a circuit breaker for protecting the energy storage system by an abnormal voltage of the grid are provided between the grid and the energy storage system. When the circuit breaker is operated, the energy storage system is separated from the grid, The separated energy storage system has a problem in that power is supplied to or not supplied to the system.

Low voltage continuous power generation prevents a grid outage accident by supplying reactive power for a predetermined time in response to the low voltage of the grid. As a system, static system support and dynamic system support are applied differently.

In other words, when the voltage fluctuation of the grid is less than 10% (grid voltage is 91 ~ 99%), it is defined as a static grid support and it is judged as a low voltage accident situation. ) is defined as dynamic system support to determine the low-voltage accident situation, and appropriate measures are taken for each situation. At this time, the international standard of the reference value for judging the voltage fluctuation 10% is defined as the rms value for the normal part of the phase voltage.

Accordingly, when the system voltage is lowered to less than 10%, the static system support is judged and appropriate support should be provided. Problems that can be judged as support may arise.

That is, when it is determined that the static system support is to be made and the reactive power is supplied to the grid, a switching ripple is generated due to an increase in the grid impedance, and the edge storage system 10 is activated by the switching operation of the breaker. can be separated from

Accordingly, in addition to accurately detecting the voltage of the system, it is necessary to detect a voltage having an accurate value.

Accordingly, in the present invention, the LVRT control device 40 is configured to maintain low-voltage continuous power generation in the event of a low-voltage fault in the system.

The LVRT controller 40 controls to supply a reactive current to the grid when a low voltage of the grid is generated based on the voltage of the grid 30 .

3 shows the configuration of the LVRT control device applied to the energy storage system for maintaining the low-voltage grid connection according to the present invention.

Referring to FIG. 3 , the LVRT controller 40 includes a voltage detector 410 , an instantaneous peak value detector 420 , and an LVRT controller 430 .

4 is a diagram showing the configuration of a voltage detection unit applied to the energy storage system for maintaining the low voltage grid connection according to the present invention.

Referring to FIG. 4 attached, the voltage detector 410 performs a function of detecting the voltage of the system, and includes a low-pass filter 411, a lead compensator 412, a comparator 413, and a root mean square (RMS). and a transducer 414 .

The low-pass filter 411 removes high-frequency components, and the cut-off frequency of the low-pass filter 411 is configured to have a relatively larger bandwidth than the switching frequency of the circuit breaker (installed between the energy storage system and the grid). That is, in order to obtain the same frequency response as the switching frequency of the circuit breaker, the cut-off frequency of the low-pass filter 411 is configured to have a relatively larger bandwidth than the switching frequency of the circuit breaker.

As the cut-off frequency of the low-pass filter 411 is configured to have a relatively larger bandwidth than the switching frequency, the switching ripple of the voltage can be partially removed, but the magnitude of the voltage is reduced or the phase is delayed by affecting the frequency response Problems may arise. Accordingly, the voltage detection unit 410 of the present invention is configured with a forward compensator 412 for compensating for the delayed phase.

5 is a graph (a) and error rate (b) showing the frequency for the voltage passing the low-pass filter, the effective voltage of the system, and the voltage for which the phase delay is compensated in the energy storage system for maintaining the low-voltage grid connection according to the present invention; is shown.

Referring to the attached Figure 5 (a), the frequency voltage (V_LPF) that has passed through the low-pass filter 411 shows a phase delay of 3% relative to the effective frequency voltage (V_real) of the grid voltage, but , it was found that the frequency voltage V_comp compensated for by the delay compensator 412 tracks the effective frequency voltage V_real of the grid voltage.

Here, the delay compensation by the advance compensator 412 is configured to have an intermediate value between the effective value frequency of the system voltage and the frequency that has passed through the low-pass filter 411, so that the reversal phenomenon by compensation is prevented.

Referring to FIG. 5B , the voltage detection according to the present invention shows an error of 11% compared to the voltage detection of the conventional method.

That is, it can be seen that the operation is insensitive in the static system support target area of less than 10%, but the voltage detection is performed quickly in the dynamic system support target area of 10% or more.

The comparator 413 compares the output frequency output from the advance compensator 412 with the steady-state voltage frequency, and the steady-state voltage frequency has a preset value.

The RMS (Root Mean Square) converter 414 converts a voltage frequency effective value (RMS: Root Mean Square) corresponding to the difference value based on the difference value between the normal voltage frequency and the output frequency compared in the comparator 413 to voltage information. is converted and output, and the output voltage information consists of a low voltage ratio to the system normal voltage.

For example, the indication of the low voltage ratio output from the RMS converter 414 may be '00%' of the system normal voltage.

That is, the power control unit 400 performs the logic for static system support when the value output from the RMS converter 414 of the voltage detection unit 410 is less than 10% based on 10%, and when it is 10% or more, the dynamic Logic for system support is configured to be executed.

In the present invention, the logic for static system support may be configured as a configuration for normal static system support. For example, if it is determined as static system support, the active power control logic may be executed.

On the other hand, in order to support low-voltage continuous generation (LVRT) for a certain period of time in a system low voltage accident situation, dynamic grid support must be performed while maintaining grid connection. As the electrical equilibrium state becomes unbalanced, an instantaneous overcurrent flows according to the impedance and voltage reduction rate. In addition, instantaneous overcurrent occurs even at the moment when the system voltage is restored to normal. That is, instantaneous overcurrent occurs at the moment when the system voltage is lowered by more than a certain level from the normal state and at the moment when the system voltage is restored to the normal voltage when the system voltage is lowered by more than a certain level.

The instantaneous overcurrent causes the circuit breaker to operate, so that the PCS is separated from the system. In the present invention, the instantaneous peak value detection unit 420 is configured to limit the overcurrent.

The instantaneous peak value detection unit 420 performs a function of detecting the instantaneous peak value of the system current when the voltage of the system detected by the voltage detection unit 410 is determined to be low voltage and when it is restored from the low voltage state to a normal state. do.

The peak current value at the moment when a low voltage accident occurs is derived differently depending on the phase angle. The highest transient peak current occurs when the current phase at the moment of voltage accident is 90° and 270°. In addition, although a protection relay such as an overcurrent circuit breaker determines the allowable range based on the rms value, the switching element determines the allowable range based on the instantaneous peak value, so it is necessary to prepare for a failure due to a current spark.

In addition, the same situation applies even when the system voltage is restored to the normal voltage. That is, even when the voltage drop is 100%, since it is not completely zero, the highest transient peak current occurs even at the time of recovery from the accident when the current phase is 90° and 270°.

Therefore, assuming that the spark ranges are similar at the time of voltage accident and at the time of voltage recovery, the peak current values occurring in the current phase of 90° and 270° and the peak current values occurring in the other current phases are detected in advance. , and using it as comparison data, it is possible to infer the magnitude of the peak current in the event of a voltage accident.

Therefore, in order to prevent the peak current due to the instantaneous drop of the grid voltage, the grid voltage must be compensated.

The grid-connected energy storage device supplies power to the grid by operating in conjunction with the grid. At this time, if the grid voltage momentarily drops, the power is reduced but an overcurrent is generated. Therefore, the PCS will be separated from the system for stability and will not satisfy the system connection maintenance condition. Therefore, when the grid voltage drops momentarily, it must be compensated to prevent overcurrent from occurring.

If the control for compensation according to the system voltage drop is not performed, the largest overcurrent occurs when the voltage drop occurs at 90° and 270° of the grid voltage.

6 is a diagram showing a diagram for detecting an instantaneous peak value applied to an instantaneous peak value detector in an energy storage system for maintaining low voltage grid connection according to the present invention

6, the pulse width (Dgrid) of the grid voltage reflecting the change in the grid voltage is the pulse width (D' _grid ) of the grid voltage before one cycle and the instantaneous grid voltage (V _Sn ) at the nth sampling point. and the instantaneous system voltage (V' _Sn ) at the nth sampling point before one cycle.

If this is expressed as an equation, it is expressed by Equations 1 and 2 below.

In Equations 1 and 2, n is the nth sampling time of the system voltage, m is the output compensation constant, V _Sn is the instantaneous system voltage measured at the nth sampling time, and V' _Sn is the nth instantaneous moment before one cycle Grid voltage, V _S(n+m) is the n+mth instantaneous system voltage before 1 cycle, D _MAX is the maximum pulse width, V _d is the DC link voltage, D' _grid is the grid before 1 cycle The pulse width of the voltage, D _grid , is the pulse width of the grid voltage reflecting the change of the grid voltage.

In this case, when the PCS outputs a voltage, it is not output in phase with the system voltage and is a constant for compensating for delayed output.

According to Equations 1 and 2, the grid voltage and the output voltage of the PCS are equally output by using the n+m-th grid voltage before one cycle in order to improve the power quality. However, it may take more than one cycle to detect the instantaneous voltage drop, and the PCS outputs the voltage one cycle before the voltage drop occurs.

If the grid voltage drops between one cycle, an overcurrent is generated in the grid, and the PCS is separated from the grid by the operation of the circuit breaker, which makes it impossible to satisfy the grid connection maintenance condition. However, as shown in FIG. 6 , by reflecting the change ratio of V _Sn and V′ _Sn to detect the instantaneous peak value, the drop of the system voltage can be quickly detected.

The LVRT control unit 430 synchronizes the phase with the grid voltage detected by the voltage detection unit 410, and supplies reactive power within the current harmonic range limited to the grid to maintain connection with the grid in a low voltage condition in the event of a grid low voltage accident. perform the control function.

Referring to FIG. 3 , the LVRT controller 430 includes a current controller 431 , a phase synchronizer 432 , and a reactive current controller 433 .

When it is determined that the voltage detected by the voltage detector 410 is a low voltage, the current controller 431 controls the output current based on the phase and the instantaneous peak value detected by the instantaneous peak value detector 420 .

In order to compensate for harmonics caused by a nonlinear load, the grid-connected solar tube inverter has a disadvantage in that it is necessary to perform current control by detecting each harmonic component. The proportional-resonance control, which has improved this disadvantage, is a controller that has a large gain value at a specific frequency. When the reference signal of the current is sinusoidal, it can be controlled without a steady state error.

7 shows a current control block diagram of a current controller applied to an energy storage system for maintaining low voltage grid connection according to the present invention.

7, the output current (i _g ) is a current controller transfer function (Gc(s)) based on a reference current (i _g-ref ), a time delay transfer function by PWM (Gpwm(s)) and the filter transfer function Gs(s).

In this case, the increase in the cut-off frequency of the current controller transfer function Gc(s) broadens the frequency width based on the control frequency, so that even a slight frequency change can be controlled. However, if the cut-off frequency is excessively increased to cope with the frequency fluctuation, it has a disadvantage that the phase margin can be reduced. Considering these advantages and disadvantages, an appropriate cut-off frequency should be selected.

The PCS applied to the present invention can be applied in consideration of only the 3rd and 5th harmonic compensation. If additional high-order harmonic compensation control is required, the bandwidth may be set in consideration of the switching frequency and the filter resonant frequency for system stability.

The phase synchronizer 432 synchronizes the output phase output from the PCS 20 with the phase of the system voltage detected by the voltage detector.

In order to improve the stable output, high efficiency, and power quality in the grid-connected PCS 20, an accurate phase synchronization technique between the grid and the system must be applied.

A phase locked loop (PLL) may be used as a synchronization method applicable to the phase synchronizer 432 . When the PLL method is applied, information on the magnitude (Vm), frequency (ω), and phase (θ) of the grid voltage can be detected, and a stable current is controlled based on the detected information, and active/reactive power control is possible.

8 shows the structure of the phase synchronizer applied to the energy storage system for maintaining the low voltage grid connection according to the present invention.

Referring to FIG. 8, the phase synchronizer 432 includes a phase detection unit 432a, a loop filter 432b, and a voltage control unit 432c.

The phase detection unit 432a generates an error signal e _pd proportional to the phase difference between the grid voltage Vm and the output voltage V'm signal.

The loop filter 432b removes a high-frequency component and noise of the error signal e _pd generated by the phase detection unit 432a.

The voltage control unit 432c outputs an output voltage V'm signal oscillated at the same fundamental frequency as the phase of the system voltage Vm by using the voltage from which the high frequency and noise are removed from the loop filter 432b.

In the above configuration, the phase (θ) of the grid voltage and the phase (θ') of the output voltage (V'm) signal output from the phase synchronizer 432 are represented by a linear small-signal model as shown in FIG. 10 .

9 is a view showing a small phase signal model for the phase synchronizer applied to the energy storage system for maintaining the low voltage grid connection according to the present invention.

The closed-loop transfer function H(s) of the phase small-signal model for the phase synchronizer is expressed by Equation 3 below.

where H(s) is the closed-loop transfer function, K ^s _p,pll is the proportional gain of the sth harmonic, K ^s _i,pll is the integral gain of the sth harmonic, s is the harmonic order, K _p,pll is the proportional gain, K _i,pll is the harmonic integral gain. Also, s is the third and fifth harmonics as the order.

A synchronous reference frame (SRF) may be applied as a small phase signal model for the phase synchronizer. SRF converts the stationary coordinate system (αβ axis) to the synchronous coordinate system (dq axis) and can quickly and accurately follow the frequency and phase of the system through direct current control. However, unlike three-phase, since coordinate transformation cannot be performed in a single phase in a single phase, a SOGI (Second order generalized integrator)-QSG (Quadrature Signal Generator) method can be applied.

The QSG has a problem in that the phase delay of the all-pass filter is not accurately performed when the frequency of the system is changed because the cut-off frequency of the all-pass filter is fixed. In order to solve this problem, the SOGI-QSG method that extracts the normal fraction in a second order generalized integrator (SOGI) method having a second-order adaptive filter characteristic in a fixed coordinate system is applied.

SOGI can generate a sync signal having the same voltage magnitude and phase and a quadrature signal having the same magnitude but delayed by 90 degrees by using one input signal.

At this time, the d-axis voltage from which the sync signal of SOGI is output can remove the DC offset voltage due to the characteristics of SOGI, but the q-axis voltage from which the orthogonal signal is output includes the DC offset voltage as it is. Accurate phase tracking is difficult because accurate synchronization is not performed.

10 shows a control block diagram of SRF using SOGI-QSG in a phase synchronizer applied to an energy storage system for maintaining low voltage grid connection according to the present invention.

10, the output transfer function of SOGI-QSG consists of a transfer function of a bandpass filter and a transfer function of a lowpass filter, and an orthogonal signal is generated based on an adaptive filter that automatically adjusts its own parameters. Because it is output, accurate coordinate transformation can be performed by generating a virtual voltage signal delayed by 90° in the phase of the grid voltage even when the frequency of the sensed grid voltage is not the basic frequency.

The reactive current controller 433 controls the output of the reactive current while maintaining the linkage with the system in a low voltage condition in case of a system accident.

That is, the reactive current controller 433 is a function capable of supporting system fault recovery by outputting a reactive current from the PCS in case of a system accident.

At this time, in order for the grid-connected PCS based on renewable energy to safely perform reactive current in the event of a grid accident, the PCS capacity must be considered.

The conventional grid-connected PCS selects the PCS capacity based on the operation with a power factor of 1. If the PCS designed with a power factor of 1 is set to output reactive current in the event of a low voltage accident, there is a risk of an accident due to overcurrent.

The three reactive current outputs that can be applied according to the capacity of the PCS are divided into Constant Average Active Power Control, Constant Active Current Control, and Constant Peak Current Control. do.

The constant average active power control and constant active current control aim to produce as much energy as possible even during LVRT, and the constant peak current control technique has the advantage of being directly applied to the PCS installed in the past. Accordingly, in the present invention, it is preferable that a constant peak current control in which the PCS output current peak value is kept constant is applied in the event of a low voltage accident.

Accordingly, the reactive current controller 433 controls the reactive current to be supplied to the grid voltage in a synchronized phase in the phase synchronizer 432 based on the output current controlled by the current controller 431 .

Here, the reactive current supplied through the reactive current controller 433 is configured to supply the reactive current to the system by dividing the fault voltage of the system and the fault phase of the system.

For example, when the grid voltage is 0.9pu ~ 1.1pu, it is determined as a normal operation section, and when the grid voltage is 0.5pu or less, the reactive current is output as 100% of the rated standard.

In addition, when it is determined that the fault phase of the system is 0 °, 45 ° and 90 °, the phase synchronizer 432 is configured to synchronize the phase of the grid voltage to the synchronized phase to supply the reactive current.

According to the present invention, since the peak current can be reduced when an abnormality of the grid voltage occurs, the stability of the grid voltage can be secured through the LVRT control, and an operation error of the PCS can be prevented due to the reduction of the peak current, and the system The voltage is detected quickly and accurately to guarantee a spare time for the LVRT control operation, so that accurate LVRT control can be derived.

In addition, since the reactive current can be effectively supplied when an abnormality of the grid voltage occurs, but the phase of the reactive power supplied to the phase of the supplied grid voltage can be synchronized and supplied, there is an advantage that can solve the problem of being separated from the energy storage system in case of a low voltage accident.

Although preferred embodiments of the present invention have been described above, the scope of the present invention is not limited thereto, and it should be understood that the scope of the present invention extends to those substantially equivalent to the embodiments of the present invention, Various modifications may be made by those of ordinary skill in the art to which the invention pertains without departing from the spirit of the invention.

10: energy storage device 20: PCS
30: system 40: LVRT control device
410: voltage detection unit 411: low-pass filter
412: advance compensator 413: comparator
414: RMS converter 420: instantaneous peak value detection unit
430: LVRT control unit 431: current controller
432: phase synchronizer 433: reactive current controller

Claims (5)

An energy storage device for charging and discharging electrical energy, a PCS that transfers electrical energy transferred from the system to the energy storage device for charging or discharging the electrical energy charged in the energy storage device and supplying it to the system, and the system of the system and an LVRT control device that controls to supply a reactive current to the system when a low voltage of the system occurs based on the voltage,
The LVRT control device,
When it is determined that the grid voltage detected in the grid is a low voltage, the PCS is controlled to determine and supply reactive power to be supplied to the grid through the PCS,
The LVRT control device,
a voltage detection unit detecting the system voltage;
an instantaneous peak value detection unit configured to detect an instantaneous peak value of the grid current when the system voltage detected by the voltage detection unit is determined to be a low voltage and when the low voltage state is restored to a normal state; and
an LVRT control unit that synchronizes the phase with the grid voltage detected by the voltage detector and controls to supply reactive power within a current harmonic range limited to the grid so as to maintain connection with the grid under a low voltage condition in the event of a low voltage accident of the grid;
includes,
The instantaneous peak value detection unit,
The peak current values occurring in the current phase of 90° and 270° and the peak current values occurring in other current phases are detected and stored in advance, and the current phase at the moment of low voltage accident is 90° and 270°. by comparing the detected peak current value with the stored comparison data to detect the magnitude of the peak current in case of a voltage accident,
The LVRT control unit,
a current controller for controlling an output current based on a phase and an instantaneous peak value detected by the instantaneous peak value detecting unit when the system voltage detected by the voltage detecting unit is determined to be a low voltage;
a phase synchronizer for synchronizing an output phase output from the PCS to a phase of a grid voltage detected by the voltage detector; and
a reactive current controller for controlling the output of reactive current while maintaining the linkage with the system in a low voltage condition during the system accident;
includes,
The current controller is
The output current is determined by the current controller transfer function based on the reference current, the time delay transfer function by PWM, and the filter transfer function, and the cut-off frequency increase of the transfer function is applied to the 3rd and 5th harmonic compensation, and additional high-order harmonics When compensation control is required, the bandwidth is set by the switching frequency and the filter resonant frequency,
The phase synchronizer,
a phase detection unit generating an error signal proportional to a phase difference between the grid voltage and the output voltage signal;
a loop filter for removing high-frequency components and noise of the error signal generated by the phase detection unit; and
a voltage control unit for outputting an output voltage signal oscillated at the same fundamental frequency as the phase of the grid voltage by using the voltage from which the high frequency and noise are removed from the loop filter;
including,
The closed-loop transfer function of the phase small-signal model for the phase synchronizer is determined by Equation 1 below,
(Equation 1)

H(s) is the closed-loop transfer function, K ^s _p,pll is the proportional gain of the sth harmonic, K ^s _i,pll is the integral gain of the sth harmonic, s is the third and fifth harmonic orders, K _p,pll is the proportional gain gain, K _{i, pll} is the harmonic integral gain,
The phase small-signal model is
SOGI-QSG, which extracts normal fractions in a second order generalized integrator (SOGI) method with second-order adaptive filter characteristics in a fixed coordinate system, is applied.
The reactive current controller is
When the grid voltage is 0.9pu ~ 1.1pu, it is determined as a normal operation section, and when the grid voltage is 0.5pu or less, it is configured to output the reactive current at 100% of the rated standard, and the fault phase of the system is 0°, 45 When it is determined that it is ° and 90 °, the energy storage system for maintaining low voltage grid connection, characterized in that it is configured to supply reactive current by synchronizing the phase of the grid voltage with the phase synchronized in the phase synchronizer.
delete The method according to claim 1,
The voltage detection unit,
a low-pass filter that removes harmonic components;
a advance compensator for compensating for a delay phase of a frequency from which harmonics are removed in the low-pass filter;
a comparator for comparing an output frequency output from the advance compensator with a normal voltage frequency; and
an RMS converter that converts a root mean square (RMS) of a voltage frequency corresponding to the difference value into voltage information based on a difference value between the normal voltage frequency and the output frequency compared in the comparator and outputs the converted voltage information;
Energy storage system for maintaining low-voltage grid connection, characterized in that it comprises a.
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