Power System Stabilizer for Voltage Source Converters
Devices and methods for mechanism-based feedback controller employed in a wind powered power system are provided. A controller can include a vector control-based voltage source converter with feedback control circuitry. The feedback control circuitry is configured to modulate either a power order or a dc-link voltage order to control coupling between voltage and power. The controller can be connected to a wind-based turbine generator of a wind farm and regulate power deployed to a power grid.
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This application claims the benefit of U.S. Provisional Application Ser. No. 62/701,029, filed Jul. 20, 2018, the disclosure of which is hereby incorporated by reference in its entirety, including all figures, tables, and drawings.
BACKGROUND4 Hz oscillations and 30 Hz oscillations have been observed in real world wind farms connected to weak grid power systems. Stability issues caused by these occurrences can limit the efficiency of the delivery of wind-based energy to a power grid.
Weak grid stability of power systems can be due to the coupling of the power delivery and the voltage at the point of common coupling (PCC). Increasing the power delivery can lead to a reduction in the PCC voltage and lead to instability in weak grid power systems. By reducing the instability in a weak grid power system, the delivery efficiency to a power grid can be enhanced.
BRIEF SUMMARYEmbodiments of the subject invention provide methods and devices for mechanism-based feedback control for vector control-based voltage source converters (VSCs) employed in wind-based power systems.
Embodiments of the subject invention provide methods and devices that reduce the coupling between power and voltage. Feedback control strategies are provided that can modulate either the power order or the dc-link voltage order with either the d-axis current or the PCC voltage as an input signal. Experiments of the PCC voltage feedback control have demonstrated the capability of the devices and methods for enhancing the stability of a power system and improved delivery of wind-based energy to a power grid.
The following disclosure and exemplary embodiments are presented to enable one of ordinary skill in the art to make and use controller system comprising a vector-based voltage source converter according to the subject invention. Various modifications to the embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the devices and methods related to the controller system comprising the vector-based voltage source converter are not intended to be limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features described herein.
A “weak grid” power system comprises a grid in which voltage level does not remain as constant as in a “strong grid” power system, such that the voltage level and fluctuations need to be taken into account. Weak grid power systems can also be characterized by low short circuit capacity, low inertia, and low fault currents.
Vector control can be based on the PCC voltage, (i.e., the dq-frame's d-axis is aligned with the PCC voltage space vector). Hence P=VPCCid and Q=−VPCCiq. For a given PCC voltage, adjusting the d-axis current can adjust the active power P and not influence the reactive power Q at the steady state. Similarly, adjusting the q-axis current can adjust the reactive power Q and not influence the active power P. The converter's outer control loops can generate dq-axis current orders and a current control effect can be represented by a first order delay.
The relationship between the wind farm currents, PCC voltage, and the grid voltage is as follows:
vPCC,d+jvPCC,q=jXg(id+jiq)+
It is assumed that
VPCC'vPCC,d=−Xgiq+Vg cos δ
0=vpCC,q=Xgid−Vg sin δ (2)
wherein δ is the angle by which
Combining the two equations in (2) leads to the following:
VPCC=−Xgiq+√{square root over (Vg2−(Xgid)2 )} (3)
ΔVPCC=−XgΔiq-−cΔid (4)
wherein
and c→∞ if id is close to the short circuit current, Vg/Xg.
Equation 4 indicates that an increase in the d-axis current leads to a reduction in the PCC voltage. Further, the linear expression of P versus VPCC and id can be found as follows:
P=VPCCid→ΔP=idΔVPCC+VPCCΔid (5)
The entire system's linear model including the circuit and vector control can be seen in
At the steady state, G(s→∞)=0. If the bandwidth of block G(s) is high, in a lower frequency range, then G(s) from equation (6) can be equivalent to 0. Faster voltage control can lead to increased bandwidth and be beneficial for stability. Additionally, slower power control is also beneficial for stability. The root locus method can verify that both faster voltage control and slower power control are beneficial to the stability of a power system.
The system in
wherein τp=Kpp/Kip.
The root loci based on L1 for two sets of voltage control parameters are shown in
The coupling between power and voltage can be suppressed by introducing feedback control to modulate the power order or the dc-link voltage order for vector control-based grid-side converters. Input signals can be either the d-axis converter current or the PCC voltage. The feedback control is implemented in both analytical models and detail model-based MATLAB/SimPowerSystems Type-3 and Type-4 wind testbeds. The analytical models verify that the feedback control can improve weak grid power system operation for VSCs in both power control and dc-link voltage control modes. The MATLAB/SimPowerSystems testbeds demonstrate that the PCC voltage-based control can significantly improve operation margins for both Type-3 and Type-4 wind farms.
Either the PCC voltage measured by PLL ΔVpll or the converter d-axis current Δi1d can be used to modulate the real power order. The output of a proportional control method using ΔVpll as an input signal is added to the real power order. The output of the proportional control method using Δi1d as an input is subtracted from the real power order.
The performance of the feedback control can be analyzed based on the eigenvalue loci generated from the analytical model (Model 1). Because the analytical model is nonlinear, an initialization procedure is required to perform a flat run. At the steady state, the output from the stability control power is zero. The parameters used in the analytical model are listed in Table I.
The system is assumed to operate at 0.9 pu power and the PCC voltage is set at 1 pu. Eigenvalue loci are plotted in
In control design, a small gain is preferred to avoid reaching system limits. A current-based stability control requires a larger gain than voltage-based stability control.
Stability control can be implemented to modulate the dc-link voltage reference instead of the power order if the VSC is in dc-link voltage control mode. To have a similar effect as modulating the power order, an integrator can be used. Experiments show that modulating the dc-link voltage reference with the output from an integrator control with PCC voltage input can lead to increase or reduction of the dc-link voltage at the steady state. Therefore, a high pass filter (HPF)
can be used after the integrator to filter out the dc component. Combining the integrator 1/s and the HPF can be equivalent to a low pass filter (LPF)
This control implementation is presented in
The eigenvalue loci for the system (Model 2), as seen in
It can be seen that the stability control can enhance the system stability. In addition, for VSCs in dc-link voltage control mode, the gain required for the current-based stability control is very large.
PCC voltage-based stability control, the wind farm can transfer more than 0.9 pu power to a very weak grid power system (SCR=1). It can be seen when P=0.97 and Xg =1, the system has two oscillation frequencies, one at 7 Hz and the other at 2 Hz. The time-domain simulation results corroborate with the eigenvalue analysis in
Final stage validation was carried out in two testbeds in MATLAB/SimPowerSystems. The testbeds aligned with the real-world system with full dynamics and converter limitations. The two testbeds were based on the demo testbeds of Type-3 wind and Type-4 wind in SimPowerSystems. The topologies of Type-3 and Type-4 wind testbeds are shown in
Both of wind farms were connected to the grid through respective 220 kV long transmission lines. The respective parameters of the two testbeds are listed in Table I and Table II.
The testbeds imposed limitations on the respective converter currents. In the Type-3 wind testbed, the limitation of the RSC current was [0 0.9] pu. In the Type-4 wind testbed, the limitation was [−1.1 1.1] pu.
In the Type-3 wind farm, the feedback control loop was implemented in a rotor-side converter (RSC) to change the power order. The wind farm power base was 100 MW, while the rated power output of the wind farm was 90 MW or 0.9 pu. At the steady state, the rotating speed of the rotor was 1.25 pu and the slip value was −0.25. With the total d-axis current from wind at 0.90 pu, the RSC d-axis current was 0.72 pu and the GSC d-axis current was 0.18 pu to the grid.
Without stability control, the system suffered 3 Hz oscillations. This performance aligns with the eigenvalue analysis presented in
The system operating limit increases with voltage-based control.
The power base of the Type-4 wind was 110 MW and the rated power was 100 MW or 0.9 pu. For Type-4 wind, the feedback control was implemented in a GSC to modulate the VDC order. In the first case study, the system dynamic responses without control, with voltage or current-based control were compared. Xg was increased from 0.5 pu to 0.61 pu at 2 seconds to emulate a parallel transmission line tripping event.
Without stability control, 3 Hz oscillations appeared after the dynamic event. Both of the voltage-based and current-based control methods made the system stable. The voltage-based control (Kvpll=2) had shorter transients and lower overshoot than the current-based control (Kid=4000).
The system operating limits were examined with stability control equipped. The values of Kid and Kvpll were set to 4000 and 2, respectively.
The methods and processes described herein can be embodied as code and/or data. The software code and data described herein can be stored on one or more machine-readable media (e.g., computer-readable media), which may include any device or medium that can store code and/or data for use by a computer system. When a computer system and/or processer reads and executes the code and/or data stored on a computer-readable medium, the computer system and/or processer performs the methods and processes embodied as data structures and code stored within the computer-readable storage medium.
It should be appreciated by those skilled in the art that computer-readable media include removable and non-removable structures/devices that can be used for storage of information, such as computer-readable instructions, data structures, program modules, and other data used by a computing system/environment. A computer-readable medium includes, but is not limited to, volatile memory such as random access memories (RAM, DRAM, SRAM); and non-volatile memory such as flash memory, various read-only-memories (ROM, PROM, EPROM, EEPROM), magnetic and ferromagnetic/ferroelectric memories (MRAM, FeRAM), and magnetic and optical storage devices (hard drives, magnetic tape, CDs, DVDs); network devices; or other media now known or later developed that are capable of storing computer-readable information/data. Computer-readable media should not be construed or interpreted to include any propagating signals. A computer-readable medium of the subject invention can be, for example, a compact disc (CD), digital video disc (DVD), flash memory device, volatile memory, or a hard disk drive (HDD), such as an external HDD or the HDD of a computing device, though embodiments are not limited thereto. A computing device can be, for example, a laptop computer, desktop computer, server, cell phone, or tablet, though embodiments are not limited thereto.
All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.
It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof can be suggested to persons skilled in the art and are to be included within the spirit and purview of this application. In addition, any elements or limitations of any invention or embodiment thereof disclosed herein can be combined with any and/or all other elements or limitations (individually or in any combination) or any other invention or embodiment thereof disclosed herein, and all such combinations are contemplated with the scope of the invention without limitation thereto.
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Claims
1. A controller system for a wind-based power system, the controller comprising:
- a vector control-based voltage source converter configured to have feedback control circuitry;
- wherein the feedback control circuitry is configured to modulate either a power order or a dc-link voltage order,
- wherein a first input signal into the controller system is an alternating current (AC) voltage, and
- wherein the feedback control circuitry is configured such that a change in magnitude of the AC voltage is used as an input into the feedback control circuitry to modulate the power order or the dc-link voltage order.
2. The controller system of claim 1, wherein the feedback control circuity is configured to reduce coupling between an active power and a voltage at a point of common coupling.
3. The controller system of claim 1, wherein a second input signal into the controller system is a d-axis converter current.
4. The controller system of claim 1, wherein the first input signal into the controller system is a voltage at a point of common coupling.
5. The controller system of claim 3, wherein the controller system is configured to modulate either the power order or the dc-link voltage order by using the d-axis converter current.
6. The controller system of claim 4, wherein the controller is configured to modulate either a power order or a dc-link voltage order by using the voltage at a point of common coupling.
7. The controller system of claim 1, further comprising:
- an integrator; and
- a high pass filter connected to the integrator,
- wherein the integrator and the high pass filter are configured to modulate the dc-link voltage order.
8. The controller system of claim 1, wherein the controller system is connected to a wind-powered turbine generator.
9. The controller system of claim 1, wherein the controller system is connected to a power grid system.
10. The controller system of claim 9, where the power grid system is a weak grid power system.
11. The controller system of claim 1, wherein the controller system is configured for Type-3 wind.
12. The controller system of claim 1, wherein the controller system is configured for Type-4 wind.
13. The controller system of claim 1, wherein the first input signal is a phase-locked-loop (PLL) voltage of the controller system.
14. The controller system of claim 4, wherein the voltage at a point of common coupling is a PLL voltage.
15. The controller system of claim 3, wherein the first input signal into the controller system is a voltage at a point of common coupling.
16. The controller system of claim 15, wherein the voltage at a point of common coupling is a PLL voltage.
17. A controller system for a wind-based power system, the controller comprising:
- a vector control-based voltage source converter configured to have feedback control circuitry;
- wherein the feedback control circuitry is configured to modulate either a power order or a dc-link voltage order,
- wherein a first input signal into the controller system is an alternating current (AC) voltage,
- wherein the feedback control circuity is configured to reduce coupling between an active power and a voltage at a point of common coupling,
- wherein a second input signal into the controller system is a d-axis converter current.
- wherein the first input signal into the controller system is a voltage at a point of common coupling,
- wherein the controller system is configured to modulate either the power order or the dc-link voltage order by using the d-axis converter current.
- wherein the controller is further configured to modulate either a power order or a dc-link voltage order by using the voltage at a point of common coupling,
- wherein the controller system further comprises: an integrator; and a high pass filter connected to the integrator,
- wherein the integrator and the high pass filter are configured to modulate the dc-link voltage order,
- wherein the controller system is connected to a wind-powered turbine generator or a weak grid power system,
- wherein the controller system is configured for Type-3 wind or Type-4 wind, and
- wherein the first input signal is a phase-locked-loop (PLL) voltage of the controller system.
Type: Application
Filed: Jan 30, 2019
Publication Date: Jan 23, 2020
Applicant: University of South Florida (Tampa, FL)
Inventors: LINGLING FAN (Tampa, FL), ZHIXIN MIAO (Tampa, FL)
Application Number: 16/261,870