Linear quadratic regulator based current control of grid connected inverter for Renewable Energy Applications
07 Apr 2016-pp 106-111
TL;DR: In this article, a grid connected renewable energy system is modeled as single phase equivalent circuit and an optimal control law is derived using Linear Quadratic Regulator (LQR), where the reference currents are generated using instantaneous real reactive (p - q) theory to have low grid current Total Harmonic Distortion (THD) and power factor closer to unity in the grid side.
Abstract: Harnessing electrical power from Renewable Energy Sources (RES) to the grid in a more efficient way requires a power electronic converter. But the power electronic devices introduce harmonics thus affecting the quality of power. A high performance controller is essential to control the real and reactive power flow between the RES and grid without affecting the quality of power. In this paper, grid connected Renewable Energy System is modeled as single phase equivalent circuit and an optimal control law is derived using Linear Quadratic Regulator (LQR). The reference currents are generated using instantaneous real - reactive (p - q) theory to have low grid current Total Harmonic Distortion (THD) and power factor closer to unity in the grid side. To show the supremacy of the LQR method, simulation results are compared with the conventional Hysteresis Controller (HC) for three phase three wire system with balanced grid voltage conditions. It is inferred from the results that LQR is superior in its tracking ability and has better stability
Citations
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TL;DR: A novel model predictive control strategy for stand-alone voltage source converters (VSCs) with constant switching frequency that allows the usage of linear theory for tuning the parameters and the superior characteristics of this control strategy compared to cascaded linear control and finite control set MPC are demonstrated experimentally.
Abstract: This paper introduces a novel model predictive control (MPC) strategy for stand-alone voltage source converters (VSCs) with constant switching frequency. Multiple control targets are incorporated in a single cost function. This allows fulfilling all the requirements for an uninterruptible power supply (UPS) inverter in terms of fast dynamic response, elimination of steady-state error, and harmonic compensation capability. The control output vector is determined as an analytical solution that corresponds to the minimum value of the cost function. The space vector modulation (SVM) is then used to synthesize the corresponding switching sequence. Therefore, this control strategy is equivalent to the linear state feedback control, which allows the usage of linear theory for tuning the parameters. The superior characteristics of this control strategy compared to cascaded linear control and finite control set MPC are, finally, demonstrated experimentally.
29 citations
Cites background from "Linear quadratic regulator based cu..."
...Recently, work in [10] has been addressing to the problem of fast regulation of UPS systems using linear quadratic regulator (LQR)....
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TL;DR: A threshold virtual impedance has been added to the state-feedback control in order to protect the inverter against overcurrent and to choose the right cost factors that allow the system to achieve the imposed specifications.
Abstract: Grid-forming inverters usually use inner cascaded controllers to regulate output AC voltage and converter output current. However, at the power transmission system level where the power inverter bandwidth is limited, i.e., low switching frequency, it is difficult to tune controller parameters to achieve the desired performances because of control loop interactions. In this paper, a direct AC voltage control-based state-feedback control is applied. Its control gains are tuned using a linear quadratic regulator. In addition, a sensitivity analysis is proposed to choose the right cost factors that allow the system to achieve the imposed specifications. Conventionally, a system based on direct AC voltage control has no restriction on the inverter current. Hence, in this paper, a threshold virtual impedance has been added to the state-feedback control in order to protect the inverter against overcurrent. The robustness of the proposed control is assessed for different short-circuit ratios using small-signal stability analysis. Then, it is checked in different grid topologies using time domain simulations. An experimental test bench is developed in order to validate the proposed control.
23 citations
TL;DR: In this article, a tracking controller for the power sharing by interlink bidirectional AC/DC converter in a hybrid alternating current/direct current (AC/DC) microgrid is proposed.
Abstract: A hybrid alternate current/direct current (AC/DC) microgrid consists of an AC subgrid and a DC subgrid, and the subgrids are connected through the interlink bidirectional AC/DC converter. In the stand-alone operation mode, it is desirable that the interlink bidirectional AC/DC converter manages proportional power sharing between the subgrids by transferring power from the under-loaded subgrid to the over-loaded one. In terms of system security, the interlink bidirectional AC/DC converter takes an important role, so proper control strategies need to be established. In addition, it is assumed that a battery energy storage system is installed in one subgrid, and the coordinated control of interlink bidirectional AC/DC converter and battery energy storage system converter is required so that the power sharing scheme between subgrids becomes more efficient. For the purpose of designing a tracking controller for the power sharing by interlink bidirectional AC/DC converter in a hybrid AC/DC microgrid, a droop control method generates a power reference for interlink bidirectional AC/DC converter based on the deviation of the system frequency and voltages first and then interlink bidirectional AC/DC converter needs to transfer the power reference to the over-loaded subgrid. For efficiency of this power transferring, a linear quadratic regulator with exponential weighting for the current regulation of interlink bidirectional AC/DC converter is designed in such a way that the resulting microgrid can operate robustly against various uncertainties and the power sharing is carried out quickly. Simulation results show that the proposed interlink bidirectional AC/DC converter control strategy provides robust and efficient power sharing scheme between the subgrids without deteriorating the secure system operation.
16 citations
Cites methods from "Linear quadratic regulator based cu..."
...In [21], LQR technique is proposed for the cu rent control of grid-co nected inverter by considering its advantages, such as g od transient response and excellent stability margins....
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...In [21], LQR technique is proposed for the current control of grid-connected inverter by considering its advantages, such as good transient response and excellent stability margins....
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TL;DR: In this paper, a hierarchical actor-critic algorithm is developed, whose computational complexity is independent of the total number of agents, and its global linear convergence to the optimal policy is proved.
Abstract: Multi-agent reinforcement learning has been successfully applied to a number of challenging problems. Despite these empirical successes, theoretical understanding of different algorithms is lacking, primarily due to the curse of dimensionality caused by the exponential growth of the state-action space with the number of agents. We study a fundamental problem of multi-agent linear quadratic regulator in a setting where the agents are partially exchangeable. In this setting, we develop a hierarchical actor-critic algorithm, whose computational complexity is independent of the total number of agents, and prove its global linear convergence to the optimal policy. As linear quadratic regulators are often used to approximate general dynamic systems, this paper provided an important step towards better understanding of general hierarchical mean-field multi-agent reinforcement learning.
13 citations
Cites background or methods from "Linear quadratic regulator based cu..."
...LQR serves as a powerful model for optimal control problems and has achieved tremendous success in real-world problems such as Unmanned Aerial Vehicle (UAV) (Zhi et al., 2017; Setyawan et al., 2019) and Power Grids (Minciardi and Sacile, 2012; Vinifa and Kavitha, 2016)....
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...As LQR models are fundamental in approximating general dynamic systems, our method readily applies to a number of practical settings, such as Unmanned Aerial Vehicle (UAV) (Zhi et al., 2017; Setyawan et al., 2019) and Power Grids (Minciardi and Sacile, 2012; Vinifa and Kavitha, 2016)....
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TL;DR: This paper presents a control to mitigate these power quality issues in addition to the control of power flow between solar photovoltaic system and non-linear load, using linear quadratic regulator.
4 citations
References
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TL;DR: In this article, the performance of linear controllers such as proportional-integral, proportional-resonant, and deadbeat (DB) controllers for grid-connected distributed power generation systems has been evaluated.
Abstract: This paper discusses the evaluation of different current controllers employed for grid-connected distributed power generation systems having variable input power, such as wind turbines and photovoltaic systems. The focus is mainly set on linear controllers such as proportional-integral, proportional-resonant, and deadbeat (DB) controllers. Additionally, an improved DB controller robust against grid impedance variation is also presented. Since the paper discusses the implementation of these controllers for grid-connected applications, their evaluation is made in three operating conditions. First, in steady-state conditions, the contribution of controllers to the total harmonic distortion of the grid current is pursued. Further on, the behavior of controllers in the case of transient conditions like input power variations and grid voltage faults is also examined. Experimental results in each case are presented in order to evaluate the performance of the controllers.
886 citations
TL;DR: In this paper, the instantaneous power theory was introduced for steady and transient states and for generic voltage and current waveforms, and an example showing how this theory can be used to design and control an active power filter is presented.
Abstract: Conventional active and reactive power theory, valid for the steady-state analysis is reviewed. The instantaneous power theory, introduced by H. Akagi et al. (1983, 1984) is also presented. This instantaneous theory is valid for steady and transient states and for generic voltage and current waveforms. Some examples explaining the physical meaning of the new concepts are presented. By using the concepts of symmetrical components together with the new theory, the powers in an unbalanced system are analyzed, including the zero-sequence instantaneous power. An example showing how this theory can be used to design and control an active power filter is presented. Some simulation results are presented and discussed. >
361 citations
TL;DR: In this paper, a current controller for voltage-source inverters in microgrids is proposed to inject a clean sinusoidal current to the grid, even in the presence of nonlinear/unbalanced loads and/or gridvoltage distortions.
Abstract: In this paper, a current-control strategy is proposed for voltage-source inverters in microgrids. The main objective of the proposed controller is to inject a clean sinusoidal current to the grid, even in the presence of nonlinear/unbalanced loads and/or grid-voltage distortions. The repetitive control technique is adopted because it can deal with a very large number of harmonics simultaneously. The proposed current controller consists of an internal model and a stabilizing compensator, which is designed by using the H∞ control theory. It turns out that the stabilizing compensator may be simply an inductor. This leads to a very low total harmonic distortion (THD) and improved tracking performance. In order to demonstrate the improvement of performance, the proposed controller is compared with the traditional proportional-resonant, proportional-integral, and predictive deadbeat controllers. The control strategies are evaluated in the grid-connected mode with experiments under different scenarios: steady-state and transient responses without local loads, and steady-state responses with unbalanced resistive and nonlinear local loads. The proposed controller significantly outperforms the other control schemes in terms of the THD level, with the price of slightly slower dynamic responses.
272 citations
"Linear quadratic regulator based cu..." refers background in this paper
...In comparison with conventional controllers, H∞ controller provides better performance by offering low grid current THD even in the presence of non linear loads, but it has slow dynamics [22]....
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TL;DR: The obtained experimental results show that the proposed adaptive SRF-PLL highly rejects the undesired harmonics even if the fundamental harmonic frequency of a highly polluted grid voltage abruptly varies.
Abstract: The proper operation of grid-connected power electronics converters needs using a synchronization technique to estimate the phase of the grid voltage. The performance of this synchronization technique is related to the quality of the consumed or delivered electric power. The synchronous-reference-frame phase-locked loop (SRF-PLL) has been widely used due to its ease of operation and robust behavior. However, the estimated phase can have a considerable amount of unwanted ripple if the grid voltage disturbances are not properly rejected. The aim of this paper is to propose an adaptive SRF-PLL which strongly rejects these disturbances even if the fundamental frequency of the grid voltage varies. This is accomplished by using several adaptive infinite-impulse-response notch filters, implemented by means of an inherently stable Schur-lattice structure. This structure is perfectly suited to be programmed in fixed-point DSPs (i.e., it has high mapping precision, low roundoff accumulation, and suppression of quantization limit cycle oscillations). The proposed adaptive SRF-PLL has been tested by means of the TI TMS320F2812 DSP. The obtained experimental results show that the proposed synchronization method highly rejects the undesired harmonics even if the fundamental harmonic frequency of a highly polluted grid voltage abruptly varies.
268 citations
TL;DR: The proposed control system (implemented digitally) provides superior performance over the conventional multiple proportional-integral and proportional-resonant control methods due to the absence of the PARK's transformation blocks as well as phase lock loop requirement in the control structure.
Abstract: In this paper, a novel current control technique, implemented in the a-b-c frame, for a three-phase inverter is proposed to control the active and reactive power flow from the renewable energy source to a three-phase generalized microgrid system. The proposed control system not only controls the grid power flow but also reduces the grid current total harmonic distortion in the presence of typical nonlinear loads. The control system shapes the grid current taking into account the grid voltage unbalance, harmonics as well as unbalance in line side inductors. The stability of the control system is ensured by the direct method of Lyapunov. A SRC is also proposed to improve the performance of the current controller by estimating the periodic disturbances of the system. The proposed control system (implemented digitally) provides superior performance over the conventional multiple proportional-integral and proportional-resonant control methods due to the absence of the PARK's transformation blocks as well as phase lock loop requirement in the control structure. A new inverter modeling technique is also presented to take care of unbalances both in grid voltages and line side inductors. Experimental results are provided to show the efficacy of the proposed control system.
105 citations