Power Control Loop Research Articles

Comparing strategies to damp electromechanical oscillations through STATCOM with multi-band controller

This paper aims to compare two strategies for damping low-frequency electromechanical oscillations in multi-machine power systems through Static Synchronous Compensators (STATCOM) with a multi-band controller. STATCOM is represented by a controllable voltage source behind an impedance and the multi-band controller acts as a power oscillation damper that modulates parameters of the voltage source in the transient period. In the first strategy, the multi-band controller acts on the voltage-control loop through the voltage modulation. In the second one, the controller acts on the real power-control loop to modulate the phase angle of the voltage source. The coordinated design of multi-band controllers and power systems stabilizers is performed through an optimization approach taking into account several operation conditions.

Incorporation of active power ancillary services into VSC-HVDC connected energy sources

This paper investigates the interaction between frequency containment services (FCS) and power oscillation damping (POD) ancillary services installed in a Voltage Source Converter High Voltage DC (VSC-HVDC) line. In VSC-HVDC systems, ancillary services can be installed to maintain appropriate levels of power system stability. Both FCS and POD controllers are typically installed in the active power control loop separately. The modulation signals generated by these two ancillary services both affect the active power injected by the converter. When operating a VSC with FCS and POD ancillary services the resultant combination of the modulation signals in the active power loop may result in reduced performance. The results in this study show that performance of controllers reduces when both ancillary services are activated, particularly in low-inertia systems. Modifications to the control structures of both controllers that improve their performance under various disturbances are proposed.

A Novel Power-Angle Control Method of DFIG-DC System Based on Regulating Air Gap Flux Vector

This article presents a novel power-angle control method of the doubly fed induction generator dc system based on regulating the air gap flux vector. The relationship between the angle of air gap electromotive force (EMF) vector and the stator output power is revealed in this article, which can be used for controlling the output power directly. Since the stator frequency is determined by the rotating speed of air gap flux vector and the output power is determined by the angle of air gap EMF vector, the stator frequency can be generated by the power control loop that can avoid the classical voltage model or current model and reduce the parameter dependence. The stator frequency can be regulated through the d-axis rotor current since the product of the stator frequency and the d-axis rotor current is constant. Furthermore, the improved direct resonant control for torque ripple and harmonic current mitigation can still be applied in this novel control method because air gap flux orientation is achieved through controlling the q-axis air gap flux to be zero. Finally, the experimental results are presented to validate the proposed control method.

Rotor Current Oriented Control Method of DFIG-DC System Without Stator Side Sensors

This letter proposes a novel stator power and frequency control method of doubly-fed induction generator (DFIG)-dc system, which is based on rotor current orientation and avoids using stator side sensors. By means of establishing the relationship of the rotor current vector, stator active power, and stator frequency, the stator active power can be controlled by the angle between the rotor current vector and stator voltage vector. The stator frequency can be controlled by the magnitude of the rotor current vector. The stator active power is calculated by dc voltage and dc current at stator side. In this way, the stator voltage and current sensors can both be eliminated. Furthermore, the stator frequency and orientation angle are acquired by the power control loop, which avoids using voltage model or current model. In this way, both parameter dependency and dc sampling uncertainties can be eliminated. Finally, experiments based on a 1-kW DFIG-dc setup were carried out to verify the proposed method.

Analysis of potential low frequency resonance between a 1GW MMC HVDC and a nearby nuclear generator

This paper presents a comprehensive analytical eigenvalue stability study of subsynchronous torsional interactions between the 1 GW Eleclink MMC HVDC and the 1.12 GW Gravelines turbine-generator in north France. It revalidates the conclusions of previous studies on VSC-based HVDC systems by showing that the risk of SSTI may be of significance when the HVDC is based on the MMC and not the two level VSC technology. The study demonstrates that in the case of the weak French grid and the rated HVDC power flow from France to England, the HVDC can destabilize the dominant low frequency, 6.3 Hz, torsional mode. The analysis also expands the work of previous studies on MMC and reveals that the case of the HVDC power flow from the English to the French grid may also introduce the risk of destabilizing the lower frequency mode. Furthermore, it also reveals a noticeable impact of the PLL and that increasing the gains has an opposite effect on the two HVDC power flow directions. Moreover, participation factors are analyzed for the 6.3 Hz mode indicating that the main cause of the torsional interactions is the HVDC power control loop. The main findings are verified on the detailed EMTP model.

Direct Power Control of Shunt Active Power Filter Using Space Vector Modulation Based on Supertwisting Sliding Mode Control

This article proposes direct power control (DPC) associated with space vector modulation (SVM) for a shunt active power filter (SAPF). This control is suggested in order to overcome the drawbacks of the conventional DPC, where the SVM is able to decrease the high active and reactive power ripples maintaining a fixed switching frequency. In addition, the performance of the proposed scheme is improved by replacing PI controllers with supertwisting second-order sliding mode controllers (ST-SMCs) in the active and reactive power control loops. This technique provides high robustness and dynamics for external perturbations. The proposed control DPC-SVM based on ST-SMC is investigated by simulation and practical implementation using MATLAB/Simulink with a real-time interface based on a dSPACE 1104 board.

Cascaded Droop and Inverse Droop Regulation for Two-Layer Coordinated Power Flow Control in Series-Connected Power Cells

To interconnect string power cells into high voltage grid in a distributed manner, a cascaded power flow regulator is proposed for simultaneous control of the injected power to the grid and flexible power sharing among string power cells. First, the traditional frequency and voltage magnitude droop control at the central controller behaves as the outer power control loop for an accurate regulation of the system power flow to main grid. Then, by setting the frequency reference and voltage magnitude reference from outer power control loop as base values for each power cells local controller, inner voltage magnitude droop for apparent power S control and the frequency inverse droop for P/E control are implemented at each power cell local controller to achieve well-decoupled real and reactive power sharing among series-connected power cells. With this control architecture, the power cells and the entire grid-tied system can obtain the flexible power regulation in a fully voltage control manner. Accordingly, seamless grid-tied to islanding operation mode transfer and power sharing of the system with other converters in autonomous islanding operation can be easily obtained.

A Discrete Time Domain-Based MILP Framework for Control Parameter Tuning

The problem of tuning a fixed optimal set of parameters in time domain for dynamic systems is inherently nonlinear due to the product of tunable variables (controller gains and time constants) and time-dependent variables (bounded to the process), involved in equality constraints. This article presents a framework to transform such nonlinear problem into a mixed-integer linear problem through a binary-disjunctive representation of the problematic product between tunable and process-bounded variables, whereas addressing alternative linear representations to other nonlinearities such as saturation. A set of linear formulations in time domain are also developed to describe typical dynamic performance requirements, such as overshoot and settling time. The proposed framework was used to determine the optimal control parameters for disturbance rejection of the active power control loop in a photovoltaic generator with battery energy storage.

A Control Approach Integrating Electric Vehicle Charger and Dynamic Response of Demand-Side Resource Aggregators

Regardless of whether level 1, level 2, or dc fast charging, several methods are under development or available as an aftermarket option for Plug-in electric vehicles (EVs). While confining the charging behavior of EVs to low-load periods is difficult, it leads to significant and uncertain load peaks and valleys in the utility power network. Therefore, this paper proposes a new method for an electric vehicle charger. The proposed charging method involves power supervisory characteristics on the basis of the paradigm of a constant-current and a constant-voltage (CC–CV) charger so that the external commands from the demand-side resource aggregator in the smart grid can be executed. In the method, first, a feedback loop is included in the controller to formulate the relationship between battery voltage and charging power as a quadratic concave downward polynomial. The controlled output power at the beginning of constant voltage charging is then set to the maximum value of the quadratic polynomial because the charging power requirement near this point is closed to the highest. Hence, if the charging power is lower than the allowable maximum power, the proposed charger is determined to have followed the traditional CC–CV charging steps. On contrast, the proposed circuit launches the power control loop when the actual charging power is beyond the maximum power limitation of aggregator. The proposed charger can keep the power consumption inside an allowable range of aggregator. That is the power consumption at the EV charging sites becomes directly programmable by their aggregators. The proposed method is examined by conducting an interactive test of a remote energy management system and the proposed controller. The results revealed that the proposed method provide an easy method to dynamically control consumption pattern of the EV charging for demand-side power management.

A Vehicle-to-Microgrid Framework With Optimization-Incorporated Distributed EV Coordination for a Commercial Neighborhood

In this paper, an improved vehicle-to-microgrid (V2M) framework is proposed for a commercial locality operating as a hybrid ac–dc microgrid, which optimally coordinates electric-vehicle (EV) storages in a distributed manner. An aggregator model is proposed that solves the economic dispatch problem of parked EV storages in a centralized fashion and generates power references in real time for the designed EV storage controllers. Unlike the conventional EV storage controller, the proposed optimization-incorporated distributed EV storage controller can switch from decentralized to distributed control mode or vice versa based on situations and utilizes a sparse vehicle-to-vehicle communication network. The power flow between the ac and dc subgrids is managed by interlinking converters (IC). The IC control structure is augmented by combining voltage- and power-based droop-control schemes in the power control loop. This modification enables simultaneous ac and dc bus voltage regulations. Case studies are carried out to validate the efficacy of the developed framework with a real commercial network and loads. The results exhibit robust performance of the overall system for generation-demand variability, transitions between islanded and grid-tied conditions, and user-preferred EV disconnections and time delay.

Control-Winding Direct Power Control Strategy for Five-Phase Dual-Stator Winding Induction Generator DC Generating System

A control-winding direct power control (CWDPC) strategy for a dc generating system based on the five-phase dual-stator winding induction generator is proposed and investigated in this article. For this generator, the flux-oriented control is usually employed in the previous work mainly for the sake of good static performance. However, the present control strategy has the limitation of the further enhancement of the dynamic properties. This article explores a CWDPC strategy to upgrade the dynamic performance of this generating system. The fundamental active power of the five-phase control winding (CW) relates to its dc bus voltage while the dc bus voltage on the five-phase power-winding (PW) side is related to the control of five-phase CW fundamental reactive power. By replacing inner current loops to power control loops, the CW instantaneous fundamental active and reactive powers are calculated through the power observer and regulated directly through two power-control loops. In the transient process, the CW fundamental active power changes slightly while the reactive power can be quickly controlled for fast recovery of the dc bus voltage on the five-phase PW side. As the system has an additional control degree of freedom, the third harmonic is also considered in the proposed strategy, and the corresponding implementation is given as well. The simulation and experimental results demonstrate that using the proposed CWDPC strategy, the system has shorter transient time and lower voltage change in the dynamic response than the flux-oriented control strategy, which means the dynamic performance is further improved.

Effect of Reactive Power Characteristic of Offshore Wind Power Plant on Low-Frequency Stability

Oscillation phenomena of offshore wind power plant (OWPP) in a wide frequency range can be caused due to impedance interactions between grid-connected inverters (GCIs) and transmission cables. In this article, impedance model of GCI with outer power control loop, inner current control loop and phase-locked loop is first established in dq reference frame. The correctness is validated by frequency scanning method. Then, the effects of active and reactive power/current references on dq impedance characteristics of GCI with/without consideration of power control loop are investigated using complex space vectors and complex transfer functions. Furthermore, RLC circuit model of transmission cable considering frequency-dependent characteristics is also established for dq-domain IBSC. On the basis of them, it's found that low-frequency oscillation phenomena of OWPP under power control mode may occur if active power reference exceeds a certain threshold value, which can be mitigated by injecting a certain amount of negative reactive power. Impacts of PLL parameters, length of transmission cable and number of paralleled GCIs on required negative reactive power for low-frequency stabilization are further investigated. Both Matlab/Simulink-based simulation and OPAL-RT-based real-time verification are implemented in an OWPP with four permanent magnet synchronous generators to validate the correctness of the reactive power characteristic analysis results and the feasibility of mitigating low-frequency oscillation phenomena by negative reactive power injection.

A Power Control Strategy for IoT Sensors Developed for 5G Networks

This research work presents a power control mechanism developed for ProSe-enabled sensors so that the sensors can be smoothly integrated into the fifth generation (5G) of mobile networks. It is strongly anticipated that 5G networks will provide an enabling environment for the 21st century innovations like the internet of things (IoT). Sensors are pivotal in IoT. The proposed power control mechanism involves an open loop power control (OLPC) mechanism that a ProSe-enabled sensor has to use to establish communication with a base station (BS) and a closed loop power control (CLPC) the BS then has use to establish transmit power levels for devices to be involved in a device to device (D2D) communication depending on the prevailing channel conditions. The results obtained demonstrate that the developed scheme does not adversely affect the quality of service (QoS) of a 5G mobile network.

Transient Oscillation Suppression Method of Modular Multilevel DC Transformer

The modular multilevel DC transformer (MMDCT) is more likely to suffer from transient oscillation, which is caused by insufficient damping of the equivalent RLC circuit with smaller arm resistance and submodule capacitance and may threaten the system stability. To tackle this issue, this paper conducts an in-depth analysis of the oscillation generating mechanism and thereby proposes a transient oscillation suppression method by incorporating an oscillation suppression angle (OSA) between the upper and lower arm switching voltages, with which the direction of the circulating current could be adjusted during this overlapped period. However, the open loop control of OSA shows indistinctive improvement of the damping ratio so that a damping compensator for regulating the circulating current is utilized. Furthermore, the compensating gain is optimally designed to achieve the critical damping state based on the small-signal model viewing OSA as an input variable. Besides, the outer capacitor voltage control loop and the power control loop are incorporated to provide the circulating current reference and to track the load power demand, respectively. Extensive simulation results from a realistically designed MMDCT model in PSIM validate the feasibility and effectiveness of the proposed transient oscillation suppression method.

Comprehensive control scheme of the interlinking converter in hybrid AC/DC microgrid

This paper presents a comprehensive control scheme for the interlinking converter (ILC) in a hybrid AC/DC microgrid consisting of the outer loop flexible power sharing control and the improved robust inner loop control. The outer loop power control of ILC is presented to achieve flexible power sharing of distributed generations (DGs) in the hybrid microgrid, depending on different power management objectives, which is realized based on the deduced balance state equation, and regulating the frequency and DC voltage at the same time. The improved robust inner loop control of ILC is also presented to suppress external disturbance and system model uncertainties with the improved dynamic response. This improved inner loop control which includes a disturbance observer link, can force the converter current to track the reference value with no steady error and improve the dynamic stability of the microgrid. With the proposed outer loop power sharing control and improved inner loop control, the comprehensive control scheme for the ILC is presented. Simulations cases show the effectiveness and superiority of the proposed comprehensive control scheme.

Grid-Forming Power Converters Tuned Through Artificial Intelligence to Damp Subsynchronous Interactions in Electrical Grids

The integration of non-synchronous generation units and energy storage through power electronics is introducing new challenges in power system dynamics. Specifically, the rotor angle stability has been identified as one of the major obstacle with regards to power electronics dominated power systems. To date, conventional power system stabilizer (PSS) devices are used for damping electromechanical oscillations, which are only tuned sporadically leading to significant deterioration in performance against the ever-changing operating conditions. In this paper, an intelligent power oscillation damper (iPOD) is proposed for grid-forming converters to attenuate electromechanical inter-area power oscillation. In particular, the iPOD is applied to a synchronous power controller (SPC) based grid-forming power converter to increases gain of the active power control loop at the oscillatory frequency. Predictions regarding the mode frequency, corresponding to the current operating points, are given by an artificial intelligence ensemble model called Random Forests. The performance of the proposed controller is verified using the two area system considering symmetrical fault for random operating points. In addition, a comparison with PSS installed in each generator reveals the individual contribution with respect to the inter-area mode damping.

Effects analysis of excitation circuit on power control for VSG: a design‐oriented study

Reactive power control loop (RPCL) in virtual synchronous generator (VSG) plays the role of excitation circuit in conventional SG. The control strategy improvement or parameters configuration method of RPCL is always the emphasis of analysis. However, the impacts of RPCL on VSG system performance are usually overlooked. On the basis of that, this study aims to investigate the effects of RPCL on power control performance. First, a simplified SG model is established and implemented to VSG. Then, VSG system modelling with state equations in power control time scale is elaborated. The excitation current in the modelling is regarded as one crucial state variable to identify the effects of the excitation circuit on VSG system. Eigenvalue locus analysis and eigenproperties analysis reveal how the excitation circuit affects power oscillation damping performance and even results in power coupling issue of power control. The effects of operating points on system power control are also taken into account. Finally, the experimental results are given to verify the theoretical analysis.

New Fast MPPT Method Based on a Power Slope Detector for Single Phase PV Inverters

This article presents a novel MPPT method for two stage PV inverters with a single phase connection to the power grid. The method takes advantage of the 100 Hz/120 Hz harmonic present on the DC-bus voltage to guide the MPP search. It consists of detecting the slope of the P-V curve and integrating it to obtain the duty-cycle. The power slope detector (PSD) is able to calculate the P-V slope to command the MPPT even at very low powers, where the amplitude of the oscillations is barely perceptible. Design equations are provided, both of the gain of the PSD and of the gain of the MPPT integrator. It also shows how this PSD-MPPT strategy can be combined with the power control, allowing regulation of powers lower than those of the MPP. The power control loop is analyzed, and its stability is related to a single gain to be designed. The PSD-MPPT is tested in a two stage PV inverter, where the step-up DC-DC converter consists of three parallel boost converters. The results show that the PSD-MPPT method can work without the measurement of the current in the boost converters, which implies a cost reduction. A PV efficiency of about 99.8% is obtained with a usual ripple in the DC-bus of 4% peak-peak. In addition, the PSD-MPPT method is characterized as being extremely fast, both in the MPP search and in the power control, with response times around 50 ms. The PSD-MPPT is a simple algorithm of constant parameters that can be solved in a low cost microcontroller at a sampling frequency of about 2 kHz, requiring only the voltage and current of the PV array.

Active Slip Frequency Based Islanding Detection Technique for Grid-Tied Inverters

In this article, an islanding detection technique (IDT) is proposed based on applying positive feedback of wide area measurement based slip frequency at the inverter reactive power control loop. The classical Q--f droop characteristic based IDT is affected due to variations in load and active power mismatch (APM) conditions that may exist during islanding condition. To overcome these issues, an adaptive reactive power injection based IDT is proposed. Adaptiveness in the proposed method is achieved by considering the gain of the reactive power control loop as a product of rate of change of susceptance with respect to frequency seen at the inverter terminals, square of per unit point of common coupling voltage and slip frequency, which is the difference between distribution generator frequency and a reference substation bus frequency. The application of slip frequency as a positive feedback parameter injects negligible reactive power during grid side frequency disturbances and also associates with under frequency and over frequency ride through requirements. The method is least effected by load quality factor variations and APM conditions. The IDT is able to detect islanding conditions with a negligible non detection zone. Real time simulation studies are performed using dSPACE DS1104 real time controller and RTDS/RSCAD.

Auxiliary signal‐assisted droop‐based secondary frequency control of inverter‐based PV microgrids for improvement in power sharing and system stability

This study proposes a modified decentralised droop controller for inverter-based autonomous photovoltaics (PV) microgrids to address the problem of instability and slower power sharing between PV inverters. The proposed controller modifies the conventional droop mechanism in transient as well as in steady state by adding an auxiliary control signal in the power control loop of the inverters. The proposed approach offers three key benefits, i.e. improves active power sharing, enhances system stability and performs the secondary control action by nullifying the frequency deviation by using only one auxiliary signal in the control loop. The accuracy of the proposed modified droop controller is verified with the help of time-domain simulations in MATLAB/SIMULINK. The results obtained show that the controller is able to improve the power sharing and stability of the microgrid as well as to restore frequency. The performance of the proposed control scheme is also validated by experimental results and small-signal analysis.