Voltage Control Method Research Articles

Most Valuable Player based selective harmonic elimination in a cascaded H-bridge inverter for wide operating range

Abstract Selective Harmonic Elimination PWM (SHEPWM) is a classical method of voltage control of the inverter while eliminating the undesirable lower order harmonics for low-frequency applications. Multivariable non-linear equations often result in SHE modulation scheme. In this work, a Most Valuable Player Algorithm (MVPA) has been applied to solve the non-linear SHE equations for the elimination of lower order harmonics in a cascaded H bridge multilevel inverter. The MVPA algorithm shows better convergence characteristics and wider modulation control compared to some popular meta-heuristic optimization methods. Optimized switching angles have been obtained for 5, 7 and 9 level inverters for different modulation index (MI) from 0.05 to 1. The Total Harmonic Distortion (THD) for the output voltage was found to be smoother compared to the powerful Differential Evolution (DE) algorithm for modulation index greater than 0.6. And for modulation index greater than 0.8, MVPA scores better than DE in terms of THD. Experimentally, the performance of MVPA has also been validated for 1-phase 5, 7 and 9 level cascaded H-bridge inverter and 5th, 5th and 7th, 5th, 7th, and 11th harmonic was eliminated respectively.

Speed sensorless direct voltage control of brushless doubly-fed machine independent generation system based on improved particle swarm optimization algorithm

The speed information is obtained by sensors in the control strategy of brushless doubly-fed machine (BDFM) independent power generation system. However, the use of sensors increases the hardware cost of the system, reduces the reliability and the sensors are difficult to adapt to the harsh environment such as high temperature and high humidity. In order to solve the above problems, a speed sensorless direct voltage control method for brushless doubly-fed machine independent generation system is proposed. At the same time, in order to improve the control quality, the improved particle swarm optimization algorithm is used to optimize the control parameters of the system. Simulation results show that the BDFM independent power generation system can still maintain excellent performance and can meet the performance requirements in various operating conditions with the control method proposed in this paper.

Highly efficient voltage-controlled magnetism in HfZrO/CoFeB hybrid film and Hall device

We investigate the voltage-controlled magnetism effect of HfZrO/CoFeB hybrid film and a Hall device with perpendicular magnetic anisotropy. The magnetization versus magnetic field experiments and anomalous Hall experiments before and after applying voltage are performed. The results exhibit that the coercive field of samples remain unchanged while the saturation magnetization shows a permanent increase (more than 60%), which is regardless of the direction of applied voltage. Different from conventional voltage-controlled magnetic anisotropy, in our work, only the saturation magnetization is enhanced by the applied voltage without trading off other magnetic parameters of CoFeB. Thus, such a finding proposes a more efficient voltage-controlled method to achieve a magnetic memory device with high thermal stability, high tunnel magnetoresistance and low switching current for magneto-resistive random-access memory under scaling beyond 2X nm.

Generic power flow algorithm for bipolar DC microgrids based on Newton–Raphson method

This paper proposes a generic power flow algorithm for bipolar DC microgrids based on the Newton–Raphson (NR) method. The bipolar DC microgrid has gained considerable attention for its effectiveness and reliability with regard to power supply. To date, however, power flow algorithms for bipolar DC networks have not been fully investigated and generalized, although power flow analysis is an essential tool, with diverse applications ranging from network design to real-time control. In this study, a current injection-based power flow algorithm is established that considers the grounding scheme and voltage control method of energy source. Six bus types are defined, depending on the grounding scheme and voltage control combination. The unknown pole voltages and equations to be iteratively solved are identified. Afterwards, the unknown pole voltages are updated via a single NR method, which makes the proposed method straightforward and efficient. The case study validates that the proposed power flow algorithm can find the bus voltages of each pole sufficiently close to the simulation results of PSCAD/EMTDC, while having a low computational burden.

An Improved LMSVM Method for Leakage Current Suppression and Neutral-Point Voltage Control in Transformerless NPC Three-Level Inverters

Transformerless three-level inverters have many advantages over two-level inverters in photovoltaic (PV) systems. However, in transformerless systems, except for the inherent unbalanced neutral voltage of the three-level inverter, the leakage current also needs to be limited strictly. Concerning the defects of the conventional common-mode voltage (CMV) suppression methods, an improved LMSVM (ILMSVM) method is proposed. Unlike the conventional CMV suppression method, the ILMSVM method no longer just abandons the high CMV small vectors to limit the range of CMV to ±1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6U_{\mathrm {dc}}$ </tex-math></inline-formula> , but limits CMV variation to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta 1/6U_{\mathrm {dc}}$ </tex-math></inline-formula> in all switching cycles. So, high CMV small vectors also can be used in some sequence to help controlling neutral point (NP) while keeping leakage current well suppressed. Moreover, medium vector sequences are also used as an independent control sequence to help achieving bidirectional control on NP potential. Compared with LMP/NSVM strategy, the ILMSVM method has advantages in regulating NP potential, and leakage current spikes of the LMP/NSVM method are also eliminated. In comparison with the conventional SVPWM (CSVPWM) method, leakage current in the ILMSVM method is also well restrained. Simulations and experiment results verify that the ILMSVM method can well restrain leakage current and maintain good control of NP in comparing with LMP/space vector modulation (SVM) method and CSVPWM method.

A Modified Neutral-point Voltage Control Strategy for Three-level Inverters Based on Decomposition of Space Vector Diagram

Capacitor voltage imbalance is a significant problem for three-level inverters. Due to the mid-point modulation of these inverter topologies, the neutral point potential moves up or down depending on the neutral point current direction creating imbalanced voltages among the two capacitors. This imbalanced capacitor voltage causes imbalanced voltage stress among the semiconductor devices and causes increase output voltage and current harmonics. This paper introduces a modified voltage balancing strategy using two-level space vector modulation. By decomposing the three-level space vector diagram into two-level space vector diagram and redistributing the dwell times of the two-level zero space vectors, the modified voltage balancing method ensures minimal NP voltage ripple. Compared to the commonly used NP voltage control method (using 3L SVM [9]), the proposed modified NP voltage control method offers a slightly higher neutral-point voltage ripple and output voltage harmonics but, it has much lower switching loss, code size and execution time.

Nonlinear Control Method of Photovoltaic Power Generation LVRT Based on Adaptive Maximum Power Tracking

With the continuous increase in the scale of photovoltaic grid connection, the impact of the safe and stable operation of photovoltaic power generation systems on the grid cannot be ignored. When a short-term failure of the power grid occurs, the large-scale disconnection of the unit will seriously affect the stability of the grid voltage and frequency. Therefore, this paper proposes a non-linear control method for photovoltaic power generation low voltage ride through (LVRT) based on adaptive maximum power tracking. This method adaptively adjusts the power tracking trajectory through the feedforward control of the voltage drop amplitude. Cooperating with the nonlinear control of the grid-connected inverter, this method can quickly and effectively control the power output of photovoltaic cells on the basis of providing appropriate reactive power support, so as to realize the rapid response of grid system. Through the simulation in PSCAD, it is verified that the control method described in this article can well realize the LVRT in different amplitudes of the photovoltaic system, especially when zero voltage drops occur.

Cluster Partition-Based Zonal Voltage Control for Distribution Networks With High Penetrated PVs

With the high penetrated photovoltaics (PVs) accessing distribution networks (DNs) in the future, the overvoltage of distribution network will be more serious, and the voltage control will be more complex. To solve this problem, a cluster partition-based zonal voltage control method for DN is proposed in this study. First, a cluster partition method using a comprehensive performance index is proposed: in terms of DN structure, a global density quality function index is introduced to measure the coupling degree of nodes in clusters. In terms of cluster function, the inconsistency coefficient index is presented to reflect the PV output characteristics, and a node membership function index is presented to limit the scale of the cluster. Then, the DN cluster partition is carried out under the fast Newman (FN) algorithm. Second, a second-order cone programming (SOCP) model of voltage control is established to maximize the PV consumption in each cluster. To achieve the coordinated optimization of clusters, an iterative optimization strategy among clusters is proposed. Finally, the effectiveness and rationality of the proposed method are verified in a 10 kV actual feeder system in Zhejiang Province, China.

Data-Driven Fast Voltage Control in Non-DPMU Distribution Networks With Microgrids

Traditional voltage control methods for distribution networks assume perfect knowledge of the power system model. Nevertheless, the extensive scale of future distribution networks makes it unrealistic to acquire the overall operation state monitoring. Moreover, with the deregulation of distribution networks, partial controllable resources belong to independent systems, such as microgrids, causing distribution system operators unable to force them to provide voltage support directly. To cope with the previously mentioned problems, a data-driven fast voltage control method for distribution networks with MGs is proposed in this article. First, voltage sensitivity matrices are estimated indirectly by identifying line parameters in a regression approach, without using measurement data of distribution phasor measurement units (DPMUs) in distribution networks. Then, an incomplete information game model is proposed to motivate MGs to provide ancillary services of voltage control. To guarantee privacy, only a little key information is shared among MGs and distribution system operators. Moreover, MGs make voltage control strategies autonomously based on the data-driven deep reinforcement learning algorithms, while maximizing their own profits. Finally, we test the method on the modified IEEE 33-node networks and IEEE 123-node networks. The results demonstrate that the proposed method can provide an accurate voltage estimation in electricity markets with non-DPMU measurement data and increase energy and asset utilization.

Coordinated Distributed Voltage Control Methods for Standalone Microgrids

A microgrid is a small-scale power grid comprising distributed generators (DGs), distributed storage systems, and loads. It will lose contribution from the main grid if it shifts to islanded mode due to pre-planned or unforeseen disturbances. To restore the terminal voltages of all the distributed generators to the reference value, this paper presents three coordinated secondary control strategies. First, motivated by the synchronization control theory of multiagent systems, a distributed control technique is developed where each of the DGs is considered an agent and they exchange information via a communication network. second, a two-level control technique is designed in which a global controller is employed to monitor the overall performance of the DGs by transmitting corrective signals to the local controllers of the agents. In this technique, all the communication is between the global controller and the local controllers without any direct communication between the agents. Third, decentralized control is provided in which each DG is separately controlled by its local controller that operates based on the local feedback measurements. Simulations are carried out on an islanded microgrid consisting of four DGs to illustrate our design approach.

Cooperative Markov Decision Process for Voltage Control of Grid-Tied Photovoltaic Generators With Reactive Power Support

The problem of voltage control (VC) by means of inverter reactive support in distribution grids with photovoltaic (PV) generation is addressed. A novel decentralized approach for VC is proposed, which consists in heuristic specification of a distributed reactive support cooperative partially observable Markov decision process (DRS-C-POMDP) whose agents control groups of PV nodes clustered according to voltage correlation. The proposed method employs offline policy optimization, which yields closed-form reactive power setpoint control laws. Hence, the online processing time required by most VC methods is avoided and controller implementation is simplified. The method is validated via simulation on a 123-bus feeder, in which it is compared to other decentralized heuristic methods from the literature. The results indicate that, for identical local inverter controls, the proposed method yields superior performance.

Control Design of a 99% Efficiency Transformerless EV Charger Providing Standardized Grid Services

A nonisolated DC charger is proposed for electric vehicle (EV) battery system. The leakage current can be reduced by the novel charging system without bulky transformer. A zero sequence voltage control method is developed to stabilize the common-mode voltage, thus, reduce the leakage current. The proposed fast charger includes two energy conversion stages: DC–DC converter for battery side control; DC–AC converter for grid interface and common-mode voltage control. The parameters of the system with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter are analyzed and optimized for a better performance of grid interface. Grid service functions are designed for the EV charger to provide grid-voltage/frequency compensations. High power (22 kW) and high efficiency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 99%) are achieved with low-leakage current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt;$</tex-math></inline-formula> 20 mA). The experiments are implemented to verify the proposed EV charger system.

Deep Reinforcement Learning-Based Adaptive Voltage Control of Active Distribution Networks with Multi-terminal Soft Open Point

The integration of highly penetrated distributed generators (DGs) aggravates the rise of voltage violations in distribution networks. Connected by multi-terminal soft open points (M−SOPs), distribution networks gradually evolve into an interconnected flexible architecture with high controllability. Distribution networks with M−SOPs can exchange active power flexibly, and M−SOPs can provide local reactive power support to alleviate voltage violations. However, conventional model-based M−SOP optimization methods cannot regulate voltage profiles adaptively owing to the rapid fluctuations of DGs. In this paper, a data-driven voltage control method is proposed for M−SOPs using a deep deterministic policy gradient network (DDPG). First, the data-driven voltage control framework is proposed for M−SOPs based on DDPG. The M−SOP−based voltage control problem is reformatted as a Markov decision process (MDP) to construct the DDPG agent. Based on real-time measurement, the DDPG agent can adaptively regulate the M−SOP operation to address the frequent DG fluctuations. Then, a multi-dimensional and dynamic boundary action masking approach is proposed to address the complex coupling in the action space of M−SOPs. Finally, the effectiveness of the proposed method was verified using the IEEE 33-node system. The results show that the proposed method can adaptively alleviate the voltage fluctuations caused by rapid DG power variations.

Voltage Control in MV Network with Distributed Generation—Possibilities of Real Quality Enhancement

Connecting an increasing number of distributed sources in MV (medium voltage) and LV (low voltage) distribution networks causes voltage problems resulting mainly from periodic power flows towards the HV/MV (HV—high voltage) transformer station. This temporarily changes the nature of distribution networks from receiving to supply networks and causes an increase in the voltage values deep within the network, often above the permissible level. Therefore, it is necessary to search for new voltage control methods that take into account the active participation of distributed sources. The article proposes a concept of such a system in which the control signals are transformer taps in the HV/LV station and the values of reactive powers generated or consumed by RES (renewable energy sources). These values can be determined either by solving the optimisation problem (according to a given quality indicator criterion) or on the basis of appropriately selected settings of the Q(U) characteristics of the inverters and the HV/LV transformer ratio. The article describes both approaches, pointing to the advantages and disadvantages of each of them.

Comparison of Quantitative Feedback Theory Dependent Controller with Conventional PID and Sliding Mode Controllers on DC-DC Boost Converter for Microgrid Applications

This paper is discussed towards the issue of variations happening in the voltage control method of DC-DC boost converter. Quantitative feedback theory (QFT) is adjusted to efficiently plan a robust PID regulator, which is acknowledged utilizing only output voltage of the plant sensed by voltage sensor. The benefits of the explained PID configuration by utilizing the QFT design (Tarakanath Kobaku et al. 25) is proved with the fact that it disposes the problem of extra work and ad-hoc tuning of PID gain utilizing the regular PID methods, current measurement isn’t needed, it provides the design for case of non-minimum phase of boost converter even if there is restrictions on the bandwidth and disturbance factors are already included in the design of QFT based controller with the help of QFT bounds and thus disturbance effect is considered in the output of system. Various simulation results are taken in each case i.e. conventional PID controller and sliding mode controller. Here results are compared with the QFT based controller to verify the importance of this controller over other two controllers in terms of output response, reference tracking, line regulation and most importantly on the basis of cost effectiveness.

Guest Editorial: Enhancing hosting capability for renewable energy generation in active distribution networks

Guest Editorial: Enhancing hosting capability for renewable energy generation in active distribution networks

Fast Coscheduling of Discrete and Continuous Resources in Active Distribution Systems

This article proposes a novel fast method for flexibility scheduling and steady-state voltage control in smart distribution systems enabled with both conventional discrete controllable devices (DCDs), e.g., capacitor banks and <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -load tap changers, and continuous and faster control devices, e.g., distributed energy resources, renewable resources, and static voltage regulators. By approximating the active and reactive losses, a linear branch flow model is presented for the optimal power flow (OPF) problem in distribution systems. Accurate models are also proposed for DCDs which suit the proposed OPF formulation and require fewer binary variables compared to those available in the literature. Conservation voltage reduction (CVR) scheme is modeled in the proposed OPF methodology. The CVR effects on energy saving, peak-load reduction, line losses, and transformer losses are analyzed in the case studies. As shown in various case studies, the proposed method is fast enough for quasi-real-time applications and is efficacious in minimizing the system operation cost, holding an acceptable level of power quality based on the available standards and evading the conflict between control actions.

Fast Power Grid Partition for Voltage Control With Balanced-Depth-Based Community Detection Algorithm

Network partition in complex power networks is essential for the var-voltage control. Traditional partition methods such as Ward method are applied in practical power networks, but they are unable to evaluate the quality of partition results. Moreover, they lack efficiency when dealing with large-scale networks. Since complex operation characteristics and topology have emerged in recent power systems, the power grid partition requires higher efficiency and quality. Therefore, this paper proposes a fast network partition method with a balanced-depth-based community detection algorithm. Its aim is to significantly improve the efficiency of partition while maintaining high quality of partition, with which the inter-zone coupling is minimized while the intra-zone coupling is maximized. In the meantime, a surrogate-optimization-based selection algorithm is proposed to select the zonal pilot bus, based on which the secondary voltage control method is used to evaluate the quality of partition. Results from four case studies conducted in various power networks with different sizes, as compared to other partition methods, validate the high efficiency and high quality of the proposed power grid partition approach.

A novel distributed secondary voltage control method for AC microgrids based on voltage observer

Since the islanded AC Microgrid is affected by the impedance of line, it is difficult to coordinate the voltage regulation and reactive load distribution. A distributed secondary voltage controller based on observer is proposed for isolated AC Microgrids, and it does not need more voltage communications. This method can guarantee the estimated average voltage equals to average of actual output voltages that can converge to the nominal values, and realize accurate proportional load sharing in a microgrid. Then, the convergence of the voltage observer is proved. Finally, the proposed control method is verified by time-domain simulation.

Single‐phase small capacitor motor drive system with high‐efficiency buck active power decoupling converter

This paper proposes a novel active power decoupling circuit (APDC) based on modified Buck converter to reduce the DC-link voltage ripple of small capacitor motor drive systems. The APDC is placed between the Boost PFC circuit and the DC-link, and has two energy flow paths, and the sum of the power decoupling capacitor and the DC-link capacitor voltage equals to the output voltage of the Boost converter. With these configurations, the voltage of the decoupling capacitor can be precisely controlled to be the same as the AC component of the output voltage of the Boost converter, thereby effectively reducing the DC-link voltage ripple. Another unique feature of the proposed APDC is that the inductor works in discontinuous current mode and only work for half of the time, which not only reduces the current stress of power switches, but also further improves the efficiency of the proposed APDC. Then, an instantaneous voltage control method is investigated to reduce the DC-link voltage drop, which significantly improve the dynamic performance of the motor. The design guideline of key components is given in detail. The feasibility of the small capacitor motor drive system based on the proposed APDC is verified by experimental results.