VSC Control Research Articles

Control strategy for flexible interconnection device considering transformer power losses

This paper proposes a flexible interconnection device control strategy. It takes transformer power loss into account to solve the problems of voltage overrun, station load imbalance, and unbalanced transformer capacity allocation that occur after the large-scale connection of distributed power sources into the low-voltage distribution network to realize the mutual aid of active power and load balancing between interconnected stations. Firstly, the VSC control strategy with capacitor current feedback active damping link is introduced to accurately execute the power commands and maintain the DC bus voltage stability. Then, the load regulation strategy with transformer power loss is proposed to generate the power commands of the flexible interconnection device. Finally, the simulation results show that the control strategy of the flexible interconnection device with transformer power loss mentioned in this paper can flexibly control the power transmitted by each port and maintain the bus voltage stability. The simulation results show that the flexible interconnection device control strategy proposed in this paper, which takes into account the transformer power loss, can flexibly control the transmission power at each port to keep the bus voltage stable. Moreover, it can effectively reduce system loss compared with the traditional load regulation strategy.

ILA Optimisation Based Control for Enhancing DC Link Voltage with Seamless and Adaptive VSC Control in a PV-BES Based AC Microgrid

The paper presents a grid-connected microgrid with a photovoltaic system and a battery as a storage element. The optimal design and control of storage elements and power quality improvement are enhanced using sigmoid-function-based variable step size (SFB-VSS) adaptive LMS control. The DC-link voltage and battery current are enhanced using an ILA-optimization-based PI controller. Comparative analysis shows that an ILA-optimized PI controller improves battery stress and DC-link voltage fluctuations, enhancing overall system stability. The relative percentage error of Vdc is only 0.5714% for ILA-optimized values as compared to GA, PSO, and manually tuned PI gains which are 0.857%, 1.14285%, and 0.86%, respectively. ILA-optimized parameters also enhance battery current, reducing stress on the battery. The system was studied under various dynamic conditions, achieving power balance in all conditions. The system has the capability of seamless transfer of control from GC mode to SA mode when the grid is disconnected. The proposed VSC control shows better performance in steady-state and dynamic conditions, maintaining a THD under 5%, which follows IEEE standard 519, and providing better DC offset rejection, fewer oscillations in the weight component of the load, and better convergence. The proposed control also enhances the frequency of the grid, ensuring a smooth transition between modes. The system is simulated in the MATLAB Simulink environment, and all the optimization techniques were carried out offline.

A Collaboration of VSC with SEIG Based Microgrid in Voltage and Frequency Control

In the generation, the most widely used generator is synchronous generator (SG) but its cost of producing per kilo watt (kW) power is higher than induction generator (IG). There are many advantages of IG over SG like, IG has small size for per kW output power production it means that the energy density of IG is higher than SG, when operates in parallel IG runs without hunting problem. IG can recover the energy with relatively simple control. IG is an alternative source to convert mechanical energy into electrical energy. Some problems are associated at distribution and generation level when IG operates in an isolated electric system. These problems are mainly caused by in linear changes in load. It results in loss of system reliability. The quality of voltage and frequency also get affected. So, to enhance reliability to control voltage & frequency, a flexible method with power electronic converter is used which control the dynamics of system and provide electrical power support as well simultaneously. When this converter emulates with its control mechanism, it works like a real synchronous generator which is called as VSG. This paper presents the distribution-generation system in collaboration with IG and VSC. VSC controller are designed on the basis of d-q model of converter. Performance of this control strategy is verified through Matlab/Simulink. The simulation result represents the actual behavior of the system against the non-linear loads. It shows the optimization of power and keeping the voltage frequency under a permissible limit.

Fuzzy induced controller for optimal power quality improvement with PVA connected UPQC

Abstract The major power quality issues in grid are voltage fluctuations and harmonics. For better power quality of the power system during disturbances on the grid, the UPQC device is utilized to maintain voltage magnitude and reduced harmonics. At the DC link of UPQC along with the capacitor, a renewable PV source is connected which contributes in voltage compensation by the series VSC and harmonics compensation by the shunt VSC. For stable DC voltage generation from PV source a modified P&O MPPT with DC reference is included controlling the boost converter connected to PV source. The controllers of VSCs are operated by feedback loop synchronized schematics with voltage reference generation in series VSC control and current reference generation in shunt VSC control. The shunt control is updated with hybrid Fuzzy-PI controller replacing the traditional PI controller further improving the power quality of the grid. The hybrid Fuzzy-PI varies the K p and K i gains as per the error generated by the DC voltage comparison concerning 25 rule-base for each gain. A comparative performance analysis is done with both the controllers in the shunt converter and the results are generated using MATLAB Simulink software.

Hybrid Control Scheme for VSC Presenting Both Grid-Forming and Grid-Following Capabilities

This work proposes the Hybrid Control Converter (HCC), a new kind of VSC control. This method aims to have one single converter mimic the behavior of two individual, virtual converters working in parallel (one with grid-following and the other with grid-forming control). This way, the hybrid converter can show behavioral characteristics of both kinds of converter control at the same time. Two possible applications for the HCC are presented. The first one is related to synthetic inertia capability of wind turbines. Results show that the HCC presents superior performance when compared to conventional techniques based on grid following converters, resulting in a smaller grid frequency drop during transients. The second application shows that the HCC allows for autonomous connection of multiple distinct AC systems using a meshed DC transmission system. This enables automatic frequency support and generation and load sharing between the AC grids without requiring any specific action by the system operator.

Grid-connected solar photovoltaic-fed brushless DC motor drive for water pumping system using colliding body optimization technique

PurposeThis paper aims to present an improvement to the power quality of the grid by using a colliding body optimization (CBO) based proportional-integral (PI) compensated design for a grid-connected solar photovoltaic-fed brushless DC motor (BLDC)-driven water pumping system with a bidirectional power flow control. The system with bidirectional power flow allows driving the pump at full proportions uninterruptedly irrespective of the weather conditions and feeding a grid when water pumping is not required.Design/methodology/approachHere, power quality issue is taken care of by the optimal generation of the duty cycle of the voltage source converter. The duty cycle is optimally generated by optimal selection of the gains of the current controller (i.e. PI), with the CBO technique resulting in a nearly unity power factor as well as lower total harmonic distortion (THD) of input current. In the CBO technique, the gains of the PI controller are considered as agents and collide with each other to obtain the best value. The system is simulated using MATLAB/Simulink and validated in real time with OPAL RT simulator, OP5700.FindingsIt was found that the power quality of grid using the CBO technique has improved much better than the particle swarm optimization and Zeigler–Nichols approach. The bidirectional flow of control of VSC allowed for optimum resource utilization and full capacity of water pumping whatever may be weather conditions.Originality/valueImproved power quality of grid by optimally generation of the duty cycle for the proposed system. A unit vector tamplate generation technique is used for bidirectional power transfer.

Fifth-Order Generalized Integrator for a Double-Stage Single-Phase Grid-Interfaced Photovoltaic Supply System

This paper presents a fifth-order generalized integrator (FOGI) as an orthogonal signal generator (OSG) to obtain a grid synchronization signal for a double-stage single-phase grid-interfaced photovoltaic supply system (GIPVSS). The proposed GIPVSS consists of a single-ended primary inductor converter (SEPIC) and a H-bridge voltage source converter (HBVSC) to transfer solar power to the grid system. The SEPIC controls maximum power delivery (MPD) from the Photovoltaic (PV) array and feeds that power to the grid via an HBVSC. In this study, all energy storage elements (inductors and capacitors) of SEPIC are considered with their effective series resistance (ESR). The dynamic modeling of SEPIC is obtained using a small signal analysis. The control of VSC allows harmonics’ mitigation and power factor improvements. A FOGI and a second-order adaptive filtering approach are used in controling VSC. The FOGI is used to obtain a grid synchronization signal from sensed grid voltage under disturbances. The second-order adaptive filtering approach is used to obtain an active load current component. The comparative analysis of FOGI with other reported algorithms is carried out under harmonics in sensed grid voltage. The effectiveness of the proposed system and its control is validated in a laboratory prototype using a Launch Pad TMS320F28379D digital signal processor (DSP).

Analysis and comparison of multi-parameter factors affecting the stability of subsynchronous oscillation of grid-connected PMSG

This article mainly focuses on DC voltage control loop of PMSG which affecting the subsynchronous oscillation (SSO) stability. Based on the established general dynamic interconnection model, the oscillation stability region of DC voltage control parameters is calculated by the eigenvalue method. It is found that a series of ’convex points’ appeared in the stability region are mainly due to strong dynamic interaction between DC voltage control of PMSG and other dynamics induced by modal coupling. Furthermore, by the proposed oscillation stability region, this article reviews the mechanism of the subsynchronous oscillation instability induced by the modal coupling, and the two types of parameters, system operating parameters and VSC control parameters, that affect the system SSO instability. In addition, effective measures to suppress SSO instability of the wind–thermal bundling system is proposed.

Research on Distribution Network Reliability and Power Supply Soft Self-healing

The reliability of distribution network is an important index to ensure high-quality power supply. In this paper, the FMEA table is established based on the traditional power supply mode consequence analysis method, and the network topology after power supply isolation is determined by querying the table according to the power supply information. Then, the control mode of VSC is adjusted according to the VSC control strategy under different power supply conditions. Finally, through network reconfiguration and island operation, the continuous spatiotemporal power grid topology power supply restoration optimization model is established. On this basis, this paper combines sequential Monte Carlo method to evaluate the reliability of continuous spatiotemporal power grid topology, and illustrates the reliability evaluation difference caused by the difference of distribution network topology information.

Effect of partial shading in grid connected solar PV system with FL controller

As conventional fossil fuel reserves shrink and the danger of climate change prevailing, the need for alternative energy sources is unparalleled. A smart approach to compensate the dependence on electricity generated by burning fossil fuels is through the power generation using grid connected PV system. Partial shading on PV array affects the quantity of the output power in photovoltaic (PV) systems. To extract maximum power from PV under variable irradiance, variable temperature and partial shading condition, various MPPT algorithms are used. Incremental conductance and fuzzy based MPPT techniques are used for maximum power extraction from PV array. Basically 11 kW Solar PV system comprising of PV array coupled with an Inverter through a dc-dc converter is considered for the analysis and output of the inverter is supplied to the load through the LCL filter. An Intelligent controller for maximum power point tracking of PV power is designed. Also, a fuzzy controller for VSC is developed to improve the system performance. The above proposed design has been simulated in the MATLAB/Simulink and analyzed the system performance under various operating conditions. Finally, the performance is evaluated with IEEE 1547 standard for showing the effectiveness of the system.

Control and stability analysis of DC microgrid system including wind and solar generation sources and grid‐connected voltage source converter

SummaryThe study system in this paper is a grid‐connected DC microgrid system comprising PV and wind generation sources, resistive and constant power loads (CPLs), and grid‐connected VSC. The VSC keeps the DC link voltage constant, and the wind and PV sources operate in maximum power point tracking (MPPT) mode, and thus, the DC–DC converters related to these sources act such as controlled current sources behaving similar to negative CPLs. The study system suffers from the stability under high CPL penetration and weak AC grid conditions. Hence, this paper investigates and enhances the system stability under both high CPL penetration and weak AC grid conditions. To do this, modal and small‐signal stability analysis of the system is carried out, and then, as the main findings, impacts of the wind and PV sources output powers, CPL power, and AC grid short‐circuit power on the system stability are studied. Next, as the main contribution, VSC control structure is modified by adding stabilizing signals to enhance the system stability under high CPL penetration, reduced output powers of the wind and PV sources, and weak AC grid conditions. At the end, several simulation results verify the theoretical results at different conditions.

Understanding the role of VSC control strategies in the limits of power electronics integration in AC grids using modal analysis

In this article, the interactions between AC grid and existing VSC control modes are analyzed. First, three VSC control modes, namely grid-feeding, grid-supporting and grid-forming, are compared with each other in order to assess VSC/grid coupling level. Then, in order to underscore the growing role of VSCs in transmission systems, an elementary benchmark network is considered. It includes an equivalent SM (Synchronous Machine), a VSC, a load and a transmission line with a varying electrical distance. Thereafter, a modal analysis is conducted on this model to study the interactions between the different VSC controls and the SM dynamics using a Matlab/Simulink environment. The converter penetration rate is represented by a variable power ratio between the converter and the machine, while the electrical distance by an RL dynamic impedance. This approach makes it possible to assess the stability limits on the VSC share imposed by the system strength. Negative interactions are identified and recommendations for novel converter control strategies proposed. Besides, the influence of the AVR with the VSC in grid-forming as well as PLL-related stability issues with the VSC in grid-feeding or grid-supporting are shown to be the key limiting factors for increasing the penetration of PE-based sources.

Improvement of Power Quality WECS using AI Based STATCOM

The major concern in a growing power quality is harmonics distortion which is caused by the non-linear nature of the loads. This problem has drawn much attention from utilities, users and industries. To reduce the harmonic distortion for improving the power quality of the system a custom power devices has been proposed. A static compensator (STATCOM) is implemented at distribution level for overcoming several power quality problems. In this paper, new control technic i.e AI is proposed on shunt compensator to estimates the weight values of load currents. The control approach is based on the convergence of the load currents and property of the input signal. A working prototype of the STATCOM is implemented using three-phase VSC and AI control technique based PWM controller approach is developed in MATLAB/SIMULINK.

Effective sensitivity-based method for N-1 contingency analysis of VSC-based MTDC power grids considering power generation droop speed controls

This paper introduces a sensitivity-based approach for the efficient calculation of the post-disturbance, steady-state conditions in VSC-based MTDC power grids subject to generation outages and VSC disconnections. This approach is useful for the N-1 contingency analysis, a crucial tool in power system control centres. In order to maintain a high degree of fidelity, the approach considers state-of-the-art VSC control strategies: converters with DC voltage regulation, power-injection control, DC voltage/power droop control, and converters imposing frequency regulation on passive grids. The speed-governing response of power plants is also considered, enabling accurate calculations of AC/DC power flows and AC system frequencies. Indeed, the derived sensitivity factors permit the primary frequency response of the AC systems and DC grid to be efficiently assessed, something that would require several hours of simulation by standard electromagnetic transient (EMT) simulators. Evidently, this novel approach allows a much faster calculation of the AC/DC network’s post-disturbance conditions than is possible with dynamic simulation tools. A formulation with such modelling versatility and practicality does not currently exist elsewhere.The usefulness of this novel formulation has been confirmed using an MTDC system comprising six VSC-connected AC grids and a 13-bus DC grid. The disconnections of VSC units and power plants have also been evaluated. The study shows that the post-disturbance, steady-state conditions computed by the sensitivity-based approach agree well with those obtained by full dynamic simulations, because the relative errors between the two fundamentally different methods are smaller than 0.1% for the AC system frequencies and smaller than 4% for the AC and DC transmission line power flows.

Voltage Control of Four-Leg VSC for Power System Applications With Nonlinear and Unbalanced Loads

Voltage source converters are presented as the key devices for the future massive integration of distributed renewable energy resources in the network. This article presents a novel approach to control a three-phase four-leg voltage source converter for grid-forming operation. The objective of the controller is to generate a balanced three-phase voltage with a given amplitude and frequency at the point of common coupling. The proposed control algorithm works over the stationary axes. It is based on full state feedback law in combination with a resonant control loop tuned at the fundamental frequency in order to guaranty zero steady-state error on the voltage. The main advantage of the controller with respect to the classical cascade controllers is that this strategy is not modified depending on the type of load connected at the point of common coupling. Moreover, a systematic methodology to compute the controller gains is presented by solving an linear quadratic regulator problem that considers an extended model. This method guarantees small signal stability and provides active damping to the system. A laboratory testbed with different type of loads is used to validate and compare the proposed algorithm with the classical one. The experimental results demonstrate the effectiveness of the proposal by achieving low levels of harmonic distortion and imbalances in steady-state as well as a fast transient response.

VSC Control of Grid Connected PV for Maintaining Power Supply During Open-Phase Condition in Distribution Network

The grid-connected photovoltaic (PV) system with battery energy storage (BESS) has been in use for many demand-related issues. This article discusses the use of grid-connected PV with BESS in case of open-phase condition in the distribution system. An open-phase condition at medium voltage (11 kV) will result in continuous voltage sag at low voltage (LV) voltage level (400 V), which may be detrimental to various kind of loads, especially for constant power loads. This article presents an approach to mitigate such a scenario by using a control strategy for voltage source converter of grid-connected PV connected to the distribution transformer terminal. The utility of the proposed configuration is demonstrated as the voltage is restored in all phases at the LV level during an open-phase fault at the MV level, and results obtained for selected possible loading conditions have been presented. For further validating the proposed control strategy in real-time conditions, experimental verification is also done on an NI-based PXI hardware setup along with an Opal-RT based hardware in the loop system, and results for the same have been presented.

Acidic Control of a Chemical Plant Implementing Non Linear Controllers

This research work concentrates on developing a non linear control technique using PID control, Fuzzy Logic controller and SMC (Sliding Mode Controller) for acidic control of a chemical plant. This control involves (Variable Structure Control) VSC control. The acidic control process includes a prototype model. This model has two inlets to the sample liquid. The two inlets supply acid and alkaline liquids into a Continuous Stirred Tank Reactor [CSTR]. Both the inflows are controlled in proper proportions, which help to maintain the desired pH of the system. The pH neutralization process is a non linear process; hence VSC serves as a better solution. One type of VSC is the Sliding Mode Control which is applied to this system. Apart from this VSC, both PID control and fuzzy control are incorporated on the same process and the results are furnished. SMC suffers a problem called chattering, but provides reliable results when compared to PID and Fuzzy Logic Controller. This research is done using LabVIEW Software, Hence all the control methods are implemented using the same. Laboratory Virtual Instrumentation Workbench is abbreviated as LabVIEW. The programming done with this software is applied to the prototype system and the results of the controllers are compared and it is found that SMC controller is better suited than PID and fuzzy controllers.

Fault Ride through Capability Augmentation of a DFIG-Based Wind Integrated VSC-HVDC System with Non-Superconducting Fault Current Limiter

This paper proposes a non-superconducting bridge-type fault current limiter (BFCL) as a potential solution to the fault problems of doubly fed induction generator (DFIG) integrated voltage source converter high-voltage DC (VSC-HVDC) transmission systems. As the VSC-HVDC and DFIG systems are vulnerable to AC/DC faults, a BFCL controller is developed to insert sizeable impedance during the inception of system disturbances. In the proposed control scheme, constant capacitor voltage is maintained by the stator VSC (SVSC) controller, while current extraction or injection is achieved by rotor VSC (RVSC) controller. Current control mode-based active and reactive power controllers for an HVDC system are developed. Balanced and different unbalanced faults are applied in the system to show the effectiveness of the proposed BFCL solution. A DFIG wind-based VSC-HVDC system, BFCL, and their controllers are implemented in a real time digital simulator (RTDS). The performance of the proposed BFCL control strategy in DFIG-based VSC-HVDC system is compared with a series dynamic braking resistor (SDBR). Comparative RTDS implementation results show that the proposed BFCL control strategy is very efficient in improving system fault ride through (FRT) capability and outperforms SDBR in all cases considered.

Investigation on sub‐synchronous oscillations in DFIG‐based transmission system based on improved complex torque coefficients method

The complex torque coefficients (CTC) method has been widely adopted to investigate the conventional sub-synchronous oscillation (SSO) issues. However, the application of the CTC method is limited when large-scale wind power generations are integrated into power systems. Therefore, in order to overcome the disadvantages of the CTC method on the aspects of frequency scan and stability determination, this paper proposes an improved CTC method based on the shaft system modal decoupling and small-signal self-excitation theory. The improved method is demonstrated and validated through the IEEE first benchmark power system (FBMPS). By establishing the small-signal model of the series-compensated DFIG-based transmission system, the proposed method is applied to examine the SSO in a grid-connected DFIG system. Moreover, the impacts of different parameters on the electrical damping characteristics of the system are deeply analysed. In addition, an effective VSC control strategy for suppression of the SSO in series-compensated DFIG-based transmission system is proposed.

Peak Detection Control Strategy based STATCOM for Suppression of Power Quality Problems

This paper explains about control of VSC using uni-polar switching based Static Compensator (STATCOM) using Peak Detection Control Strategy for elimination of PQ Problems. The proposed STATCOM are operated in PFC (Power Factor Correction) mode and ZVR (Zero Voltage Regulation) mode. Under these operation modes of STATCOM is able to suppression of harmonics, PFC at source, reactive power compensation, load balancing and compensation of neutral current. A non-isolated star/three individual single- phase transformers are connected at PCC (Point of Common Coupling) to mitigate neutral current. The Peak Detection Control Strategy is simple to implement for estimating switching pulses for VSC. This peak detection control strategy based STATCOM is modelled, simulated results are validated using MATLAB R2013b.