Power Flow Regulation Research Articles

Toward Distributed Energy Services: Decentralizing Optimal Power Flow With Machine Learning

The implementation of optimal power flow (OPF) methods to perform voltage and power flow regulation in electric networks is generally believed to require extensive communication. We consider distribution systems with multiple controllable Distributed Energy Resources (DERs) and present a data-driven approach to learn control policies for each DER to reconstruct and mimic the solution to a centralized OPF problem from solely locally available information. Collectively, all local controllers closely match the centralized OPF solution, providing near-optimal performance and satisfaction of system constraints. A rate distortion framework enables the analysis of how well the resulting fully decentralized control policies are able to reconstruct the OPF solution. The methodology provides a natural extension to decide what nodes a DER should communicate with to improve the reconstruction of its individual policy. The method is applied on both single- and three-phase test feeder networks using data from real loads and distributed generators, focusing on DERs that do not exhibit intertemporal dependencies. It provides a framework for Distribution System Operators to efficiently plan and operate the contributions of DERs to achieve Distributed Energy Services in distribution networks.

Fault Diagnosis of On-load Tap-changer Based on Vibration Signal

On-load tap-changer (OLTC) is an important part of large-scale load transformers, and its fault will directly affect the reactive power flow regulation of the power system and the stability of the load center voltage. In this paper, a fault state diagnosis method based on vibration signal is proposed for fault diagnosis of tap changer. The vibration signals are decomposed by wavelet packet transform and calculated energy entropy to obtain the state information of the tap changer. The experimental data shows that the state characteristics are obvious under different faults, which lays a foundation for in-depth study of fault diagnosis.

A Distributed Cooperative Control Algorithm for Optimal Power Flow and Voltage Regulation in DC Power System

Power loss and voltage regulation are two fundamental and challenging issues in DC distributed generation system. In this paper, a multi-objective optimization problem is established to achieve trade-offs between reducing power loss and decreasing voltage deterioration by adjusting the weighting coefficients. This problem is proven to be a convex optimization under constant impedance load and constant current load. A distributed algorithm based on random coordinate descent method is developed to solve this problem without acquiring the information of line impedance. Thus, the optimal operating point of the system can be found automatically in real-time, and the deviation of optimal solution caused by the fluctuation of network parameters is avoided. In addition, the complexity of this algorithm is low and its computational requirement is reduced in practice. Finally, case studies are implemented to verify the effectiveness of the proposed algorithm under the scenarios of line parameter fluctuation, load variation, communication line failure, and plug-play capabilities.

Distributionally Robust Chance-Constrained AC-OPF for Integrating Wind Energy Through Multi-Terminal VSC-HVDC

Using the multi-terminal VSC-HVdc (MTDC) system to collect far-distance offshore wind power into the main onshore ac grid has become one of the ideal transmission approaches for renewable energy integration. However, with traditional fixed dc droop control in MTDC system, the lack of the capability of regulating power flow might pose a great challenge to the system operation. Accordingly, a novel MTDC model has been established in this paper which establishes that the output power of onshore converters can be formulated as a linear function of the control variables, namely power initial set point of each onshore converter. Based on this model, considering the uncertainty from wind power forecast error, a distributionally robust chance-constrained AC-OPF model is proposed for the hybrid AC/MTDC system. Without any presumption on the probability distribution of uncertainty, this model ensures the secure operation by enforcing chance constraints under the worst-case probability distribution over an ambiguity set based on the Wasserstein-Moment metric. Aiming at obtaining a tractable chance-constraint reformulation, we first employ the partial linearization of ac power flow equations to yield a linear model which represents the system response under uncertainties. We then develop an efficient and scalable approach to derive the linear analytical reformulation of distributionally robust chance constraints. The performance of the proposed model is firstly tested on the 14-bus system for an illustrative purpose, and then on the 1354-bus system for demonstrating scalability.

A Dynamic Load Controller for a Standalone Solar PV System Employing a Dual Input/Output Biphase dc–dc Converter

A load matching system which utilizes a time-sharing controller for an standalone solar PV system employing a Dual Input/Output Biphase Buck dc–dc Converter is presented here. The load matching system consists of an integrated load voltage and MPPT controller to regulate power flow between a standalone dc load, solar PV and a battery. The Dual Input/Output Biphase Buck dc–dc Converter incorporates the advantages of both multiport converters used for standalone systems and the multiphase converters used for low voltage and high-current applications. The steady-state analysis of the proposed system is performed by deriving the appropriate analytical equations and computing using the MATLAB programming language. A dynamic model of the Dual Input/Output Biphase Buck dc–dc Converter is developed and has been used for simulating the dynamic performance of the proposed load matching system. The efficacy of the proposed load matching scheme was also validated through experimentation under various operating conditions.

Coordinative Control Strategy for Power Electronic Transformer with Microgrid Connection

Power electronic transformer integrates rich forms of electrical interfaces and is able to realize a flexible, accurate and continuous regulation of the network power flow. It’s a promising power equipment to act as the ‘energy router’ in the energy internet, achieving a large-scale access and efficient utilization of distributed energy resources. To solve the DC-bus voltage fluctuation of PET caused by the randomness of the output power of microgrid, a coordinative control strategy between the input stage, isolation stage of PET and the microgrid is proposed. The frequency-domain analysing method is used compare the characteristics of the power fluctuation response under the proposed coordinative control strategy and a traditional control strategy. The simulation results indicate that the proposed control strategy can achieve the stability and fast convergence of DC bus voltage, which proves the correctness and advancement of the coordinative control strategy.

Control Algorithm for UPFC Based on Non-linear Model of Power System

This article proposes a state-variable stabilizing control algorithm for unified power flow controller in a multi-machine power system. This stabilizing control is activated in the transient state of the power system and is complementary to the main steady-state control designed for power flow regulation. The stabilizing control algorithm proposed in this article is derived for a non-linear multi-machine system model in order to maximize the damping of power swings in a power system. This article also shows that the effects of multiple control devices on power swing damping are additive, i.e. each new device added to power system network increases damping.

Power-Based Droop Control in DC Microgrids Enabling Seamless Disconnection From Upstream Grids

This paper proposes a local power-based droop controller for distributed energy resource converters in dc microgrids that are connected to upstream grids by grid-interface converters. During normal operation, the grid-interface converter imposes the microgrid bus voltage, and the proposed controller allows power flow regulation at distributed energy resource converters’ output. On the other hand, during abnormal operation of the grid-interface converter (e.g., due to faults in the upstream grid), the proposed controller allows bus voltage regulation by droop control. Notably, the controller can autonomously convert from power flow control to droop control, without any need of bus voltage variation detection schemes or communication with other microgrid components, which enables seamless transitions between these two modes of operation. Considering distributed energy resource converters employing the power-based droop control, the operation modes of a single converter and of the whole microgrid are defined and investigated herein. The controller design is also introduced. Furthermore, the power sharing performance of this control approach is analyzed and compared with that of classical droop control. The experimental results from a laboratory-scale dc microgrid prototype are reported to show the final performances of the proposed power-based droop control.

Three-Phase Custom Power Active Transformer for Power Flow Control Applications

This paper presents the three-phase custom power active transformer (CPAT), characterized by the integration of power electronics in a transformer to facilitate grid services. Such integration enables step-up/step-down transformation between primary and secondary as well as shunt and series compensation services to the power system through a single transformer. The CPAT can empower the grid with flexible ac transmission system and power quality services such as power flow control, reactive power compensation, active filter, and voltage regulation through a single monolithic transformer. In this paper, designs of the three-phase CPAT are realized and analyzed based on their equivalent magnetic circuit as well as their structure requirements and constraints. Simulation analysis of the three-phase CPAT clarifies its capability to actively regulate power flow between the primary and secondary windings as well as achieve grid harmonic current compensation. Moreover, through real-time simulations and an experimental prototype, the merits and performance of the three-phase CPAT were further validated.

Quantified analysis method for operational flexibility of active distribution networks with high penetration of distributed generators

With the integration of high shares of distributed generators (DGs), it is increasingly difficult to cope with the various uncertainties and puts forward higher requirements for the operational flexibility in active distribution networks (ADNs). Controllability of ADNs has been significantly improved by the application of energy storage and power electronic devices. However, due to the difficulties in effective coordination of various controllable resources, the controllability cannot be fully translated into the system flexibility. In this paper, an analytical framework for quantifying the flexibility of ADNs is proposed, including the quantification of node flexibility, the matching of system flexibility, and the flexibility of network transmission. The proposed framework provides a novel perspective of flexibility to reinterpret operation issues of distribution networks. As the power imbalance and voltage deviation are even more severe caused by the high shares of DG integration, the indexes of flexibility from magnitude, frequency and intensity dimensions are deteriorated in ADNs operation. Through the spatial and temporal regulation of power flow, various controllable devices, such as soft open points (SOPs) and energy storage system (ESS), can effectively mitigate the power imbalance and voltage deviation to improve the indexes of system flexibility. Thus, under the unified analytical framework, the potential benefits of system controllability are fully utilized to provide effective countermeasures for the flexible operation of ADNs. Finally, case studies are performed on the modified IEEE 33-node system to quantify the operational flexibility of ADNs and verify the flexibility enhancement brought by controllable resources.

Coordination control strategy for three‐port SNOP based on FCS‐MPC

As a new type of power electronic device, Soft Normally Open Point (SNOP) can replace the traditional tie switch in distribution network to realise uninterrupted flexible regulation of power flow and optimise voltage profile. Several possible joining topologies and working modes of the three-port SNOP in the distribution network is discussed here. For the back-to-back three-port SNOP topology, a three-port SNOP coordination control strategy based on Finite-Control-Set Model-Predictive Control (FCS-MPC) is proposed, which has many advantages of clear principle, simple implementation, and fast dynamic response. The dynamic mathematical model of three-port SNOP is established, and the working mode set is proposed. Based on FCS-MPC, the control algorithms of UdcQ mode, PQ mode, and CVCf mode are implemented. Accordingly, coordination control strategy between the modes of three ports is presented. In order to verify the feasibility of the proposed scheme, a three-port back-to-back SNOP simulation model was built using Matlab/Simulink. The simulation results show that the proposed coordination control strategy can effectively achieve multi-port coordinated control which can provide a flexible solution for applications of multi-port SNOP.

Research on an Improved Hybrid Unified Power Flow Controller

An improved hybrid unified power flow controller (IHUPFC) is developed in this paper to control the power flow and increase the utilization rate of the existing transmission lines. The IHUPFC is composed of an improved “Sen” transformer (IST) and a small capacity unified power flow controller (UPFC). After a detailed introduction of the structure and voltage regulation performance of the IST, this paper derives the capacity relationship between the IST and the UPFC. Then, this paper introduces the technology of the IHUPFC in power flow regulation and transmission capability improvement. Besides, the advantages of the IHUPFC in performance and structure are analyzed. The IST has simpler structure and larger power flow control area than a “Sen” transformer, and the IHUPFC provides a continuous active and reactive power flow control. Simulation and experiment studies are used to verify the practicability and superiority of the IHUPFC.

A new topology of the distributed power flow controller and its electromagnetic transient characteristics

In this paper, the modular multilevel converter (MMC) is introduced into the shunt side of the distributed power flow controller (DPFC), and a new topology of distributed power flow controller is constructed. The mathematical models are developed, in these models the internal energy relationship between the MMC module and the single-phase converter in the shunt side, as well as between the shunt-side single-phase converter and series-side single-phase converter are considered, and the corresponding control strategies are proposed based on the mathematical models. The internal electromagnetic transient characteristics of the MMC based DPFC are studied in PSCAD/EMTDC. The simulation results show that the MMC based DPFC is able to adjust the control variables such as voltage amplitude, phase angle and line reactance of the power system individually or simultaneously, and also the DPFC is able to respond to the power flow regulation demand effectively.

Implementation of Web Context to Monitoring Load Frequency Control in a Deregulated Environment with Consideration of the Governor and Boiler Dynamics

Load frequency control in a deregulated environment for an interconnected thermal reheat power system network using conventional and fuzzy based control is analyzed in this paper. In order to emulate the system behavior, the dead zone non-linearity is taken into account for the formulated objective function with the inclusion of boiler dynamics and generation rate constraints. Regulation of power flow between different areas in an interconnected power system network with frequency being constant is the key concept in load frequency control. All of that type of utilizes integral control in order to restore the frequency and thereby the power flow before there is change in load. The performance analysis for PI controller based control and fuzzy tuned PI controller for a step load change using load frequency control of thermal reheat turbine are comparatively analyzed. Using the modelling tool MATLAB/SIMULINK performances of the controllers are simulated to study for its dynamic response including the governor non-linearity and boiler dynamics. With peak overshoot as the performance index the fuzzy controller showed an improvement of 18.75% over PI controller. The settling time of the fuzzy logic controller is found to be 75% better in comparison to the PI Controller. Power–frequency information data has been transmitted to server through internet with live updates and tele-monitoring system enables the various experts to ensure high security of system operation (Qiu et al. in IEEE-2001, 2001). The proposed controller exhibit better performance and satisfy the load frequency control requirements in a non-linearity environment and the same was monitored by web context.

Improved UPFC Controller Based on Lyapunov Theory for Power Flow Control and DC Voltage Regulation

In this paper, an improve robust UPFC controller is designed based on Lyapunov direct method which enhances the power flow and DC voltage regulation of a power system. The proposed controller is robust and produces an optimal response in the presence of unknown bounds of system parameter uncertainty and disturbances. The magnitudes of the uncertainty and disturbances are identified online and the information obtained is then used to tune the controller. The performance of the proposed controller is simulated on a Kundur power system and compared with a state feedback controller with integral control, called state feedback PI controller. Comparative results indicate satisfactory performance of the proposed controller in dealing with the uncertainties and disturbances considered.

Distributed Hierarchical Control of AC Microgrid Operating in Grid-Connected, Islanded and Their Transition Modes

In this paper, a distributed hierarchical control is proposed for ac microgrid, which could apply to both grid-connected (GC) mode and islanded (IS) mode as well as mode transitions. The control includes three control levels: 1) the basic droop control is adopted as the primary control; 2) the secondary control is based on the distributed control with a leaderâĂ”follower consensus protocol; and 3) the tertiary level is a mode-supervisory control, which manages the different control targets of four operation modes. Under the proposed control framework, the following targets are achieved: 1) the frequency/voltage recovery and accurate power sharing in IS mode; 2) flexible power flow regulation between utility-grid and microgrid in GC mode; 3) universal control strategy from GC to IS modes without control switching; and 4) smooth active-synchronization from IS mode to GC mode. In this sense, the proposed method can adapt to all four operation modes of microgrid. Compared with central-standard hierarchical control, the proposed method only requires local neighbor-to-neighbor interaction with a sparse distributed communication network. Thus, the scalability, flexibility, reliability, and robustness are greatly improved in practical application. In addition, stability analysis is added to facilitate the control parameter designs, and substantial simulation cases are provided to validate the control feasibility, link-failure-resiliency, and plug-and-play capability.

Hierarchical SCOPF Considering Wind Energy Integration Through Multiterminal VSC-HVDC Grids

In this paper, a hierarchical security-constrained optimal power flow (SCOPF) model is proposed for a meshed ac/multiterminal HVDC (MTDC) system with high wind penetration. The two interacting levels in the proposed model are as follows: 1) the high level is a traditional SCOPF problem in an ac system that aims to minimize total generation and security control costs; and 2) the low level is a dynamic power dispatching problem, which regulates power flow in an MTDC grid according to reference signals from the high level. Thus, the proposed method utilizes an MTDC system to provide support for the ac system by redistributing power flow across the entire grid and reducing control costs. Two modified IEEE meshed ac/dc systems are used to demonstrate the performance of the proposed method.

Nine‐switch ac/ac current source converter for microgrid application with model predictive control

In this study, a current source nine-switch ac/ac converter for microgrid application is presented. In contrast to traditional back-to-back configuration, the number of switching devices in the proposed converter decreases from 12 to 9 with three insulated gate bipolar transistors and three diodes reduced, which achieves simpler structure and lower cost on switching devices. In addition, in comparison with its voltage source counterparts, the proposed converter inherits the advantages of sinusoidal output voltage and current waveform and no use of dc-link electrolytic capacitor. Moreover, a model predictive control (MPC) without weighting factor selection issue is designed for the proposed converter to realise both sides’ power flow regulation. Simulation and experiments have been conducted, which validate the effectiveness of the proposed converter and MPC scheme.

Direct power control based virtual flux using SOGI-FLL estimator for BDFIG-WEC system

This paper deals with the control aspect of the brushless doubly fed induction generator, 'BDFIG' incorporating in a wind energy conversion system, 'WECS'. The control process may divide in two main parts control of machine side converter, MSC' and grid side converter, 'GSC'. Control process adopts direct power control, DPC approach thanks to its robustness and implementation simplicity direct power control permits decoupled regulation for active and reactive powers and as well pretty good robustness and performance during transient and steady state operation without PI regulators and rotating coordinate transformations. The regulation of power flow toward supply network via stator is performed using conventional direct power control at level the machine side converter where the active power reference signal is tuning depending to the output of the MPPT unit and the reactive power reference signal is set to zero to ensure unity power factor operation. Meanwhile, control of power flow between grid and grid side converter, 'GSC' is done using improved direct power control based virtual flux and Second order generalized integrator - frequency locked loop, 'SOGI-FLL' estimator in order to estimate frequency and angular position of voltage grid vector and as well power flow has been analysed during various operation conditions, 'synchronous, sub synchronous and super synchronous speeds'. The obtained simulation results show satisfactory system operation in terms of robustness and optimum power tracking behaviour and good steady state stability and finally reasonable settling time during transients.

Non-isolated stacked bidirectional soft-switching DC-DC converter with PWM plus phase-shift control scheme

In this paper, a non-isolated stacked bidirectional DC-DC converter with zero-voltage-switching (ZVS) is introduced for the high step-up/step-down conversion systems. The extremely narrow turn-on and/or turn-off duty cycle existing in the conventional bidirectional buck-boost converters can be extended due to the stacked module configuration for large voltage conversion ratio applications. Furthermore, the switch voltage stress is halved because of the series connection of half bridge modules. The PWM plus phase-shift control strategy is employed, where the duty cycle is adopted to regulate the voltages between the input and output sides and the phase-shift angle is applied to achieve the power flow regulation. This decoupled control scheme can not only realize seam-less bidirectional transition operation, but also achieve adaptive voltage balance for the power switches. In addition, ZVS soft-switching operation for all active switches is realized to minimize the switching losses. Finally, a prototype of 1 kW operating at 100 kHz is built and tested to demonstrate the effectiveness of the proposed converter and the control strategy.