Droop Slope Research Articles

Flexible load control of new energy based on improved genetic algorithm

In the medium and low voltage distribution network, the load form of users is complex and changeable. There are a large number of single-phase and two-phase loads connected to the distribution network, resulting in a three-phase unbalanced operation of the distribution network. With the development of the new energy, the high proportion of distributed new energy will further aggravate the three-phase imbalance of the distribution network. Therefore, this paper proposes a coordinated optimization framework of droop parameters based on the multi-converter droop control, which takes the minimum loss of the distribution network as the optimization objective, and optimizes the reference point and the slope of the VSC droop hierarchically. A small-signal stability optimization dispatching method for the VSC droop slope in the DC distribution network is proposed. By adding small-signal stability constraints to the slope optimization model, the optimal slope command and slope stability region which can ensure the small-signal stable operation of the system are obtained. Experiments show that the optimization model of the VSC small-signal stability slope can make the droop control instruction significantly improve the small-signal stability of the system to adapt to the intra-day source load power fluctuations with a small economic cost. The slope stability region pre-optimization model can provide a reliable stability slope upper limit for the slope optimization problem based on ensuring the system operation economy. The research in this paper can make full use of the flexible control ability of power electronic equipment, and then suppress the three-phase imbalance, which is of great significance to improve the security and economy of the distribution system operation.

Voltage control and power sharing in DC Microgrids based on voltage-shifting and droop slope-adjusting strategy

• Designed a dynamic droop controller based on average of instantaneous power, droop control and voltage. • Proposed a new distributed control algorithm for DC microgrids. • Accurate power sharing and fast voltage recovery. To overcome the disadvantages of droop control, this paper proposes a novel method of hierarchical distributed secondary control for DC microgrids. With the load changes, distributed energy resources can achieve power sharing and restore the DC bus voltage. Based on the rated voltage and instantaneous power of the DC grid (DG), a hierarchical distributed control method including shift voltage and droop slope adjustment has been developed. By increasing the shifted voltage and adjusting the droop gain, it can effectively compensate the voltage drop caused by the droop controller. In this way, the DC bus voltage can always be adjusted automatically, regardless the load changes. In addition, this paper introduces the design process of the controller in detail and proves the stability of the system. Finally, simulations and experiments have verified the effectiveness and superior performance of the proposed control method.

Multi-Objective Hierarchically-Coordinated Volt/Var Control for Active Distribution Networks With Droop-Controlled PV Inverters

Due to increasing installation of photovoltaic (PV) units, reactive power compensation from PV inverters contributes significantly to Volt/Var control (VVC) for active distribution networks. While PV inverters support VVC functions, lack of systematic coordination and heavily varying PV power generation lead to low control efficiency. To maximize benefits of the inverter-based VVC, this paper proposes a multi-objective hierarchically-coordinated VVC method with droop-controlled PV inverters. This method aims to minimize both average bus voltage deviation and network power loss, by simultaneously optimizing PV inverter reactive power setpoints for central control and droop control functions for local control. The droop control characteristics of PV inverters are fully modeled, including voltage ranges of the dead band and control zones, as well as droop slope gradients. In addition, this paper applies a Taguchi’s orthogonal array testing technique to handle random variations of PV power generation in the optimization problem. Moreover, to efficiently solve this optimization problem with integer variables, this paper proposes a solution algorithm based on model relaxation and a feasibility pump method. The proposed method is tested on two distribution systems, and simulation results verify highly efficient control performance in comparison with existing methods.

Inequality Constraints Based Method for Fast Estimation of Droop Slope Stability Regions for MMC-Based MTDC Systems

This paper proposes an inequality constraints based method to efficiently and quickly estimate stability regions of droop control slopes for modular multilevel converter (MMC)-based multi-terminal dc (MTDC) systems. At first, a general small-signal model of the MMC-MTDC system is developed, which consists of the dc network and the MMCs with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dq$</tex-math></inline-formula> controllers and multiple droop controllers. When deriving corresponding nonlinear state-space models, the edge-node matrix is introduced for the dc network modeling with arbitrary grid topology and transmission line model. Then, based on the eigenvalues sensitivity and the Taylor Series of eigenvalues, a set of inequality constraints are proposed to expeditiously estimate the suprema of the droop slopes and identify the droop slope stability regions for the MMC-MTDC system. To verify the state-space model, a comparison of dynamic responses between the math model calculation in MATLAB and the EMT simulation in PSCAD/EMTDC is conducted, which demonstrates the accuracy and the correctness of the developed small-signal model. The effectiveness of the proposed parameter stability region estimation method is demonstrated by several examinations including the supremum tests of droop slopes, the stability region sketches on accuracy, and the predicted unstable operations in PSCAD/EMTDC.

Residential Distribution System Harmonic Compensation Using Priority Driven Droop Controller

As renewable energy based distributed generation (DG) units are being increasingly connected throughout today's distribution system, they can be used to mitigate harmonics caused by the wide adoption of nonlinear residential loads. To make the best use of all these DGs' ratings, it is important to develop a method to coordinate DGs' participation efforts in harmonic compensation according to their ratings and locations. Due to the low droop slope for the harmonic controller in DGs, traditional harmonic droop control methods can lead to significant harmonic sharing errors. Also, very limited work has been carried out in literature so far to identify the DGs' compensation priorities according to their locations and power rating. To address this issue, a novel priority-driven, droop-based, selective harmonic compensation scheme is developed in this work. The proposed control scheme improves the harmonic sharing accuracy. The compensation priority design and the way to integrate with droop control is studied. To ensure stability, a virtual impedance modelbased stability analysis is also discussed. Analysis, comparisons, and simulation results are used to verify the improvement of compensation performance.

Optimizing wind turbine participation for frequency regulation in large scale power systems

This paper proposes an optimization approach for the participation of wind turbines in large scale power systems. The wind turbine participation optimization aims to regulate the droop slopes and the synthetic inertia of the aggregated wind turbines to command the power contribution of wind turbines proportional to their power ratings in the primary frequency response schemes. A reduced second order model of power system is firstly presented to simplify the analysis of large scale power system. Meanwhile, the internal dynamics of general wind turbines are analyzed and the concept of stability margin is proposed to describe the wind turbines' capability of enduring frequency dips at the bus node. A convex optimization based approach is then presented to regulate the droop slopes and synthetic inertia of the wind turbines incorporating their stability margin. Five case studies are conducted in New England 39-bus system, IEEE 118-bus power system and the Polish 3375-bus system. The simulation results indicate that the proposed approach is capable of transiting the bus frequency to the new steady state in a smoother and more stable manner.

Power sharing strategy in islanded microgrids using improved droop control

This paper presents a power sharing control method for paralleled three-phase inverters in an islanded microgrid. In this study, the mismatch of power sharing when the line impedances have significant differences for inverters connected to a microgrid has been solved, the accuracy of power sharing in an islanded microgrid is improved, the voltage droop slope is tuned to compensate for the mismatch in the voltage drops across line impedances by using communication links. The method will ensure in accurate power sharing even if the communication is interrupted. If the load changes while the communication is interrupted, the accuracy of power sharing is reduced but the proposed method is better than the conventional droop control method. In addition, the accuracy of power sharing base on the proposed method is not affected by the time delay in the communication channel and local loads at the output of the inverters. The control model has been simulated in Matlab with two or three inverters are connected in parallel. Simulation results demonstrate the accuracy of the proposed control method. Futhermore, an experimental setup was built in the laboratory. Results obtained from the experimental setup verify the effectiveness of the proposed method.

Impact of Harmonics and Unbalance on the Dynamics of Grid-Forming, Frequency-Droop-Controlled Inverters

Due to its multiple advantages, the grid-forming, droop-controlled (GFDC) inverter is a strong contender for large-scale deployment in the future power systems. However, it is still unclear whether higher harmonics can play an important role in the dynamics of GFDC inverter networks. The major objective of this article is to gain insight into whether certain higher harmonics influence or interact with GFDC inverter network dynamics and to characterize this harmonic interaction. For this purpose, a multiple-harmonic dynamic phasor model (DPM) of a representative GFDC network is derived, which can be conveniently extended to hundreds of DPM equations in order to rigorously investigate the effects of dc offsets and/or second-harmonic content, six-step switching harmonics, and unbalance. For a representative network, the results from eigenvalue migration studies show that GFDC dynamics are not strongly influenced by the presence of six-step switching harmonics nor unbalance. For the first time, it is shown that under conditions of high droop slope and high-bandwidth power filtering, dc offsets and second-harmonic content can excite a resonance within the network and even influence the location of the eigenvalues of the linearized DPM. All DPM results are thoroughly validated using Simulink, and the selected results are validated experimentally using the Wisconsin Energy Institute (WEI) microgrid testbed.

Consensus-Based Cooperative Droop Control for Accurate Reactive Power Sharing in Islanded AC Microgrid

The conventional droop control technology, which can achieve autonomous power sharing among the distributed generation units, suffers from inaccurate reactive power sharing due to its dependence on line impedances. In this article, a novel consensus-based cooperative droop control technique where the droop slope gain is adaptively adjusted is proposed to improve the reactive power-sharing accuracy with the help of a sparse communication network. Through detailed small-signal analysis, it is verified that the proposed droop technique presents excellent immunity against communication delay. Laboratory test results are presented and the performances of the proposed control technique under different operation scenarios are verified, including the load change, communication link failure, and plug-and-play operation.

Stability Evaluation on the Droop Controller Parameters of Multi-Terminal DC Transmission Systems Using Small-Signal Model

For a complex multi-terminal DC (MTDC) transmission system, controller parameter settings exert tremendous influence on the system dynamics and transient stability. The traditional droop control scheme, which aims to locally control the converter's power by regulating its DC voltage according to the pre-defined droop slopes, does not normally employ communications for coordination with other terminals. The droop scheme is designed for power sharing purpose and its controller gains' impact on the MTDC network voltage is usually ignored. Moreover, the droop controller parameters are selected without considering various operating states of the MTDC terminals. This paper proposes a droop controller parameters select the criterion to cater to MTDC network voltage stability. To do so, a detailed small-signal model for a four-terminal MTDC system candidate with typical network topology, considering each converter station's electrical characteristics and cascaded control loops, is established. In order to maintain the fidelity for DC network modeling, the model also takes account of the DC network electrical dynamics and losses. A time-domain simulation model is built in Matlab Simulink to verify the correctness of the small-signal model.

Control Power Sharing of Parallel Inverters in Microgrid with Consideration of Line Impedance Effect

This paper presents a power sharing control method for use between paralleled three-phase inverters in an islanded microgrid. In this study, the mismatch of power sharing when the line impedances have significant differences for inverters connected to a microgrid has been solved, the accuracy of power sharing in an islanded microgrid is improved, the voltage droop slope is tuned to compensate for the mismatch in the voltage drops across line impedances by using communication links. The method will ensure in accurate power sharing even if the communication is interrupted. If the load changes while the communication is interrupted, the accuracy of power sharing is reduced but the proposed method is better than the conventional droop control method. In addition, the accuracy of power sharing base on the proposed method is not affected by the time delay in the communication channel and local loads at the output of the inverters. The control model has been simulated in Matlab/Simulink with two or three inverters are connected in parallel. Simulation results demonstrate the accuracy of the proposed control method. Futhermore, in order to validate the theoretical analysis and simulation results, an experimental setup was built in the laboratory. Results obtained from the experimental setup verify the effectiveness of the proposed method.

Optimizing Power–Frequency Droop Characteristics of Distributed Energy Resources

This paper outlines a procedure to design power–frequency droop slopes for distributed energy resources (DERs) installed in distribution networks to optimally participate in primary frequency response. In particular, the droop slopes are engineered such that DERs respond in proportion to their power ratings and they are not unfairly penalized in power provisioning based on their location in the distribution network. The main contribution of our approach is that a guaranteed level of frequency regulation can be obtained at the feeder head, while ensuring that the outputs of individual DERs conform to some well-defined notion of fairness. The approach we adopt leverages an optimization-based perspective and suitable linearizations of the power-flow equations to embed notions of fairness and information regarding the physics of the power flows within the distribution network into the droop slopes. Time-domain simulations from a differential algebraic equation model of the 39-bus New England test-case system augmented with three instances of the IEEE 37-node distribution network with frequency-sensitive DERs are provided to validate our approach.

Equivalence of Primary Control Strategies for AC and DC Microgrids

Microgrid frequency and voltage regulation is a challenging task, as classical generators with rotational inertia are usually replaced by converter-interfaced systems that inherently do not provide any inertial response. The aim of this paper is to analyse and compare autonomous primary control techniques for alternating current (AC) and direct current (DC) microgrids that improve this transient behaviour. In this context, a virtual synchronous machine (VSM) technique is investigated for AC microgrids, and its behaviour for different values of emulated inertia and droop slopes is tested. Regarding DC microgrids, a virtual-impedance-based algorithm inspired by the operation concept of VSMs is proposed. The results demonstrate that the proposed strategy can be configured to have an analogous behaviour to VSM techniques by varying the control parameters of the integrated virtual-impedances. This means that the steady-state and transient behaviour of converters employing these strategies can be configured independently. As shown in the simulations, this is an interesting feature that could be, for instance, employed for the integration of different dynamic generation or storage systems, such as batteries or supercapacitors.

Instantaneous penetration level limits of non‐synchronous devices in the British power system

The installed capacity of non-synchronous devices (NSD), including renewable energy generation and other converter-interfaced equipment such as energy storage, bi-directional transfer links, electric vehicles, etc., is expected to increase and contribute a large proportion of total generation capacity in future power systems. Concerns have been expressed relating to operability and stability of systems with high penetrations of NSD, since NSD are typically decoupled from the grid via power electronic devices and consequently reduce the “natural” inertia, short-circuit levels and damping effects which are inherently provided by synchronous machines. It is therefore crucial to ensure secure and stable operation of power systems with high penetrations of NSD. This paper will show and quantify the instantaneous penetration level (IPL) limits of NSD connected to a simple example power system in terms of steady-state stability beyond which the system can become unstable or unacceptable, defined as “unviable”. The NSD used in this example will be a conventional dq-axis current injection (DQCI) convertor model. The paper will introduce a set of criteria relating to locking signal in converter phase-locked loop, frequency, rate of change of frequency and voltage magnitude, which will be used to determine the system viability and the IPL limit. It will also be shown that there are several factors that can potentially affect the IPL limits. Frequency and voltage droop slopes and filter time-constant for DQCI converter are varied and it is shown how these settings influence the IPL limits. Finally, to provide additional insight into network viability under high penetrations of NSD, a visualisation method referred here as “network frequency perturbation” is introduced to investigate responses of individual generators to a change in network frequency.

Synchronous Power Controller With Flexible Droop Characteristics for Renewable Power Generation Systems

The increasing amount of renewable power generation systems is a challenging issue for the control and operation of the electrical networks. One of the main issues is their lack of inertia, which is becoming a greater problem as much as the share of the power plants based on traditional synchronous generators gets reduced. In this regard, the new grid codes ask these plants to provide new functionalities such as the frequency support and inertia emulation. In this paper, a synchronous power controller for grid-connected converters is proposed as a good solution for the renewable generation systems with energy storage. It provides inertia, damping, and flexible droop characteristics. Different from the faithful replication of the swing equation of synchronous machines, an alternative control structure is proposed, by which the damping and inherent droop slope can be configured independently to meet the requirements in both dynamics and frequency regulations. Analysis and experimental results are both shown to validate the proposed controller.

Grid-Friendly Matching of Synchronous Machines by Tapping into the DC Storage

Abstract: We propose a novel control strategy for grid-forming converters in low-inertia power grids. Our strategy is inspired by identifying the structural similarities between the 3-phase DC/AC converter and the synchronous machine model. We explicitly match these models through modulation control so that they become structurally equivalent. Compared to standard emulation of virtual synchronous machines, our controller relies solely on readily available DC-side measurements and takes into account the natural DC and AC storage elements which are usually neglected. As a result, our controller is generally faster and less vulnerable to delays and measurement inaccuracies. We provide a virtual adaptive oscillator interpretation of our controller various plug-and-play properties of the closed loop, such as passivity with respect to the DC and AC ports as well as the steady-state droop slopes, which we illustrate in simulations.

Reactive Power Sharing in Islanded Microgrids Using Adaptive Voltage Droop Control

In this paper, a strategy that employs an adaptive voltage droop control to achieve accurate reactive power sharing is investigated. Instead of controlling the output voltage of the inverter directly, the voltage droop slope is tuned to compensate for the mismatch in the voltage drops across feeders by using communication links. If the communication channel is disrupted, the controller will operate with the last tuned droop coefficient, which is shown to still outperform the controller with the initial fixed droop coefficient. Also, the net control action of the adaptive droop terms is demonstrated to have a negligible effect on the microgrid bus voltage. Since communication is not used within the tuning control loop, the strategy is inherently immune to delays in communication links. A small-signal model of the proposed controller is presented, and the effectiveness of the proposed strategy is demonstrated on a 1.2 kVA prototype microgrid.

A Study on Integrated Performance of Tap-Changing Transformer and SVC in Association with Power System Voltage Stability

Abstract—Electricity market activities and a growing demand for electricity have led to heavily stressed power systems. This requires operation of the networks closer to their stability limits. Power system operation is affected by stability related problems, leading to unpredictable system behavior. Voltage stability refers to the ability of a power system to sustain appropriate voltage levels through large and small disturbances. Steady-state voltage stability is concerned with limits on the existence of steady-state operating points for the network. FACTS devices can be utilized to increase the transmission capacity, the stability margin and dynamic behavior or serve to ensure improved power quality. Their main capabilities are reactive power compensation, voltage control and power flow control. Among the FACTS controllers, Static Var Compensator (SVC) provides fast acting dynamic reactive compensation for voltage support during contingency events. In this paper, voltage stability assessment with appropriate representations of tap-changer transformers and SVC is investigated. Integrating both of these devices is the main topic of this paper. Effect of the presence of tap-changing transformers on static VAR compensator controller parameters and ratings necessary to stabilize load voltages at certain values are highlighted. The interrelation between transformer off nominal tap ratios and the SVC controller gains and droop slopes and the SVC rating are found. P-V curves are constructed to calculate loadability margins.

Potential of Type-1 Wind Turbines for Assisting With Frequency Support in Storage-Less Diesel Hybrid Mini-Grids

Diesel hybrid autonomous power systems (mini-grids) capable of operating with high penetration of wind usually require additional means to mitigate the impact of fluctuating wind energy on stability and power quality. The use of energy storage units and sophisticated wind turbines (WTs) with pitch angle control and/or a power electronics interface are deemed essential. They become costly and not economically attractive for many small remote communities with limited resources. This paper investigates the potential of inexpensive fixed-pitch directly coupled squirrel-cage-induction-generator-based (type 1) WTs for assisting with frequency stability and support in a storage-less diesel hybrid mini-grid. The impact of some parameters of the WT on the magnitude of the slope of its power-versus-frequency (droop) characteristics is examined. The results show that a type-1 WT with an increased droop slope can reduce frequency variations in the mini-grid as well as variations of power and torque in the diesel engine generator set.

Combined use of tap-changing transformer and static VAR compensator for enhancement of steady-state voltage stabilities

Both tap-changer transformers and static VAR compensators can contribute to power systems voltage stabilities. Combining these two methods is the subject of this paper. Effect of the presence of tap-changing transformers on static VAR compensator controller parameters and ratings required to stabilize load voltages at certain values are highlighted. The interrelation between transformer off nominal tap ratios and the SVC controller gains and droop slopes and the SVC rating are found. For any large power system represented by its equivalent two nodes system, the power/voltage nose curves are found and their influence on the maximum power/critical voltage are studied.