Droop Constants Research Articles

Maximum span determination and optimal sizing of cable for improved performance of droop-controlled DC microgrid

DC microgrids are seen as smart solutions to interface DC loads and distributed energy resources (DERs). However, in DC microgrids, the placement of sources and loads as well as the size of cable impact the system’s span, regulation of voltage, and losses. This paper proposes novel algorithms to determine the maximum span and optimal size of cable for the improved performance of a droop-controlled DC microgrid. The proposed algorithms utilize an improved power flow analysis (IPFA) method based on Newton-Raphson technique to determine the maximum span and the optimal size of cable, enhancing the voltage regulation and reducing the cost. Additionally, the impacts of power rating, droop constants, and size of cable on the DC microgrid’s maximum span are investigated and reported. Owing to the intricate nature of the problem concerning the ideal size of cable for the droop-controlled DC microgrid, a heuristic optimization approach is employed. Further to enhance the rate of convergence and computational performance, an improved particle swarm optimization (IPSO) is also proposed. The constraint of keeping the bus voltage variations below the allowable voltage regulation limits is applied to the objective function of the optimization problem. In the case of a droop-controlled, DC microgrid having a specified configuration, the suggested algorithm can determine the ideal size of cable, guaranteeing both the least cost and enhanced voltage regulation. Comprehensive numerical and modelling results have been presented for a droop-controlled DC microgrid with different loads and DERs to verify the effectiveness of the proposed methods. Furthermore, the analysis reveals that configuration III of the DC microgrid with an optimal cable size of 19 mm2 lowers the absolute voltage regulation to as low as 1.06 V. The results of the detailed analysis validate the enhanced performance of the proposed algorithms and are highly useful to both system designers and consumers of the DC microgrids, eventually paving the way for the widespread use of DC microgrids in the future.

Reinforcement Learning-Based Integrated Control to Improve the Efficiency of DC Microgrids

The hierarchical control of the DC microgrid regulates the terminal voltages of the interfacing converter to achieve proportional load sharing and good voltage regulation at the DC bus. In doing so, the difference of the voltage at different nodes increases which results in higher circulating current and leads to higher losses. In this paper, a Reinforcement Learning Based Integrated Control (RLIC) is proposed which will minimize the circulating current and power losses in the transmission. The proposed RLIC consists of a primary and secondary controller. The primary controller is a robust sliding mode controller which receives the voltage references from secondary controller and regulates the terminal voltage and source current accordingly. The secondary control consists of a proportional integral control (PI), and a Deep Neural Network (DNN) surrogate model with implementation of Q-Learning as reinforcement method. A novel DNN based surrogate model uses the droop value from the PI controller and estimates the power loss and the local and global loading difference for a particular node, for a set of operating condition. This surrogate model is used by the Q-Learning based reinforcement technique which adjusts the droop constants and provide the voltage reference to primary controller to maintain load sharing and reduce the power losses, there-by leading to an improved overall efficiency. The proposed control structure is verified to improve efficiency while maintaining the load sharing and bus voltage regulation.

A New Communication-Free Grid Frequency and AC Voltage Control of Hybrid LCC-VSC-HVDC Systems for Offshore Wind Farm Integration

We propose frequency and voltage control method for a hybrid high-voltage direct current (HVDC) system for integrating an offshore wind farm into transmission networks. Conventionally, DC voltage level is maintained as constant regardless of the wind velocity and the grid frequency, so AC voltage fluctuates with active power variation in a hybrid HVDC system due to the innate reactive power absorption of inverter side line-commutated converter (LCC). However, in the proposed method, the DC voltage changes according to the wind velocity. Furthermore, DC voltage is simultaneously regulated to participate in frequency control in a communication-free manner. These two characteristics can be achieved by using five droop control methods proposed here. Additionally, by determining droop constants using the proposed determination methods, AC voltage can be maintained as constant during active power fluctuations. Therefore, using the proposed method, an offshore wind farm can be stably connected to mainland transmission networks and successfully used as frequency supporting resources, because voltage characteristics of the AC network are not destabilized. A small-signal state-space (SS) model of the target system was also developed to investigate stability of the proposed control. The utility of the proposed method is demonstrated by case studies using PSCAD and SS models.

Frequency response support assessment from uncertain wind generation

Large share of uncertain generation from Wind Power Plants (WPPs) in the power systems has a detrimental impact on system frequency stability, as WPPs inherently do not provide inertial response support. Currently, extensive research is being carried out to envisage Synthetic Inertia (SI) & Primary Frequency Response (PFR) support from WPPs. Modelling of inertial response support from WPPs and its consideration in stochastic scheduling requires investigation. System inertia condition varies over a day and hence, accurate estimation of PFR requirement is a challenging task. This paper develops a novel approach to model SI & PFR support from WPPs and simultaneously estimate the system inertia condition & PFR requirement in a day-ahead stochastic scheduling framework. Wind generation uncertainty is characterized by probabilistic scenarios. IEEE Reliability Test System (RTS) is used to carry out the case studies at different wind penetration levels. Numerical results demonstrate that the SI and PFR support from WPPs reduce the overall cost by 15% and wind curtailment reduction up to 35%. SI support from WPP adds around 12% to overall system inertia. The proposed model could be enhanced for the optimal tuning of inertial, droop constants, and modelling of multiple uncertainties.

Performance improvement of an islanded AC microgrid in presence of Plug-in Hybrid Electric vehicles, load and renewable generation uncertainties

In this paper, a performance improvement scheme for an autonomous AC microgrid in the presence of dispatchable and renewable units, and Plug-in Hybrid Electric vehicles has been proposed. In a standalone mode of operation, a droop-control strategy is implemented at the primary control level for a communication-free accurate power-sharing among multiple dispatchable generators. For a network that is neither highly reactive nor highly resistive, active and reactive powers are coupled to both voltage magnitudes and phase angles. A mixed-droop feature is incorporated at the primary control level for autonomous power-sharing among the units for such networks. The mixed-droop feature is considered in this paper. Further, stochastic models of load demands, renewable generation, and plug-in hybrid electric vehicle charging loads are considered. Objectives are to reduce the cost of operation and minimize the emission of greenhouse gases from generation activities. The bi-objective problem is solved in the fuzzy domain. Performance improvement is achieved in two steps, first, by choosing optimal static droop constants using the particle swarm optimization, and then a further performance improvement is achieved by network reconfiguration. The proposed algorithm is tested on a thirty-three bus autonomous microgrid and gives satisfactory results.

Parameter Adjustment for the Droop Control Operating a Discharge PEC in PMG-Based WECSs With Generator-Charged Battery Units

Permanent magnet generator (PMG)-based wind energy conversion systems (WECSs) with battery units, have become a popular class of distributed generation units. These distributed generation units are typically operated using various types of controllers, including droop controllers. Existing droop controllers are designed to operate grid-side dc-ac power electronic converters (PEC) to ensure stable and reliable power production by a PMG-based WECS. The employment of battery storage units (to mitigate fluctuations in the power produced by a PMG-based WECS) introduces additional considerations for the design of droop controllers. Such considerations are due to the power available from battery units that is dependent on the state-of-charge (SOC). This paper proposes adjustments in the parameters (droop constants) of the droop control (operate the the discharge PEC) based on the SOC of the battery units. These adjustments are made to further support stable and reliable power delivery of the PMG-based WECS into the point-of-common-coupling (PCC). The proposed adjustments of droop constants are evaluated using a 7.5 kW grid-connected PMG-based WECS with 3.52 kW generator-charged battery storage units. Performance tests are carried out for step changes in the active and reactive power demands, changes in the wind speed, and grid-side disturbances. Test results show that the proposed correction of the droop constants is critical for maintaining a stable, effective, and accurate power delivery by the battery units, thus supporting the voltage/frequency stability at the PCC under different operating conditions.

Method for Determining the Droop Coefficients of Hybrid Multi-Terminal HVDC Systems to Suppress AC Voltage Fluctuations

Different from voltage-source converter (VSC), reactive power absorption of line-commutated converters (LCCs) changes depending on DC voltage level. Therefore, the amount of AC voltage fluctuations at inverter side are different according to $I$ – $V$ droop constants in hybrid multi-terminal high-voltage direct current (MT-HVDC) system. This paper proposes a method for calculating the $I$ – $V$ droop constants in a hybrid MT-HVDC system to suppress these AC voltage fluctuations, based on the sensitivity of AC and DC networks. Simulations performed using PSCAD demonstrate the effectiveness of the proposed method.

A Robust $H_\infty$ Multivariable Stabilizer Design for Droop Based Autonomous AC Microgrid

The autonomous microgrid (MG) with multiple voltage source inverters (VSI) is operated in droop mode to share the load proportionally. These VSI controllers require higher droop constants if the feeder lines connecting the VSI to microgrid has a low X/R ratio. The higher droop gains and large loading conditions in the microgrid may exhibit instability with the low frequency power modes (LFPM). This paper details a systematic multivariable plant model of the MG using dynamic phasor modeling of power flow transfer functions to analyze the LFPM. Further, a robust supplementary multivariable controller is developed to stabilize the developed plant model, damp LFPM and improve transient power sharing performance of VSIs without affecting their steady-state power sharing. This stabilizer is developed based on Glover-McFarlane H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> loop shaping robust stabilization technique with an additional pole placement constraints formulated in linear matrix inequality framework. A laboratory scale experimental validation is presented to evaluate the performance of proposed stabilizer under various operating conditions and uncertainties in the autonomous MG. Moreover, the transient power sharing performance of DGs with proposed controller was compared with that of widely used derivative droop controller.

A Procedure to Determine the Droop Constants of Voltage Controllers Coping with Multiple DG Interactions in Active Distribution Systems

In modern distribution systems, the presence of an increasing amount of Distributed Generation (DG) systems causes over-/under-voltage problems, due to the reverse power flows. To face these problems, the voltage-reactive power droop controllers of DG systems are commonly used for their simplicity and are required by international standards. On the other hand, the interaction among voltage droop controllers of different DG systems may introduce instability. The paper presents an effective procedure to determine the droop constants of voltage-reactive power controllers for multiple DG systems. Firstly, a multi-input multi-output model of the distribution system is introduced. Then, using the concept of the interaction measure under decentralized control, a simple constraint is added to the single-input single-output design of each droop controller. Such a constraint guarantees stability with respect to the interaction among the voltage droop controllers of all the DG systems. Eventually, the proposed procedure is applied to an LV test system with 24 nodes and six photovoltaic systems; the results of numerical simulations are presented, giving evidence of the effectiveness of the proposed procedure in various operating conditions of the distribution system.

A Coordinated Control Strategy for LCC HVDC Systems for Frequency Support with Suppression of AC Voltage Fluctuations

In this paper, a new control method for a line-commutated converter-based (LCC) high-voltage direct current (HVDC) system to support the frequency and AC voltage was proposed. For coordinated control of the inverter and rectifier, droop-based controllers are adopted for the outer loop of conventional controllers, and DC voltage is used to transfer the frequency information so that communication is not required. A method to determine the droop constants of the proposed controller is presented to emulate the conventional $f$ / $P$ droop characteristic and suppress the AC voltage fluctuations. Furthermore, a small-signal state-space model of the LCC HVDC system with the proposed scheme is derived, and root locus analysis is carried out to evaluate its stability. The effects of the proposed method are verified based on case studies, using the well-known CIGRE BENCHMARK model. The advantages of the proposed method can be summarized as follows: 1) system reliability is enhanced because there is no risk of communication failure during frequency information transfer and 2) voltage stability at the inverter side is enhanced as AC voltage fluctuation during frequency support is minimized.

A Novel Frequency Support Control Method for PV Plants Using Tracking LQR

Growing level of Photovoltaic (PV) generation challenges the conventional generation control in power systems, as the current PV systems do not possess inertia or governor system. However, a supervisory control can be adopted for such PV systems to provide frequency support. In this article, a novel controller for large-scale PV plants is proposed for this purpose. Different than other approaches which only focus on emulating droop and inertial response of local PV, the proposed approach uses a tracking linear quadratic regulator (LQR) based controller to help the system frequency effectively track that of a designed reference system with given inertia and droop constants. To design the LQR, an unknown input observer (UIO) is adopted to estimate the system states, as well as the unknown disturbance. We also introduce a small-signal PV system model which is suitable for designing frequency support controls. The model is validated against the detailed nonlinear PV system model, and the effectiveness of the proposed controller is demonstrated using a standard test system. Test results also show that the proposed method achieves the desired frequency response more effectively than the alternative method from literature, while the later method is liable to over-compensation of frequency response at the price of requiring more power reserve in PV.

Impact of forced and unforced system parameter variations on network stability and system economics of radial MTDC networks

This paper focuses on investigating the impact of system parameters uncertainty on voltage droop-controlled Multi-Terminal High-Voltage DC (MTDC) networks. The voltage droop typically controls the power-sharing among the receiving end-nodes for minimum transmission power loss. The hierarchical control layers are used to update the droop constants to cope with the system power/parameter variations. In this paper, a study is presented elucidating the effect of unforced system parameter variations, as transmission line resistance uncertainty and the power injection variation, on the droop characteristic settings for radial MTDC networks, from system economics and network steady-state and dynamic stability perspectives. Moreover, forced system parameter variations, such as those due to cyber-attacks, are addressed in this paper. Cyber threat with a malicious attack on the communication system in the hierarchical control layers is considered particularly during power variations to investigate the impact of falsifying the DC voltage droop characteristics. Towards generalization, two different droop-controlled radial MTDC networks, with optimal droop gains adjusted for minimum transmission loss, are employed for explicating and assessing the presented concept.

Optimal power dispatch considering load and renewable generation uncertainties in an AC–DC hybrid microgrid

An AC-DC hybrid microgrid is gradually becoming popular. For economic viability and environmental sustainability, an AC-DC microgrid should be operated optimally. This study introduces an optimal power dispatch strategy for simultaneous reduction of cost and emission from generation activities in an AC-DC hybrid microgrid under load and generation uncertainties. The operational attributes of an AC-DC hybrid microgrid, and load and renewable generation uncertainties are incorporated in the optimal scheduling problem by using a customised power-flow technique, and by modelling uncertainties by Hong's two-point estimate method, respectively. The economic and environmental objectives are modelled in the fuzzy domain by fuzzy membership functions. A combination of particle swarm optimisation and fuzzy max-min technique is then employed for obtaining the optimal solution. The static active power droop constants of the dispatchable units are the control variables. Simulation results on a 6-bus AC-DC hybrid microgrid system demonstrate that optimal scheduling results in 4.26% reduction of operating cost and 13.91% reduction of emission in comparison to capacity based droop settings. Further, a comparison between the proposed method and the elitist multi-objective GA indicates that the optimised solution lies on the Pareto-front, thereby validating the proposed technique.

Unified Power Flow Algorithm for Standalone AC/DC Hybrid Microgrids

Recent studies on power flow analysis of islanded microgrids have become increasingly important and different solutions have been proposed. However, they are often limited to only ac or only dc microgrids and hence, the power flow must be solved separately. This paper proposes a straightforward and efficient method to solve power flows of hybrid ac/dc microgrids simultaneously, based on the well-established Newton–Raphson approach. It considers the lack of slack bus during islanded operation, the coupling of ac frequency and dc bus voltage, as well as the droop control of distributed generators. To achieve power sharing between the ac and dc subgrids, this method incorporates the interlinking converter droop constants in the equation. The algorithm was tested on modified hybrid microgrids involving multiple ac–dc subgrids. The results are compared against results from time-domain simulations to validate the algorithm’s accuracy. The proposed technique can be easily implemented to aid the design and planning process of hybrid microgrids.

An Efficient Method of Determining Operating Points of Droop-Controlled Microgrids

Traditional methods of determining steady-state power system operating points are not applicable to droop-controlled microgrids. The conditions of an islanded microgrid, including the absence of a slack bus and inherent coupling of complex power, frequency, and bus voltage, require new tools to properly analyze. In this study, a method of determining the operating points of droop-controlled microgrids is proposed. The procedure is similar in form and function to a traditional power flow analysis, but is performed in the synchronous reference frame to ensure compatibility with conventional inverter models. A quasi-Newton iterative process is used to solve the nonlinear equations pertaining to each generation unit and load bus. The method provides a structured approach to determining the consistent system-level linearization points required for large-scale microgrid studies. Grid-forming, grid-feeding, and grid-supporting generation units are supported, along with both constant impedance and constant power loads. The method is validated in hardware experiments, simulations, and comparisons to results of existing power flow algorithms. As an example of the method's potential applications, a procedure for determining droop constants that ensure equal Q sharing between generation units is constructed around the proposed method's basic functionality.

DC Grid Control Through the Pilot Voltage Droop Concept—Methodology for Establishing Droop Constants

Converter control is an important aspect of voltage-source-converter high-voltage dc (VSC-HVDC) based multi-terminal dc grids. Variants of power drooped against dc voltage control have previously been proposed for this purpose. However, there is at this time no clear methodology available on how to select the size of the droop constants for distributing a power mismatch, following, e.g., a converter trip, on several converters. This paper proposes a methodology for establishing droop constants for a variant of power drooped against dc voltage called pilot voltage droop control. The methodology is derived from the basic control laws in a straight forward fashion and the accuracy is exemplified by time domain simulations in Digsilent's Powerfactory platform.

Reduced-Order Model and Stability Analysis of Low-Voltage DC Microgrid

Depleting fossil fuels, increasing energy demand, and need for high-reliability power supply motivate the use of dc microgrids. This paper analyzes the stability of low-voltage dc microgrid systems. Sources are controlled using a droop-based decentralized controller. Various components of the system have been modeled. A linearized system model is derived using small-signal approximation. The stability of the system is analyzed by identifying the eigenvalues of the system matrix. The sufficiency condition for stable operation of the system is derived. It provides upper bound on droop constants and is useful during planning and designing of dc microgrids. Furthermore, the sensitivity of system poles to variation in cable resistance and inductance is identified. It is proved that the poles move further inside the negative real plane with a decrease in inductance or an increase in resistance. The method proposed in this paper is applicable to any interconnecting structure of sources and loads. The results obtained by analysis are verified by detailed simulation study. Root locus plots are included to confirm the movement of system poles. The viability of the model is confirmed by experimental results from a scaled-down laboratory prototype of a dc microgrid developed for the purpose.

Impact of DC Line Voltage Drops on Power Flow of MTDC Using Droop Control

This paper discusses the impact of dc transmission voltage drops on the distribution of dc grid balancing power when dc voltage droop control is applied. DC line voltage drops in a multiterminal VSC-HVDC (MTDC) system result in nonuniform variations of dc bus voltages when changes in dc grid power flow occur. This in turn affects the distribution of instantaneous balancing power in a MTDC that uses dc voltage droop control. The values of dc voltage droop constants determine the degree of impact that dc voltage drops will have on the sharing of balancing power in the dc grid. In this paper, an analytical expression for estimating the distribution of balancing power which accounts for dc line voltage drops is derived. A five-terminal MTDC was modelled in PSCAD for demonstrating the effects of dc line voltage drops as well as for validating the proposed analytical expression which estimates balancing power distribution.