Modified Droop Method Based on Master Current Control for Parallel-Connected DC-DC Boost Converters

Load current sharing between parallel-connected DC-DC boost converters is very important for system reliability. This paper proposes a modified droop method based on master current control for parallel-connected DC-DC boost converters. The modified droop method uses an algorithm for parallel-connected DC-DC boost converters to adaptively adjust the reference voltage for each converter according to the load regulation characteristics of the droop method. Unlike the conventional droop method, the current feedback signal (master current) for one of the parallel-connected converters is used in the inner loop controller for all converters to avoid any differences in the time delay of the control loops for the parallel-connected converters. The algorithm ensures that the load current sharing is identical to the load regulation characteristics of the droop method. The proposed algorithm is tested with a mismatch in the parameters of the parallel converters. The effectiveness of the proposed algorithm is verified using Matlab/Simulink simulation

A Novel Power Sharing Control Method for Distributed Generators in DC Networks

The power sharing control method is a desirable solution to integrate multiple renewable energy generators into the grid and to keep them working synchronously. Power sharing control between different distributed generators is an important consideration for the stabilized operation of the power grid network. In this thesis work, a novel method is used with the concept of droop control technique and is designed to control power from each individual generator in DC network particularly. The proposed power sharing control method can be widely applied to grid connected network and to islanded power grid network for obtaining high efficiency of power distribution and also provides higher stability. An efficient power control method to share the load demand power is designed based on the concept of droop control. This method does not follow sequential or predefined topology of power sharing but uses the availability of power from each generator as a factor of control. The proposed controller can be applied to an individual distributed generator to regulate its output power quickly and accurately. The power sharing control method was formulated, modeled and verified by simulation studies of steady state and transient stability tests. The optimal coupling resistance for power sharing was also identified. The interaction of the controller and the communication delay was also studied. The interference of communication delay is negligible for the power sharing controller. The system is simulated in MATLAB/SIMULINK environment

A Photovoltaic-Fed DC-Bus Islanded Electric Vehicles Charging System Based on a Hybrid Control Scheme

Electric vehicle (EV) charging stations fed by photovoltaic (PV) panels allow integration of various low-carbon technologies, and are gaining increasing attention as a mean to locally manage power generation and demand. This paper presents novel control schemes to improve coordination of an islanded PV-fed DC bus EV charging system during various disturbances, including rapid changes of irradiance, EV connection and disconnection, or energy storage unit (ESU) charging and discharging. A new hybrid control scheme combining the advantages of both master–slave control and droop control is proposed for a charging station supplying 20 EVs for a total power of 890 kW. In addition, a three-level (3L) boost converter with capacitor voltage balance control is designed for PV generation, with the aim to provide high voltage gain while employing a small inductor. The control techniques are implemented in a simulation environment. Various case studies are presented and analysed, confirming the effectiveness and stability of the control strategies proposed for the islanded charging system. For all tested conditions, the operating voltage is maintained within 5% of the rated value

Development of an alternative droop strategy for controlling parallel converters in standalone DC microgrid

Most of parallel-connected DC-DC converters schemes are based on a high-bandwidth communication network to achieve minimum circulating current, proper load current sharing, and acceptable voltage regulation. However, in DC microgrids, the use of communication network can be costly and unsuitable considering the data reliability and cost investment because the load and renewable energy sources are connected to the point of common coupling. Therefore, the droop control as a decentralized method has gained more attention. However, the challenge for the conventional droop method is to overcome the issue of circulating current, poor load current sharing, and the drop in DC grid voltage due to the droop action. This thesis develops and tests an approach for minimizing the circulating current, as well as improving the voltage regulation and the load current sharing for the droop method. The developed approach is based on the concept of synchronized switching, which is implemented using an alternative droop strategy for controlling different sizes of parallel-connected DC-DC boost converters. In this thesis, synchronous switching, based on an optimized controller, is presented to eliminate the initiation of circulating current and minimize the ripple in the output current for parallel-connected boost converters. Furthermore, a modified droop method, including the cable resistance, is introduced. The modified droop method uses the measurements of the voltage and current at the point of common coupling to estimate the voltage set point for each converter locally. The communication network is eliminated by utilizing the modified droop method because, in the proposed method, there is no current and voltage measurement data transmitted from one converter to the other converter. Additional loop control is also applied for equal current sharing between parallel converters to overcome the issue of mismatch in parameters of the parallel converters. The additional loop control is added to improve the load current sharing in the modified droop control. The modified droop control method with additional loop control is verified using MATLAB/SIMULINK and validated with experimental results. However, the droop action of the modified droop and different cable resistances degrades the voltage regulation and load current sharing. Therefore, an improved droop method, which utilizes the virtual droop gain and voltage droop control gain, is proposed to overcome the problem of load current sharing and voltage regulation. The virtual droop gain compensates the differences in the cable resistances, and the voltage droop control gain regulates the voltage at the point of common coupling. This maintains the common DC bus at its rated value. The effectiveness of the improved droop method is demonstrated by MATLAB/Simulink and Laboratory prototype results. Finally, the proposed method is utilized in a standalone DC microgrid. An example of a DC microgrid of a residential building powered by a PV solar system illustrates the feasibility and the effectiveness of the proposed methods

Control Algorithm for Equal Current Sharing between Parallel-Connected Boost Converters in a DC Microgrid

DC microgrids are gaining more attention compared to AC microgrids due to their high efficiency and uncomplicated interconnection of renewable sources. In standalone DC microgrid, parallel-connected converters connect the storage system to the load. To achieve equal current sharing among parallel converters, several methods have been presented, but they vary in their current sharing performance, complexity, cost, and reliability. In DC microgrid, the conventional droop control method is preferred because it is more competitive in terms of cost, suitability, and reliability compared to the master-slave control method. However, the conventional droop method cannot ensure equal current sharing due to the mismatches in parameters of parallel-connected converters. To address this limitation, a control algorithm that supervises a modified droop method to achieve precise current sharing between parallel modules is proposed in this paper. The control algorithm is based on the percentage of current sharing for each module to the total load current. The output current measurement of each converter is compared to the total load current and is used to modify the nominal voltage for each converter. The effectiveness of the proposed algorithm is verified by MATLAB simulation model and experimental results

Comparison between alternative droop control strategy, modified droop method and control algorithm technique for parallel-connected converters

Most of the active current sharing methods are based on a communication network. The communication link is also used with the improved droop control methods to achieve a precise load current sharing and regulate the voltage at the common DC bus. Conversely, the conventional droop method that is considered a decentralized method becomes more attractive for controlling parallel-connected converters in DC microgrids. The conventional droop methods' main drawbacks are associated with the unequal load current sharing and voltage deviation at the common DC bus. In this paper, the modified droop method as a conventional droop method is augmented with a virtual droop and adaptive voltage control gains to improve the load current sharing and the voltage regulation, respectively. In contrast with other improved droop approaches, the control approach proposed in the paper does not require a communication link to exchange information between parallel modules. Instead, it uses the converters' theoretical load regulation characteristics to estimate the voltage set point for each converter locally. The proposed virtual resistive gain manipulates the modified droop method to regulate each module's droop gain, which ensures equal current sharing. The proposed method also eliminates the tradeoff between current sharing difference and voltage regulation by implementing the adaptive voltage control, which compares the estimated voltage at the point of common coupling with the rated bus value and adjusts the droop gains based on the compared values to ensure a constant voltage at various load conditions. The load current sharing and voltage restoration improvements of the proposed method versus the modified droop method and the control algorithm technique are observed in this paper. The proposed method's effectiveness is demonstrated by MATLAB/Simulink simulation and validated by an experimental prototype

Hierarchical control of dc microgrids with constant power loads

This dissertation proposes general methodologies for designing hierarchical control schemes for dc microgrids loaded by constant power loads (CPLs). CPLs form a major proportion of the system loads in many microgrids. Without proper control, CPLs present destabilizing effect at the dc microgrid. In addition to stable operation of microgrid, proper current sharing among paralleled sources is essential. The proposed hierarchical control strategy consists of two control levels. The lower level consists of droop-based primary controllers which enables current-sharing among paralleled sources and also damps limit cycle oscillations due to CPLs. The higher level consists of secondary controller which compensates for voltage deviations due to primary controller. This higher level is implemented either as autonomous controllers or as a centralized controller. In the case of autonomous secondary controllers, they operate alongside of primary controllers in each of the paralleled converters. In the case of centralized secondary controller, a remote secondary controller uses a high speed communication link to communicate to local controllers. Interfacing sources with different characteristics and voltage ranges necessitates the use of complex converter topologies. As an initial step towards implementing hierarchical control scheme for such microgrids with CPLs, a linear controller is proposed for dc microgrids with standalone SEPIC, Cuk and Zeta converters. During the first stage of the two stage controller, limit cycle oscillations are damped by inserting a virtual resistance in series with the converter input inductor. During the second stage, an integral controller is added to the first stage to compensate for voltage deviations. For microgrids containing different converter topologies, stability of equilibrium points is examined and stability conditions are derived and explained. Experiments performed on a prototype microgrid are used to verify the proposed control laws. Expanding study on stability of microgrids, the maximum real power load in a dc microgrid bus is traced geometrically. The generalized circle diagram approach used in a conventional power system is modified for this purpose. The different types of buses present in a dc microgrid are described and the locus of operating points is obtained. The proposed method is verified by simulations on an example dc microgrid.Electrical and Computer Engineerin

CONTROL STRATEGIES OF DC MICROGRID TO ENABLE A MORE WIDE-SCALE ADOPTION

Microgrids are gaining popularity in part for their ability to support increased penetration of distributed renewable energy sources, aiming to meet energy demand and overcome global warming concerns. DC microgrid, though appears promising, introduces many challenges in the design of control systems in order to ensure a reliable, secure and economical operation. To enable a wider adoption of DC microgrid, this dissertation examines to combine the characteristics and advantages of model predictive control (MPC) and distributed droop control into a hierarchy and fully autonomous control of the DC microgrid. In addition, new maximum power point tracking technique (MPPT) for solar power and active power decoupling technique for the inverter are presented to improve the efficiency and reliability of the DC microgrid. With the purpose of eliminating the oscillation around the maximum power point (MPP), an improved MPPT technique was proposed by adding a steady state MPP determination algorithm after the adaptive perturb and observe method. This control method is proved independent with the environmental conditions and has much smaller oscillations around the MPP compared to existing ones. Therefore, it helps increase the energy harvest efficiency of the DC microgrid with less continuous DC power ripple. A novel hierarchy strategy consisting of two control loops is proposed to the DC microgrid in study, which is composed of two PV boost converters, two battery bi-directional converters and one multi-level packed-u-cell inverter with grid connected. The primary loop task is the control of each energy unit in the DC microgrid based on model predictive current control. Compared with traditional PI controllers, MPC speeds up the control loop since it predicts error before the switching signal is applied to the converter. It is also free of tuning through the minimization of a flexible user-defined cost function. Thus, the proposed primary loop enables the system to be expandable by adding additional energy generation units without affecting the existing ones. Moreover, the maximum power point tracking and battery energy management of each energy unit are included in this loop. The proposed MPC also achieves unity power factor, low grid current total harmonics distortion. The secondary loop based on the proposed autonomous droop control identifies the operation modes for each converter: current source converter (CSC) or voltage source converter (VSC). To reduce the dependence on the high bandwidth communication line, the DC bus voltage is utilized as the trigger signal to the change of operation modes. With the sacrifice of small variations of bus voltage, a fully autonomous control can be realized. The proposed distributed droop control of different unit converters also eliminates the potential conflicts when more than two converters compete for the VSC mode. Single-phase inverter systems in the DC microgrid have low frequency power ripple, which adversely affects the system reliability and performance. A power decoupling circuit based on the proposed dual buck converters are proposed to address the challenges. The topology is free of shoot-through and deadtime concern and the control is independent with that of the main power stage circuit, which makes the design simpler and more reliable. Moreover, the design of both PI and MPC controllers are discussed and compared. While, both methods present satisfied decoupling performances on the system, the proposed MPC is simpler to be implemented. In conclusion, the DC microgrid may be more widely adopted in the future with the proposed control strategies to address the current challenges that hinder its further development