Yibo Wang

Frequency Stability of Hierarchically Controlled Hybrid Photovoltaic-Battery-Hydropower Microgrids

Hybrid photovoltaic (PV)-battery-hydropower microgrids (MGs) can be considered to enhance electricity accessibility and availability in remote areas. However, the coexistence of different renewable-energy sources with different inertias and control strategies may affect system stability. In this paper, a hierarchical controller for a hybrid PV-battery-hydropower MG is proposed in order to achieve the parallel operation of the hydropower and PV-battery system with different rates and to guarantee power sharing performance among PV voltage-controlled inverters, while the required power to the hydropower-based local grid is supplied. In this case, the PV-battery system will operate as a PQ bus to inject the desired active and reactive powers to the local grid, while the hydropower station will act as a slack bus which maintains its voltage amplitude and frequency. An integrated small-signal state-space model is derived to analyze the system stability of the hybrid MG. The simulation results show system frequency and voltage stability for a hybrid MG demonstration which includes the 2-MWp PV installations, a 15.2-MWh battery system, and a 12.8-MVA hydropower plant. The experimental results on a small-scale laboratory prototype verify the validity of the theoretical analysis and proposed control strategy.

Control strategy for autonomous operation of three-phase inverters dominated microgrid under different line impedance

The traditional power droop controllers are commonly used in distributed generation (DG) units to share the loads among the parallel inverters. However, the parameters of the power transmission and distribution line in microgrid are small and generally presented high resistance-to-inductance (R/X) ratio which will seriously influence the performance of the traditional power droop controller, even leading to instability. In this paper, the control strategies for parallel inverters under the inductive and resistance line impedance are researched contrastively based on the small signal models of the power loop. The employed droop scheme can maintain the load sharing capability under different line impedance. These theoretical findings are eventually verified experimentally on laboratory prototypes.

Frequency stability of hierarchically controlled hybrid photovoltaic-battery-hydropower microgrids

Hybrid photovolvaic-battery-hydropower microgrids can increase electricity accessibility and availability in remote areas. In those microgrids with grid-connected and islanded modes capabilities, seamless transition between both modes is needed as well. However, the different resources with conventional constant P/Q and P/V controls coexisting in the microgrid may affect frequency stability. In this paper, a hierarchical control is proposed to perform power sharing among PV voltage source inverters (VSIs), while injecting the dispatched power to the main grid. Further, frequency stability analysis is presented based on small signal models of the hybrid PV-HP microgrid, including 2 MWp PV station, 15.2 MWh battery storage system, and 12.8 MVA hydropower plant. Simulation results of the microgrid and experimental results on a scaled-down laboratory prototype verify the effectiveness of the proposed hierarchical controller.

An MPC-Based ESS Control Method for PV Power Smoothing Applications

Random fluctuation in photovoltaic (PV) power plants is becoming a serious problem affecting the power quality and stability of the grid along with the increasing penetration of PVs. In order to solve this problem, by the adding of energy storage systems (ESS), a grid-connected microgrid system can be performed. To make this system feasible, this paper proposes a model predictive control (MPC) based on power/voltage smoothing strategy. With the receding horizon optimization performed by MPC, the system parameters can be estimated with high accuracy, and at the same time the optimal ESS power reference is obtained. The critical parameters, such as state of charge, are also taken into account in order to ensure the health and stability of the ESSs. In this proposed control strategy, communication between PVs and ESS is not needed, since control command can be calculated with local measured data. At the same time, MPC can make a great contribution to the accuracy and timeliness of the control. Finally, experimental results from a grid-connected lab-scale microgrid system are presented to prove effectiveness and robustness of the proposed approach.

Design of energy storage control strategy to improve the PV system power quality

Random fluctuation of PV power is becoming a more and more serious problem affecting the power quality and stability of grid as the PV penetration keeps increasing recent years. Aiming at this problem, this paper proposed a control strategy of energy storage system based on Model Predictive Control (MPC). By the continuous optimizing of MPC, we can obtain the system parameters accurately, and then calculate the energy storage power. At the same time, this control took state of charge (SOC) and other parameters into account to ensure the health and stability of the energy storage units. Finally, simulation results proved the proposed control strategy had good performance and robustness.

A novel frequency restoring strategy of hydro-PV hybrid microgrid

The conventional PV systems based on the voltage inverters only inject dispatched power to the utility grid when they work at a grid-connected mode in the hydro-PV hybrid microgrid. Due to the droop method employed for load sharing between generators, as well as the enormous inertia of system. The existence of frequency steady-state error and the slow active power/frequency dynamic response are inevitable. Therefore, a novel frequency restoring strategy for the hydro-PV hybrid microgrid based on the improved hierarchical control of PV systems is proposed in this paper. The output active power of PV systems is controlled by an extra frequency restoring controller resided in the tertiary control level. The frequency steady-state error is eliminated through regulating and rebalancing the power flow between the hydropower and the PV system. The proposed strategy has verified through simulations and experiments.

A novel semiconductor switch test platform for power converter optimization

This paper presents a novel semiconductor switch test platform for power converter optimization which allows to test power semiconductors under different switching current waveforms. As switching current has a major influence on the switching behavior of power semiconductors and thus on switching losses. Compared with conventional double pulse test circuit which can only test power semiconductors under linear current waveform, the proposed test platform can provides programmable current source by controlling the switches. Experimental results measured from SiC MOSFET have verified the effectiveness of the test platform over conventional circuits.

Design techniques of distributed photovoltaic/energy storage system

The intermittent and fluctuating energy sources such as photovoltaic power generation system may cause impact on the power grid. In this paper, the key technologies and control methods of distributed photovoltaic / storage system are systematically studied. This paper introduces the overall design scheme and main function of the integrated system include energy storage and distributed photovoltaic, then discusses the design principle of different module and subsystem, such as storage battery banks, bi-directional converters, measurement and monitor system and energy management system. The novel estimate algorithm of state of charge in battery and control method of photovoltaic / storage system were proposed. The field operation data show that photovoltaic / storage system can be used to stabilize the power fluctuations from photovoltaic to grid, adjust grid voltage of common connection point and increase income base of the power tariff in peak and valley.

State of charge estimation of lead-carbon batteries in actual engineering

Lead-carbon Batteries as an energy storage device, its state of charge is an important parameter of the entire battery energy storage system. This paper uses the Improved Thevenin model as the battery mathematical model, and establishes the state-space equations. First of all, it fits the function relationships between the parameters and the SOC. And then it establishes a set of equations combined with the electrical characteristics of the model. Finally it searches out the best parameter estimation according with unconstrained nonlinear optimization methods. It simulates the battery performance very well to use the Battery mathematical models and parameters, the error within 1%. This paper uses Kalman filter based on the wavelet transform estimation method to estimate the SOC, and it finally verifies this estimation method has a high accuracy of estimation, the error within 2%.

Large-scale photovoltaic generation system connected to HVDC grid with centralized high voltage and high power DC/DC converter

Large-scale photovoltaic(PV) generation system connected to HVDC grid has many advantages compared to its counterpart of AC grid. DC connection can solve many problems that AC connection faces, such as the grid-connection and power transmission, and DC connection is the tendency. DC/DC converter as the most important device in the system, which has become one of the hot spots recently. There are many DC/DC converter topologies, but none of them can realize high voltage and high capacity of DC/DC converter requirements for PV system connected to HVDC grid. The paper proposes a centralized DC/DC converter which uses Boost Full Bridge Isolated DC/DC Converter (BFBIC) topology as basic power module and combines through input parallel output series(IPOS) form to improve power capacity and output voltage to match with the HVDC grid. The BFBIC topology has a unique future: it allows the converter work at an extra switching state — the shoot-through state to pre-boost the voltage of the converter, and then the voltage is step up by the high frequency isolation transformer. Through controlling the shoot-through duty cycle, it can realize PV maximum power point tracking (MPPT). Meanwhile it adopts input current sharing control strategy to realize input current and output voltage balancing between the parallel/series DC/DC converter modules. A ±10kV/200kW DC/DC converter platform is built to verify the proposed topology and control strategy.