Lipei Huang

State-of-Charge Balance Using Adaptive Droop Control for Distributed Energy Storage Systems in DC Microgrid Applications

This paper presents the coordinated control of distributed energy storage systems in dc microgrids. In order to balance the state-of-charge (SoC) of each energy storage unit (ESU), an SoC-based adaptive droop control method is proposed. In this decentralized control method, the droop coefficient is inversely proportional to the nth order of SoC. By using a SoC-based droop method, the ESUs with higher SoC deliver more power, whereas the ones with lower SoC deliver less power. Therefore, the energy stored in the ESU with higher SoC decreases faster than that with lower SoC. The SoC difference between each ESU gradually becomes smaller, and finally, the load power is equally shared between the distributed ESUs. Meanwhile, the load sharing speed can be adjusted by changing the exponent of SoC in the adaptive droop control. The model of the SoC-based adaptive droop control system is established, and the system stability is thereby analyzed by using this model. Simulation and experimental results from a 2 × 2.2 kW parallel converter system are presented in order to validate the proposed approach.

Hierarchical Control of Parallel AC-DC Converter Interfaces for Hybrid Microgrids

In this paper, a hierarchical control system for parallel power electronics interfaces between ac bus and dc bus in a hybrid microgrid is presented. Both standalone and grid-connected operation modes in the dc side of the microgrid are analyzed. Concretely, a three-level hierarchical control system is implemented. In the primary control level, the decentralized control is realized by using the droop method. Local ac current proportional-resonant controller and dc voltage proportional-integral controller are employed. When the local load is connected to the dc bus, dc droop control is applied to obtain equal or proportional dc load current sharing. The common secondary control level is designed to eliminate the dc bus voltage deviation produced by the droop control, with dc bus voltage in the hybrid microgrid boosted to an acceptable range. After guaranteeing the performance of the dc side standalone operation by means of the primary and secondary control levels, the tertiary control level is thereafter employed to perform the connection to an external dc system. Meanwhile, the impact of the bandwidth of the secondary and tertiary control levels is discussed. The closed-loop model including all the three control levels is developed in order to adjust the main control parameters and study the system stability. Experimental results of a 2 ×2.2 kW parallel ac-dc converter system have shown satisfactory realization of the designed system.

Online Identification of Permanent Magnet Flux Based on Extended Kalman Filter for IPMSM Drive With Position Sensorless Control

In order to realize precise rotor position/speed control of ac motor drives under sensorless operation, motor parameters should be online estimated. In this paper, the identification on permanent magnet flux of interior permanent magnet synchronous motor (IPMSM) under position sensorless control is investigated. A rotor-flux-oriented vector control with model reference adaptive system (MRAS)-based rotor position/speed estimation is employed as the basic control strategy for IPMSM drive. An identification scheme based on extended Kalman filter for the permanent magnet flux of IPMSM is proposed. By using this scheme, the identification problems due to low-order state equations of IPMSM can be avoided. Based on these works, the online permanent magnet flux identification of IPMSM with rotor position/speed senseless control is realized. Simulation and experimental results demonstrate the feasibility and effectiveness of the proposed control methods, which shows that the estimation error of rotor position based on the identified permanent magnet flux is limited within a very low level.

Power loss and junction temperature analysis of power semiconductor devices

A new developed electro-thermal calculation method is implemented to estimate the power loss and working temperature of IGBT devices. Based on the measurement of IGBT characteristics, the exact estimation of power loss considering the junction temperature is introduced. Then the thermal network is used to calculate the working temperature. The comparison between experiment and calculation results shows that this method is effective as a designing step with only the time domain voltage and current data obtained from simulation results.

A new robust algorithm to improve the dynamic performance on the speed control of induction motor drive

A nonlinear auto-disturbance rejection controller (ADRC) has been developed to ensure high dynamic performance of induction motors in this paper. By using the extended state observer (ESO), ADRC can accurately estimate the derivative signals and precise decoupling of induction motors is achieved. In addition, the proposed strategy realizes the disturbance compensation without accurate knowledge of induction motor parameters. The simulation and experimental results show that the proposed controller ensures good robustness and adaptability under modeling uncertainty and external disturbance. It is concluded that the proposed topology produces better dynamic performance, such as small overshoot and fast transient time, than the conventional proportional/integral/derivative (PID) controller in its overall operating conditions.

Fundamental characteristics of five-level double converters with adjustable dc voltages for induction motor drives

In this paper, two kinds of control strategies for a three-phase five-level double converter are described on the assumption that the converter is applied to an induction motor drive system. The purposes of the proposed control strategies are to correct voltage imbalance of the DC-bus capacitors, to keep the input power factor at near unity, and to achieve an adjustable-speed drive. Characteristics of the converter operated by each of the two control strategies are examined and the validity is verified by experiments using a 3.7-kW induction motor.

Double-Quadrant State-of-Charge-Based Droop Control Method for Distributed Energy Storage Systems in Autonomous DC Microgrids

In this paper, a double-quadrant state-of-charge (SoC)-based droop control method for distributed energy storage system is proposed to reach the proper power distribution in autonomous dc microgrids. In order to prolong the lifetime of the energy storage units (ESUs) and avoid the overuse of a certain unit, the SoC of each unit should be balanced and the injected/output power should be gradually equalized. Droop control as a decentralized approach is used as the basis of the power sharing method for distributed energy storage units. In the charging process, the droop coefficient is set to be proportional to the nth order of SoC, while in the discharging process, the droop coefficient is set to be inversely proportional to the nth order of SoC. Since the injected/output power is inversely proportional to the droop coefficient, it is obtained that in the charging process the ESU with higher SoC absorbs less power, while the one with lower SoC absorbs more power. Meanwhile, in the discharging process, the ESU with higher SoC delivers more power and the one with lower SoC delivers less power. Hence, SoC balancing and injected/output power equalization can be gradually realized. The exponent n of SoC is employed in the control diagram to regulate the speed of SoC balancing. It is found that with larger exponent n, the balancing speed is higher. MATLAB/simulink model comprised of three ESUs is implemented and the simulation results are shown to verify the proposed approach.

DC voltage control strategy for a five-level converter

This paper describes a control method for a three-phase five-level diode-clamp pulse width modulation (PWM) converter considering DC-link capacitor voltage balancing problem. The proposed control circuit uses multiband hysteresis comparators (MHCs) to simplify the control of the main circuit. The DC-link capacitor voltage balancing problem is solved by changing the shape of the MHC. The proposed method can (1) overcome voltage imbalance at the DC-link capacitors; (2) achieve a unity power factor; (3) generate nearly sinusoidal input currents; and (4) regenerate electric power back to the power system. Simulation and experimental results demonstrate the validity of the proposed method.

An Overmodulation Method for PWM-Inverter-Fed IPMSM Drive With Single Current Sensor

In order to lower the cost and expand the working range of pulsewidth modulation (PWM)-inverter-fed interior permanent magnet synchronous motor drives, the control methods under the overmodulation mode with only a dc-link current sensor are investigated. The space vector modulation of the PWM inverter is analyzed in detail, and an adjustment scheme for the reference voltage vectors is developed to fulfill the requirements by phase current reconstruction under overmodulation. Furthermore, in order to simplify the control process and reduce the control errors, a specified overmodulation method based on the superposition principle is proposed for the PWM inverter with phase current reconstruction, which overcomes the shortcoming of the PWM adjustment scheme. Experimental results demonstrate the feasibility and the effectiveness of the proposed control method, which realizes the single current sensor overmodulation control with maximum voltage transfer ratio.

Improved Modeling of Medium Voltage SiC MOSFET Within Wide Temperature Range

An improved model of medium voltage (1200 V) silicon carbide (SiC) MOSFET based on PSpice is proposed in this paper, which is suitable for wide temperature range applications especially at low temperature. The static characteristics of SiC MOSFET are described by introducing temperature-dependent voltage source and current source. The effect of negative turn-off gate drive voltage is also taken into account in the modeling. In order to reflect the low-temperature characteristics of SiC MOSFET accurately, low temperature (-25 °C) measurements are carried out, which provide the modeling basis. The determinations of key parameters in the model are analyzed in detail, including the on-state resistor, internal gate resistor, temperature dependent sources, and some capacitors. The proposed model is verified by the experimental tests on a buck converter prototype at different input voltages, input currents, and temperatures. Simulation results on the proposed model coincide well with the experimental test results, in terms of switching waveforms and power losses even at low temperature (-25 °C). These results demonstrate that the proposed model exhibits high accuracy within wide temperature range.