A Method for Balancing the Supply and Demand in an Isolated System Consisting of Voltage Control Type Inverters in FRIENDS

The authors have proposed Flexible, Reliable and Intelligent ENergy Delivery System (FRIENDS) as a future power distribution system. In the FRIENDS, new facilities called Quality Control Centers (QCCs) which consist of power electronics devices, distributed generators (DGs), energy storage systems (ESSs), etc. are installed between distribution substations and customers. By controlling and operating those devices in QCCs adequately, various functions can be realized. Particularly, when a power supply from the transmission network is interrupted, isolated local networks which consist of some QCCs are composed in order to realize uninterruptible power supply. In the isolated local network, the DG and ESS in QCCs are employed as backup generators. This paper proposes a novel distributed and autonomous method for balancing the supply and demand during isolated operation. This paper also investigates the effectiveness of the proposed method through some computational simulations using PSCAD/EMTDC

Simplified power quality conditioner for dynamic voltage restoration and fault current limitation

Power quality improvement has become one of the important issues because of increasing sensitive loads. For example, dynamic voltage restoration is one of the major issues about the power quality. On the other hand, a large current caused by some system fault may be unwillingly large because of many interconnections of distributed generators. A Unified Power Quality Conditioner (UPQC) is well known apparatus, which consists of two self-commutated converters. This paper proposes a simplified power quality conditioner by replacing the shunt converter of the UPQC by two diode-bridge rectifiers. By the simplification of the UPQC, the overall system reliability is increased and installation cost is reduced. Power conditioning is provided by the series converter, while the d.c. voltage control is realized by the diode bridge rectifier. Although the active power flow through the rectifier must be only one direction, the amount of active power can be regulated by the d.c. voltage control of the series converter. In case some excessive energy is stored in the d.c. capacitor, it can be returned to the power system by an appropriate control of the series converter. In this paper, the effectiveness of the Simplified Power Quality Conditioner (SPQC) as a dynamic voltage restorer and a fault current limiter is verified by simulation

Sensorless Control of Dual-Active-Bridge Converter with Reduced-Order Proportional-Integral Observer

When controlling a Dual-Active-Bridge (DAB) DC/DC converter, the high frequency terminal current is usually measured for use in the current feedback controller. In order to measure that current, a wide bandwidth sensor accompanied with high-speed amplifiers are required. Furthermore, a high Analog-to-Digital sampling rate is also necessary for sampling and processing the measured data. To avoid those expensive requirements, this paper proposes an alternative control method for the DAB converter. In the proposed method, the terminal current is estimated by a reduced-order proportional integral observer. A technique is also proposed to reduce the phase drift effect when the voltages at two terminals are not matched. Afterwards, a combined current feedforward—voltage feedback control system is developed to enhance the system dynamics and to regulate the output voltage. This control system needs only the information of the terminal voltages and no current sensor is required. Experimental results show that the observer can estimate the terminal current very quickly with the accuracy of more than 98 % . In addition, the output voltage is well regulated with a fluctuation of less than ± 2.6 % and a settling time of less than 6.5 ms in the presence of a 30 % load change