D. Ranamuka

Online Voltage Control in Distribution Systems With Multiple Voltage Regulating Devices

Voltage regulation in distribution systems is typically performed with the aid of multiple voltage regulating devices, such as on-load tap changer and step voltage regulators. These devices are conventionally tuned and locally coordinated using Volt/VAR optimization strategies in accordance with the time-graded operation. However, in case of distribution systems with distributed generation (DG), there could be a possibility of simultaneous responses of DG and multiple voltage regulators for correcting the target bus voltage, thereby resulting in operational conflicts. This paper proposes an online voltage control strategy for a realistic distribution system containing a synchronous machine-based renewable DG unit and other voltage regulating devices. The proposed strategy minimizes the operational conflicts by prioritizing the operations of different regulating devices while maximizing the voltage regulation support by the DG. It is tested on an interconnected medium voltage distribution system, present in New South Wales, Australia, through time-domain simulation studies. The results have demonstrated that voltage control for a distribution feeder can effectively be achieved on a real-time basis through the application of the proposed control strategy.

Online Coordinated Voltage Control in Distribution Systems Subjected to Structural Changes and DG Availability

The responses of multiple distributed generation (DG) units and voltage regulating devices, such as tap changers and capacitor banks, for correcting the voltage may lead to operational conflicts and oscillatory transients, where distribution systems are subjected to network reconfiguration and availability of the DG units. Therefore, coordinated voltage control is required to minimize control interactions, while accounting for the impact of structural changes associated with the network. This paper proposes a strategy for coordinating the operation of multiple voltage regulating devices and DG units in medium voltage (MV) distribution systems, under structural changes and DG availability, for effective voltage control. The proposed strategy aids to minimize the operational conflicts by allowing voltage regulating devices to operate in accordance with the priority scheme designed based on the electrical-distance between voltage regulating devices and DG units, while maximizing the voltage support by the DG units. The proposed coordination scheme is designed to enact with an aid of a substation centered distribution management system for online voltage control. The control actions of proposed coordination strategy are tested on a MV distribution system, derived from the state of New South Wales, Australia, through simulations, and results are reported.

Dynamic adjustment of OLTC parameters using voltage sensitivity while utilizing DG for Volt/VAr support

Voltage regulation in medium voltage (MV) distribution systems is still widely performed with the aid of substation on-load tap changer (OLTC). An OLTC can be operated with or without enacting the line drop compensation (LDC) module embedded in the OLTC control. However, in case of distribution systems with higher penetration of renewable and distributed generation (DG), it is well known that the DG reverse power flow can increase the number of OLTC tap operations under certain system conditions. On the other hand, voltage correction by DG is one of the promising concepts highly regarded by the researchers and engineers. In this paper, a methodology is proposed for Volt/VAr support by means of DG and dynamic adjustment of OLTC parameters. As an assessment tool, first order sensitivity of the regulating point voltage, estimated by LDC scheme, to change in reactive power support provided by voltage support DG is derived. The simulation studies are carried out on a test system using MATLAB software, and results have demonstrated the accuracy of proposed sensitivity and its application for dynamically updating the OLTC parameters for ensuring effective voltage control with the aid of synchronous machine based voltage support DG.

Examining the Interactions between DG Units and Voltage Regulating Devices for Effective Voltage Control in Distribution Systems

Voltage regulation by means of coordinated voltage control is one of the challenging aspects in an active distribution system operation. Integration of distributed generation (DG), which can also be operated in Volt/VAr control mode, may introduce adverse effects including control interactions, operational conflicts, steady-state voltage variations, and oscillations. Therefore, examining and analyzing the phenomenon (both steady state and dynamic) of the control interactions among multiple Volt/VAr support DG units and voltage regulating devices such as tap changers and capacitor banks would be essential for effective voltage control in active distribution systems. In this paper, the interactions among DG units and voltage regulating devices are identified using their simultaneous and nonsimultaneous responses through time-domain simulations. For this task, an analytical technique is proposed and small signal modeling studies have been conducted. The proposed methodology could be beneficial to distribution network planners and operators to ensure seamless network operation from voltage control perspective with increasing penetration of DG units.

Examining the interactions between DG units and voltage regulating devices for effective voltage control in distribution systems

Voltage regulation by means of coordinated voltage control is one of the challenging aspects in an active distribution system operation. Integration of distributed generation (DG), which can also be operated in Volt/VAr control mode, may introduce adverse effects including control interactions, operational conflicts, steady-state voltage variations, and oscillations. Therefore, examining and analyzing the phenomenon (both steady state and dynamic) of the control interactions among multiple Volt/VAr support DG units and voltage regulating devices such as tap changers and capacitor banks would be essential for effective voltage control in active distribution systems. In this paper, the interactions among DG units and voltage regulating devices are identified using their simultaneous and nonsimultaneous responses through time-domain simulations. For this task, an analytical technique is proposed and small signal modeling studies have been conducted. The proposed methodology could be beneficial to distribution network planners and operators to ensure seamless network operation from voltage control perspective with increasing penetration of DG units.

Mitigating tap-changer limit cycles in modern electricity networks embedded with local generation units

Cascaded on-load tap changers (OLTCs) are widely used for coarse control of voltages in largely interconnected electric power systems. There could be interactions between load dynamics and OLTC control under certain system operating conditions which may lead to the OLTC limit cycle phenomena, thereby resulting into long-term voltage oscillations in the system. In recent years, renewable and nonrenewable local generation (LG) units have been getting interconnected in modern power systems. The existence of OLTC limit cycles in the presence of LG has not been addressed in the literature in greater details. In this paper, the OLTC limit cycle phenomenon, which can occur due to interactions among load dynamics, OLTC controls, and LG operation in electricity networks, has been investigated. Also, a novel strategy is explored for mitigating the power system oscillations introduced by OLTC limit cycles, especially for a network embedded with LG. The proposed mitigation strategy including detailed investigations and analyses has been verified for a two-bus test system and successfully tested on a multibus system using MATLAB.

Novel strategy of updating Volt/VAr control set-points for eliminating interactions between different voltage regulating devices and DG units

ABSTRACT In modern medium voltage distribution grids, voltage and reactive power (Volt/VAr) control can effectively be achieved using the time delayed responses of on-load tap changer, line voltage regulators, capacitor banks and distributed generation (DG) units. The timely coordination among different Volt/VAr control (VVC) devices can be assured through online control processes and updating the control parameters (i.e. set-points) of VVC devices. This is essential for efficient operation of the distribution grids while achieving steady-state voltage recovery and other control objectives such as loss minimisation; as the control interactions among VVC devices and Volt/VAr support DG units caused by poor coordination can lead to operational conflicts. In this article, a novel control approach is proposed for updating VVC set-points for eliminating operational interactions between different voltage regulating devices; thereby, minimising power losses in a network. The proposed strategy has been tested on a practical distribution system derived from the state of New South Wales, Australia, and the simulation results are reported. The results have demonstrated that the interactions among VVC devices and Volt/VAr support DG units can be avoided while minimising power losses, with the implementation of the proposed strategy.

Mitigating Tap Changer Limit Cycles in Modern Electricity Networks Embedded With Local Generation Units

Cascaded on-load tap changers (OLTCs) are widely used for coarse control of voltages in largely interconnected electric power systems. There could be interactions between load dynamics and OLTC control under certain system operating conditions which may lead to the OLTC limit cycle phenomena, thereby resulting into long-term voltage oscillations in the system. In recent years, renewable and nonrenewable local generation (LG) units have been getting interconnected in modern power systems. The existence of OLTC limit cycles in the presence of LG has not been addressed in the literature in greater details. In this paper, the OLTC limit cycle phenomenon, which can occur due to interactions among load dynamics, OLTC controls, and LG operation in electricity networks, has been investigated. Also, a novel strategy is explored for mitigating the power system oscillations introduced by OLTC limit cycles, especially for a network embedded with LG. The proposed mitigation strategy including detailed investigations and analyses has been verified for a two-bus test system and successfully tested on a multibus system using MATLAB.

Simulink model for examining dynamic interactions involving electro-mechanical oscillations in distribution systems

In Australian medium voltage (MV) distribution networks, the majority of embedded local generation (LG) uses synchronous machine based technology. LG units with a synchronous generator can easily be dispatched and controlled to provide or absorb reactive power and thereby locally supporting the system voltage and increasing the voltage stability margin. The fast response from a synchronous machine based LG unit exciter will ensure fast voltage recovery. This will significantly improve the transient voltage stability. Also, the synchronous machine based LG units can provide inertial support to the system. Furthermore, some of these LG technologies work as standby power supplies all over the world. However, there can be dynamic interactions between nearby synchronous machine based LG units leading to significant rotor swings. It is mainly because of (a) low inertia coefficient associated with the machines and (b) the operation of excitation system deteriorates the damping of oscillations that follow the first rotor swing after the perturbation where a fast responding excitation system can reduce the damping torque component. This paper demonstrates modelling MV distribution systems using MATLAB-SimuLink and conventional modelling technique for investigating inter-unit electro-mechanical oscillations in nearby LG units.