Kalid Yunus

Impacts of Stochastic Residential Plug-In Electric Vehicle Charging on Distribution Grid

In this paper, the impacts of residential Plug-In Electric Vehicles (PEVs) charging on a distribution grid are investigated. A stochastic charging model is developed and used to study the impacts on distribution transformer loading, hotspot temperature variation and Accelerated Aging Factor (AAF) of the transformer. Different penetration levels of PEVs are considered in a typical distribution system. Furthermore, distribution of State of Charge (SOC) is discussed which can be used to optimize battery capacity and required charging infrastructure. Distribution of parking time interval is also discussed which can be used to evaluate availability of PEVs for overnight charging. The merit of stochastic approach compared with deterministic approach is also illustrated. The main contribution of this paper is the stochastic approach to evaluate the impact of residential PEV charging on the distribution grid.

A combined zone-3 relay blocking and sensitivity-based load shedding for voltage collapse prevention

A typical power system voltage collapse scenarios is often ended with the undesirable operation of the Zone-3 distance relay of the transmission lines. This paper presents a protection scheme to avoid power system voltage collapse using a combined method of distance relays Zone-3 blocking scheme and a sensitivity-based load shedding selection. The Zone-3 distance relay blocking is based on the proper differentiation between transmission line overloading and line faulted conditions, using a fast estimation of power flow based on Line Outage Distribution Factor (LODF) and Generation Shift Factor (GSF). The Zone-3 distance relay of the transmission line would be blocked if the power flow change over the line is determined to be due to an overload so that more time would be available for the system to take necessary control actions. One of the important control actions is the emergency load shedding. A method based on the calculated sensitivities GSF to identify the most effective load shedding positions and amounts is proposed. The proposed method has been implemented in the Advanced Real-Time Interactive Simulator for Training and Operation (ARISTO) software with the Nordic 32-bus test system. ARISTO offers the possibility to test the proposed scheme since it can be seen as the virtual power system with all live information. The analyses of power system voltage collapse scenarios with and without the proposed scheme implemented have shown the effectiveness of the scheme to prevent the voltage collapses.

Investigating the impact of wake effect on wind farm aggregation

Aggregation methodologies for creating equivalent wind farm models are needed for power system transient stability studies involving large wind farms. One strong argument in the literature suggests single machine equivalent representation of wind farm with the assumption that all wind turbines receive the same incoming wind speed and thus operate at the same loading condition. In this paper, we examine how the validity of such single machine equivalent is affected under different wind speeds across a feeder in a wind farm with many WTGs. In particular, we compare the total active power output from the WTGs which indicates significant difference between single machine equivalent and full wind farm model under certain wind speed ranges and distances between the WTGs. We then conclude the need to use multi-machine equivalent representation for large wind farm in order to achieve adequate accuracy under the full range of wind conditions.

ARIMA-Based Frequency-Decomposed Modeling of Wind Speed Time Series

In this paper, a modified auto regressive integrated moving average (ARIMA) modeling procedure that can capture time correlation and probability distribution of observed wind-speed time-series data is presented. The procedure introduces frequency decomposition (splitting the wind-speed data into high frequency (HF) and low-frequency (LF) components), shifting, and limiting in addition to differencing and power transformation which are used in the standard ARIMA modeling procedure. The modified modeling procedure is applied to model 10 minute average measured wind-speed data from three locations in the Baltic Sea area and the results show that the procedure can capture time correlation and probability distribution of the data. In addition, it is shown that, for 10-min average wind-speed data in the Baltic Sea area, it could be sufficient to use ARIMA(6,0,0) and ARIMA(0,1,6) to model the HF and LF components of the data, respectively. It is also shown that, in the Baltic Sea area, a model developed for an observed wind-speed data at one location could be used to simulate wind-speed data at a nearby location where only the average wind-speed is known.

Droop based steady state control algorithm for a meshed HVDC grid

In this paper, a steady state model of a power flow control algorithm for a meshed HVDC grid proposed. The objective of a power flow control in a DC grid, as in a point-to-point transmission, concerns controlling the DC voltage in the system within its limits and fulfill the power input/output at the PCC (Point of Common Coupling) to the AC system, without exceeding maximum currents through the valves, lines or cables. The best power flow control can be achieved with the use of DC-DC converters at the nodes of the meshed HVDC grid. However, the use of the DC-DC converters in such a DC grid is quite costly, especially if implemented in an offshore environment. The main contribution of this paper is to demonstrate how the control of steady state power flow in the DC grid can be achieved without using the DC-DC converters. The control algorithm is derived based on the principles of droop control.

Primary — Secondary control strategy for meshed HVDC transmission grids at steady state

HVDC (High Voltage DC) transmission systems are believed to be a key to integrate more renewable energy sources, especially from remote area and offshore environment, into power systems. In this paper, a steady state control strategy that can be used in a meshed HVDC grids is presented. The control strategy combines local primary controllers and a central secondary or supervisory controller. The objective of the primary controllers is to enable shared management of required changes in bus powers as a result of a disturbance in the system. The objective of the secondary controller is to change voltage and power settings of the primary controllers so that power flows in the transmission cables of the system are kept within the thermal limits after the disturbance. The primary controllers react first when there are disturbances in the system followed by the secondary controller which acts only when necessary.