Shahrokh Akhlaghi

PMU placement for state estimation considering measurement redundancy and controlled islanding

To select optimal locations for placing phasor measurement units (PMU), in this paper, a two-step convex optimization approach to attain the full network observability of a power system in both the normal operation conditions and controlled islanding operation conditions is proposed. The objective is to get the minimum number of PMUs which guarantees the system full observability with the maximum measurement redundancy. In the first step, a minimization model is applied to convex programing (cvx) to determine the minimum number of PMUs and all the possible candidate buses, which ensures the full network observability. In the second step, in the cases of multiple solutions, a maximization model is applied to cvx to maximize the measurement redundancy. Furthermore, the effect of zero-injection buses is considered to further reduce the number of required PMUs. The proposed approach is tested on three IEEE test systems, i.e. IEEE 14-bus, 30-bus and 118-bus, to demonstrate its effectiveness.

Performance analysis of the Slip mode frequency shift islanding detection method under different inverter interface control strategies

This paper investigates the performance of Slip mode frequency shift (SMS) islanding detection method (IDM) in the presence of different inverter interface control strategies (IICS). The Impact of the two constant-current and constant-power IICS when the inverters are equipped with SMS-IDM, are compared and analyzed from detecting islanding point of view. This study is based on time-domain simulations in the MATLAB/Simulink. Performance of the SMS-IDM is tested under UL-1741 test condition. The results of study prove that, SMS-IDM efficiency can be degraded by the power regulator of the constant-power control strategy. So, efficiency of the SMS-IDM on islanding detection with constant-current control strategy is much higher than the constant-power control. Therefore, to improve the performance of the SMS-IDM with constant-power IICS, A new hybrid islanding detection method combination of the SMS-IDM and reactive power deviation is proposed. Finally, performance of the proposed method is evaluated under the UL-1741 anti-islanding test condition, load switching and loads with high quality factor.

A Multi-Step Adaptive Interpolation Approach to Mitigating the Impact of Nonlinearity on Dynamic State Estimation

Accurate estimation of dynamic states (such as synchronous machine rotor angle and speed) is critical for monitoring and controlling transient stability. Extended Kalman filter (EKF)-based approaches have been developed for estimating dynamic states. In order to improve the EKF’s performance in estimating dynamic states of a synchronous machine, this paper proposes a multi-step adaptive interpolation (MSAI) approach to achieve a balance between estimation accuracy and computational time. This approach consists of three major steps. First, two indexes are calculated to quantify the nonlinearity of the state transition function and measurement function, respectively. Second, based on the nonlinearity indexes, the interpolation factor is determined using a finite state machine model. And finally, to mitigate the negative impact of nonlinearity on the estimation accuracy, pseudo-measurements are added between consecutive measurements through linear interpolation. A simple example is used to validate the proposed nonlinear indexes. The two-area four-machine system and 16-machine 68-bus system are used to evaluate the effectiveness of the proposed MSAI approach. It is shown through the Monte-Carlo method that the estimation accuracy can be improved through interpolation. In addition, a good tradeoff between estimation accuracy and computational time can be achieved effectively through the proposed MSAI approach.

A novel hybrid approach using sms and ROCOF for islanding detection of inverter-based DGs

This paper proposes a novel hybrid approach for islanding detection of inverter-based distributed generations (DG) based on combination of the Slip mode frequency-shift (SMS) as an active and rate of change of frequency (ROCOF) relay and over/under frequency relay as passive methods. This approach is utilized to force the DG to lose its stable operation and drift the frequency out of the allowed range of the frequency threshold. Performance of the proposed approach is evaluated under the IEEE 1547, UL 1741 and multiple-DG operation. The simulation results demonstrate the effectiveness of the proposed approach for detection of islanding, especially for loads with high quality factor. It operates accurately under the condition of load switching and does not interfere with the power system operation during normal condition. In other words, not only it holds the benefits of both SMS and ROCOF, but also it removes their drawbacks by having less non-detection zone and faster response.

Study of sufficient number of optimal tilt angle adjustment to maximize residential solar panels yield

In this paper a comprehensive study is carried out on the optimal number of intervals for adjusting the optimal tilt angle of a solar panel. The tilt angle of solar panels is important for capturing solar radiation and it depends on the path of the sun in the location of the solar panel. A Bee Algorithm is used to compute the optimal tilt angle for a given period. The main goal is to show instead of using a tracking system, which is costly for residential usage, manual adjustment of the tilt angle of a solar panel for a certain number of times during a year is sufficient to receive most of the solar radiation. This study is performed for different locations across the US with different latitudes and longitudes. The optimal number of intervals for each location is computed and the effects of the changes in the longitude and latitude are investigated.

Optimal PMU placement considering contingency-constraints for power system observability and measurement redundancy

This paper proposes a two-step optimization approach for optimal placement of phasor measurement unit (PMU) to obtain complete observability of power system in the case of preinstalled PMUs. The complete observability of the system in the case of normal operation and pre-installed PMUs is formulated and then, different contingency conditions in the system are considered, i.e. single line outage and single bus outage. At the first step of the proposed two-step optimization approach, a minimization model is applied to convex programing (cvx) to achieve the minimum number of PMUs which guarantees the complete observability of the system. At the second step, simulated annealing (SA) is applied to maximize the measurement redundancy. Additionally, to further reduce the number of required PMUs the zero-injection bus effect is considered. At last, the proposed approach is tested on several IEEE standard systems, i.e. IEEE 14-bus, 30-bus, 39-bus, IEEE 16-machine 68-bus and 118-bus, to demonstrate the effectiveness of the proposed approach.

Back-to-back converter control of grid-connected wind turbine to mitigate voltage drop caused by faults

Power electronic converters enable wind turbines, operating at variable speed, to generate electricity more efficiently. Among variable speed operating turbine generators, permanent magnetic synchronous generator (PMSG) has got more attentions due to low cost and maintenance requirements. In addition, the converter in a wind turbine with PMSG decouples the turbine from the power grid, which favors them for grid codes. In this paper, the performance of back-to-back (B2B) converter control of a wind turbine system with PMSG is investigated on a faulty grid. The switching strategy of the grid side converter is designed to improve voltage drop caused by the fault in the grid while maximum available active power of wind turbine system is injected to the grid and the DC link voltage in the converter is regulated. The methodology of the converter control is elaborated in details and its performance on a sample faulty grid is assessed through simulation.

Locating sources of forced oscillations using transfer functions

This paper proposes a transfer function method to locate sources of forced oscillations using phasor measurement unit (PMU) data. Forced oscillations herald problematic controls and device failure. They also incur stability concerns of power grid operators. To mitigate the negative impacts of forced oscillations, it is essential to locate their sources in time. This paper proposes a method to locate oscillation sources by building transfer functions between bus frequencies using PMU data. It is shown through analytical and simulation studies that the relative location of an oscillation source with respect to measurement points can be determined using the magnitude responses and phase responses of transfer functions. The proposed method does not heavily rely on a dynamic model and therefore is easy to implement and apply.

Adaptive multi-step prediction based EKF to power system dynamic state estimation

Power system dynamic state estimation is essential to monitoring and controlling power system stability. Kalman filtering approaches are predominant in estimation of synchronous machine dynamic states (i.e. rotor angle and rotor speed). This paper proposes an adaptive multi-step prediction (AMSP) approach to improve the extended Kalman filters (EKF) performance in estimating the dynamic states of a synchronous machine. The proposed approach consists of three major steps. First, two indexes are defined to quantify the nonlinearity levels of the state transition function and measurement function, respectively. Second, based on the nonlinearity indexes, a multi prediction factor (Mp) is defined to determine the number of prediction steps. And finally, to mitigate the nonlinearity impact on dynamic state estimation (DSE) accuracy, the prediction step repeats a few time based on Mp before performing the correction step. The two-area four-machine system is used to evaluate the effectiveness of the proposed AMSP approach. It is shown through the Monte-Carlo method that a good trade-off between estimation accuracy and computational time can be achieved effectively through the proposed AMSP approach.