Wilsun Xu

Voltage stability analysis using generic dynamic load models

This paper investigates the dynamic nature of voltage stability. Using a generic load model representing the dynamic behaviour of aggregate loads, the mechanism of voltage collapse is revealed by examining the interaction of load dynamics with the supply network. The relationship between power flow based static techniques and time simulation based dynamic techniques for voltage stability analysis is established. Guidelines for voltage stability control are developed. >

Comparative analysis between ROCOF and vector surge relays for distributed generation applications

This paper presents a comprehensive comparative analysis between rate-of-change-of-frequency (ROCOF) and vector-surge (VS) relays for distributed generation islanding detection. The analysis is based on the concepts of detection-time versus active power-imbalance curves and critical active power imbalance. Such curves are obtained through dynamic simulations. The performance of these devices considering different scenarios is determined and compared. Factors such as voltage-dependent loads, generator inertia constant, and multidistributed generator systems are analyzed. False operation of these relays due to faults in adjacent feeders is also addressed. Results show that ROCOF relays are more reliable to detect islanding than vector surge relays when the active power imbalance in the islanded system is small. However, ROCOF relays are more susceptible to false operation than VS relays.

A method for determining customer and utility harmonic contributions at the point of common coupling

A new method is proposed in this paper to determine the harmonic contributions of a customer at the point of common coupling. The method can quantify customer and utility responsibilities for limit violations caused by either harmonic source changes or harmonic impedance changes. It can be implemented in current power quality monitors and digital revenue meters. The method makes it possible to develop fair and consistent billing schemes for harmonic distortion control.

A Power Line Signaling Based Technique for Anti-Islanding Protection of Distributed Generators—Part I: Scheme and Analysis

Anti-islanding protection of distributed generators (DG) is a significant technical barrier to the emerging DG industry. This paper presents an innovative power line signaling based anti-islanding scheme developed in response to the challenge. The scheme broadcasts a signal from a substation to the DG sites using the distribution feeders as the signal paths. A DG is considered as islanded from the upstream system if the signal cannot be detected at the DG site. The proposed scheme has been evaluated using analytical, simulation and field tests. The results are very promising. This paper presents the main ideas of the scheme and its design considerations. Methods to create and detect the signals are shown and their performances are analyzed.

SVC placement using critical modes of voltage instability

The location of SVC (static VAr compensators) and other types of shunt compensation devices for voltage support is an important practical question. This paper considers a tool based on the determination of critical modes. Critical modes are computed by studying the system modes in the vicinity of the point of collapse. System participation factors for the critical mode are used to determine the most suitable sites for system reinforcement. Because the method does not rely on base case linearizations, the method is able to properly consider all system limits and nonlinear effects. The paper tests the proposed method by performing an assessment of the impact of the addition of static VAr compensators to a 1380 bus model of the BC Hydro system. >

Economic Load Dispatch Constrained by Wind Power Availability: A Here-and-Now Approach

In this paper, a load dispatch model is developed for the system consisting of both thermal generators and wind turbines. The probability of stochastic wind power is included in the model as a constraint. This strategy, referred to as the here-and-now approach, avoids the probabilistic infeasibility appearing in conventional models. It is shown that, based on the presented model, the impacts of stochastic wind speed on the generated power can be readily assessed.

Minimum Emission Dispatch Constrained by Stochastic Wind Power Availability and Cost

In this paper we develop a load dispatch model to minimize the emission due to oxides of nitrogen (NOx). This model takes into account both thermal generators and wind turbines. We derive a closed-form in terms of the incomplete gamma function (IGF) to characterize the impact of wind power. Accordingly, the effects of wind power on emission control are investigated. The model is implemented in a computer program and a set of numerical experiments for a standard test system is reported.

An investigation on the validity of power-direction method for harmonic source determination

Power direction method has been used widely to identify the locations of harmonic sources in a power system. A number of utility-customer disputes over who is responsible for harmonic distortions have been settled with the help of the method. A closer examination of the method, however, reveals that it is unable to fulfill the task of harmonic source detection. Case studies can easily show that the method gives incorrect results. In this paper, problems associated with the method are investigated using case studies and mathematical analysis. The results show that the power direction method is theoretically incorrect and should not be used to determine harmonic source locations. Main cause of the problem is that the direction of active power flow is a function of the phase angle difference between the two sources. The direction of reactive power flow, on the other hand, has a better correlation with the source magnitudes.

A practical method for assessing the effectiveness of vector surge relays for distributed generation applications

This work presents a simple and reliable method for predicting the islanding detection performance of vector surge relays. The relay performance is characterized by a tripping-time versus power-imbalance curve. With the curve, one can determine the time taken by a vector surge relay to detect islanding for any generation-load mismatch level. The main contribution of this paper is the development of analytical formulas for directly determining the behavior of vector surge relays. As a result, efforts needed to asses the relay performance for a given distributed generation scheme can be simplified significantly. The accuracy of the formulas has been verified by extensive simulation study results.

Performance of frequency relays for distributed generation protection

This paper investigates the efficiency of under/over frequency relays for protection of distributed synchronous generators considering islanding detection and frequency-tripping requirements. Comparison between frequency and vector surge relays, which are islanding detection-dedicated devices, shows that the former can be as effective as the latter. This feature supports the idea that frequency relays can replace vector surge relays for islanding detection purpose. In this case, frequency relays must also meet the generator manufacturer recommendations and the utility frequency-tripping requirements. In order to investigate the relay capability to meet both necessities, it is proposed the concept of application region, which defines a region in the detection time versus active power imbalance space where frequency-based relays can be adjusted to satisfy the anti-islanding and frequency-tripping requirements simultaneously. The paper also presents a set of formulas to determine directly the application region.