Showing papers on "Power-flow study published in 2004"

FACTS: Modelling and Simulation in Power Networks

Abstract: Preface. Introduction. Modelling of FACTS Controllers. Modelling of Conventional Power Plant. Conventional Power Flow. Power Flow including FACTS Controllers. Three--phase Power Flow. Optimal Power Flow. Power Flow Tracing. Appendix A: Jacobian Elements for FACTS Controllers in Positive Sequence Power Flow. Appendix B: Gradient and Hessian Elements for Optimal Power Flow Newtona s Method. Appendix C: Matlab(R) Computer Program for Optimal Power Flow Solutions using Newtona s Method. Index.

A comparison of the AC and DC power flow models for LMP calculations

Abstract: The paper examines the tradeoffs between using a full ac model versus the less exact, but much faster, dc power flow model for LMP-based market calculations. The paper first provides a general discussion of the approximations associated with using a dc model, with an emphasis on the impact these approximations will have on security constrained OPF (SCOPF) results and LMP values. Then, since the impact of the approximations can be quite system specific, the paper provides case studies using both a small 37 bus system and a somewhat larger 12,965 bus model of the Midwest U.S. transmission grid. Results are provided comparing both the accuracy and the computational requirements of the two models. The general conclusion is that while there is some loss of accuracy using the dc approximation, the results actually match fairly closely with the full ac solution.

On some misinterpretations of the instantaneous reactive power p-q theory

Abstract: The main features of the instantaneous reactive power (IRP) p-q theory, considered as a power theory of three-phase systems, are analyzed in this paper using the theory of the currents' physical components (CPC). This analysis shows that the p and q powers are not associated with separate power phenomena, but with multiple phenomena. Moreover, the results of the IRP p-q theory contradict some common interpretations of power phenomena in three-phase circuits. Namely, according to the IRP p-q Theory the instantaneous reactive current can occur even if a load has zero reactive power, Q. Similarly, the instantaneous active current can occur even if a load has zero active power, P. Moreover, these two currents in circuits with a sinusoidal supply voltage can be nonsinusoidal even if there is no source of current distortion in the load. The analysis shows that a pair of values of instantaneous active and reactive p and q powers does not enable us to draw any conclusion with respect to the power properties of three-phase unbalanced loads even in a sinusoidal situation. Thus, the instantaneous reactive power p-q theory does not identify power properties of such loads instantaneously. This conclusion may have an importance for control algorithms of active power filters. The paper reveals the relationship between the p and q powers and the active, reactive and unbalanced powers, P, Q, and D and specifies the required energy storage capability of active power filters operated under sinusoidal unbalanced conditions.

A novel optimal reactive power dispatch method based on an improved hybrid evolutionary programming technique

Abstract: This paper proposes an improved evolutionary programming (IEP) and its hybrid version combined with the nonlinear interior point (IP) technique to solve the optimal reactive power dispatch (ORPD) problems. In an IEP method, the common practices in regulating reactive power are followed in adjusting the mutation direction of control variables in order to increase the possibility of keeping state variables within bounds. The IEP is also hybridized with the IP method to obtain a fast initial solution, which is then used as a highly evolved individual in the initial population of the improved EP method. Numerical tests of the proposed algorithm on the IEEE 118-bus system and a realistic power system in Western China are very encouraging compared with the existing ORPD algorithms in term of computational efficiency and optimality.

Out-of-step protection fundamentals and advancements

Abstract: Power systems are subjected to a wide range of small or larger disturbances during operating conditions. Small changes in loading conditions occur continually. The power system must adjust to these changing conditions and continue to operate satisfactorily and within the desired bounds of voltage and frequency. The power system should be designed to survive larger types of disturbances, such as faults, loss of a large generator, or line switching. Certain system disturbances may cause loss of synchronism between a generator and the rest of the utility system, or between interconnected power systems of neighboring utilities. If such a loss of synchronism occurs, it is imperative that the generator or system areas operating asynchronously are separated immediately to avoid widespread outages and equipment damage. In this paper we describe the philosophy and application fundamentals of out-of-step protection in transmission systems. We also discuss recent enhancements in the design of out-of-step tripping and blocking protection functions that improve the security and reliability of the power system. In addition, we demonstrate the out-of-step phenomena and distance relay element behavior using EMTP and Matlab simulations.

Multiterminal voltage-sourced converter-based HVDC models for power flow analysis

Abstract: Two mathematical models for multiterminal voltage-sourced converter (VSC)-based HVDC are proposed in this paper. The first model assumes that all the converters are co-located in the same substation while the second model is a general one, in which DC network can be explicitly represented. For both models proposed, primary converters basically have the ability to independently control either active and reactive power or active power and voltage of the terminals while the secondary converter of the multiterminal VSC HVDC can be used to control terminal bus voltage and balance power exchange among the VSC converters. In addition, theoretic and numerical comparisons between the multiterminal VSC HVDC and the VSC FACTS controller-the Generalized Unified Power Flow Controller are also presented. Numerical examples are given on the IEEE 30-bus system, IEEE 118-bus system and IEEE 300-bus system.

Evaluation of uncertainty in dynamic simulations of power system models: The probabilistic collocation method

Abstract: This paper explores the use of a new technique, the probabilistic collocation method (PCM), to enable the evaluation of uncertainty in power system simulations. The PCM allows the uncertainty in transient behavior of power systems to be studied using only a handful of simulations. The relevant theory is outlined here and simple examples are used to illustrate the application of PCM in a power systems setting. In addition, an index for identification of key uncertain parameters, as well as an example with a more realistic power system, are presented.

Multiple bi-directional input/output power control system

Abstract: A multiple bi-directional input/output power control system includes a network of functional blocks housed in a single enclosure, providing DC power to one or more DC loads, and providing control and internal pathways, sharing one or more AC and/or DC power inputs. The system feeds back AC power from the DC power source into an AC input connection, and the fed-back AC power is shared by other AC loads. The system operates at least one alternative source of DC in a dynamic manner, allowing maximization of power generating capability at respective specific operating conditions of the moment. Power isolation may be handled by an AC isolation block right at a power input. Therefore all other blocks within a multi-function power control unit (MFPCU) are isolated from AC ground.

Modeling of power electronic loads in AC distribution systems using the generalized State-space averaging method

Abstract: Most of the loads in ac distribution systems have positive incremental impedance characteristic. However, power electronic loads, when tightly regulated, sink constant power from the system. Therefore, they have negative incremental impedance characteristic. This can cause negative impedance instability. Power electronic loads usually have a controlled or uncontrolled rectifier at the front end. In this paper, these loads are modeled using the generalized state-space averaging method. An assessment of their effects in ac distribution systems is also presented. Experimental results are presented to verify the proposed analysis.

Method and apparatus for tracking maximum power point for inverters, for example, in photovoltaic applications

Abstract: A power system employs an outer voltage feedback loop and an inner current feedback loop to control a power converter, such as a DC to AC inverter for transferring electrical power between a power source, for example a photovoltaic array, and a load, for example a power grid. The outer loop accommodates variations in the output of the power source, for example accommodating anomalies in IV characteristics such as IV droop characteristic associated with photovoltaic cells. The outer loop may employ a first control regime or a second control regime, for example, dependent on whether a DC bus voltage or power is smaller than a value corresponding to measurement resolution or expected noise.

Real and reactive power coordination for a unified power flow controller

Abstract: This paper proposes a new real and reactive power coordination controller for a unified power flow controller (UPFC). The basic control for the UPFC is such that the series converter of the UPFC controls the transmission line real/reactive power flow and the shunt converter of the UPFC controls the UPFC bus voltage/shunt reactive power and the DC link capacitor voltage. In steady state, the real power demand of the series converter is supplied by the shunt converter of the UPFC. To avoid instability/loss of DC link capacitor voltage during transient conditions, a new real power coordination controller has been designed. The need for reactive power coordination controller for UPFC arises from the fact that excessive bus voltage (the bus to which the shunt converter is connected) excursions occur during reactive power transfers. A new reactive power coordination controller has been designed to limit excessive voltage excursions during reactive power transfers. PSCAD-EMTDC simulation results have been presented to show the improvement in the performance of the UPFC control with the proposed real power and reactive power coordination controller.

Auto-master-slave control technique of parallel inverters in distributed AC power systems and UPS

Abstract: This work presents a new control scheme: auto-master-slave control technique of parallel connected SPWM inverters. The control scheme ensures a fast dynamic response and precise load current sharing capability. Based on auto-master-slave control technique, the control circuitry employs the real power share bus and reactive power share bus interconnecting all the paralleled inverters. With the power share buses and the power deviation controllers, the paralleled inverter with highest output power becomes the master inverter; other inverters become slave inverters. Master inverter drive the power bus, and the power deviations in slave inverter module are calculated by the power sharing controller. Hereafter, the inverters compensate for the frequency and amplitude of output voltage, and then the unbalance power was eliminated. An experiment system was built to verify this control scheme, and the results indicate the precise power balance performance for parallel operation.

A new concept of voltage-collapse protection based on local phasors

Abstract: A new algorithm for protection against voltage collapse is proposed. The algorithm makes use of the magnitudes and angles of the local phasors (i.e., bus voltages and load currents). The change in an apparent power-line flow during a time interval is exploited for computing the voltage-collapse criterion. The criterion is based on the fact that the line losses in the vicinity of the voltage collapse increase faster than the delivery of the apparent power and, at the voltage-collapse point, the losses consume all of the increased power. The selected criterion equals 0 when a voltage collapse occurs. The proposed algorithm could be easily implemented in a numerical relay. The information obtained by the relay can be used at two levels-for the coordinated system-wide control action or for automatic action on the local level. The algorithm is simple and computationally very fast. It was tested on the IEEE 118-bus test system.

Improved power flow control for contactless moving sensor applications

Abstract: An improved power flow control method for contactless moving sensor applications is proposed. The method allows the design of a system where sensors with different power ratings or a wide range of load variations can be implemented. A phase-controlled variable inductor is used to tune the resonant circuit of the power pickups of an inductively coupled power transfer (ICPT) system according to the actual power requirements of the sensors, thereby, helping to reduce the power losses without affecting the maximum power transfer capacity. Soft switching is achieved in the variable inductor control, and the effect of the equivalent tuning parameters on the power flow is analyzed theoretically. Simulation results show that a significant improvement of the existing controllers is achieved at no load or very lightly loaded conditions.

Allocating the costs of reactive power purchased in an ancillary service market by modified Y-bus matrix method

Abstract: In an open accessed transmission system, the costs of each ancillary service will be unbundled. This paper proposes a straightforward method of allocating the costs of reactive power purchased by contract or from a bidding market. This method uses basic circuit theory and partitions the Y-bus matrix to decompose the voltage of the load buses with a view to calculating the reactive power sharing. This method is derived from the system equations without such assumptions as the proportional flow or lossless transmission line.

Unified power flow controller using the cascade multilevel inverter

Abstract: Power flow control is important in power systems and recently becomes more urgent because of the deregulation and 2003 blackout. This paper presents a novel configuration of unified power flow controller. The proposed configuration has several unique features and advantages over the conventional configuration. As a result, cascade multilevel inverter can be used in the new configuration to lower the cost, volume, and increase the reliability of the system. Analysis and simulation results will be presented.

Improvements in the representation of PV buses on three-phase distribution power flow

Abstract: This letter describes a new voltage controlled bus (PV bus) representation in connection with the three-phase current injection method-TCIM of power flow analysis. This representation requires an augmented system of linearized equations to incorporate the reactive power as a dependent variable. Tests and comparisons between the former and the new approach are described and the improved stiffness of this new proposition is demonstrated.

A common modeling framework of voltage-sourced converters for load flow, sensitivity, and dispatch analysis

Abstract: This paper discusses the use of injected shunt and series voltage sources to model voltage-sourced converter (VSC)-based flexible ac transmission systems (FACTS) controllers such as the unified power-flow controller (UPFC) and the interline power-flow controller (IPFC). Compared to traditional power network models including only shunt voltage injections, the inclusion of series voltage injections provides a new capability of power system models and offers a common modeling framework in which a Newton-Raphson load-flow solution and a network sensitivity analysis can be readily developed. The sensitivities can be used for optimal dispatch of FACTS controllers.

Reflections on apparent power and power factor in nonsinusoidal and polyphase situations

Abstract: In this paper it is shown that it is not possible to generalize the classical concepts of apparent power and power factor to the general polyphase nonsinusoidal case by unique concepts if one wants to retain the meaning with respect to efficiency of the electrical energy transfer as well as with respect to power oscillations. Separate generalizations are proposed and discussed.

Hybrid clean-energy power-supply framework

Abstract: A hybrid clean-energy power-supply framework integrates a fuel cell, solar cell, and wind energy, applies a max power tracking rule, raises the output power of a solar cell and wind energy to supply a power load and transfer the surplus electrical energy to a water-electrolyzing apparatus for producing hydrogen and oxygen, and provides a fuel for a fuel cell power generating system. Furthermore, the present invention utilizes features of each clean-energy power generating system, depends on the powerful calculation capacity of a central processing unit to monitor and dispatch each power generation and supply system, and thus ensures the reliability of supply power and reduces the power generation cost. Such a framework can selectively grid-connect with the utility power or run as a stand-alone power supply system and has a mechanism for preventing the island effect.

A processing unit for symmetrical components and harmonics estimation based on a new adaptive linear combiner structure

Abstract: Symmetrical components as well as harmonic tracking are of great importance in many applications in power systems such as power quality and protection. This paper introduces a novel Adaptive linear combiner (ADALINE) structure for symmetrical components estimation. This structure is capable of dealing with multi-output systems for parameter tracking/estimation rather than the existing ADALINE, which deals only with single output systems. As the new topology deals with Multi-Output systems, it is called MO-ADALINE. Moreover, the paper presents a new processing unit, which can estimate symmetrical and harmonic components from the measured current signals. The advantages of this proposed unit are its independence of the voltage waveform and its ability to give information about the reactive component of the resolved current. Simulation results are given to validate the proposed algorithms.

Implementing transformer nodal admittance matrices into backward/forward sweep-based power flow analysis for unbalanced radial distribution systems

Abstract: This paper proposes a new method to handle distribution transformers of various winding connections in the backward/forward sweep-based power flow analysis for unbalanced radial distribution systems. The method takes advantage of available nodal admittance matrices of distribution transformers, and can automatically solve the problem of conductively isolated subnetworks to obtain their equivalent phase-to-reference voltages. In addition, this paper presents a limitation of backward/forward sweeps, and an extension of the power summation method for distribution power flow analysis from single-phase to unbalanced three-phase. The validity and effectiveness of the proposed method can be demonstrated by numerous examples and real systems.

Power converter and power unit

Abstract: Control means is provided, in which the whole of the phase difference of a power waveform of each phase is matched to a value dividing a cycle of the power waveform into n-equal portions, and at the same time, the whole of the power value of each phase is controlled to be identical, so that the power conversion unit having a n-alternating current output (n is an integer numeral serving as n≧2) connected to the direct current power source and smoothing means provided between the direct current power source and the power conversion unit are made small in capacity.

Reliability block diagram simulation techniques applied to the IEEE Std. 493 standard network

Abstract: This is one of a series of papers discussing the application and accuracy of different analysis techniques supporting the determination of industrial and commercial power system reliability and availability. There is a need recognized in the power industry to identify and utilize a standard tool, or a set of tools, to analyze the reliability of power systems. Historically, the results of applying different reliability methodologies and tools varied significantly, and comparisons were difficult. The Reliability Analysis Techniques Working Group of the Gold Book (IEEE Std. 493-1997) developed a standard network to enable comparison of analytical techniques. This paper describes the approach of simulations via reliability block diagrams as applied to the Gold Book standard network. Reliability indexes of the load points are presented, and are compared with ones obtained from other techniques in the series to determine the accuracy.

Practical robust PSS design through identification of low-order transfer functions

Abstract: This paper presents a low-order identification technique, which is based on standard Prony analysis. The technique has the ability to extract crucial dynamic characteristics from a system of any size (e.g., a large-scale power system), which then can be used for developing a robust controller. In this paper the proposed identification technique is coupled with a robust controller design technique based on genetic algorithm to simultaneously tune power system stabilizers (PSSs) for multiple operating conditions. To illustrate the proposed identification and controller design techniques three case studies are presented, including a two-area benchmark system, a 50-machine test system and a large-scale (23 300 bus) Eastern Interconnection power system. To allow the proposed tools to be applied to the large-scale power system, the authors developed an interface with a standard production grade transient stability analysis software package PSS/E.

The role of digital modeling and simulation in power engineering education

Abstract: This paper resulted from the research aimed at introducing modeling and simulation as a major methodological approach for enhancing power engineering education. The reasons for such an approach are explained first. Different options and uses of the modeling and simulation tools are discussed next. Several implementations of the teaching examples are outlined. The paper ends with the conclusions reached based on the study.

Power utility maximization for multiple-supply systems by a load-matching switch

Abstract: For embedded systems that rely on multiple power sources (MPS), power management must distribute the power by matching the supply and demand in conjunction with the traditional power management tasks. Proper load matching is especially critical for renewable power sources such as solar panels and wind generators, because it directly affects the utility of the available power. This paper proposes a power distribution switch and a source-consumption matching algorithm that maximizes the total utility of the available power from these ambient power sources. Our method yields over 30% more usable power than conventional MPS designs.

Towards interval analysis of the load uncertainty in power electric systems

Abstract: The aim of this paper is to present a methodology based on interval mathematics to deal with load uncertainty in power electric systems. The approach is suitable to model two types of uncertainty in power flow analysis, namely: (i) the influence of the load measurement errors at all system buses on the voltage profile; (ii) the voltage profile behavior under a load variation during a specific period of time. This methodology entails an interval power flow model and the resulting interval nonlinear system is solved using Krawczyk's method. We implemented the algorithm in the Matlab environment using the IntLab toolbox. Results for IEEE test-systems and a comparison with related work are presented.

Analysis of angle stability problems: a transmission protection systems perspective

Abstract: Postfault rotor angle oscillations lead to power swings. Both unstable and stable swings can induce distance relay tripping. For unstable swings, a new computational procedure to locate all of the electrical centers is developed. It simplifies the work associated with visual screening of all the R-X plots. For stable swings, a generic three-tier hierarchy of stability-related norms defined by branch norm, fault norm, and system norm is proposed. Ranking by branch norm leads to ranking of power swings. Ranking by fault norm leads to ranking of faults or contingencies. Magnitude and rate of change of system norm can be used to detect an out-of-step condition. Results on a ten-machine system and a utility system with detailed models are also presented.