AN ON-LINE ALGORITHM FOR PREDICTING THE TEMPERATURE OF POWER TRANSFORMER WINDINGS

The paper presents an on-line algorithm for predicting the average temperature of power transformer winding according to its load current. The algorithm has accounted for the whole history of load current and variations of ambient temperature. The overload capabilities of transformer can be increased by monitoring and controlling the winding temperature and the necessary durability of the transformer can be ensured. The algorithm can also forecast the future winding temperature and the time of reaching the max, permitted the assumed future load current and oil temperature.     

Multiple Load Flow Solutions in Electric Power Systems Based on Genetic Algorithm

Methods for satisfying the power balance requirement and the voltage magnitude costraint are developed and incorporated into the genetic algorithm method to form a constrained genetic algorithm for solving the load flow problem.The robustness of the load flow algorithm is enhanced by the dynamic population.the technique for accelerating the convergence of the optimisation process and the network node sequencing procedure described in the paper.The efficiency and feasibility of the developed CGALF algorithm have been tested using KK 11 node system under light load and heavy load conditions.     

THE DECOUPLJNG CALCULATION OF MULTI-AREA POWER SYSTEM LOAD FREQUENCY CONTROL

The calculating burden of the decoupling judgement of multi-area power system load frequeney centrel is very large by tranditional methods. This paper changcs the decoupling judgement problem into a minimum cost network flow problem. The solution algorithm is out-of-kilter . algorithm which is characterized by simple manipaultion, fast and reliable convergence. In the paper, we discuss and calculate the decoupling problem of two arca power system load frequency control. The results show that the proposed approach is feasible and effective.     

Optimal Load Curtailment in Bulk Power System Reliability Evaluation

This paper studies the question for considering optimal load curtailment in bulk power system adequacy evaluation. A minimization model of load curtailment is utilized for the contingency states which may have load curtailment. In order to calculate realistic bus indexes, two practical load curtailment philosophies are incorporated in the minimization model of load curtailment, which is achieved in computer program developed for bulk power system reliability evaluation. The minimization model of load curtailment, correlated algorithms, and flow chart of program are given in this paper. The effectiveness and availability of the model are verified from two case studies.     

THE NEW PHASE SELECTION SCHEMES ON USE OE FAULT COMPONENT

In accordance with the characteristic of digital relay and fault locatoin four methods of phase selection on fault component are presented in this paper.These methods are simple in principle, reliable in action, and immune from load flow,system oscillation.Hardly affected by fault location, source impedance and fault resistance, they can select fault phase reliably for any power system and fault type.As phase selection schemes, they can be used for perfect digital relay.     

Application of Linear Programming in the Steady-State Real Power Security Region

This paper presents a new rnethod for analysis and computation of steady-statereal power security region by linear programming.In this method based on the D.C load flow,theupper and lower bounds of the hyper-cuboid explicit steady-state security region are chosen asunknown variables.The objective function is the total amount of the adjustable generations withweighting factors.The model of the steady-state real power security region is changed in to the proposed linearprogramming methed for ealculation.The numerieal exarnples are tested on IEEE 6-bus and 30-bussample systems.The computational resuIts showed the proposed model a nd algorithm feasible andeffective.     

An optimal equivalent subchannel algorithm for low voltage power line communication system

According to the low voltage power line channel features,combined with adaptive bit loading and power allocation algorithm,an optimal equivalent subchannel algorithm is proposed.Under the condition of fixed amount of transmitted bits and the minimum transmitted power model,impact of optimal equivalent subchannel on the system performance is studied.The optimization algorithm sorts subchannels with similar transmission features into groups,thus the signaling load can be reduced,and the spectrum can be better used.The simulation results show that system can get lower bit error rate(BER) by adopting optimal equivalent subchannel algorithm.     

A Hybrid Filter Composed of Shunt Passive and Series Active Filter on DC Side in HVDC System

For requirements of suppressing harmonics on the dc side of HVDC system,a new hybrid filtering system a shunt passive and a small rated reactor transformer coupling series active filter is presented,and the harmonic compensation and control principles are discussed theoretically in this paper.Digital simulation results show excellent filtering characteristics of harmonic current and very high performance/cost by using a new hybrid filtering system proposed int this paper.     

Reliability Evaluation Model of Composite Generation and Transmission System Incorporating FACTS Devices

The utilization of Flexible AC Transimssion System(FACTS)technology has significant impact on power system reliability performance.Series FACTS devices such as Thyristor Controlled Phase Shifting Transformers(TCPSTs) and Thyristor Controlled Series Compensators(TCSCs) can change the natural allocation of the power flow and effectively alleviate transmission congestion.Parallel FACTS devices such as Static Var Compensators(SVCs)can provide reactive power support and maintain voltage stability.So it is important to quantitatively evaluate their benefits to the power system reliability.The traditional models of the composite generation and transmission system reliability evaluation are mainly focused on conventional generation and transmission facilities.This paper researches the power flow model of TCPST and TCSC,and then presents the optimal load shedding model incorporating TCPSTs,TCSCs and SVCs and effectively incorporates them into the reliability evaluation model.The proposed method is tested on the RBTS and the obtained results show that the system reliability can be improved effectually by FACTS devices.     

Heuristic research and fuzzy evaluation for-post-faultrestoration in distribution networks

A heuristic research and fuzzy evaluation fault restoration algorithm for distribution network is proposed to quickly restore power supply for the regions where power supply is interrupted. The algorithm searches power supply paths by finding effective tie-switches and sectionalizing switches, and eliminates improper switch pair according to power flow calculation. During service restoration process, the problem is divided into self-healing restoration, whole-region restoration, sub-region restoration, load-transfer restoration and load shedding restoration. The algorithm takes switch operation times and customer priority into account, and consequently, obtains initial feasible schemes. Fuzzy technique is used to evaluate the schemes from minimal switch operation times, load transfer, feeder margin and maximal voltage drop. Weighted sum of four indices provides a good guideline for choosing the optimal scheme for restoration operation. Calculation results of a certain distribution network show that this algorithm is feasible.     

AUTOMATIC CONTINGENCY SELECTION AND RANKING USING AN UNIFIED MODEL AND ALGORITHM FOR P - AND Q - SUBPROBLEM

A new approach of the automatic contingency selection and ranking with the network flow programming (NFP) is presented in this paper. NFP is adaptable to change the network topology as the transmission or generation branches arc in single or multiple outage. Thus the fast calculation of the contingency states by NFP cm provide the urgent information in real time N- 1 security analysis. In the paper, automatic contingency selection and ranking forP- and Q-type subproblem arc solved by an unified network flow model and algorithm. It is based on the existence of weak coupling between real and reactive quantities in power systems. The performance indices to assess the severity of contingencies are defined as the total real and reactive load required to be curtailed. The proposed ACS technique including the model and its algorithm are examined with IEEE 5-, 14-, 30-, 57- and 118- bus test systems on M-340. And the encouraging results are given in the paper.     

An improved dynamic model of transformer hotspot temperature based on top oil temperature

The winding hot spot temperature of the oil immersed power transformer is an important parameter. It has a great influence on the transformer insulation aging and a close relationship with the transformer top oil temperature. The changes of non linear thermal resistance, open circuit impedance, and oil viscosity and winding losses with temperature are also taken into account. An improved model added on the transformer top oil temperature to calculate transformer winding hot spot temperature by using the viscosity and loss correction factors is proposed. Model parameters are estimated by Levenberg Marquardt method. In the end, by comparing with the measured data tested under different conditions, the model shows good consistency under underload (90%), rated load (100%) and overload (110%) conditions respectively, and describes the temperature variation more accurately in the dynamic loading profiles.     

Full bridge LLC series resonant converter for X-Ray high voltage DC power supply

To prevent the duty cycle loss of full-bridge converter controlled by the phase-shifting method, a high frequency high voltage X-ray power supply is proposed. It combines full bridge LLC(FB-LLC) series resonant converter with bipolar single-phase symmetrical voltage multiplier. FB-LLC series resonant, high voltage transformer and bipolar single-phase symmetrical voltage multiplier are used in the main circuit. The LLC series resonant circuit and working conditions of soft switch are analyzed. The fundamental harmonic approximation model is established. The parameters of the main circuit are designed. The simulation results show that the output voltage adjusted continuously within 40~120 kV and without duty cycle loss. The X-ray power supply is proposed to achieve a quick rise and low ripple of output voltage. According to the simulation results, the proposed topology is correct and reliable.     

A PROBABILISTIC PRODUCTION COSTING FOR MULTISTATES AND MULTIBLOCKS OF UNITS

A probabilistic production costing method for multistates and multiblocks of units is presented in this paper. The mixtures of normal distributions are used to represent the power of the system and the load. The energy, reliability and production costs are calculated directly from the analytical formulae. Numeral examples demonstrate that the algorithm presented in the paper is accurate and efficient.     

Application of Combination Grey Model in Power Load Forecasting

Based on the gray forecast theory, this paper studies the principle and deficiency in power load forecasting by the basic grey model and other improved models, and introduces a new method -the combination grey model to forecast the long-medium power load. Based on an example, the basic grey model, other improved models and combination grey model are used to forecast power load and results of all models are analyzed and compared. The calculation results show that forecasting power load by grey theory is credible and simple. For this type of complex problems such as forecasting the long-medium power load, the combination grey model is specially useful because of it's high precision and facility. The method can be used as one of the tools of forecasting the long-medium power load.     

A NONLINEAR PROGRAMMING MODEL AND ITS ALGORITHM FOR TRANSMISSION NETWORK PLANNING

The problem of transmission network planning is studied thoroughly by applying the mathmatical optimal tecqhniue in this paper. A novel approach is put forward for computer aided transmission network planning, which is quite different from what is based on D.C. load flow equations. A nonlinear transmission planning model is set up by regarding A.C. load flow equations as the system model. A practical model which can be solved more easily, and an united algorithm are obtained by applying the SUMT to the nonlinear model. The results of some practical examples indicate that this method can not only make the planning results more accurate, but also provide the voltage qualities and produce planning plans autometically and seek the plans optimally and fast.     

Recognizing the reliability non-coherence components ofdistribution substation typical configuration

Reliability non-coherence components (RNC) may exist in a power system due to the effects of distribution substation connection modes, components reliability performance, load level, and other factors. Recognizing RNCs and removing them from a power system can improve system reliability. The reliability non-coherence phenomenon is analyzed and the reliability non-coherence probability, frequency and energy indices, which can describe the degree of the system reliability non-coherence feature, are proposed. Based on the reliability evaluation theory of substation configuration, recognizing technique of RNCs of distribution substation is proposed. The theoretical analysis and example show that the breaker connects high voltage buses may be the RNC when the rated capacity of one transformer can meet the load requirement, and the reliability parameters of breakers have some effects on the reliability non-coherence features of the typical substation configuration.     

AN A. C. LOAD FLOW ANALYSIS OF THE RELIABILITY EVALVATION OF COMPOSITE GENERATION AND TRANSMISSION SYSTEMS

An A. C. load flow analysis of the reliability evaluation of composite generation and transmission systems is presented on the basis of a fast outage simulation algorithm and an optimal model of active and reactive power adjustment after outage contingencies. The information of the first -level outage events is utilized for simulating higher -level outage events in the fast outage simulation algorithm ; the optimal model is satisfactory in speed and accuracy with the application of concentric relaxation and linear increment constraint on the comprehensive adjustment of active and reactive power. As an example. the IEEE Reliability Test System is tested and the results show that the method is accurate and well-matched in speed with the method studied by R. Bllinton, et al.     

REAL POWER ECONOMIC RE-DISPATCH WITH SECURITY CONSTRAINS OF POWER SYSTEM

This paper reports on application of network flow programming to the real power economic redispatch with security constraints. For the contingency state which violates the line security constraints , the re-dispatching is performed by eliminating the curtailment of real loads. The network flow model is incremental, and is solved by OKA. Therefore, it is very simple, fast and accurate. The model and its solution algorithm are tested on JEEE 30 bus system and the encouraging results are obtained.     

Analysis on the Effect of Harmonic on the Instruments with Average Value Principle

The application of non-linear load produces a mass of harmonics. The AC voltage meter and the AC current meter used in power system are usually rectifying instruments, which adopt principle of responding to the average value of voltage and current and get the virtual value by using of the proportion between the average value and it. Because they are designed based on the 50 Hz standard sinuous wave, the principle error will occur when the voltageof power system and the current of loads include harmonic. Based on their working principle, this kind of error is analyzed quantificationally. And the result shows that it can not be neglected. The voltage and current meters with real virtual value are proposed.