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The conventional UVLS (undervoltage load shedding) relay and the UFLS (underfrequency load shedding) relay, designed separately without considerations on load characters and mutual influences of the voltage and the frequency, may result in excessive or insufficient load curtailments. This paper proposes a new adaptive load shedding method based on the underfrequency and undervoltage combined relay...
Recently, the worldwide blackouts have shown that disadvantages of under-frequency load shedding (UFLS) devices such as settled threshold, constant time delay and determined load curtailments may result in frequency instability by executing insufficient or excessive load curtailments. Based on the analysis of frequency dynamics, this paper analyzes the influences of the time delay on the effect of...
Power transformer plays a pivotal role in the power transmission, whose failures have serious impact on reliability of power supply and operation stability of power system. Partial insulation aging caused by thermal effects in large-scale transformers is one of significant factors which have a negative effect on operation stability. Just because temperature change of in large power transformers is...
As a transformer main protection, gas protection is an important and traditional non-electricity protection. For internal faults of transformer, gas protection can directly response to non-electricity characteristic such as oil flow velocity, and then has higher sensitivity and reliability compared with electric protection in principle. However, for external faults, great electromagnetic force and...
Double-circuit transmission lines and even multi circuit transmission lines are trends for development of power system. The paper analyzes the wire layouts of double-circuit line and characteristics of its impedance matrix. And then the sequence currents of double-circuit line are investigated. The paper points out that besides unbalanced net through current, there are distinct circulating currents...
The detection and classification of transient signals are widely applied in many fields of power system. The study of transient signal detection and classification is a sustaining focus of researchers as well as a difficult issue. There are still many problems needed to be solved in this area. Based on the wavelet transform (WT), the idea of entropy and weight coefficient is introduced, and the wavelet...
In this thesis, existing instability criterion and its stability indicator τ for autonomous single machine infinite-bus system (SMIB) are firstly given, which reflect convex-concave characteristics of the trajectory inΔω-δ phase-plane. On this basis, it is discussed that there are some problems when the instability criterion of autonomous SMIB system is used in non-autonomous SMIB system. Proceeding...
An improved differential equation method for earth fault location in non-effectively earthed system is proposed. Karrenbauer transformation is used to construct the compound mode network of single phase earth fault. Based on the mode network, the fault equation including mode voltage, mode current, line parameters and earthed resistance is derived .The fault distance can be gotten by solving the equation...
Based on the qualitative analysis of different fault transients in EHV (extra high voltage) transmission line, the features of different transient components were given out. Taking various transient features into account and combining with the information theory, this paper proposed a new algorithm for fault classification based on multi-information measurements of fault transients, which took advantage...
A novel dynamic instantaneous power-system frequency estimation method for phaser measurement units (PMUs) was proposed for generalized sinusoidal models, which was intending to simulate the dynamic condition via modulating phase and considering magnitude damping. Frequency estimation from traditional discrete Fourier transform (DFT) could be attained based on frequency and damping character of voltage...
With further complication of power system, the problem of low frequency oscillations in large electric network becomes more and more serious. Unified power flow controller (UPFC) is the most valuable and universal device in flexible AC power transmission system, which can improve system stability and inhibit low frequency oscillations under proper control. This paper intends to analyze the single-unit...
Fast and accurate fault diagnosis is the primary prerequisite for separating the faulty devices and restoring the power supply, therefore it is of great importance to develop an advanced diagnosis method to meet the power system requirements. With the perspective of information fusion, this paper proposes a novel algorithm for fault diagnosis in power system via the fusion of several different wavelet...
UHV transmission can transmit huge capacity energy over long distance. The paper proposes a new negative-sequence distance protection based on Bergeron model. Since Bergeron model has taken distribution parameters into account, the proposed principle is valid for providing protection in the case of remote or close faults, and it has high sensitivity for fault with fault-path resistance. Furthermore,...
Automatic reclosure (AR) is extremely effective in improving the reliability, stability and security of the overall power system. However, the present autoreclosure operates blindly and practices employing a fixed time. In the case of permanent fault, a second shock to the system can cause irreparable damage to expensive equipments. According to characters of transient faults and permanent faults,...
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