Liao Qingfen

A new on-load tap changing system with power electronic elements for power transformers

In order to avoid the disadvantages of conventional mechanical tap changing system and its influence on power system, a new on-load tap changing system with power electronic elements for power transformers is presented in this paper. The system is composed of main circuit, signal detection and triggering circuit design, choice of the thyristor switching, protecting transformer from overvoltage both in case of switching on and switching off no-load transformer. It possesses the inherent attributes of greater efficiency and less maintenance.

Power quality disturbances detection and location using mathematical morphology and complex wavelet transformation

Bused on mathematical morphology (MM) and complex wavelet, a novel method on power quality (PQ) disturbance detection and location is presented in this paper. At first, a generalized morphology filter with horizontal linear structure elements is designed to filter impulses and random white noises in PQ disturbance signals. Numerical simulation results show that the characteristics of the original signal can be well retained and the noise interferences are suppressed effectively with the proposed filter method. Then, to the filtered results, the complex wavelet derived by Daubechies real wavelet and its compound information are applied to detect and locate the start and end time that the disturbance occurs. The voltage interruption, voltage sag, voltage swell, high frequency oscillation and harmonic distortion are used to verify the validity of the filter-location approach. Simulation results show the integrated detection approach of morphology-complex wavelet is valid and effective. Besides, it can reduce calculation time and can be implemented easily in the available hardware.

An islanding surface searching approach based on Floyd-warshall and DFS algorithm

Reasonable islanding strategy is significant for avoiding blackout when the power system suffers from major fault. On the basis of this, this paper analyzes the electrical connection between nodes and proposes a new method of islanding strategy searching based on Floyd-warshall and DFS algorithm. By comparing the degree of electrical coupling relationship between load nodes and generator groups, the problem of islanding strategy searching can be mapped to the partition of the shared nodes. Afterwards, the shared nodes can be divided into proper island by further search which takes the minimum unbalanced power in each island after islanding as the objective function. After the above steps, the optimal islanding strategy can be obtained. The proposed method owns the advantage of low computation complexity and high fitness of operational modes. The effectiveness and rapidity are demonstrated through case studies on IEEE-39 test system and IEEE-118 test system.

Dynamic optimization of adaptive under-frequency load shedding based on WAMS

The rule No.599 of power grids in China indicates that the shedding load is equivalent to the failure loss load, which requires less load-shedding as much as possible under the premise of security and stability, to reduce the cost of frequency accident control. Therefore, in this paper, the load-shedding contribution factor based on frequency regulation effect, P-V characteristic and the importance of the load is designed to indicate the placement of load shedding and the allocation of the load to be cut. According to the influence of voltage leap on unbalanced power, the calculation accuracy is improved. A dynamic correction scheme is described to adjust load-shedding amount of each steps according to change rate of frequency derivative, to fully utilize the self-recovery capacity of system frequency. Simulation based on IEEE 39-bus system shows that the new strategy could reduce the load shedding amount and improve frequency recovery process, in which both the reliability and economy are taken into consideration.

Analysis of substation noise prediction based on equivalent source method

When predicting the noise level of substation, the model of transformers and reactors as noise sources is very important. A new acoustic mode of the transformers and reactors is built in ultra-high voltage substation. Each of the transformers and reactors is equivalent to several point sources based on equivalent source method. And in the process of the equivalent, the measurement and calculation are made easier by using sound power level to represent the equivalent sources intensity. A model of 9 equivalent point sources is chosen to verify the accuracy of the model. This acoustic model of the transformers and reactors is combined with outdoor point sound sources attenuation theory to predict the substation noise value. Simulation of a 1000kV substation has been down which shows that the model is feasible to calculate the noise level anywhere inside the substation.

Study on an optimal controlled islanding scheme based on active semi-supervised spectral clustering algorithm

Controlled islanding is a timely process which can split the power system into electrical islands in place based on the real-time operating condition and fault information. The key to avoid the collapse of the system is how to locate the out-of-step splitting sections rapidly and accurately. However, with the larger system scale, the computation complexity of optimal controlled islanding is increasing geometric exponentially which has to be faced with NP-hard problem. This paper proposes an active semi-supervised clustering algorithm (ASSC) to solve the problem. Firstly, the model for optimal controlled islanding of power systems is constructed with the minimal composite power-flow disruption as the objective function and generator coherency got from the active learning strategy as the supervised constraint which is suited to be clustered. Secondly, the solving process can be transformed into the relaxation solution of constraint spectral clustering aimed at partition of the static graph. Finally, the optimal controlled islanding schedule can be selected based on the improved K-means algorithm. The computation efficiency can be improved without losing the power grid possible solution. Moreover, the accuracy, effectiveness and rapidity are verified in the simulations of IEEE 118 standard system and Sichuan Province and Chongqing City Power Grid.