Baohui Zhang

Unified Power Flow Algorithm Based on the NR Method for Hybrid AC/DC Grids Incorporating VSCs

This paper proposes a unified power flow algorithm based on the Newton-Raphson (NR) method to obtain a feasible solution for hybrid AC/DC grids incorporating voltage source converters (VSCs). The power flow model of hybrid AC/DC grids is composed of an AC grid model, a VSC station model, and a DC grid model. Detailed converter losses are involved in VSC station modeling. In the DC grid model, a unified dc control expression is applied to adapt to different control methods. To obtain sparse Jacobian matrix, the mismatch equations and unknown variables are classified into three categories corresponding to AC, VSC, and DC models. Then the proposed algorithm achieved quadratic convergence with the NR method. Once the solution is infeasible (variable out-of-limit) or diverging, the constrained load flow (CLF) method is applied automatically to achieve a feasible solution by revising the references of ac/dc grids. Simulations were carried out for the hybrid AC/DC test system composed of two AC grids and two DC grids. The results under different DC voltage control modes show that the proposed algorithm is able to obtain a feasible solution with high accuracy and convergence speed.

A Novel Traveling-Wave-Based Main Protection Scheme for

The UHVDC transmission system has been increasingly used because of the advantages in long distance and large capacity transmission. However, the existing traveling-wave-based protection for UHVDC transmission lines is not sensitive enough to properly detect the internal fault with high fault resistance. A novel traveling-wave-based protection scheme for bipolar UHVDC lines is proposed in this paper. The discriminative characteristic between the internal and external fault, embodied in the backward traveling wave, is extracted to formulate the main protection criterion. The faulty line is identified by the polarity of the voltage fault component. In addition, only the single-pole voltage and current of rectifier side are required for the proposed protection scheme. The simulation and field data tests show that the proposed protection scheme can make a correct identification of the fault section and selection of the faulty line in the ± 800 kV UHVDC system and ±500 kV HVDC system. Besides, the proposed protection can correctly detect the grounding fault with fault resistance up to 500 Ω.

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The early traveling wave faulted-phase selectors, due to lack of effective tool to process transient signals, had to directly used instantaneous values of signals so that they cannot overcome such bad influence as noise disturbance. Fortunately, wavelet analysis, with its time-frequency localization ability and the wavelet transform modulus maxima (WTMM) concept, is well suited to treat with singularity of fault-generated traveling waves in EHV/UHV transmission lines. This paper presents a novel approach to fast and accurate phase-selection, which used the WTMM of initial model current traveling waves according to the fault characteristic relations deduced from the boundary conditions of various types of faults. The criterion is explicit in characteristics and physical concepts, and is apt to be realized. A large number of EMTP simulations demonstrated the new faulted-phase selection algorithm.

800 kV UHVDC Bipolar Transmission Lines

With the increasing consumption of traditional energy and rising of environment pollution, as well as the gradually improving requirements of users for power supply, the application of distributed generation based on renewable energy and clean energy technology has been widely used in power system. As an effective form for distributed generation access to distribution network, microgrid combines the distributed power, energy storage equipments and loads through power control and energy management to make up a flexible, controllable autonomous system, which can operate in both grid-connected and islanded states and transfer neatly between these two states. Since autonomous operation of microgrid cannot do without effective control strategy, how to control the operation state of micro grid in grid-connected and islanded states and realize smooth switching of the microgrid is an important content of study on control strategy of microgrid. In the light of the problems exist in microgrid peer to peer control based on local information and modified droop controller, a master slave-peer to peer integration control strategy based on communication is proposed, which combines advantages of master slave control and peer to peer control, achieves steady operation of the micro grid in grid-connected and islanded states as well as smooth switching between these two states through coordination control on the composite energy storage system and micro sources.

A novel approach to faulted-phase selection using current traveling waves and wavelet analysis

Renewable energys grid-integration will strongly influence 3-sectional overcurrent protections correct operation in distribution network because of its in feed current effect or shunt effect. Firstly, the fault current characteristics of different types of renewable energy sources are simulated, and their impacts on overcurrent protection are analyzed. Secondly, the detailed influence factors, including the renewable energy capacity, the integration location, the fault location, the reliability coefficient and the transmission line length, are thoroughly discussed in the distribution system model. Especially, by changing the renewable energy source capacity, the relationship between the short-circuit capacity ratio at the integration point and the fault current flowing through the protection is investigated. Lastly, simulation results on PSCAD/EMTDC show that when the short-circuit capacity ratio at the integration point is superior to 11.72%, the instantaneous overcurrent protection (1st section) will mal-operate, and when the ratio surpasses 21.8%, the time-delay instantaneous overcurrent protection (2nd section) will mis-operate.

A master slave peer to peer integration microgrid control strategy based on communication

The high-energy arcing faults in power transformer can cause overpressure which would break the tank open, lead to oil-spill and even catching on fire. Facing to the great hazard of transformer overpressure during internal arcing faults duration, this paper has investigated the physical process of tank internal overpressure and revealed the relationship between the electrical faults and overpressure characteristics from the energy conversion perspective. Due to the fact that the arcing faults tests inside power transformers are extraordinarily difficult, destructive and costly, the theoretical model has been proposed and numerical simulation approaches has been employed. At first, the transformer internal arcing fault energy model, the fault pressure source and the overpressure wave propagation have been mathematically modeled considering the whole phenomenon as an acoustic problem. Then, according to the real full-scale 240 MVA/220 kV oil-immersed power transformer structure parameters, a transformer simulator has been set up. Thirdly, by employing 3D FEM, the transient overpressure characteristics such as pressure rise curves and effective pressure indexes at different positions have been explored. The simulation results illuminate that the chief effect to overpressure inside tank is the arc energy, the following is the positions where arcing faults happen. Additionally, both the theoretical model and the simulation method proposed in this paper can be extended to simulate the overpressure phenomenon during arcing faults in other types of oil-immersed transformers and reactors instead of costly and unpractical field tests.

Impact of renewable energy integration on overcurrent protection in distribution network

In this paper, a simplified method for channel capacity estimation has been derived based on the transmission-line theory under the condition of high-bandwidth utilization. This method decouples the channel capacity into several independent components which may help to investigate the effects of backbone and branch, respectively. Analysis of backbone effects with respect to the available bandwidth reveals the applicable condition for the simplified method. Single branch effects are investigated considering branch length together with the loads. Besides, coupling effects caused by branch interactions are studied in a statistical way. The simulation results show great consistencies by using the proposed method and the conventional one and reveal the way that branches affect the channel capacity in different power networks.

Simulation and analysis of power transformer internal arcing faults overpressure characteristics

This paper addresses the issue in critical machines identification for real-time assessment and control of power system transient stability. To satisfy both accuracy and rapidity, on the basis of clustering study on a 3-unit 9-bus system, a novel real-time critical machines identification method is proposed. First, three candidate decomposition patterns are selected by the top three largest predicted angle variation distance; And then an index is proposed to choose the pattern whose equivalent OMIB system operating state goes farthest from the equilibrium point as the best decomposition pattern. The scheme only needs real-time machine dynamic information, and has the advantage of little computation, rapid identification speed and excellent accuracy. Simulations on IEEE 39-bus 10-unit system validate the efficiency of proposed scheme.

Branch Effects on Channel Capacity for Broadband Power-Line Communications

Transformer is a key component of power system and its steady and safe running is of much importance to the whole power system. So electrical engineers develop many protection device to detect transformer abnormal state and fault state. Non-electrical parameter relay protection mainly reflects nonelectrical parameter changes between transformer normal state and abnormal state. Pressure relief valve is one these devices and it is used to protect transformer itself, which feels oil pressure increase and act to inject hot oil to surroundings. It is set according to its pressure on the wall of tank, usually half or 0.6 times of its inherent oil pressure. However, in practice, oil pressure valve always act in external short circuit fault. Towards it, there is few research on it. There is no relevant test research to study non-electrical parameter changes during transformer fault, which is still nearly empty. So we designed a transformer non-electrical parameter measure system and used it to measure non-electrical parameter changes during transformer external short circuit fault, and analyze data obtained. Through these results, we figured out something as following, 1. Oil pressure will increase sharply and fluctuate fiercely during external short circuit, which may arrive beyond open pressure 2. The second harmonic component counts most in oil pressure and it will increase sharply when transformer meet an external short circuit fault after an external short circuit impact. By using these conclusions, we can figure out the reason why oil pressure relief valve misact during external short circuit fault and the difference between external short circuit fault and internal tank fault. Thus, we can come up some measures to improve oil pressure relief valve and coordination of every nonelectrical protection components. Moreover, we can use these conclusions to build new fast and reliable transformer relay protection.

Study on real-time clustering method for power system transient stability assessment

The hybrid high-voltage direct current (HVDC) transmission system comprising a line commutated converter (LCC) at the sending end and a voltage source converter (VSC) at the receiving end has broad prospects in many application fields. At present, the protection of HVDC transmission line usually utilizes the transient signals to discriminate faults, and the line boundary characteristics are the key influence factors to the postfault transients. In this paper, the different boundary characteristics at each end of the hybrid HVDC system transmission line are investigated, in both frequency domain and time domain. And suggestions for the transmission line protection are proposed.