Influence of MV Distribution Networks Structure on Voltage Dips Propagation G. Ala, A. Barone Barone, S. Favuzza, M. Inzerillo
[email protected] Department of Electrical Engineering University of Palermo Palermo, Italy
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Abstract In this paper, transient behaviour due to voltage dips which occur in electric power MV distribution networks as a consequence of a fault by using an improved ATP circuitbreaker model are reported. The circuit-breaker is simulated by using a tool written in MODELS language and by arranging a time-variable differential arc-conductance based on the combination of Mayr's and Cassie's equations. An accurate analysis of the transient subsequent to circuitbreaker operation, in different fault conditions, are reported in order to show how the amplitude of the dip depends on the fault type, location and network structure.
the power system plant in relation with possible and realistic fault conditions. In order to accurately analyze transients, it is of basic importance to employ a realistic model of the circuit-breaker and of the related arcdynamics. The simulation carried out in the paper employs an equivalent circuit of a MV distribution network enclosing power transformer, ingoing and outgoing lines and SF6 circuit-breakers. 11. CIRCUIT-BREAKER SIMULATION MODEL
Keywords: MV Distribution Network, Voltage Dips, Transient, Circuit-Breaker, Arc-Dynamics. I. INTRODUCTION
The massive diffusion of electronic equipment in electric power systems, both for protective and control apparatus, has originated the need of an higher standard of quality both in terms of energy supply continuity and of waveform quality for the line voltage in MV and LV distribution networks. Many are the causes that can influence the quality of electrical supply for industrial installations and all of them can be set in the wide field of the electromagnetic compatibility (EMC). Particularly, with reference to conducted disturbances, the voltage dip is one of the most dangerous since, depending on its amplitude and duration, it can influence the regular operation of several, widely diffised, sensitive loads categories. As an example, voltage dips can disable electric motors because of the undesirable operating of the protective relay, can determine the shutdown of the gas-discharge lamps, or can cause malfunctions in microprocessor control systems, programmable controllers and induction heating furnaces. Moreover, since the industrial equipment connected to the power network are aimed to industrial production, the effects of this malfunctions could have repercussions on material damage, production loss and even problems with safety. In the vast majority of cases, the effect of a fault is to produce a short-circuit current which must be eliminated by disconnecting the defective component of the system, by opening the circuit-breaker. In the majority of the cases, the circuit-breaker opens after a delay of few hundred of milliseconds. Between the fault occurring and its elimination the line voltage fall down depending on the amplitude of the short circuit current. In this paper, transient behaviour due to voltage dips which occurs in MV networks as a consequence of a fault, is analysed. Since particular measurements are not often available, especially in the case of planned networks that do not already exist, it is necessary to simulate the behaviour of
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Each phase of the circuit-breaker is modelled in ATPEMTP code, by using a tool written in MODELS language which employs a representation of the variable arcconductance based on the combination of the Mayr's and Cassie's differential arc equations. The circuit-breakermodel consists of two series-connected ohmic resistors representing the Mayr's and Cassie's parts, respectively. In this case for high values currents, practically all the arc voltage is supplied by Cassie's portion; Mayr's portion increases just before current zero and takes over all the recovery voltage after the current interruption, while Cassie's portion goes to zero. 111. APPLICATION
The application example consists of a network with ring structure. It is constituted of two ring, one of HV voltage (1 50 kV) and one of MV voltage (20 kV). The MV ring can be opened in different sections, in order to change the network structure. The SF6 circuit-breakers are simulated by means of the previously described differential model. Two kind of faults has been considered: a three phase fault and a bi-phase fault on a customer load. For each kind of fault, three configuration of the network are considered: two open-ring configurations and a ring configuration. The voltage time profiles in several HV and MV nodes of the network are reported. IV. CONCLUSIONS
Transient behaviour due to voltage dips which occurs in MV networks as a consequence of a fault condition has been analysed, with reference to a typical ring configuration of MV distribution network. Two kind of faults have been considered: a three-phase fault and a biphase fault on a load connected to a MV cable-line. A realistic model of the arc dynamics related to the circuitbreaker opening operation has been employed by using EMTP-ATP code. The simulations carried out have shown how the amplitude of the voltage dip depends on the type and location of the fault and how it is strictly related to the network structure.
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