PRAOGEN, A TOOL FOR STUDYING CONNECTION OF GENERATING PLANT ONTO THE MEDIUM- VOLTAGE NETWORK

Transcription

1 PRAOGEN, A TOOL FOR STUDYING CONNECTION OF GENERATING PLANT ONTO THE MEDIUM- VOLTAGE NETWORK J.L. Fraisse, F. Boulanger, Ph. Juston, P. Lemerle, O. Jeannin EDF-DEGS; EDF-R&D, France Since early in 1990 following price conditions changes, independent production connected to the medium voltage power system developed to a considerable extent. Distribution networks were not designed to take into account the connection of dispersed generation. Connection of the generators to MV networks can raise a certain number of problems, as follows : overshoot of thermal limits of conductors, increase in the voltage level of the MV system, overshoot of short circuit current limits, misoperation of protection system. Therefore Ministry of Industry published technical rules for the connection of embedding generation to MV distribution networks. According to this rules, EDF defined studies which must be done for connecting a new generator. The delay of the examination is short (three months). By consequent, EDF developed a new software which : detects thermal and voltage constraints caused by the connection of new producers, detects short-circuit constraints and check the right set of protections. In this paper, we present this second application of the new software PRAOGEN. Short-circuit currents limits Grid equipment (switch, circuit breaker) and conductors (overhead lines or cables) are specified to resist to a short-circuit level. The increase of the short-circuit current, caused by the connection of a new power plant, must not induce an overshoot of the grid equipment limits. The most difficulty comes from the fact we don t know previously which is the equipment under constraint. It is necessary therefore to calculate the short-circuit current for each equipment of the distribution network. It is also necessary to consider both the normal and contingency operation diagrams : by consequent, there are a several hundreds of simulation to carry out. PRAOGEN compute automatically these calculations of short-circuits currents and detect the possible constraints. Protection system The protection against multiphase faults used by EDF is definite time current protection. It is set at 0.8 times the two-phase short-circuit current calculated at the feeder end. Under these conditions, it is necessary to ensure that the short-circuit current injected by the generating units connected to a sound feeder remains lower than this setting for a fault located on an adjacent feeder. If not, unwanted tripping will take place. PRAOGEN calculates the protection thresholds taking into account the contingency schemes; it also detects risk for protection unexpected tripping and propose, if necessary, to install a directional overcurrent protection on the feeder. PRAOGEN is available on PC version under Windows operating system. Data necessary to this kind of calculation are available either in the supplier data bases (network data) or introduced by the user (producer data). The presentation of the results uses standard office tools (like EXCEL). Their integration in the study reports is consequently easier. Thanks to this new tool, EDF is able to quickly carry out the studies concerning new power plant connections on the Medium Voltage network and to fully comply with one of its functions of distribution system operator.

2 PRAOGEN, A TOOL FOR STUDYING CONNECTION OF GENERATING PLANT ONTO THE MEDIUM- VOLTAGE NETWORK J.L. Fraisse, F. Boulanger, Ph. Juston, P. Lemerle, O. Jeannin EDF-DEGS; EDF-R&D, France OVERVIEW In France, the medium-voltage distribution network has been traditionally free of anything but the smallest generating plants, chiefly in the form of hydroelectric generators rated at just a few megawatts. But changing price conditions in 1992 brought about a sharp rise in the number of independent generating plants connected onto the MV distribution network, first with peak demand plants (running 396 hours per year at power ratings from 4 to 8 MW), and then with rapid growth in two types of plant designed to benefit from the attractive price offers (involving distributor company s commitment to buy predetermined amounts of electricity) that were launched to stimulate rational energy utilization practices : combined heat and power plants meeting specified performance criteria, waste incineration plants. Then in 1995, EDF, with backing from the French Ministry for Industry, decided to go ahead with a programme for wind-power plants that would supply a total of 250 to 500 MW on the mediumvoltage network by And following the European directive on development of renewable energy sources, these initial figures are to be increased. All these developments explain the sharp rise in applications to connect generating plants (occasionally topping 10 MW) onto the MV network as from A recent survey (1999) found that generation on the MV network already totalled 3800 MVA, which is the equivalent of three nuclear units! And projects for forthcoming MV-network connections amount to an additional 2600 MVA. The phenomenon concerns one substation in two, and for 20% of these substations, MV power injection exceeds a quarter of the total transformer rating. As MV generation rose above marginal proportions, it became clear that the technical requirements for connection to the distribution network would require careful study. The French Ministry for Industry accordingly published technical decrees defining conditions for connecting generating plant [1]. When a generator company applies for connection, the distributor company has three months to check compliance with the technical conditions set out in these decrees. EDF therefore wrote up a technical reference manual defining computation methods, assumptions and modelling principles for carrying out MV-network connection studies. Connection studies cover the following main points: Steady state thermal constraints Power injected onto the network must not overload the system components (lines, cables, HV/MV transformer). Modification to network voltage profile The voltage profile of an MV feeder will rise when generating plant is connected, and the higher the power injected, the higher this rise will be. The rise will affect all LV networks powered by the feeder, and during slack periods it may cause the system to exceed its contractual MV or LV voltage thresholds. Short-circuit current When a network fault occurs, the generating plants on the network will contribute to feeding current through the fault, thus increasing the short-circuit current throughout the MV network. The total resulting short-circuit current must not exceed the limit specified for the network equipment and conductors. Protection system Because of the earthing policy adopted, protection against phase-to-earth faults will not be affected by connection of generating plants onto the MV network. But protection against phase-to-phase faults will be affected, on two counts : When a fault occurs on the generator feeder, the fault current contributed by the HV/MV transformer will decrease, and if it drops below

3 the protection threshold, the fault condition will escape detection. When a fault occurs on an adjacent feeder, the short-circuit injected by the generators may exceed the protection threshold, in which we will experience nuisance tripping on the generator feeder. Attenuation of 175 Hz ripple control signal Owing to the series injection mode used, a generator plant on the MV network will attenuate the 175 Hz ripple control signal received by customers [2]. This attenuation must remain within acceptable limits. centres during We set out the main features of this new module below. Short-circuit current The idea is to determine the network components whose short-circuit current rating is liable to be exceeded owing to connection of generating plants. The first difficulty stems from the fact that different parts of the network have different short-circuit current ratings. There is a degree of standardization among equipment like circuitbreakers and switches (8kA and 12.5kA thresholds), but conductor short-circuit ratings vary considerably, as we can see from the table below. TOOLS FOR CONNECTION STUDIES Though the individual principles involved are fairly simple, a full connection study is a complex task, that must be completed in a short space of time (three months). For this reason, it was decided that the teams responsible for examining connection applications should be provided with computer tools capable of automating the process. We already had a software tool Harmonique [3] for analysing propagation of the 175 Hz signal, so all we would have to do here was specify operating conditions (assumptions and modelling principles) to analyse the impact of generator connection. But there was no existing tool for computing overload, voltage-profile, short-circuit current and protection aspects of generation on the MV network. Here, EDF would have to develop a specific tool. PRAOGEN The Praogen software was developed in two stages. An initial module for computing overload and voltage aspects was completed in 1998 [4] and is currently operational at EDF distribution centres. This was followed by development of a second module covering short-circuit current and impact on protection systems. As with the first module, detailed functional specifications were drawn up to provide the basis for coding, which was performed in C++ language on the Mosaic software platform [5], a Windows PC system that provides developers with modular software building-block facilities to minimize development costs. Following an acceptance testing phase, Praogen module 2 is scheduled for deployment across user Almelec overhead line I scmax HN33 cable I scmax 34 mm ka 50 mm ka 75 mm ka 95 mm ka 117 mm ka 150 mm ka 148 mm ka 240 mm ka This means there is no way of knowing beforehand which part of the network will reach its short-circuit threshold first : it might be the substation feeder circuit-breakers, or the overhead line sections at the feeder end. So we are required to evaluate the short-circuit current at all points throughout the network. For a substation with ten feeders each feeding a hundred line sections, this means a thousand current computations. And as well as studying this for normal operating conditions, we must also examine backup scenarios, and this gives us several thousand short-circuit current calculations in all. A second self-imposed constraint was compliance with international standard IEC-909 [6], to avoid any risk of dispute on the results produced by the software. To summarize the method specified in IEC-909, we note that short-circuit currents are measured on the network under no-load conditions at 1.1 times the nominal network voltage. All network components (lines, transformers) are modelled by impedances (ignoring lateral capacitances), and generators are modelled by their subtransient reactances x d. This gives us the initial symmetrical short-circuit current I k, which is then transformed using formulas specified in the standard to obtain the breaking current Ib, for comparison with the equipment ratings.

4 Very briefly, we can summarize the software algorithm as follows : Import network data from databases existing at the centres (topology and component impedances), Enter data on generator plants. There are relatively few items here : apparent power, subtransient reactances of machines, reactances of step-up transformers, and impedance of connection line, Select required operational diagrams, Set three-phase fault condition for each diagram and each network component, Reduce network down to source nodes (MV network, generating plants), fault node and branch nodes, Compute short-circuit current on network component by inverting the bus impedance matrix. Computations give us short-circuit current values for each network component under each operational diagram. These values can then be compared with the rated short-circuit current thresholds. Results can be presented to the user in two forms, with on-screen identification of critically affected network components, and a summary table in Excel. In addition to a full result listing, the Excel table also logs all the computation assumptions and all the input data, ready for direct inclusion in the study report. Protection systems sensitivity may be decreased and zero- - sequence wattmetric protection installed. The MV busbar on the substation is protected by a phase overcurrent relay and a residual overcurrent relay on the incoming feeder. This protection system also performs a backup function in the event of failure of outgoing feeder protection. In addition, a plant connected to the network will be disconnected in the event of internal fault by a protection installed at the point of common coupling. Again, this protection consists of a zerosequence overcurrent relay and a phase overcurrent relay. Impact of generating plant on sensitivity and selectivity of protection system The protection against phase-to-earth faults is not affected by generating plant connected to the MV feeders, because connection is via a transformer with star-delta coupling (star on generator side and delta on network side). This means there will be no change in the zero- sequence currents detected by protection devices on the feeder under phase-toearth fault conditions. However, the protection system against phase-to-phase faults is affected by generating plant connected to the MV feeders, since the additional short-circuit current contributed by the generating plant does change the shortcircuit currents detected by the protection devices on the feeder. Fault on generator connection feeder. The fault current injected to the substation by the transformer will tend to decrease when the generating plant is running. The other feature of Praogen module 2 concerns setting of protection systems. EDF distribution networks consist of radial feeders (open-ring mode) powered by an HV/MV transformer. Each feeder is protected by the following devices : HV/MV transformer I Protection device Branch PI Phase overcurrent protection against phase-tophase faults, with a detection threshold set at 0.8 Iscbi, where Iscbi is the phase-to-phase short-circuit current at the point most electrically distant from the feeder. This setting will clear all two-phase or three-phase faults occurring on the feeder. Residual overcurrent protection (three times zero- sequence current) against phase-to-earth faults, set at 1.2 Ic (where Ic is the residual capacitive current on the feeder) to prevent nuisance tripping on fault-free feeders. For highly capacitive feeders, the detection Phase-to-phase fault Figure 1 : Fault on generator connection feeder It may therefore be necessary to adjust the setting of the protection device on the generator connection feeder. Fault on adjacent feeder. When a fault occurs on an adjacent feeder, the generators inject a shortcircuit current that is detected by the protection device on the connection feeder. If this current

5 exceeds the detection threshold, we will observe nuisance tripping on the connection feeder, in which case it will be necessary to replace this device by a directional overcurrent device. overcurrent, overvoltage, shirt-circuit current, HV/MV transformer I Protection device PI protection system. By enabling EDF to rapidly process applications for connecting generating plants onto the MV networks, this new tool will facilitate efficient accomplishment of an important network management function. Three-phase fault Figure 2 : Fault on adjacent feeder The Praogen software adjusts the protection plan automatically, by applying the following algorithm : Input user-specified operating diagrams, For each operating diagram, compute minimum phase-to-phase short-circuit current detected by protection device on generator connection feeder, by simulating phase-tophase on each feeder end, For each operating diagram, compute maximum short-circuit current detected by protection device on generator connection feeder for fault occurring on another feeder, by simulating three-phase fault on substation MV busbar. When computation is completed, Praogen proposes settings for the protection devices on the generator connection feeders, and may also recommend installation of directional protection devices. It also checks the setting of protection device installed at the point of common coupling, which is also liable to nuisance tripping when faults occur on the distribution network. As with the shortcircuit test, the software outputs setting results in two forms, with an on-screen display and an Excel file. CONCLUSION In the past, studies into the connection of generating plant onto the distribution network have suffered from oversimplification in certain aspects, owing to a lack of tools capable of performing the high number of computations required. The Praogen software solves this problem. Combined with the Harmonique software, it evaluates all the critical issues involved in studying connectability : Upgrades are planned to extend coverage of the Praogen software beyond the examination of connection applications. Future releases might, for example, compute maximum connectable power values at different points on the network, thus facilitating evaluation of connectability capacities for existing networks. REFERENCES [1] Conditions techniques de raccordement au réseau public des installations de production autonome d énergie électrique. Decrees of 21 July 1997 and 3 June 1998, Journal Officiel de la République Française [2] J.L. Fraisse, P. Michalak, P. Juston and A. Grandet. Technical conditions for the connection of generating facilities to the medium voltage network. Development of a 175Hz active filter for independent power producers, in Proceedings of CIRED 97, Vol. 1, Article N 5.14, Birmingham, UK, 2 to 5 June 1997 [3] S. Arson, J. Martinon, P. Rioual. «Intégration et utilisation d une méthode d analyse statistique dans un logiciel de simulation des réseaux électriques en régime harmonique». Electrimacs [4] D. Cortinas, Ph. Juston «Assessing the Impact of Dispersed Generation on Medium Voltage Networks: Analysis Methods». in IEEE PowerTech 99 Budapest. [5] D. Voirin, O. Jeannin and P. Haberstich. A configurable software platform for the distribution network research. in Proceedings of CIRED 99, Article N 5.22, Nice, France, 1 to 4 June [6] Calculation of short-circuit currents in threephase AC networks. International standard IEC_909, first edition 1988.