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Transcript
UNIVERSITÀ DEGLI STUDI DI PADOVA ALMA MATER STUDIORUM UNIVERSITÀ DI BOLOGNA A. Borghetti, R. Caldon, S. Guerrieri, F. Rossetto Dispersed Generators Interfaced with Distribution Systems: Dynamic Responce to Faults and Perturbations AIM OF THE WORK Analysis of the impact of distributed generators connected to distribution networks either directly (rotating generators) or by means of power electronic interfaces (static generators). Assessing the adequacy of the power system protections. DISTRIBUTED GENERATORS AND NETWORK INTERFACE MODEL Microturbine unit model Voltage Controller Active and reactive Active and Reactive Power Controller power controller Photovoltaic system model Fuel cell unit model Pref P-Q Controller Vdc-set Controller (Voltage and) Speed Controller P-Q Controller . V P-Q Controller . id_ref 1 1 s Tp 1 1 s Tm id I Vdc Gen bus Network bus Inv bus Synchronous Rectifier Generator Booster PWM-VSI inverter Inverter bus Fuel Cell Vdc-source LC-Filter Photovoltaic Vdc-source LC-Filter Boost Converter PWM-VSI inverter iq Network bus Inverter bus Network bus PWM-VSI inverter abc/dq Vdc 1 1 s Tm Kd 1 1 / s Td vmd 1 1 s Tm 1 1 s Tm vmr dq/abc 1 1 s1Tm 1 s Tm vmi LC-Filter Qref V . . - 1 1 s Tq Kq 1 1 / s Tq vmq iq_ref RESPONSE TO FAULTS AND PERTURBATIONS OF DISTRIBUTION NETWORKS INCLUDING DG External MV Grid MEDIUM VOLTAGE NETWORK Examined system configurations: i) two diesel units (rotating generators); ii) two sets of 10 microturbines (static generators); iii) without DG units. d b a LOW VOLTAGE NETWORK with five small powerful DG units: - two microturbines; - two fuel-cells; - one photoltaic unit. Location 3 c DG-plant 5 Location 2 DG-plant 4 Location 4 Location 1 DG-plant 1 DG-plant 2 DG-plant 3 THREE-PHASE SHORT CIRCUIT AT THE COMMON MEDIUM VOLTAGE BUS PHASE-NEUTRAL SHORT CIRCUIT SHORT CIRCUIT CURRENTS FOR CONFIGURATIONS I), II) AND III) AND RATIO BETWEEN DIFFERENT TYPES OF GENERATION CONTRIBUTION TO THE SHORT CIRCUIT CURRENT FOR TWO DIFFERENT VALUES OF THE SHORT CIRCUIT POWER Fault current with rotating generators [A] 103 3937 104 4480 Fault current with static generators [A] 3827 4375 Fault current without DG units [A] 3810 4358 7.56 7.18 Pcc [MVA] Rotating generation contribution Inverter contribution INCREASE OF THE MAXIMUM CURRENT VALUE IN THE FAULTED PHASE AT LOCATIONS 1 AND 2. Location 1 Location 2 If increase% kVA 0.28 1.1 DG STATIC CONTRIBUTIONS TO THE THREEPHASE FAULTS AT LOCATIONS 3 AND 4 VOLTAGE AND CURRENT BEHAVIOUR AT THE MV-SIDE OF THE TRANFORMER Tr-DG1 FOR CASES I) AND II) Fault location Location 3 Location 4 Static DG Contribution [%] 1.71 48 Three-pahse breakdown to the low voltage bus:DG-plant 2 inverter ouput current with (solid line) and without (dashed line) current limitation. The DG presence results in a three-phase fault current increase THREE-PHASE SHORT CIRCUIT AT DIFFERENT FAULT LOCATIONS Advantage of static DG still significant If increase [%] (Static Dg contribution) 44.5 24.9 THREE-PHASE SHORT CIRCUIT DG contribution to fault current and transformer stress reduced by about seven times ! ROTATING AND STATIC GENERATOR CONTRIBUTIONS TO THE SHORT CIRCUIT CURRENT Break point PDG connected to the faulted branch [kVA] 156 22.4 Short circuit location Common medium voltage bus N7 N8 N9 Distance DG1-short circuit [km] 6 4 2 0 Rotating generator contribution [%] 2.79 2.97 3.19 3.29 Inverter generator contribution [%] 0.39 0.44 0.53 0.62 Rotating generator contribution Invertercontribution 7.18 6.69 6.04 5.28 Fault location Sending end of feeder b Sending end of feeder c Ig increase [%] PDG connected to other branches [kVA] 1.46 179 1.21 179 Current reference limiters action: most protection interventions can be avoided ! FAULT LOCATION VERY NEAR TO THE COMMON LV BUS: INCREASES (%) OF THE OUTGOING SECONDARY STATION CURRENTS. DG contribution to fault current of the order of about 1%. REMARKS Negligible contribution to fault current of P-Q controlled interfaced DGs Possibility for many generators to be embedded without need of re-designing the feeder protection schemes In case of static generators does not appear to exist the typical problems arising from the connection of rotating generators to distribution networks, namely increase of fault current levels and inappropriate protection device operation when there is a short circuit on an adjacent line It is more straightforward to guarantee the protection system selectivity