comparison of practical fault ride-through capability for mv - PSCC

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2.1 Dynamic Model of a CHP-plant Modern CHP-plants have a turbo charged gas engine as a prime mover. When studying the ride-through capabilities of such CHP-plants it is necessary to study the dynamics of the gas engine. Gas engine modeling is very complex due to the difficulties in describing the internal processes of the engine by equations. Most models which are available in the literature are too specific and not developed for power system dynamical studies. In [4] a model is developed, which is suitable to represent the gas engine dynamics. In Figure 1 a block scheme of the gas engine model is given. This block scheme is implemented in Matlab/Simulink and dynamic simulations are performed. In [4] a detailed description of the applied model as well as the used figures can be found. Throttle α 1-α Engine Turbo Charger Figure 1: Block scheme of a turbo charged gas engine Torque The dynamics of the gas engine is demonstrated with a simulation of a torque step. At t = 0 s the engine is started up and at t = 5 s a torque step from 0.8 p.u. to 0.3 p.u. is applied. The result is depicted in Figure 2 and shows that the dynamics of the gas engine are mainly determined by the dynamics of the throttle. 2.2 Dynamic Model of a DFIG The scheme of a DFIG used for the simulations is shown in Figure 3. This concept includes a wound rotor asynchronous generator with the stator directly connected to the grid, while the rotor is coupled with the network using an AC to AC power converter with a DClink. Wind Turbine DFIG 1.5 MW 2 MVA 575 V / 10 kV Pitch Control RSC ~ RSC Control GSC ~ GSC Control 150 kvar Figure 3: Model of the DFIG for stability assessment The simulations of the DFIG dynamic behavior were performed with the Matlab/SimPowerSystems toolbox using the DFIG model available there [5]. The model includes the following main features: • The wind turbine is modeled using steady-state aerodynamic relationship between wind speed and the mechanical power of the turbine. • The power electronic converters are represented by equivalent voltage sources generating the AC voltage averaged over one cycle of the switching frequency. • Pitch control, rotor-side converter (RSC) and grid-side converter (GSC) controls are modeled in full detail. During the simulation the RSC control operates in power factor regulation mode keeping the generator reactive power exchange equal to zero. Figure 2: Dynamic response of a gas engine In comparison with typical fault clearing times of the protection systems and critical clearing time of generators, the dynamics of the gas engine are slow. Due to these slow dynamics the gas engine is represented by its inertia only. This inertia is incorporated with the generator inertia. The generator model of the CHP-plant, used in the dynamic simulations, is a regular model of the synchronous machine which is available in PowerFactory. • The wound rotor asynchronous generator is represented by 5th order model in the dq-reference frame. • The model does not include crowbar protection. 3 TRANSIENT STABILITY OF DG-UNITS The aim of ride-through criteria is to keep DG-units connected to the grid during the disturbance and recover the power balance after fault clearing. Keeping DG-units connected to the grid means that the units have to withstand certain voltage dips. The effect of voltage dips on stability of CHP-plants and wind turbines is studied by 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 2
means of a study grid having a variable voltage source. The voltage source is able to produce a voltage dip of a certain depth and duration. The defined dynamic models are implemented in the test grid and the stability of the DG-units as a function of the depth and duration of the voltage dip is determined. In Figure 4 the study grid is shown. It consist of two 10 kV busbars, a swing-bus which represents the external grid, a 3 km XLPE 630 mm 2 Al cable connection and a generator. Parallel at the swing-bus the variable voltage source is connected. Variable Voltage Source Swing Bus V Bus 1 Bus 2 Figure 4: Study grid including a variable voltage source G 3.1 Dynamic Behavior of CHP-plants With the aid of the defined test grid the dynamic behavior of the CHP-plant is studied. In the simulations the rotor angle is used as a stability index. To find the stability limit at a certain depth of the voltage dip the duration of the voltage dip is adjusted. In this way the CCT as a function of the voltage dip can be found. In Table 1 the figures for which the simulations are carried out are displayed. The results of the simulation are depicted in Figure 5. S nom [MVA] P [MW] p.f. H [s] 3 2.4 1.0 2 3 2.4 0.8 2 3 2.4 1.0 4 3 2.4 0.8 4 Table 1: Simulation figures of the CHP-plant u [p.u.] 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 H = 2 s, p.f. = 1 H = 2 s, p.f. = 0.8 0.05 H = 4 s, p.f. = 1 H = 4 s, p.f. = 0.8 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 t [s] Figure 5: CCT of a CHP-plant as a function of the voltage dip (remains voltage) 3.2 Dynamic Behavior of a DFIG Transient stability of a DFIG can be evaluated by monitoring the DC-link voltage. Its behavior for threephase fault durations of 100 and 300 ms at wind speed of 10 m/s (voltage dip to 0.6 p.u. of the nominal voltage) is illustrated on Figure 6. 2800 2600 2400 2200 2000 1800 1600 1400 1200 Fault duration = 300 ms Fault duration = 100 ms DC-link voltage Vdc [V] 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 t [s] Figure 6: DC-link voltage of a DFIG for different fault durations In an unstable situation the DC-link voltage starts to increase significantly immediately after the fault. The other variables (active and reactive powers, mechanical speed of the machine, etc.) exhibit not so clear unstable behavior, although as it can be seen from Figure 7, active and reactive powers start to oscillate, and the mechanical speed begins to grow slowly. 2 0 Active power [p.u.] -2 0 0.1 0.2 0.3 0.4 0.5 0.6 1 0 Reactive power [p.u.] -1 0 0.1 0.2 0.3 0.4 0.5 0.6 1.11 1.1 1.09 Fault duration=300 ms Fault duration=100 ms Mechanical speed [p.u.] 0 0.1 0.2 0.3 0.4 0.5 0.6 t [s] Figure 7: Mechanical speed, active and reactive powers of a DFIG for different fault durations To determine the CCT of the DFIG the same approach as described in section 3.1 is used. The simulations are carried out for various wind speeds which lead to the results depicted in Figure 8. 16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 3
Page 1: COMPARISON OF PRACTICAL FAULT RIDE-
Page 5 and 6: 4.2 Voltage Dips Profile As mention

comparison of practical fault ride-through capability for mv - PSCC

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