Power electronic converters in power systems 1
Typical application of grid connected converters Active rectifier (sinusoidal line current, bi-directional power flow, adjustable power factor) Grid interface for power sources (fuel cells, wind power, high speed generators, solar cells) Grid interface for energy storage technologies like super capacitors, batteries, flywheels, electrolyser-fuel-cell systems Reactive power compensation (STATCOM) Active filter Shunt configured online UPS Power line conditioners Series configured UPS systems for isolated networks (voltage controlled output were converter supplies both power and voltage quality to an isolated or weak AC-power system DC-transmission 2
Power electronic converters for integration of new energy sources and energy storages Isolated or weakly connected remote area with local energy production Ih End users Production / storage Transmission Converters P P Q distribution transmission Q P energy storage P Ih Control of: Q Q Active power fuel cell P Q Reactive power (VAr compensation) and/or active filter Reactive power Power quality 3
Converter design experience: SINTEF has the required skills and experience for design og grid connected converters. Experience of high importance: Mechanical layout for compact design with low parasitic capacitances and inductances Design of magnetic components (inductors) Methods for calculations of losses and temperature cycling effects Cooling methods Control electronics and gate drivers Implementation of micro controller based control system Design of fast control electronics using micro controller and field programmable grid array (FPGA) Modelling and simulation 4
Investigated control methods for grid connected converters SINTEF has focused on control topics like: Methods for synchronisation / phase locking to AC-network Control of active rectifier with DC-link voltage control Control of reactive power / power factor PWM and tolerance band current control Voltage controlled output for UPS-type applications (isolated or weak AC networks) Active damping of oscillations between grid reactance and filter capacitor of grid connected converters 5
Specifying converter performance during transients and faults More controllability than a traditional synchronous generator. Possible to specify the behaviour of the converter during and after a transient or a line fault: Current limit Instantaneous change of reactive power flow Active damping of transients Compatibility between existing AC grid and new converter connected power sources: Failure of protection relays due to insufficient short circuit capability. Risk for interaction between different power sources during and after faults and transients A broad experience and understanding of both power systems and power electronics are needed! 6
Competence on converter behaviour in weak power systems Examples of weak power systems: Power systems on offshore installations, especially in emergency mode of operation whit reduced electric power production. Propulsion systems for ships Stand alone power systems with distributed energy production where the distributed production is connected via converters. Challenging interaction problems : Active and reactive power flow Frequency control Voltage transients Harmonics Oscillations between grid reactance and filter capacitor of grid connected converters Converters initiating mechanical resonance in generator trains 7
AC-side filtering Filtering on AC side is usually required Introduces risk of poorly damped oscillations between filter and grid (especially in weak grids) Optionally connected to DC-source / DC-load U DC + i a i b AC-grid i c + + + u a u b u c SINTEF has developed methods for active damping of such oscillations. 8
Demonstration by simulation Application: PWM converter connected to the AC-grid at a bus with low short circuit capacity compared to converter kva rating Demonstration of damping of oscillations after a step reversal of the reference for reactive current (next slide) PWM converter I measure LC-filter I mains Ideal AC Network 1 : 1 V DC = 450 V S conv = 20 kva f sw = 5470 Hz - 40 db at f sw L=0.82mH C=100µ F R C =1.0mΩ R L =6.4mΩ S transf = 30 kva x transf = 10 % L=0.6mH, R=100mΩ f grid = 50 Hz v grid = 230 V rms (l-l) 9
Active damping disabled Active damping enabled +0.25 +0.15 +0.05 ua ia,mains +0.25 +0.15 +0.05 ua ia,mains Phase a filter capacitor voltage (black) -0.05-0.15-0.05-0.15 Phase a current into AC-grid (red) -0.25 0.13 0.142 0.154 0.166 0.178 0.19 Time (sec) -0.25 0.13 0.142 0.154 0.166 0.178 0.19 Time (sec) +0.25 ua ub uc +0.25 ua ub uc +0.15 +0.05-0.05-0.15 +0.15 +0.05-0.05-0.15 Filter capacitor voltages (phase a, b, c) -0.25 0.14 0.144 0.148 0.152 0.156 0.16 Time (sec) -0.25 0.14 0.144 0.148 0.152 0.156 0.16 Time (sec) 10