Modular High Current Systems based on Supercapacitors As Pulsed Power Sources

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1 OCEM POWER ELECTRONICS Modular High Current Systems based on Supercapacitors As Pulsed Power Sources Sandro Tenconi Giusepe Taddia OCEM Power Electronics

2 2 Characteristics of pulsed power Common characteristics of pulsed power applications: High DC voltages (several kv) or High DC currents (tens of ka) High peak power (from tens to hundreds of MWs) but Short times and/or low duty cycle (typically some seconds every minutes) Moderate average power a system to store energy is required!

3 Possible AC sources of pulsed power 400 kvrms network (e.g. RFX in Padua) High voltage switchyard necessary High to medium voltage transformers MV VAR compensation often required

4 Possible AC sources of pulsed power Mechanical energy storage in a flywheel Stored energy in the GJ range. Big alternating current generator are used, with pony motor for launch and variable frequency high currents rectifiers. Examples: FTU in Frascati JET in UK JT60 in Japan The hydraulic flywheel alternator used at JET Mechanical storage and HV Power Networks Require huge investments Shall be designed for the maximum energy /power requirements Difficult to be upgraded

5 Possible DC sources of pulsed power Supercapacitors Energy Storage Modular construction possible Scalability in power and energy and eventually up-grading Compact, high peak power sources are possible, with limited system investments and power requirements Charging system power equal to the average power Supercapacitors technology and costs still quickly improving but Voltage capability is presently limited Short high current pulses possible only for Tp ~ 1s Pulsed power applications promising but still at the experimental stage

6 Supercapacitors for Pulsed Power Capacitance value strongly depends on the frequency This property depends on the porous electrode structure and the limited electrolyte's ion mobility Switching regulators with frequencies over Hz, if used, should not be directly connected to supercapacitors

7 Supercapacitors for Pulsed Power A suitable decoupling filter is required between Supercapacitors modules and switching regulators Supercapacitors LC Filter (from S.Basu and T.M. Undeland,, EPE 2009)

8 Supercapacitor DC Energy Storage Supercapacitors s: String of many cells in series integrated with over voltage protection Single cell V C ESR ac ESR dc Irms 2,70 V 3000F 0,2 mω 0,3 mω >150 A Supercapacitor : 32 cells in series : C = 94 F, Vnom = 75 V, ESR dc = 13 mω Non-repetitive peak current = 1,9 ka

9 Supercapacitor DC Energy Storage s can be stacked in series and in parallel But the max voltage is still limited... Possible scenario for 1 MW - 60 s (60 MJ) configuration in a standard 20 container with 120 BMOD0130 modules. Maxwell BMOD130 Capacitance Voltage ESR DC Leakage Current Total Energy 130 F 56 V 8.1 mohm 120 ma 56.6 Wh (0,204 MJ) Configuration 10 series x 50 parallel Total Number of s 500 Capacitance 650 F Voltage 560 V Floor Space 106 ft

10 Supercapacitor DC Energy Storage MODULE SKELETON 168V75F C: 75 F Vnom: 168 V ESRdc: 7 mω Non-repetitive peak current = 3.3 ka Weight: see below... Total Energy 294 Wh (1.06 MJ) (Low ESR) unit Natural air Forced air Liquid cooled cooled cooled (200 m 3 /h) (3 l/min) Dimensions LxWxH mm 800 x 450 x x 450 x x 450 x 170 Weight kg Therma Resistance C/W 0,16 0,08 0,012 Thermal Capacitance kj/ C Thermal Time constant min

11 Supercapacitor s for Pulsed Power 336 s: 4 s, 84 p Very High Current Modular Pulse Generator Each string has its own decoupling inductor Pulse Source without current control: each module string has its own switch

12 12 Example 1: Very High Current Pulse Generator Ipeak = 250 ka for 350 ms Duty Cycle: < 0,2 % Starting voltage: 630 V Load: 600 µh, 1 mω 336 modules (4s, 84p) Total Energy Stored 312 MJ

13 Pulsed Power s with current feedback Each string has its own decoupling inductor 368 s: 4 s, 92 p Multistage Chopper Regulator But the filter capacitor is preferably common to all modules

14 14 Example 2: Medium Current application (1 of 2) DC filter and supercapacitor bank share the same source (120V) I = 2 ka for 1 s Duty Cycle: 1s / 600 s (< 0,2 %) Starting voltage = 120 V Load: 600 µh, 10 mω 2 modules (1s, 2p)

15 15 Example 2: Medium Current application (2 of 2) DC filter and supercapacitor bank share the same source(120 V dc) Start of pulse Requested rise and fall times 10 ms End of pulse

16 16 Example 3: High Current application (25 ka) DC filter and supercapacitor bank at ~ 400 V starting voltage for 25 ka pulse (96 modules: 4 s, 24 p) Load Coils: 630 µh, 4 m Ω s: 125 V, 63 F

17 17 Example 3: High Current application (25 ka) DC filter and supercapacitor bank at ~ 200 V starting voltage for 25 ka pulse (36 modules: 2 s, 18 p) Load Coils: 630 µh, 4 m Ω s: 125 V, 63 F (11 MJ)

18 Example 4: 100 ka Current for Toroidal Coil Flat top at 100 ka with chopper regulation Chopper frequency must be filtered (368 s: 4 s x 92 p) (400 V starting voltage) Chopper regulation at 100 ka flat top

19 19 Some points about SC modules application LIFETIME RULE OF THUMB: Ref.: 1,000,000 Tcell = 25 and Vcell = Vnom (e.g.: 2,75 V) Every - 0,20 V doubles lifecycles Every + 10 halves lifecycles Tcell = Tamb + ΔT, with ΔT = I 2 x ESR x Rth AT THE END OF LIFE: reduction of the capacity - end of life: - 20 % increase of internal resistance (ESR) - end of life: + 100% - capability to handle high currents - heating (cooling) - power losses (efficency)

20 20 Conclusions about use of SC in pulsed power Supercapacitor promise to be a good alternative for energy storage up to tens and perhaps hundred of MJ Use as source voltages up to V is already viable. Voltages > 1000 V require insulating structures designed on purpose. At the very low repetition rate of tokamak pulses, a single module can achieve peak currents of 1 ka or more, so very high output currents (tens of ka) are possible with multi modular structures in parallel.

21 21 Conclusions about use of SC in pulsed power But attention shall be paid to the following points: Warm up during pulses should be controlled. Supercap modules have high thermal resistance to ambient, and high thermal capacity. So the thermal time constant may be very long (tens of minutes). Air cooling might not be sufficent to avoid temperature build-up between pulses. Since temperature affects module life, this aspect shall be accurately evaluated. SAFETY: Voltage may be present across terminals tens of minutes after power supply has been switched off. Since capacitances are high, even at low voltages the energy stored in modules is also remarkably high.