ZnO-Arresters for Overvoltage Protection in Pulsed Power Circuits

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Roland Tiedeman
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1 European Electromagnetic Launch Society 12th Topical Meeting 10th to 12th September 2001, Ayr ZnO-Arresters for Overvoltage Protection in Pulsed Power Circuits Ulrich Braunsberger Institut für Hochspannungstechnik und Elektrische Energieanlagen Technische Universität Braunschweig Pic 1

2 ZnO-Arresters for Overvoltage Protection in Pulsed Power Circuits 1. Introduction 2. Properties of ZnO-varistors 3. Design criteria for ZnO-arresters 4. Examples of ZnO-Arresters 4.1 Arrester for 30 kv, 600 kj 4.2 Arrester for 5 kv, 8 MJ 5. Summary Pic 2

3 1. Introduction (1) Capacitor voltage Switch ts Inductance Current Load resistance Capacitor variable Load Load voltage Overvoltage generation in a pulsed power circuit ts t Pic 3

4 1. Introduction (2) Capacitor voltage Switch Inductance Load Current ts Load resistance Capacitor Voltage-Arrester Arrester voltage Overvoltage protection in a pulsed power circuit Arrester current ts t Pic 4

5 2. Properties of ZnO-varistors (1) 21 mm 76 mm ZnO-varistor-disk, U(10 ka) = 6 kv Pic 5

6 2. Properties of ZnO-varistors (2) 15 I 10 5 Voltage Current Double-diode-characteristic of the ZnO-varistor Pic 6

7 2. Properties of ZnO-varistors (3) Current-voltage-characteristic of one varistor-disk, U(10 ka) = 6 kv Pic 7

8 2. Properties of ZnO-varistors (4) High 21 mm Diameter 76 mm Disk volume 95 cm³ Specific mass 5.5 g/cm³ Thermal capacity 2.75 J/(cm³ * K) Energy for 1 K temperature rise 261 J Nominal temperature rise 170 K Energy absorption 44.3 kj Physical properties of one varistor-disk, U(10 ka) = 6 kv Pic 8

9 3. Design criteria for ZnO-arresters (1) Voltage-limiting-level - determines the number (ns) of varistors to be connected in series Maximum current - determines the number (np) of varistors or varistor columns to be connected in parallel Pic 9

10 3. Design criteria for ZnO-arresters (2) Energy to be absorbed in one pulse - determines the total number of varistors needed for the Arrester (nw) The number of disks necessary for an arrester is n(disk) = MAX (( ns*np), nw ) Pic 10

11 4. Examples of ZnO-arresters (1) Varistor disk Contact plates Contact disk-spring Basic elements of a ZnO-arrester Pic 11

12 4. Examples of ZnO-arrestors (2) Varistor disks Contact plates Contact pressure screw spring Assembled varistor column Parts of a varistor column Pic 12

13 4.1. Arrester for 30 kv, 600 kj (1) Screw spring for contact pressure Cast resin insulator ZnO-varistor Contact plates U-steel frame Voltage arrester: Umax = 30 kv, Wmax = 600 kj Pic 13

14 4.1. Arrester for 30 kv, 600 kj (2) 300 mm 440 mm Voltage arrester: Umax = 30 kv, Wmax = 600 kj Pic 14

15 4.2. Arrester for 5 kv, 8 MJ (1) Variation of current and absorbed energy in one varistordisk during a nominal fault in an arrester-unit of 200 disks Pic 15

16 4.2. Arrester for 5 kv, 8 MJ (2) 1 Screw spring 2 Varistor disk 3 Insulating frame bar 4 U-steel frame 5 Connection plate Sub-unit frame with 50 varistor disks Pic 16

17 4.2. Arrester for 5 kv, 8 MJ (3) 1 Welded angle bar frame 2 Insulating angle bar supporter 3 Arrester sub-unit (front view) 4 Cast resin insulator 5 Current monitoring transformer 6 Amplifier box Front view of a complete arrester-unit Pic 17

18 4.2. Arrester for 5 kv, 8 MJ (4) Measured current and voltage waveforms during a test of an arresterunit with 50 ZnOvaristors in parallel Pic 18

19 4.2. Arrester for 5 kv, 8 MJ (5) Simulation results: Current and voltage waveform of an arrester-unit with 50 parallel ZnO-varistors Pic 19

20 4.2. Arrester for 5 kv, 8 MJ (6) Photograph of the 8 MJ-Arresters Pic 20

21 5. Summary - Necessity for overvoltage-arresters in pulsed power circuits was discussed - ZnO-varistors are inherent save elements - Properties of one ZnO-varistor (5 kv, 40 kj) were treated - Arresters can be matched to all conditions by series and parallel connection of varistors - Two examples of realised arresters were described: 30 kv, 600 kj and 5 kv, 8 MJ Pic 21

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