Neptune BU Switching

1. Neptune BU Switching Circuit diagram and operations

2. Modes of Operation • Normal • Under fault • Fault Locating • Restoration • Default mode of the BU switch: • Restoration mode for negative voltage • Fault locating mode in the positive voltage

3. Normal Mode

4. NO Latch Latch NO NC • S3 is open • Circuit is energized from either end • Voltage across Z1 or Z2 is the power supply for the controller • S5 or S6 are close, and S1 or S2 are also closed.

5. Under Fault Mode

6. Options • Option 1: Do nothing • Fault causes a drop in the voltage of several science nodes. • Science nodes send signals to shore stations. • Shore stations shut down the system. • Power interruption sets the BU controller to the default restoration mode. • System goes into fault locating mode. • Option 2: BU clears the fault by opening S1 and S2

7. Option 1 • Merits: • Simplifies the BU hardware • No need for pre-insertion resistance to limit the fault current. • No need to worry about restrikes • No need for additional control actions. • Demerits: • Entire system is shut down • Zeners must withstand the maximum possible fault current for extended time. • The duration of fault depends on • The speed of communications between nodes and shore stations. • The shut down speed of the shore station

8. Option 2 • Merits: • Faults can be cleared faster than Option 1 • Only a portion of the system is disabled • Demerits: • Pre-insertion resistance, and restrike prevention are needed inside the BU. • After several BUs are open, it is difficult to locate the fault without communications, and the system will have to shut down anyway.

9. Fault Locating Mode

10. Shore station voltage is positive to isolate all loads (is +voltage really necessary?) • S4 is closed • Circuit could be energized from either end • Voltage across Z1 or Z2 is the power supply for the controller • S5 or S6 will close S1 or S2 • Voltage across Z3 opens S4 after a delay • S1 and S2 remain closed regardless of the magnitude of the current.

11. Fault locating mode: Issues • Do we really need a positive voltage? • If the negative voltage is kept below 5kV, all science node are disconnected • If we don’t use the positive voltage circuit, we can reduce the BU components • No need for back-to-back zeners except for Z3 • No need for the extra diodes and dummy loads • One advantage of energizing the system with positive voltage is that the local BU intelligence will be able to differentiate between fault locating mode and restoration mode (depending on the direction of current). • Could we use two different voltage levels for fault locating and restoration modes?

12. Restoration Mode

13. Sequence • The voltage of the shore station is negative, and below 5kV to isolate all science nodes • BUs are closed in sequential manner • Delay and/or current measurements are needed • Any coordination?

14. If Fault is not present • The BUs remain closed • Shore station voltage increased gradually to the rated level. • Science nodes become active

15. Closing on Fault • Objective: open the breaker and keep it open until the system shuts down and restarts. • Process: open S1 and S2. S3 will close • controller memory keeps the breaker of the BU open until the system is shut down. • Shutting down the system will erase the memory, and the breaker goes into the default mode. • The system can operate with one section open

16. Problem 1: • Dummy load of the BU will be energized until the fault is fixed and the breaker is closed. • The dummy load can consume substantial power if kept on for long period • Can we put a relay circuit in series with the dummy load? • The power supply for the BU will be lost. So what?

17. Problem 2: • After S1 and S2 are opened, and before S3 is fully closed, there is no current in the zeners and the power supply for the controller is lost. • The controller sets the breaker to the default mode, and the breaker will attempt to close. This is an oscillatory process. • We may need small energy storage device for the controller • Keep S3 closed for longer period during restoration.

18. Delays and synchronization • Delay1: In sequential closing of BUs • how long should S3 remain closed? • How many subsequent BUs should be energized before S3 is opened? • Delay2: When S5 and S6 close, S1 and S2 close. There should be a delay after the power supply becomes available and before S5 and S6 are closed.

19. Delays and synchronization • Reasons for Delay2: Assume two BUs • We should close BU2 only when we know that the previous cable section has been completely energized and has achieved steady-state. • If BU1 sees no fault after completion of its own delay, it will be latched in the close position. • If BU2 sees fault during its own delay interval, it will open. However, BU1 will remain closed.

20. Closing on Fault: Issues • How can we differentiate between a fault current and cable energization current? • Closing the BUs should be sequential. • How to coordinate between neighboring BUs while closing on fault. • Can a simple time delay do it? • How shore stations know if we close on fault? (science nodes are isolated!)

21. Complete Circuit diagram

23. General Design Issues

24. Other Important considerations • Response time of shore station is crucial in ‘Option 1’ of the fault mode operation. • How to set the fault limit for shore station? • Zeners can withstand 500A peak transient current and 10A steady state current. Will they survive for extended fault time (until shore station respond)?

25. Issue for Normal operation • Current reversal in the steady state • Temporary zero current • Control circuit is rest to the default restoration mode. • Energy storage?

26. Conclusion • The BU design is viable. • Components need to withstand fault currents for extended time. • Lack of communications between the BUs may require the reliance on delay circuits for the sequential switching of the system. • Threshold for fault current needs to be carefully determined. • Positive and negative energization may need to be rethought.

27. Lab Demo

28. Circuit diagram

29. Demo steps • Energize the model from its left side with negative source • (1) Close Breaker 1+ and Breaker 2+. Bulb 1+ and Bulb 2+ won’t be on, showing that they are isolated from the power source. • (2) Close either Breaker 2 or Breaker 3 or both of them, Bulb 2 and Bulb 3 won’t be on, showing that BU1 is open and no power can reach BU2. • (3) Close Breaker 1, Bulb 1 and LED1 will be on. Now measure the voltage across S1 and S2, it will be 0, showing one of them is closed. • (4)Close Breaker 2, Bulb 2, LED 2 and LED 4 will be on, showing there is current in the circuit between BU1 and BU2. Measure the voltage across S4 and S5, it will be 0, showing one of them is closed.

30. Demo steps • (Continued) • (5) Open Breaker 1, Bulb 1 is off (of course). Except for Bulb 1, no other things change their state in the circuit, showing that once BU2 is energized, the dummy load of BU 1 (Bulb 1) can be drop off. It is no use any more. • (6) Close Breaker 3, Bulb 3 and LED 5 will be on. • (7) Open Breaker 2, Bulb 2 is off (of course). Except for Bulb 2, no other things change their state in the circuit, showing that once a load is connected to the backbone, all the dummy loads of BUs (Bulb 1 and 2) can be drop off. They are no use any more. • (8) Open Breaker 3, Bulb 3 and all the LEDs are off, showing that no current will be in the circuit if no any load is connected to the backbone.

31. Demo steps • Energize the model from its left side with positive source • (1) Close Breaker 1 and Breaker 2. Bulb 1 and Bulb 2 won’t be on, showing that they are isolated from the power source. • (2)The following operations can be the same with those in the negative energized situation. • The circuit can also be energized from the right side. The phenomenon will be the same since it is symmetrical.