Neptune BU Switching

Circuit diagram and operations

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

Normal Mode

NO

NO

NC

LatchLatch

1. S3 is open2. Circuit is energized from either end3. Voltage across Z1 or Z2 is the power

supply for the controller4. S5 or S6 are close, and S1 or S2 are

also closed.

Under Fault Mode

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

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

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 locat

e the fault without communications, and the system will have to shut down anyway.

Fault Locating Mode

1. Shore station voltage is positive to isolate all loads (is +voltage really necessary?)

2. S4 is closed3. Circuit could be energized from either end4. Voltage across Z1 or Z2 is the power supply

for the controller5. S5 or S6 will close S1 or S26. Voltage across Z3 opens S4 after a delay 7. S1 and S2 remain closed regardless of the

magnitude of the current.

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?

Restoration Mode

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 need

ed Any coordination?

If Fault is not present

The BUs remain closedShore station voltage increased gradually t

o the rated level.Science nodes become active

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

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?

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.

Delays and synchronization

o Delay1: In sequential closing of BUso how long should S3 remain closed? o How many subsequent BUs should be

energized before S3 is opened?

o 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.

Delays and synchronization

Reasons for Delay2: Assume two BUso We should close BU2 only when we know that the

previous cable section has been completely energized and has achieved steady-state.

o If BU1 sees no fault after completion of its own delay, it will be latched in the close position.

o If BU2 sees fault during its own delay interval, it will open. However, BU1 will remain closed.

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!)

Complete Circuit diagram

General Design Issues

Other Important considerations

o Response time of shore station is crucial in ‘Option 1’ of the fault mode operation.

o How to set the fault limit for shore station?

o Zeners can withstand 500A peak transient current and 10A steady state current. Will they survive for extended fault time (until shore station respond)?

Issue for Normal operation

Current reversal in the steady state Temporary zero current Control circuit is rest to the default restoration

mode. Energy storage?

Conclusion

The BU design is viable. Components need to withstand fault currents for

extended time. Lack of communications between the BUs may r

equire 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.

Lab Demo

Circuit diagram

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.

Demo steps

(Continued) (5) Open Breaker 1, Bulb 1 is off (of course). Except for Bulb 1, n

o 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, n

o 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.

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.