North East Pacific Time-series Underwater Networked Experiment (NEPTUNE): Power System Design, Modeling and Analysis Aditya Upadhye

Outline NEPTUNE Power system requirements Two design alternatives Version 1 Version 2 Cable analysis Models Simulation results Conclusions and future work

NEPTUNE

Science requirements Communication bandwidth - Gb/s Power – 200kW Reliability Robustness of design Thirty year lifetime Maintenance and support

Power System Design Basic tradeoffs Frequency: ac versus dc Network: radial versus interconnected Loads: series versus parallel Shore station supply at 10kV, 200kW Max. current-carrying capacity = 10A User voltage = 400V / 48V Max. power at each node = 10kW

Power System Design Protection Monitoring and control Sectionalizing circuit breaker Breaker control Monitoring and control Current – voltage measurements State estimation Shore station control hardware / software

Power System Design: Version 1

Version 1 Circuit

DC Circuit Breaker Need Required features During initial energization For fault isolation Required features To force a current zero and minimize arcing To prevent breaker restrikes

DC Circuit Breaker Open Circuit R1 R2 S1 S2 S3 S4 C

DC Circuit Breaker Soft Closing R1 R2 S1 S2 S3 C S4

DC Circuit Breaker Closed circuit S2 S3 R1 R2 S1 S4 C

DC Circuit Breaker Capacitor charging S2 R1 R2 S1 S4 C S3

DC Circuit Breaker Capacitor discharging R1 R2 S1 S2 S3 S4 C

DC Circuit Breaker Hardware prototype 125V, 5A breaker circuit Breaker control MOSFETs drive the switch solenoids Opto-isolator between logic circuit and driver circuit Control logic has a counter, which continuously cycles through the breaker operations

DC Circuit Breaker Hardware prototype test results Continuous Voltage: 125V Continuous Current: 4.5A Total Breaker Cycles: 125,000 Normal cycle switching frequency: 20Hz Maximum cycle switching frequency: 100Hz Maximum tested voltage: 200V Maximum tested current: 5A

Power System Design: Version 2

Version 2 Circuit

Branching Unit

Series Power Supply Indigenous power supply for each BU Less reliance on node converter Use of zener diodes in reverse region Back-to-back zener diodes

Modes of Operation Normal Fault Fault-locating Restoration Special case System startup

Normal Mode Objective: System voltage at 10kV Science loads at all nodes are provided power System voltage at 10kV BU backbone switches are closed BU dummy load switches are open No fault within the network PMACS (Protection, Monitoring and Control System) performs various measurements and estimates the system state

Fault Mode Objective: To cause system shutdown in the event of a fault and protect system components Due to the reactive nature of the cable, any fault will cause large transients with large values of di/dt and dv/dt BU controller does not respond to faults The response to fault is at system level by shore station controls, which is a complete system shutdown

Fault-locating Mode Objective: Initial state: To locate the fault without communications between the BUs or the BUs and the shore Initial state: All BU backbone switches are open All BU dummy load switches are closed System is energized at 3kV level BUs are energized sequentially When a BU backbone switch closes on a fault, it remains closed

Fault-locating Mode BU dummy load switch opens after a certain delay following BU energization Final State: All BU backbone switches are closed All BU dummy load switches are open Current in network is fault current PMACS performs calculations for locating the fault System is shut down

Restoration Mode Objective: Initial state: To energize the system and isolate the faulted cable section autonomously Initial state: All BU backbone switches are open All BU dummy load switches are closed System is energized at 5kV level BUs are energized sequentially

Restoration Mode When a BU backbone switch closes on a fault, BU opens the backbone switch to isolate the fault Final State: All BU backbone switches are closed All BU dummy load switches are open The faulted cable section is isolated The system voltage is raised to 10kV and the system re-enters normal mode

Comparison of Version 1 and Version 2

Version 1 Version 2 Conventional approach to power system design Based on the philosophy that cable faults are rare but possible Response to a fault is at the system level by the shore station controls Response to a fault is at the local level by the nearest circuit breaker Circuit breaker is complicated with many components Complexity of circuit breaker is greatly reduced Fault current is interrupted; arcing and restrikes are possible Fault current is not interrupted; arcing and restrikes are not possible Single node failure can cause failure in a large section of the network Single node failure is not catastrophic for the system as that node only will be out of service Reliability is low Reliability is increased

Electromagnetic Transients Program (EMTP)

Alternate Transients Program

ATP Theory ATP is a universal program system for digital simulation of transient phenomena of electromagnetic as well as electromechanical nature With this digital program, complex networks and control systems of arbitrary structure can be simulated Trapezoidal rule of integration

Cable Parameters

ALCATEL OALC4 Cable

Inductance Calculations The generalized formulae were applied to the OALC4 cable The core (steel) current caused flux linkages within a) the core b) the sheath c) the insulation The sheath (copper) current caused magnetic flux linkages within: a) the sheath b) the insulation

Inductance Calculations Where T is the total flux linkage associated with the conductor, i is the flux linkage internal to the conductor, and e is the flux linkage external to the conductor Where icable is the total current in the cable

Resistance Calculations The resistance per unit length of a tubular conductor is given by: The total cable resistance is given by:

Capacitance Calculations The cable capacitance per unit length can be calculated by the formula: F/m Where,  is the permittivity of the insulator. d is the outer radius of insulator c is the inner radius of insulator.

ATP Cable Modeling In ATP the cable was modeled using the concept of composite conductor. The steel core and copper sheath were treated as one composite conductor with the following properties: comp = 5.1753*10-8 m. comp = 9.0788

Results

Simulation Models

Version 1: Opening of Circuit Breaker t = topen Switch open: initial arcing t =( topen +t) Capacitor charging t = (topen-t) Switch closed

Simulation of Restrikes topen Vmax RESTRIKE!!! Initial Arcing Period

Restrikes: Simulation Circuit

Capacitor Current Restrike No Restrike

Capacitor Voltage Restrike No Restrike

Simulation Results

Current Limiting Operation The shore station power supplies are rated at 200kW, 10kV The steady-state system current = 10A Under certain conditions, the system current may increase due to Cable faults Topology changes Load fluctuations

Current Limiting Operation The system current is limited to a value below 10A using the control circuitry in the shore station This is done by dropping the shore voltage which in turn reduces the current The control action is initiated only for steady-state overcurrents and not transient overcurrents.

Fault Analysis

Version1: Simulation Circuit

Results of Current Limiting: Shore Output voltage and Current

Voltage and Current at Node 2: No Current Limiting

Capacitor Current of Node 2

Version 2: Fault Studies A pre-insertion resistance may be placed at the shore station to limit the fault current This resistance will limit the fault current before the shore controls take the appropriate mode-dependant control action Three controllable parameters in simulations: Value of pre-insertion resistance Response time of control circuitry Distance of fault from the shore station

Simulation Circuit X=100km/1200km

Results: Vary Response Time

Results: Vary Fault Distance

Conclusions A sub sea observatory NEPTUNE is the first of its kind and will open up new and exciting areas of scientific research The NEPTUNE power system implements a ‘dc network’ Version 1 dc breaker is designed and a hardware prototype was built in lab Version 2, the preferred design choice is philosophically different from conventional terrestrial power systems Transient studies of the system is performed using EMTP for worst-case scenarios from the point of view of component design and fault analysis Theoretical analysis of the cable was performed and EMTP models were developed for the above

Future Work DC breaker prototype for Version 2 Control and monitoring systems for the above using microcontroller and/or array logic A comprehensive transient model for the entire NEPTUNE network which is generic enough to simulate any fault type and any operating scenario