HVDC Power Buoy Concept
The HVDC Power Buoy, shown in Figure 1, affords a oneatmosphere stable environment located just below the waveline and local to the subsea completion it will be servicing. This enables local placement of energy conversion equipment (VFDs, etc.) without the use of massive subsea one-atmosphere enclosures.
HVDC Conceptual Power Path
The HVDC Power Buoy bridges the gap between subsea power transmission distances accommodated via high voltage AC (HVAC) and those presently addressed via HVDC transmission networks. The HVDC Conceptual Power Path (Reference Figure 2) for providing Medium Voltage AC (MVAC) power to the subsea field test case comprises four (4) main components: 1. Host Rectifier 2. HVDC Power Transmission Cable System with Cable Guillotine 3. HVDC Power Buoy 4. Local Power Distribution Cables with Re-usable Weaklinks
Host Rectifier
Four-quadrant rectifier operation is not required as there will only be consumer loads associated with this system. This affords a simple passive diode front-end easily configured to achieve a harmonic friendly multi-pulse IEEE-519 compliant input. However application of Silicon Controlled Rectifier Thyristers (SCRs) in lieu of diodes will afford an active front-end which will accommodate the following (Reference Figure 3): Ó Current Limit Function (limit of fault currents) Ó Power Limit Function (interface with vessel PMS) Ó DC Pre-Charge Function (soft-start capacitor precharging) Feeding the multi-pulse rectifier is a multi-winding rectifier duty transformer. To mitigate the need for inter-phase transformers on the rectifier side, series connection of the rectifier is envisioned. Output DC contactors isolate the transmission line from the rectifier during service.
DC Transmission Cable
For the conceptual power level (10.5 MW = 15MVA @ 0.7PF), the DC Transmission Cable is envisioned to be a 185 mm2 armored coaxial cable rated 75 kVDC. Preliminary calculations determine the 150 km, 185 mm2, 75kVDC coaxial power cable to be transmitting, 10.5 MW at 150 Adc, dropping 5.76% (Vin = 75000 Vdc, Vout = 70678 Vdc)
voltage from input to output with approximately 4.5 W/m full load transmission line losses (Reference Figure 4).
DC Cable Guillotine
A mandate for Arctic and sub-Arctic environments requires iceberg mitigation strategies for both topside and subsea equipment. A Cable Guillotine is envisioned to serve as the point of HVDC disconnection during an ice management event. This will enable disconnecting the 75 kVDC Cable from the Power Buoy without the use of HVDC Wet-mateable Connectors as the connectors would need substantial development and qualification programs. One such shearing method is described by Williams [2] in US Patent 6,397,948 B1 with the primary mechanism detailed in Figure 5 below.
HVDC Power Buoy
The HVDC Power Buoy will enclose a High Voltage Inverter System feeding a medium voltage distribution network to service the local SAPT equipment. The HVDC Power Buoy is envisioned to comprise, but is not limited to, the following: Ó Power Buoy Hull with Mooring System Ó HVDC Inverter Ó L-C-L Sinusoidal Filter / Step-down Transformer Network Ó Medium Voltage Distribution Switchgear Ó Medium Voltage DC Rectifiers with Load Share Reactors Ó Medium Voltage Inverters with Sinusoidal Output Filters Ó General Services Switchboard Ó Emergency Switchboard Ó UPS System Power Buoy Hull and Mooring System The HVDC Power Buoy is a taut moored facility, consisting of a floating buoyant structure moored by multiple tethers to a gravity base on the seabed. The floating buoyant structure is a series of stiffened carbon steel shell tubular sections of varying diameters. These sections form a column or spar, narrow at the surface, with a wide Buoyant Equipment Chamber (BEC) below the surface. The BEC is the main equipment space and provides a climate controlled oneatmosphere environment for the contingency of power conversion equipment and switchgear. It also supports physical connections to the tether and umbilical systems (Reference Figure 1 above). HVDC Inverter The HVDC Inverter (Reference Figure 6) is the “weakest
Petroleum Today - June
2017
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