Sverre Gilje of BP Norge AS and Lars Carlsson of ABB explain how HVDC Light transmission will deliver 78 MW of power to the Valhall North Sea oil and gas field, 300 km offshore.
BP has designated the redevelopment of its Valhall North Sea oil and gas field as a ‘flagship for the field of the future’, placing it at the forefront of the implementation of advanced technologies. A key innovation will be the first use of high voltage direct current (HVDC) to supply an entire offshore AC system. So the offshore gas turbines will be decommissioned, and all the field’s 78 MW of power will be delivered over a distance of 300 km from the Norwegian coast using ABB’s HVDC Light technology.
The existing Valhall complex consists of five bridge-linked platforms. Three wellhead platforms have also been installed some 6 km from the main complex. The production and compression platform and the living quarters platform will be replaced with a new facility, subject to final partner and authority approvals.
This new facility, together with the entire Valhall field, will be supplied with electric power from shore (PFS) by ABB’s HVDC Light HVDC transmission system that uses the latest development in power electronics and computerized control and protection systems. The HVDC Light system will include onshore and offshore converter stations connected by a 292 km cable. The system will convert AC power from Elkem’s 300 kV substation at Lista to DC power at 150 kV, transmit it through the subsea DC cable and convert it back to AC at 11 kV at the new platform.
Figure 1. BP has designated the redevelopment of its Valhall North Sea oil and gas field as a ‘flagship for the field of the future’, placing it at the forefront of advanced technology implementation
Applications to feed power to or from offshore installations, using HVDC, have been discussed for many years. However, due to the nature of conventional HVDC, which requires a certain strength of AC system to operate, this has not been feasible. The situation changed around ten years ago, when ABB developed its HVDC Light technology based on transistors, rather than the thyristors used by conventional HVDC. This makes the new converters self commutated, i.e., they do not require an existing AC voltage to operate but can feed into a completely passive load.
The first offshore application of HVDC Light was commissioned in 2005 to feed the new compressors on the Troll A platform outside Bergen, Norway. The Valhall PFS is different from Troll in one very important aspect. This is the first time a complete offshore power system for an entire field will be fed with electric power from the mainland using HVDC.
Lower costs and emissions
PFS is cost efficient and saves space and weight on the platform. On most offshore installations, power supply generators and large compressors are driven by onboard gas turbines or diesel engines. Many of these have total efficiencies as low as 20-25 per cent. The result is emissions of large amounts of CO2 and unnecessarily high fuel consumption.
On the Norwegian shelf, CO2 taxation is already in effect, making emissions costly even without emissions trading. If electrical power can be supplied from shore – for power supply as well as compressor drivers – CO2 emissions from offshore installations are eliminated. This leads to a cost saving for the operators. In addition, transmission of electrical energy from shore involves less maintenance, longer lifetime and higher availability than gas turbines and diesel engines. If the transmission equipment can be located on decommissioned installations offshore, the postponed removal cost for the installation can also be an important factor. In addition, considerable amounts of greenhouse gas emissions can be saved by using efficient onshore generation instead of low-efficiency offshore power stations. Even with up to ten per cent power losses over a long transmission distance to an offshore installation, the savings will be significant for most cases.
It is estimated that for the Valhall project, annual emissions of some 300 000 tonnes of CO2 and 250 tonnes of NOx will be prevented by the switch to PFS, compared with a combined cycle power plant with low NOx gas turbines.
With HVDC Light, the use of series-connected power transistors allows the connection of voltage-source converters to networks at voltage levels previously beyond reach. This can be used for power transmission, for reactive power compensation and for harmonic/flicker compensation. With fast ‘vector control’, this converter offers the ability to control active and reactive power independently while imposing low levels of harmonics, even in weak grids. The powerful and robust HVDC control, Mach 2, proven in multiple HVDC and SVC installations to date, governs the converters. In HVDC Light, pulse width modulation (PWM) is used for generation of the fundamental voltage.
Using PWM, the magnitude and phase of the voltage can be controlled freely and almost instantaneously within certain limits. This allows independent and very fast control of active and reactive power flows. The PWM VSC (Voltage Source Converter) is therefore a close to ideal component in the transmission network. From a system point of view, it acts as a zero-inertia motor or generator that can control active and reactive power almost instantaneously. Furthermore, it gives only a limited contribution to the short-circuit power, as the AC current can be controlled.
The only quantity that needs to be detected in both ends of the transmission is the DC link voltage, eliminating the need for communication between the rectifier control on land and the inverter control on the platform.
Figure 2. The entire Valhall field will be supplied with electric power from shore by ABB’s HVDC Light transmission system
The HVDC Light converter design for Valhall is based on the two-level bridge but with the midpoint of the capacitor floating. The switching of the bridge between 0 kV and -150 kV makes optimal use of the coaxial HVDC cable design with the centre conductor at high voltage and the return conductor close to the grounded screen. The design philosophy enables both steady state and dynamic operation, with extremely low levels of induced ground currents. This feature is one of the critical factors for implementing an HVDC system in an offshore environment.
Operation with fixed 60 Hz frequency in the offshore end and fixed 50 Hz grid frequency in the onshore end does not require main circuit equipment that differs from the normal design. The design principles adopted for normal transmission system applications can also be used to feed a local offshore AC network such as the Valhall complex. Some of the more important benefits with an HVDC transmission feeding platform are:
- Control of AC voltage and frequency
- Direct on-line start of large asynchronous machines
- Ride through of mainland AC system disturbances.
Space and weight are scarce resources on offshore installations. In light of these constraints, the compact HVDC Light filters offer important advantages. The offshore environment also places a number of other demands on the converter station and equipment including:
- Safety for personnel in a production and processing environment,
- Reliability and availability is of utmost importance since a shutdown means shutting down of the whole production at Valhall,
- The offshore environment is very tough with salt and humid air which imposes severe requirements on the choice of materials and surface treatment,
- Integration of the control system towards the process control and shut down systems on the platform.
The high voltage equipment will be installed inside a module offshore and in a building onshore. The ventilation system in the module/building will be designed to protect the high voltage equipment and the electronics from salt and humid air. The main circuit equipment is therefore exposed to lower environmental requirements than a normal outdoor installation, which allows for a more compact design.
The ventilation also has to take care of the airborne losses. An advantage of being offshore in the North Sea is that cold (5-11à‚°C) water for cooling is readily available. Another requirement on the ventilation system comes from possible presence of gas in the area. The installation offshore will be over pressurized to ensure that no gas can enter high voltage areas. If gas is detected, the system will be tripped and de-energized directly.
The HVDC module will be built in two storeys with the AC filters and phase reactors on the top floor and the converter valves and the DC equipment below. This is also where the 150 kV HVDC cable will terminate. The converter transformers will be located in a separate room with the bushings penetrating through the walls to the phase reactors and to the 11 kV AC side respectively. AC filters are located on both sides of the transformer to ensure that harmonics to the platform AC system are kept to a minimum.
The onshore converter station will be located at Lista and connected to the Norwegian 300 kV power grid through a double circuit line to Feda. The principal layout for the converter at Lista is essentially the same as on the Valhall platform, but with two major exceptions.
The AC voltage at Valhall is 11 kV while it is 300 kV at Lista. The AC switchyard will therefore be built as a conventional outdoor installation with one AC breaker but with two disconnectors to make it possible to connect Valhall to either of the two buses in the existing switchyard. The other major difference is the cooling system for the valves, which will feature dry air-cooling towers.
Adapting the control system for the platform process control introduces another dimension in reliability compared to normal transmission systems. In an area exposed to gas, safe and guaranteed tripping is more important than continuous operation. The control system will therefore be complemented with circuits for external tripping from emergency shut down systems.
The inverter control software will be adapted to perform voltage and frequency control – while the control hardware will be identical for rectifier and inverter converters. Protection and monitoring of the converter is also included in the same controller.
One of the important design issues has been the harmonics, ranging from low order harmonics up to very high frequency. The desire to have low weight on an offshore platform gives the optimum solution that the platform should not be designed for excessive harmonics. The design philosophy is therefore to install filters with the same filtering capacity that is normally required in a high voltage installation and not allow for large harmonics.
The harmonic generation from a HVDC Light converter is primarily at the switching frequency (1620 Hz) and above while there is almost no generation of harmonics in the low frequency range (5, 7, 11 and 13th harmonic), which is common in a classic HVDC converter.