SFC for Power from Shore application
In this post we will describe a specific type of SFC grid intertie: Power from Shore (PfS) application. First we explain the background and motivation for the electrification of offshore platforms. Following the introduction a short description of a typical power from shore system is provided. Next section discusses specific requirements on the SFC and challenges associated with this type of application. The post is rounded with a summary and remarks on future potential.
What is power from shore?
Power from shore is an electrification system that brings electric power from the shore to oil and gas platforms located offshore. The power transmission is realized with high voltage subsea cable over a distance of up to 200 km or even more. The nature of such power system is quite different than a conventional grid. it is associated with several challenges. However, engineers like challenges and can develop appropriate solutions for power from shore applications.
Why to bring power from shore?
Many of the offshore platforms operate since decades with their own local power generation (off grid). So why should the concept suddently change? It is a significant investment and the realization is technically challenging. However, the key driver is the reduction of carbon footprint in upstream oil & gas extraction and processing. Traditionally, most of the platforms use a local power generation with gas turbine located directly on the platform. These turbines have relatively low efficiency and produce considerable amount of emissions.
Some critics may call power from shore a greenwashing. Well, everyone has the right for own judgement. In our view the application can have a net positive impact. Especially in a country like Norway which has abundant water resources and surplus of renewable hydroelectric power. Not talking about the potential of offshore wind power generation.
When is SFC needed?
The microgrid on the platform may have different frequency than the one of the power grid. Norway is a 50 Hz country but many platforms use 60 Hz. That has historical background. The reasons for 60 Hz might be:
- The equipment originated from US manufacturers and thus was designed for 60 Hz.
- Equipment designed for 60 Hz has slightly smaller size and weight compared to 50 Hz thus an advantage for offshore installation.
As long as the platform was an island on its own including the power generation the choice of power frequency had to relevance. However, when electrifying a platform from shore a static frequency converter (SFC) is suddenly needed.
Challenges of Power from Shore
As mentioned, power from shore application is linked with several challenges when designing the system. Table 1 summarizes the most relevant items:
Table 1: Challenges in Power from Shore intertie
Types of power transmission
Three variants of long cable transmission principally exist:
- AC transmission at normal power frequency (50 Hz or 60 Hz)
- Low frequency AC transmission
- HVDC power transmission
AC power transmission at frequency of 50 Hz or 60 Hz is probably the choice for most offshore plants. It requires just one SFC system. However, the drawback is the largest amount of reactive power in the system and likely also the highest transmission losses. The SFC (if required) can be located either onshore or offshore – see next section.
An alternative is low frequency AC transmission system by reducing the frequency to 20-30 Hz the reactive power is reduced as well. Such system allows larger cable distance, lower rating of reactive power compensation equipment and less transmission losses. The drawback is that an SFC is required on each side of the subsea cable: in the onshore substation and at the receiving end on the platform. Moreover, the step-down transformer offshore becomes bulky with reduced power frequency compared to “conventional” 50 or 60 Hz design.
Finally, the third option is to use an HVDC link between the shore and the platform. This option is superior from reactive power point of view. Disadvantage is again the need for power electronic converters both onshore and offshore. Generally, HVDC link may be too large for most platforms (high cost, larger footprint). However, in the higher power range this option may be viable. There are existing platforms in operation that are electrified via HVDC transmission system.
Long AC cable and reactive power
When using AC transmission the submarine power cable partly acts as a very long capacitor. The system has a surplus of reactive power that needs to flow somewhere. This is a key difference to an overhead line that has an inductive character.
The long cable connection presents several challenges to be considered:
- Non-uniform current distribution along the cable
- Surplus of reactive power to be compensated
- Load dependent voltage variation
The selection of cable transmission voltage is a task of multi-objective optimization (transmission losses, amount of reactive power in the system, insulation margin for transients, investment cost etc).
The current distribution in the cable is illustrated in Figure 2 on a model example with 200 km long power from shore. Due to the nature of the system the current loading may be higher in no-load condition than at full load. That is especially the case when there is no or just small reactive power compensation offshore.
SFC onshore versus offshore
The SFC can principally be installed either onshore or offshore. Location offshore may have some technical advantages. Nevertheless, the biggest obstacle is usually the space and weight restriction on the platform itself. As we talk about existing platforms, the available space is mostly given. Sometimes, space can be freed up when removing some of the gas turbines and generators. Theoretically it is possible to reinforce and extend the platform with additional modules. However, such option is extremely costly and risky and thus mostly not preferred.
What are the pros and cons of having the SFC either onshore or offshore? Each location has specific advantages and drawbacks.
For example, the SFC onshore is usually less restricted in terms of footprint and weight is probably no issue. On the other hand the SFC will be larger in MW rating as it needs to cover the load offshore plus the power losses in the subsea cable. Also, SFC onshore means power transmission at 60 Hz resulting in 20% more reactive power at the same voltage.
In contrast, installing the SFC offshore is very critical in terms of space and weight. However, this solution typically results in lower losses and less reactive power. Moreover, the SFC may also provide the reactive power compensation capability thus eliminating the need for a shunt reactor offshore.
The summary comparing onshore and offshore SFC installation is given in Table 2.
Table 2: Comparison of SFC installed onshore and offshore
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SFC functionality
The SFC in power from shore applications provides several functions:
- Transfer of active power from shore to the loads on the platform
- Frequency conversion from 50 to 60 Hz (theoretically also other frequencies)
- Reactive power compensation (“STATCOM” functionality – static and dynamic)
- Voltage control / reactive power control
Most of the time the SFC is the only source for the offshore loads and thus operates as grid forming unit. In some cases parallel operation with other sources may be required as well.
Summary
Power from shore is a system that connects offshore platforms to the power grid on shore. Main motivation is the reduction of carbon footprint of the oil and gas industry. Especially in Norway the ball is already rolling with some projects in realization phase and many other projects in various study stage. It is expected that other countries will follow as well.
Outlook
Considering the continuously increasing environmental concerns and pressure on the oil & gas industry, we expect a growing interest in this application. And the vision goes further. In short term the power from shore as described in this post will be realized. However, there are already projects to fully electrify such platforms from offshore wind farms. In mid and long term the design and operational experience can very well be utilized for offshore wind, large scale tidal or similar systems.
References
[1] SFC as grid intertie, https://mb-drive-services.com/sfc-as-grid-intertie/
[2] Oil platform (Wikipedia), https://en.wikipedia.org/wiki/Oil_platform
