SFC as Grid Intertie

VFD hardware can be used for other applications as well. One of them is SFC used as grid intertie, i.e. interconnection of two grids with different frequency and sometimes also different number of phases. Let’s explore what these applications are, how the frequency converters for such applications look like and what are the main difference to the industrial motor drives.

Grid interties interconnect two grids with different frequency or different number of phases. Of course, the voltage level of each grid is generally different, too. In special case, the voltage and frequency may be identical on both side and the SFC serves as decoupling element.

Types of grid interties

Grid intertie is a very broad term. It can be an interconnection of utility grids with different power frequency, a link between utility network and industrial consumers, connection of systems with different number of phases etc. Let us have a look at some typical applications of grid interties.

A) Utility type of grid intertie

Utility type grid intertie connects two utility grids that usually have different power frequencies. Such situation applies to interconnection between two countries or even within one country that is using two different frequencies (e.g. Japan or Saudi Arabia). A list of countries according to their mains frequency is provided in [1]. The transmission capacity of such power link ranges from medium/high up to very high power. For the highest capacity and high transmission voltage an HVDC link can be used. The motivation for HVDC is mainly reduction of transmission losses and increase of transmission capacity.

B) Interconnection of utility grid and industrial loads

In this application the SFC connects an industrial site with utility grid. An example is a conversion of a plant supplied from its local power generation to a grid connected plant. Many owners consider the decommissioning of the local generation (reasons such as fuel cost, logistical challenges, reliability, local emissions etc) and connection of the plant to the utility grid. When the local power generation has a different nominal frequency than the utility grid, an SFC is required. Especially when the local power generation approaches end of life it is a legitim question whether to upgrade the existing power plant or invest into the interconnection with existing power grid.

C) Shore to ship

A specific type of grid intertie is SFC in port application, known under names such as shore to ship. Passenger and cargo ships supply their own loads from on-board power generation. When docking in the port the on-board generation would keep running to cover the power demand. The generators are mostly driven by diesel motors and thus not very eco-friendly. The concentration of multiple ships in the port represents a significant source of pollution. Thus, the idea is to connect the ship to onshore grid while staying in the port. However, the ships are either 50 Hz or 60 Hz depending on o their origin. An SFC provides a power supply of complementary frequency allowing all ships to turn off their on-board generators while dwelling in the harbour. The solution is primarily driven by environmental aspects but can be cost effective in addition.

D) Power from shore

This case refers to electrification of offshore consumers, such as oil & gas platforms. These platforms require substantial power to run compressors, pumps, boilers/heaters and other electric loads. Many of them use local power generation based on diesel-, steam- or gas turbine generators. As part of the efforts to reduce the carbon emissions there is the aim to decommission those fossil-fuel based generation and bring the power from shore. The most advanced country in this regard is Norway. Several power from shore projects are currently under execution and many more are being studied. Also other oil and gas producing countries are considering such solution.

E) Railway interties

Railway grid is a single-phase system. Moreover, some countries operate railway system of a different frequency than the frequency of their power grids. It creates a need for power conversion system.

The most commonly used railway electrification systems in Europe are:

15 kV / 16.7 Hz
25 kV / 50 Hz
3 kV DC
1.5 kV DC

We focus on the first two systems in above list. The DC electrification is more limited in terms of power and less suitable for corridors with heavy transport or high density of trains. Some countries thus consider retrofitting the DC railway systems to AC, at least on the main transit routes.

a) 16.7 Hz railway systems

The 16.7 Hz railway electrification has historical reasons and is the primary system in countries such as Austria, Germany and Switzerland as well as Norway and Sweden. Such system cannot be supplied from a 3-phase grid without performing a frequency conversion. In the past rotating frequency converters (RFC) have been used. These systems basically consist of two electrical machines (3-phase motor and 1-phase generator) that are mechanically coupled and have the ratio of their pole pairs 1:3. Later on, the power conversion started to be realized with static frequency converters. The railway grids in these countries are nowadays powered by both RFCs and SFCs, often operating in parallel. The trend is going towards SFC that dominate the new installations are gradually replace RFCs.

The 16.7 Hz railway system is a bit special. Some railway operators (Swiss Federal Railways, German Railways, Austrian Federal Railways) operate their own railway transmission grid (e.g. 110 kV/16.7 Hz in Germany, 132 kV/16.7 Hz in Switzerland). The SFC is then interconnecting the 3-phase utility grid with 1-phase railway grid.

The other option is that the substation with SFCs directly supplies the railway catenary system which is typically 15 kV or +/-15 kV.

b) 50 Hz or 60 Hz railway systems

50 (60) Hz railway systems can principally be supplied directly from the 3-phase grid without any need for SFC. That is the traditional solution being used for decades. It is relatively simple and well-proven. The railway substation then draws power from two phases. That causes unbalance in the 3-phase grid. To minimize the asymmetry, the railway is sectioned. One section of the traction system is supplied from a substation connected between utility phases A and B, next substation is connected between phases B and C etc. Special transformer connections aim to reduce the unbalance further. When not satisfactory, a load balancer (special type of STATCOM) can be used.

An alternative approach is to use an SFC to convert the 50 Hz 3-phase power into 50 Hz 1-phase power. The use of SFC ultimately solves the problem of unbalanced loading and improves the power quality.

Differences between grid intertie and VFD

Power hardware of a VFD in motor drive application and SFC in grid intertie application can be almost identical, particularly in case of 3-phase to 3-phase intertie. Hence the main difference is in the control concept and in some dimensioning aspects.

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Dimensioning of SFC grid intertie

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References

[1] Mains electricity by country – Wikipedia, https://en.wikipedia.org/wiki/Mains_electricity_by_country

SFC as Grid Intertie

Published by admin on November 1, 2022November 1, 2022