Transmission & distribution |
Synchronous condensers rediscovered – a new way to strengthen grids
After decades out of favour, utilities and industrial operators around the world have started to place major orders for synchronous condensers to address grid stability issues associated with the increased penetration of renewables. Why has the technology found a new role in the 21st century?
Christian Payerl ABB area sales manager for synchronous condensers
The world’s power grids have been built for well over a century on a centralised basis. They provide a linear flow of electricity from large rotating, fossil fuelled generators through transmission and distribution lines to consumers. Utilities and network operators have a deep understanding of these systems, combined with the experience and know-how to control them to ensure continuity of supply. The situation has started to change fast in recent years due to the need to decarbonise power production and increase the use of renewable sources. Networks are evolving in response to these changes. And the networks of the future, as shown on the right of Figure 1, will look very different in structure from the traditional model on the left.
The main driver for these changes is the rapid uptake of renewable energy, usually wind and solar power. At the same time, large fossil fuel power plants have been decommissioned, cutting the amount of spinning mass or kinetic reserve in the network, reducing frequency stability in the grid.
These renewable power plants tend to be geographically remote and are inherently unpredictable and intermittent due to their reliance on weather conditions. Almost without exception they generate “non-synchronous” or synthesised power. That means, between the
Traditional grid
generation source and the power system, are power electronic devices such as inverters or converters.
Inverters are power electronic devices that cannot effectively support the network in case of faults or other unfavourable network events. This is because these semiconductor devices operate like switches, with no inherent power reserve, and have limited overloadability. These new networks require intelligent sensors for monitoring inertia, fault level, loadability, phase angle and other parameters. Additionally, the changes in network structure and the continuously changing power generation mix require more advanced control algorithms. To address the new challenges, attention has turned to synchronous condensers (SCs). While these synchronous rotating machines were once widely used in the power industry, they have seen little use in recent times. This is because their former function – reactive power compensation – is now handled by modern semiconductor-based equipment. However, grid stabilisation issues are becoming more acute with the increased penetration of large-scale renewable energy resources. This is driving the return to rotating devices with physical inertia. These can provide instantaneous support to maintain stability irrespective of the upstream network voltage or frequency.
What is a synchronous condenser? A synchronous condenser is a synchronous machine. But it is not a motor, since it does not drive anything. Neither is it a generator, as there is no prime mover. However, it is very similar to a generator in its design and behaviour. And it does produce reactive power. As a rotating electric machine, the SC is a very traditional device. In the past, SCs were deployed as compensators to produce reactive power, balancing out highly inductive loads, like electric motors.
The typical users of SCs were electrical utilities and heavy industry enterprises operating transmission, distribution or industrial power grids. After many years when there was little interest, grid operators and utilities are turning back to SCs (see Figure 2). This is to handle the large-scale integration of wind and solar power generation as well as the introduction of smart grid technologies.
Reducing network risks with SCs There are three main ways that synchronous condensers can help to reduce the risk associated with future networks:
● Inertia support for frequency stability The balance between supply and demand is critical to maintaining a stable grid frequency.
Future grid
– Centralised generation – Generation with spinning mass – Few power-electronics-based generators – Strong grid (high fault level and short circuit ratio)
Strong Generation
– Distributed generation – Less generation with spinning mass
– Increased amount of power-electronics-based generators – Weaker grid (lower fault level and short circuit ratio)
Weak Transmission Distribution Above: Figure 1. Power grids are becoming decentralised 12 | May 2021 |
www.modernpowersystems.com Very weak Industrial Residential
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