It is a known fact that there is a growing shift towards renewable energy sources as well as buying and selling electrical power which most often necessitates the use of HVDC transmission systems.
Both the solar power generation and HVDC transmission systems make extensive use of power electronics devices.
In future, more-and-more grid-forming converters (GFMCs) will be introduced as the scale of microgrids also increase. These structural changes would undoubtedly change the system operation paradigms, which in turn, necessitating a deeper understanding how these new devices function and how it would affect network stability and particularly, power quality.
In many studies, it has been found that an extensive use of power electronics devices plays a major role in power quality issues, specifically voltage sags and swells.
The trend of retiring rotating generation plants could lead to grid instability if it is not carefully monitored and actioned well ahead of time before major issues develop. Currently, gas and steam turbine power generation play a vital role in terms of grid inertia and stability.
Thus, with the shift towards renewable energy sources and the trend of retiring rotating generation plants, national grid companies need to consider other alternatives to help compensate for power quality issues caused by power electronics devices. This includes capacitors, static VAR compensators, and static compensators.
The installations of capacitor banks at substations are not new. It is relatively cheap, reliable, and easy to install but it takes up a lot of space and require special controlling devices. An ordinary circuit breaker simply does not work with capacitor banks. Another disadvantage is that they can only supply reactive power and not absorb it. When load rapidly increases and voltage drops, the effectiveness of capacitors diminishes.
Static VAR Compensators (SVCs) consisting of shunt capacitors and reactors and may offer a greater degree of voltage control, but it is of little use for rapid voltage changes.
Although static synchronous compensators (StatComs) with its use of sophisticated power electronics is a far better option, it has a major drawback; it is far pricier than the more basic equipment.
A synchronous compensator is a large piece of machinery. It is spinning generator and flywheel combination. Having it connected to the high-voltage transmission network via a step-up transformer it is kept in sync with grid frequency and thus contributes to network stability, dampening any fluctuations in frequency. Again, this concept is “not new”. In the late 1970s Eskom installed several Synchronous Condensers.
The intermittency of wind energy and the extensive use of power electronics devices, such as found in HVDC transmission systems and solar power generation equipment, emphasizes that grid stabilization and equipped with AC filters to avoid the harmonic impact on the AC network performance, have an increasingly important role in a successful energy transition.
Coal-fired power stations can be repurposed whereby the steam turbine is removed and replaced with a large-mass flywheel and Synchro-Self-Shifting (SSS) clutch. The electrical rotating equipment, such as the existing generators, is then reconfigured to become synchronous condensers. Since it is already connected to the high-voltage transmission network via a step-up transformer, it will become a stabilizing device.
With the rapid shift towards renewable energy sources, we should have started thinking about ways to stabilize the network by installing more synchronous condensers and AC filters to avoid the harmonic impact on the AC network performance. In an article published in the Engineering News of 20 November 2023, it is stated that “Eskom is preparing to introduce 11 synchronous condensers – seven new and four repurposed – across its transmission system to support grid stability as the penetration of variable renewable-energy generators rises”. On 22 June 2023, the Australian Renewable Energy Agency (ARENA) published and supported a report in which the installation of synchronous condensers is also addressed.
A timely commitment to install synchronous condensers is needed because “long queue for new SCs is already forming” [my interpretation based on what I have read about this topic]. Considering that there is already a significant rise in reports about poor power quality issues, my question is: Was this unforeseen by the industry? The large-scale use of power electronics in solar power generation and HVDC power transmission has been around for many years.
