JOINT WG A1/C4.66 - GUIDE ON THE ASSESSMENT, SPECIFICATION AND DESIGN OF SYNCHRONOUS CONDENSERS FOR POWER SYSTEMS WITH PREDOMINANCE OF LOW OR ZERO INERTIA GENERATORS.

A new joint working group between A1 (Rotating Machines) and C4  (System Technical Performance) on the use of Synchronous Condensers in power systems with a predominance of low or zero inertia generators has just been initiated.  The objective of the working group is to produce an application guide dealing with rotating machine and power system issues for synchronous condensers.

This is a topic that is very relevant to the Australian power industry.  For example, in South Australia, with the high level of wind and solar generation combined with the decommissioning of the coal fired Northern Power Station, AEMO studies have shown that the power system has low levels of inertia and fault levels, which can adversely affect its operation.  Installation of Synchronous Condensers to provide inertia and improve fault levels is now being considered.  Similar problems are starting to appear in other parts of Australia. 

As part of carbon emission reduction mechanisms, many countries have adopted policies to increase the installed capacity of renewable generation, based on Solar and Wind technologies, to either supplement or replace existing thermal generation. These have been deployed en-masse at both transmission and distribution voltage levels.

The displacement of traditional synchronous machines with various forms of power electronic (PE) interfaced energy sources is contributing to fundamental changes in power system steady-state and dynamic behaviour, including:

• Reducing levels of system inertia
• Reducing levels of ‘system strength’ (with fault level / short circuit power typically used as a general proxy)
• Reducing levels of synchronising and damping torque components which impact on the interaction of remaining synchronous machines.

Significant reductions in system inertia will impact network frequency control capability including observable rates of change of frequency following credible and non-credible contingency events. At low inertia levels, there is likely to be a need to increase and possibly improve primary frequency control capabilities. The ability of under frequency load shedding and/or over frequency generation shedding schemes to operate correctly following non-credible events is also likely to be impeded if ‘minimum’ levels of inertia are not maintained.

An inability to maintain adequate ‘system strength’ will have multiple negative impacts on the power system including:

• Reduced voltage regulation capability (higher sensitivity to changes in real and reactive power flows throughout the network).
• Operation of power electronic based equipment closer to minimum acceptable short circuit ratios, potentially impacting on fault ride through performance and other system critical dynamic behaviours.
• Significant alterations to the harmonic impedance profile of the network, which may result in undesirable power quality outcomes.
• Insufficient fault current to reliably operate impedance and over-current based protection schemes.

One mitigation strategy is the use of synchronous condensers, which can address many of the issues outlined above as well as provide significant reactive power contributions. Modern design of synchronous condensers can be optimised to address a select or broad range of network needs. However, ‘off the shelf designs’ may not be sufficient to address specific network issues.  When considering the synchronous condenser solution, it is important to understand what is technically possible as well as the trade-offs that may be present in the design and construction of plant, including the corresponding cost impacts.

The Terms of Reference for this WG can be downloaded HERE.