System strength, inertia and network loss factors. What are the implications for power networks and renewable generation?
Power network operators are faced with the increasing complexity of managing power networks designed to deliver power from centralised generators to loads now being required to integrate distributed generation at both the transmission and distribution level. Equally, renewable energy project developers are met with increasing regulation and standards required to be met to enable their project to connect to power networks.
Mitigating risk
Recently there have been significant increases and volatility in regulatory costs for renewable energy projects. This has arisen for a number of reasons, such as significant falls in Marginal Loss Factors (a direct multiplier on sent out generation revenue) and increased Frequency Control Auxiliary Services charges. Following the assessment of the causes of the black out in South Australia, the Australian Energy Market Commission, Energy Market Operator and transmission network system operators have identified that system strength issues are emerging as a major challenge to the ongoing development of renewable energy projects.
System Strength
System strength has reduced following the retirement of fossil fuel plant synchronous generation replaced by renewable energy non-synchronous generation.
System strength is a local phenomenon on the network; with the propagation of system strength contributed from system strength providing technologies, hampered by the natural impedance of the network. That is, unlike network voltage and frequency control that can propagate over significant distances, system strength provision is limited to the network area proximate to the system strength providing technology.
Fault currents vary around the grid both by location and voltage level. Perceived wisdom is that fault levels (i.e. system strength) contribution generally must be supplied locally within an identified weak network. However, this may not be the most efficient way to maintain system strength. Coupled with fault current is the concept of system inertia – typically provided by steam turbine driven synchronous generators.
Obligations for renewable developers
The introduction of ‘do no harm’ provisions place an obligation on renewable energy developers to make sure their project does not result in network strength falling below a pre-defined minimum level. This may result in the need to incorporate additional capital equipment costs to meet grid strength support requirements, placing further downward pressure on the commercial viability of new projects.
Fossil fuel synchronous generators automatically contribute to system strength as a ‘by-product’ of generation. Renewable energy generation is typically asynchronous (induction motors or inverters) contributing negatively to system strength (i.e. reduces it). To compensate for the reduction in synchronous generation, additional equipment is required at key points on the network such as synchronous condensers and grid-forming inverters to increase system strength.
Steps to take
- Evaluate loss factors, FCAS (Frequency Control Auxiliary Services) and system strength requirements early
- Collaborate with other developers/network operator to improve efficiency of system strength provision
- Update studies regularly – the market is in a significant state of flux.
Recommendations
