Power System Transformation for Dummies
Power system transformation
understanding the challenges
vRES capacity is spatially and electrically dispersed.
What is the point?
Typical generation capacity of vRES technologies is in the range of kW or some MW, rather than 1000 MW like a large nuclear power plant.
Small capacities are connected to the networks at lower voltage levels and the distance between the project site and the connection point with the network should be as short as possible. Larger projects are connected to higher voltages, still with the shortest possible connection. For that reason, vRES often can be found spread all over the country and over all network levels, from low voltage distribution up to extra high voltage transmission.
Distributed generation is not new. Combined heat and power has been used for decades in district heating and greenhouses. The total capacity, however, was low compared to the generation capacity at transmission level. Additionally, these projects were always related to the built environment and thus embedded in existing infrastructure.

Why is this important?
From an electrical engineering point of view, distributed generation is not particularly challenging. But the shift from large scale, transmission-connected generation to distributed generation changes a number of things:
  • Transmission-connected power plants are closely monitored and managed by the system operator. Small scale distributed generation, like a residential PV rooftop system, usually is not monitored, certainly not directly by the system operator. With transmission-connected power plants being steadily displaced by vRES, a growing part of the generation becomes invisible and - if not addressed - uncontrollable.
  • When vRES generation exceeds peak load, distribution system operators have reinforce their networks. In rural areas, the difference in ratings of network assets may be huge. Adaptation of networks may be an incremental process covering years to decades.
  • Large amounts of distributed generation do not only affect distribution network planning, but also network operation and protection. The issues are not fundamentally challenging. But there are many new aspects and details. Organisations need to invest in assets and competencies. This takes time.
  • Distribution system operators, much more than in the past, have to coordinate their operational procedures and decisions with the transmission system operator. Implementing this also takes time.
Where is this relevant? - Country characteristics
Typical plant capacities, their geographical distribution and the connection voltages depend on various factors.
Policy frameworks differ per country. Often, they specifically encourage different vRES technologies in different economic segments at different scales.
Of course, the geographical distribution of the resource has an influence too.

When is this relevant? - Stage of development
This is different for planning and operation.
Network planning: The 'right to connect' and to 'right to feed in' are fundamental components of any successful renewables policy. Without these guarantees development of distributed vRES simply will not take off. These legal foundations have to be implemented already early in phase 0.
Any individual project potentially triggers network reinforcements. This is not related to a certain phase in development. Similarly, protection schemes may need to be reviewed on a per-project base.
With growing number of distributed generation sites, reinforcements may be required not only close to the individual connections but also deeper in the network and at higher voltage levels.
In transformation phase 3 and phase 4, distributed generation has to contribute to system ancillary services and to support system restoration after major disturbances. This asks for extended technical capabilities of vRES plants as well as full integration in system operators restoration concepts and plans.

Network and system operation: Operation of distributed vRES usually is not coordinated in early phases.
Once distributed (vRES) generation achieves levels influencing power flows at transmission level or affecting power plant dispatch and system balancing, the system operator needs to know the instantaneous output at least at an regionally aggregated level. Usually, this need becomes relevant from transformation phase 2 or phase 3. This also means, that daily planning processes and actions need to be coordinated between (transmission) system operators and distribution system operators.

How to approach? - Addressing the challenges
Because development of vRES usually takes shorter periods than planning and permitting of network extension, network development preferably anticipates growth scenarios of renewables and fixes this in network development plans. Time frames of 5 to 10 years allow a strategic perspective while keeping uncertainties of the scenarios manageable. The latter is important to limit the risks of stranded network assets in case development deviates from expectations.
The sound concept for monitoring and potentially controlling distributed generation depends on the structure of the plant population. If the country's vRES portfolio consists mainly of multi-MW windfarms, it may be justified to take real time data from every plant. This is less logical if a major share of distributed generation comes from millions of rooftop PV plants with just a few kW per installation. A generally suitable approach is monitoring of representative sample projects, potentially in combination with now-casts provided by specialists in energy-meteorology.
How the collaboration between network operators ideally is organised depends very much on the regulative framework and the mechanisms of power markets. The needs will change and the rules will evolve with growth of distributed generation. Important is that network companies are operating in a collaborative environment and they can build upon robust expectations regarding vRES development. Both aspects depend on a clear commitment of policy makers to power system transformation and stable policy frameworks and targets.
Fluctuate vRES output is fluctuating.
vRES forecasts are never perfect.
Remote vRES plants may be located remotely.
vRES generate using power electronics.
small vRES plants run without professional operator.
vRES generate at zero marginal costs.