ENTSO-E - High Penetration of Power Electronic Interfaced Power Sources

High Penetration of Power Electronic Interfaced Power Sources and the Potential Contribution of Grid Forming Converters

Download High Penetration of Power Electronic Interfaced Power Sources and the Potential Contribution of Grid Forming Converters



Executive Summary

Europe, in common with substantial parts of the world is increasingly moving towards generating electricity from Renewable Energy Sources (RES) , which are predominantly connected to the power system via power electronic converters. In addition, power electronic interfaces are added on the demand side (smart loads, motor drive systems and electrical chargers for Electric Vehicles [EVs]) and also at the transmission level (e.g. bulk High Voltage Direct Current [HVDC] as
well as offshore HVDC links).
At times of high RES generation delivered at low marginal cost (due to no fuel cost), conventional thermal generation uses large Synchronous Generators (SGs) that are `out of merit` and are therefore potentially disconnected. Periods in which RES have the potential to supply all, or close to all the system demand are increasing and becoming a reality for an increasing number of countries. Ireland is one extreme European example, where constraining/substituting wind above 65% of demand is already being experienced with installed wind capacity of 4 GW, and yet another 10 GW of new wind capacity is expected by 2030 . Such a high uptake of RES is linked to dramatic reductions in installation costs of new wind and solar Photovoltaic (PV), making these sources increasingly competitive with thermal energy .
In parallel, the power systems in several European countries are undergoing structural changes, whereby business as usual is being challenged, most notably by larger and more volatile power flows over greater distances .
This report collates analysis since the end of 2016 by a group of experts from across the power industry called Technical Group High Penetration (TG HP), focused on the power electronic converters that are being used as an interface towards the Alternating Current (AC) power system in a wide range of applications covering power generation, transmission and consumption/demand, and which have the effect of changing the dynamic properties of the AC power system. The analysis is concentrated on one of the potential solutions for the extreme case of Power Electronic Interfaced Power Sources (PEIPS) contributing between 60 and 100% of the total instantaneous power supply. It defines the characteristics of one emerging solution, that of applying Grid Forming Converters (GFC).
It does not address adequacy issues of low RES generation compared to demand. The TG HP has been reporting the developments via ENTSO-E to the Grid Connection European Stakeholder Committee (GC ESC), reflecting its initiation.
Extensive research on the topic of this report is being undertaken elsewhere, including the European R&D project MIGRATE which is currently releasing its reports . Several national and international organisations (worldwide) are engaged in current activity on this topic . This report concentrates on the potential contribution of GFC to the secure operation of the power system
where its generation is dominated by PEIPS. The report does not claim to provide extensive cover of alternatives means to GFC to deliver the capabilities identified, although it does introduce the reader to one of them, the use of synchronous condensers, including the possible addition of flywheels.
Previous work developing Connection Network Codes (CNCs) has defined system needs and how to successfully satisfy such needs for lower PEIPS penetration, approximately up to 60% in a Synchronous Area (SA), as expressed in these CNCs at European level and at national level in Grid Codes or equivalent documents. However, in the operating range of PEIPS of 60 to 100% of the total demand at the time, fewer or no SGs (in the case of 100%) are connected based on economical `in merit` energy market reasons.

The traditional electrical power system and electricity markets have been designed to work with SGs, and so these have traditionally provided various `inherent` capabilities to the system critical to ensure the stable operation of the power systems during severe faults and even basic system survival during rare system splits. Due to the potential total absence of SGs approaches during periods of high penetration (HP) of PEIPS infeed, the wider industry has engaged in a closer examination of the lack of these system capabilities . Traditionally, the focus in the context of PEIPS has been on steady state and a limited number of dynamic (faster) aspects recently expanded to include PEIPS contributing fast fault current during system faults and extended contribution to frequency
management (although this latter capability has been required from RES for more than 10 years in some countries). Demand side contributions in these contexts are emerging and have significant potential.
However, the analysis of situations of High Penetration of Power Electronics Power Sources (HPoPEIPS) has identified further System Operator (SO) concerns, including the low or potentially inadequate supply of:
• Total System Inertia (TSI)
• Fault Current Infeed described as Fault Level (FL) and also affecting Short Circuit Ratio (SCR)
The combination of the above two elements is summarised by the term low system strength. The seven topics of concern in this context examined by TG HP are:
• Creating system voltage
• Contributing to fault level
• Sink for harmonics
• Sink for unbalance
• Contribution to inertia
• System survival to allow effective operation of Low Frequency Demand Disconnection (LFDD)
• Preventing adverse control interactions
These critical capabilities or behavioural characteristics must continue to be adequately delivered, even when operating close to a 100% penetration of converter interfaced power sources, in order to continue to ensure stable voltage, frequency and system angle under all operating conditions (steady
state and disturbed).
Each of the above aspects could be treated in isolation, or alternatively solutions to these challenges could be sought in an integrated or holistic manner. In the view of some Transmission System Operators (TSOs), there is a risk associated with treating the challenges individually, as a positive contribution to one aspect may be detrimental to another. An example of this is that a pure form of contribution to synthetic inertia may be detrimental to control interactions by making these worse rather than better, and has therefore not been adopted .
Although analysis of the power system characteristics and needs should be done in a holistic manner, some of the critical capabilities indicated above could be provided in a shared manner, e.g. while some converters create system voltage and contribute to fault level in the positive sequence, other converters in the power system could serve as sinks for unbalance. The value of a holistic approach versus shared delivery has not been fully analysed by this group.