Opinion Article on Flexibility of Energy Infrastructure

This opinion article presents the author's point of view on the modern technical solution to enable grid flexibility and points out that before the flex market arises and starts acting it is necessary to specify flexibility products. The article depicts two of them: dynamic line rating supporting flexibility of the high voltage line capacity and MV/LV transformer equipped with the on-load tap changer (OLTC), which facilitates enlarging the share of the inverter-based generating sources (mainly PV installations) in the grid and thus create the possibility to offer the flexibility service. Both solutions are elaborated in the scope of the Polish demonstrator EU project – EUniversal.

October 2023

Flexibility services based on flexible features of the power network infrastructure and on optimal RES utilization

Adam Babs, Institute of Power Engineering, Poland

The flexibility of a power system refers to its ability to adapt to changes in demand and generation in a way that ensures the security of energy supply and optimal utilization of energy sources, particularly renewable sources.
In a centralized system, where sources with planned and stable generation play a dominant role, this requirement is typically done using a power system frequency controller, Central Power and Frequency Control (CPFC), which adjusts the output of generators in response to changes in demand. However, as the energy system shifts towards decentralization and greater reliance on renewable sources, a more flexible and intelligent system is required, utilizing consumer participation, improved interconnection, more efficient large-scale energy storage, and demand response and management utilizing digital technologies. Managing such a system, in addition to the active participation of consumers and energy producers, requires the implementation of new services and products. One such service is the flexibility service, which can be offered on the planned flexibility services market by both system operators and energy producers. Flexibility services offered may concern both dynamically changing conditions of energy transmission through network elements and adaptation of energy production to current needs.
The demonstrators in the EUniversal project: DSO ENERGA together with the Institute of Power Engineering elaborate a practical approach to solve the problem of constraints in the transmission infrastructure mainly in HV lines, and problems with the negative influence of massive installation in the LV grid, the inverter-based generations i.e., PV generation. The Congestion Management Service (CMS) is a flexibility service that addresses constraints inflicted by thermal, voltage, and stability limit violations on network elements such as transmission lines, cables, and transformers. Its purpose is to mitigate the risk of boundary violations and restore the network to within acceptable operational limits.
Implementation of congestion management with the use of dynamic line load capacity enables technical and cost-effectively ensure safe operation of the distribution system both under current operating conditions and for expected (forecasted) conditions. The use of the dynamic line load capacity in the operation of the distribution network leads to better and more effective use of the transmission capacity of the line. The increasing prevalence of PV micro-installations in Low Voltage (LV) electrical grids has resulted in voltage issues due to the inadequacy of current grid infrastructure. Specifically, PV energy production is suspended as voltage levels exceed permissible limits. To address this issue, efforts are being made to enhance the flexibility of LV grids to accommodate a greater number of PV micro-installations. The investigation regarding the mitigation of interrupted operation of the PV installation caused by overvoltage protection has led to elaborating the solution for voltage control using an MV/LV transformer with an on-load tap changer (OLTC). OLTC is controlled based on the voltage measurement in the depth of the LV network via the Advanced Metering Infrastructure (AMI) system, mostly at the Points of Connection (POC) of the PV, thus protecting against voltage limit violation and providing the possibility of continuous operation of the PV.
PV installations have a significant impact on the power grid, especially on the low-voltage network, but also on the MV grid because energy is transferred ‘up’ the power grid. The finding of the demonstration is combined voltage control by OLTC in the MV/LV substation with voltage regulation in HV/MV substations. The voltage controller algorithm of the HV/MV transformer should consider the power generation level in the MV and LV network. Instead of the day/night voltage set value the voltage setting might be dependent on the value and flow direction of the active power.
The basic, key challenges related to the growing production potential of photovoltaic power
plants are:

Technical challenge related to the increasing amount of energy generated from photovoltaics.
Naturally, some of this energy will not be able to be absorbed – it will be necessary to manage its amount directly at the sources of its production.

Economic challenge – at the peak of photovoltaic energy generation, energy prices on the stock exchange will fall, with the possibility of negative prices.
Meeting the technical challenges is the responsibility of the distribution network operators (DSOs) and the transmission network operator.
EU-funded research initiatives like EUniversal, focusing on multiple member nations, have the potential to greatly shape a unified system for managing distributed flexibility across Europe. The basic findings of the Polish demonstrator EUniversal project can be shared with others and adopted by other DSOs.