When solar inverter work after sunset
Akhtar Hussain Javed defended his PhD thesis at the Department of Electrical Engineering on January 29th.
Modern power grids are running into a new kind of problem — not a lack of electricity, but too much of the wrong kind. This research of Akhtar Hussain Javed shows that capacitive reactive power is increasingly flowing from distribution networks back into the transmission grid, creating real operational challenges in the Dutch power system. Using extensive measurement data, it demonstrates that this issue is already forcing corrective actions by system operators. Crucially, the work also shows that smart PV inverters, with minimal modification, can support grid stability both during the day and at night, while new methods to quantify and aggregate flexibility help prepare the system for a renewable-heavy future.
Electricity systems are changing faster than ever. Large, centralized fossil-fuel plants are being replaced by decentralized renewable sources like solar PV, while electricity demand is shifting due to electric vehicles, heat pumps, and data centers. These changes make the system cleaner and more sustainable, but they also alter how power flows through the grid — especially at the distribution level, which was never designed for this role.
One consequence of this transition is the growing importance of reactive power. Although reactive power does not deliver usable energy, it is essential for voltage control and system stability. In today’s cable-heavy and power-electronics-dominated networks, capacitive reactive power (CRP) flows are becoming increasingly dominant.
Evidence from the Dutch power system
A key contribution of this research is its empirical foundation. Using measurement data from Dutch Distribution System Operator Enexis and Transmission System Operator TenneT, the thesis of Akhtar Hussain Javed shows that reactive power is increasingly flowing upward from distribution networks into the transmission system — the opposite of traditional assumptions.
These CRP flows can lead to elevated voltages, increased losses, and operational constraints. In some cases, they even force the transmission operator to temporarily disconnect circuits to maintain system security. The findings make clear that reactive power management is no longer a theoretical concern, but an active operational challenge in real-world networks.
Letting PV inverters do more
One of the most promising solutions explored in this work is the use of smart PV inverters. While PV inverters are designed to convert solar energy during daylight hours, they are typically underutilized from a grid-support perspective — especially at night.
This research proposes control strategies that allow PV inverters to act as reactive power compensators both during the day and after sunset. With little or no hardware modification, these inverters can help absorb excess capacitive reactive power, supporting voltage control and reducing stress on the network. This turns existing renewable assets into flexible grid-support devices, offering a cost-effective and scalable solution for operators.
Making sense of flexibility at scale
Beyond reactive power, the thesis addresses a broader challenge: how to measure and use flexibility in active distribution networks. Individual flexible resources — such as PV systems or controllable loads — have limited impact on their own. Their true value emerges when they are aggregated.
To capture this, the research applies and extends the Feasible Operation Region (FOR) concept, which describes the combined active and reactive power flexibility of a network while respecting technical constraints. The work shows how network characteristics like topology, voltage limits, transformer tap positions, and cable capacities directly shape the size and boundaries of the FOR.
New analytical methods are developed to describe these boundaries accurately, helping system operators understand not just how much flexibility exists, but under what conditions it can be used.
Aggregation without giving away the grid
Finally, the thesis introduces a method to aggregate flexibility across multiple distribution networks without requiring detailed network data. This addresses practical concerns around data availability, privacy, and scalability — key barriers to real-world implementation.
By enabling network-aware flexibility aggregation while preserving confidentiality, the approach supports closer coordination between distribution and transmission operators in future power systems.
Power systems, ready for what comes next
Overall, this research delivers both evidence and solutions. It shows that capacitive reactive power is already reshaping grid operations, demonstrates how existing PV infrastructure can help manage it, and provides analytical tools to unlock flexibility at scale. Together, these contributions support a more efficient, reliable, and resilient electricity system — one capable of meeting growing demand while accelerating the transition to renewable energy.
Title of PhD thesis: . Supervisors: Dr. Phuong Nguyen, Prof. Han Slootweg, and Prof. Johan Morren.