Tech Insights

Renewable Integration Issues: Can Virtual Synchronous Generators Help?

May 16, 2024 by Liam Critchley

Renewable energy requires both AC and DC interconnections. Researchers are investigating how virtual synchronous generators can resolve reliability issues.

Developments in power electronics technology have opened the door for applications integrating direct current (DC) systems into the power grid, which uses alternating current (AC). For example, renewable sources generate DC that must be converted to AC to enter the wider grid network.

Wind farms paved the way for AC-DC interconnections, which enabled high-power transmission over long distances. Additionally, the growing number of renewable energy technologies at the distribution grid level has led to the development of multiterminal DC microgrids. 

Still, DC systems must better support AC systems to ensure greater grid reliability. Integrating AC and DC networks has advantages, but AC and DC grid coexistence still poses challenges if not managed correctly. Virtual synchronous generators (VSGs) can improve reliability in hybrid AC-DC grids. 

 

Grid-connected wind energy.

Grid-connected wind energy. Image used courtesy of Adobe Stock

 

How Can Virtual Synchronous Generators Help?

If the grid adopts a hybrid AC-DC network, integrating the two energy networks will require extensive interconnections between AC and DC systems at all levels.

While DC-AC integration offers some advantages, challenges persist. These challenges are more obvious in intermittent generation units based on renewable energy, which reduce AC grid inertia.  Renewable systems affect the frequency regulation in AC networks and voltage regulation in DC networks. Combining the two systems hinges on bidirectional DC-AC power conversion systems that can simultaneously support DC voltage regulation and AC frequency regulation. This requires emulating inertia to reduce inertia loss effects from renewable energy sources. 

One promising approach is to combine DC voltage droop regulators with virtual synchronous generators. VSGs are power converters operated with a control system allowing emulation of conventional synchronous generators’ dynamics. VSGs enable AC voltage and frequency regulation, while droop controllers regulate DC voltage. However, many systems using VSGs have focused only on AC frequency regulation capabilities and neglected DC-bus voltage dynamics.

 

Virtual Synchronous Generators Accommodate AC and DC 

A research team has developed a control scheme enabling VSGs to efficiently interconnect AC and DC networks. The scheme uses a bypass control unit inside the DC voltage droop regulator. The control unit inputs the set-point value of reactive power from the operator and the active power generated by the DC voltage droop regulator. The voltage magnitude and phase are used at set points for the inner virtual impedance control loop. This achieves the desired value of active and reactive power at the converter’s output terminals. The unit inside the DC voltage droop regulator works in parallel with the power synchronization loop to emulate inertia.

 

VSG structure.

VSG structure. Image used courtesy of the authors

 

The control scheme simultaneously exhibits fast dynamics for DC voltage regulation and slow dynamics for AC inertia emulation, accommodating the contrasting requirements of both DC voltage and AC frequency regulation in these interconnects. The fast DC dynamics are for variations in the active setpoint from the droop DC voltage regulator, whereas the slow AC dynamics are for emulating inertia when grid frequency deviations occur. 

The researchers performed two experiments with an 8 kVA three-phase converter prototype to evaluate the technique’s effectiveness. The first experiment simulated a DC power outage by inducing a step-load transient in the converter’s DC bus. The results showed the interconnect could regulate the DC voltage via droop control. The second experiment emulated an AC grid frequency variation using the VSG driven by a DC motor. It measured the converter’s inertial response.

 

Experimental setup for the VSG.

Experimental setup for the VSG. Image used courtesy of the authors

 

The researchers observed that the VSG scheme could emulate an inertia response, and the AC grid could cope with power variations occurring on the DC grid due to disruptive events. Disruptions are common with renewable sources due to intermittent energy generation. This control technique works best when active power can cancel out any perturbation in the DC and AC grids. When the load on the DC grid changes, the AC grid can act as a reserve of active power for the DC grid. On the flip side, the active power from the DC could be used to stabilize the AC grid when needed.

Special cases can also fall outside the normal interconnect stabilization operations, especially when disruptive events occur on the interconnect's DC and AC sides. These events render the control unit ineffective, not due to limitations in the controller but because the power network lacks available active power. These scenarios sometimes occur when simultaneous disruptions arise, resulting in a power system blackout. While it’s unrealistic to design a power system with a backup to cover these events, the study notes that disabling the VSG control structure will avoid any potential cascade of instability conditions from one grid to the other.

However, some mitigating factors must be considered in the scheme's current form. The experiments don’t replicate exactly what would happen in a grid environment. The real-world grid experiences more active power variations in the grid (not reactive) due to changing loads. This causes a dynamic variation in the grid frequency that depends on the global inertia of all the generators connected to the grid, including the converters. The researchers deemed it too risky to emulate this situation in the lab and decided instead to perform the experiments in a more controlled fashion.

 

Coping With AC-DC Challenges

The proposed scheme enables VSG operations with fast dynamics to cope with the DC voltage regulation requirements while simultaneously providing slow dynamics to help emulate AC inertia. The approach addresses the contrasting requirements from the DC and AC sides and allows more efficient interconnects to cope with the two grids’ varying power requirements. 

The study has shown that the main AC grid could cope with the variations arising from disturbances on the DC side when the two grids are connected, and the DC grid could be used to stabilize the AC grid when disturbances occur within the main grid. The ability to better control the interplay between the two grids through efficient interconnects will help to further integrate DC-based renewable energy sources into the grid in the coming years.