Grid Forming Inverters - The Future
Technology Insight

Grid Forming Inverters – The Future

Introduction

Traditionally, synchronous machines have been the primary providers of essential reliability services such as inertia, short-circuit current, steady-state and dynamic reactive power support, primary frequency response, power oscillation damping, and system restoration capabilities in power systems across the world. However, with the decreasing share of synchronous generation and the rapid increase in inverter-based resources (IBRs), maintaining these functions has become increasingly challenging. Issues such as reduced static and dynamic reactive power support, failure to ride through disturbances, declining system strength and inertia, and increased oscillations have emerged as key operational concerns.

The Indian power system is also encountering similar challenges. With the anticipated large-scale addition of inverter-interfaced bulk loads such as data centers in the coming years, these challenges are expected to intensify further, if not timely addressed. Consequently, the responsibility for providing essential reliability services must progressively shift from synchronous machines to IBRs.

Existing Infrastructure

In India and most of the power systems across the world, the grid-connected IBRs installed are mostly of grid-following (GFL) type. These IBRs operate as current sources, injecting controlled current into the grid based on external voltage and frequency. This mature technology has immensely helped in the large-scale integration of renewables and has performed reliably under strong grid conditions. However, the performance of grid following IBR is also observed to deteriorate under weak grid conditions.

Grid following Inverters

A Grid-Following (GFL) inverter is a power electronic converter that injects controlled current into an existing AC grid by synchronizing to the grid’s voltage and frequency. It does not establish grid voltage or frequency; instead, it follows them.

Control Philosophy

GFL inverters operate as controlled current sources. The standard control hierarchy is:

  1. Phase-Locked Loop (PLL)
  2. Outer power control loop (P–Q or V–Q)
  3. Inner current control loop (dq-frame)
  4. PWM modulation
Block diagram of Grid following Control
Figure 1: Block diagram of Grid following Control

The PLL estimates grid angle

δ = ∫ ω dt

Under weak grid conditions, PLL dynamics couple with network impedance, creating instability mechanisms.

The inverter injects current:

id = (2/3) × P / Vd
iq = (2/3) × Q / Vd

This control assumes a stiff voltage reference.

Limitations of Grid following inverters

Dependence on External Voltage Source

GFL inverters cannot establish voltage or frequency. If grid voltage collapses:

  • PLL loses synchronization
  • Current reference becomes invalid
  • Inverter trips

Weak Grid Instability (Low SCR)

  • PLL oscillations occur
  • Negative damping may appear

The impedance-based stability framework shows that GFL inverters exhibit non-passive behaviour in weak grids.

Lack of Inertia

Frequency response depends entirely on external synchronous machines. Inverter decouples DC source from AC frequency.

No inherent swing equation exists.

Limited Fault Current

GFL inverters are current-limited (typically 1.1–1.2 p.u.), reducing:

  • Fault level
  • Protection selectivity
  • Distance relay accuracy

Control Interaction

High penetration of PLL-based inverters leads to:

  • Sub-synchronous control interactions
  • Poor damping of inter-area modes
  • Harmonic resonance risks

GFL inverters were appropriate in systems dominated by synchronous machines. They are not structurally suited for inverter-dominated systems.

Grid Forming Inverters

The inverters which are based on grid-forming (GFM) control operate as voltage sources and are capable of independently establishing voltage and frequency references, are expected to perform reliably under weak grid conditions also. Black start capability is another advantage of this type of control.

Block diagram of Grid forming control
Figure 2: Block diagram of Grid forming control

Control Philosophy

Droop Control

Based on steady-state power flow relationships:

P = (E × V / X) × sin δ

Frequency is adjusted according to active power:

ω = ω0 − Kp(P − Pref)

Voltage magnitude is adjusted according to reactive power:

V = V0 − Kq(Q − Qref)

Virtual Synchronous Machine (VSM)

Implements the swing equation:

J (dω/dt) = Pm − Pe − D(ω − ω0)

Where:

  • J = virtual inertia
  • D = damping coefficient

Virtual Oscillator Control (VOC)

Nonlinear oscillator-based synchronization:

  • Naturally synchronizes with grid
  • No PLL required
  • Strong nonlinear stability

High robustness but mathematically complex.

General Block diagram of GFM Inverters with Voltage and Current loop
Figure 3: General Block diagram of GFM Inverters with Voltage and Current loop

Advantages of Grid forming control

  • Stable operation in regions with low system strength / low proportion of synchronous machines
  • Fast frequency response and dynamic reactive power / voltage support
  • Resistance to phase angle jumps resulting in improved transient stability
  • Improved system damping, thereby enhancing small signal stability
  • Regulation of voltage harmonics
  • Inherent negative sequence current injection and balanced internal voltage under unbalanced conditions leading to reliable protection performance

References

R. Rosso, X. Wang, M. Liserre, X. Lu, and S. Engelken, “Grid-forming converters: an overview of control approaches and future trends,” pp. 4292–4299, Oct. 2020, doi: 10.1109/ECCE44975.2020.9236211.

R. Rosso, X. Wang, M. Liserre, X. Lu, and S. Engelken, “Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review,” vol. 2, pp. 93–109, Apr. 2021, doi: 10.1109/OJIA.2021.3074028

A discussion paper titled “Grid Forming Technology and Possible Applications in the Indian Power System” published by GRID-INDIA on Dec 2025.

FacebookXPinterestRedditEmailShortlink

Related Posts

Comments are disabled