Building a Fault-Tolerant Renewable Grid: Lessons from Fortescue's Solar-Battery Blackout Survival

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Overview

When a bushfire caused a transmission line failure at Fortescue's green grid in Western Australia, conventional wisdom said the system should collapse. Instead, it rode through the fault using only solar photovoltaic (PV) generation and battery energy storage—no spinning machines, no fossil fuel backup. This event, described by experts as "impossible," proves that 100% inverter-based grids can maintain stability during severe disturbances. This tutorial translates that achievement into actionable design principles for energy professionals.

Building a Fault-Tolerant Renewable Grid: Lessons from Fortescue's Solar-Battery Blackout Survival — Source: reneweconomy.com.au

Prerequisites

Before diving into the step-by-step guide, ensure you have:

Basic understanding of power system stability – including frequency and voltage control concepts.
Familiarity with inverter-based resources (IBRs) – solar inverters and battery energy storage systems (BESS).
Knowledge of grid-forming vs. grid-following inverters – the former can set voltage and frequency; the latter follow.
Access to simulation tools – e.g., PSS®E, PSCAD, or DIgSILENT PowerFactory for verifying designs.

Step-by-Step Instructions

1. Assess the Base System Configuration

Fortescue's grid consisted of a large solar farm, a BESS, and a single 220 kV transmission line to the main grid. Start by modeling your system with similar characteristics: no synchronous machines, only IBRs. Key parameters to document include:

Total installed solar capacity (e.g., 150 MW)
BESS power rating and energy capacity (e.g., 35 MW / 35 MWh)
Short-circuit ratio (SCR) at the point of interconnection – this was very low in Fortescue’s case.

2. Implement Grid-Forming Control for Batteries

The critical enabler was the BESS operating in grid-forming (GFM) mode. Unlike traditional grid-following inverters, GFM inverters actively create a voltage reference and can synchronize with the network without a stiff grid. Steps:

Configure the BESS controls to emulate a synchronous machine's droop characteristics – both active power-frequency (P-f) and reactive power-voltage (Q-V).
Set the virtual inertia constant (H) in the inverter control algorithm. For Fortescue, a value around 2–4 seconds was used (typical for a 35 MW BESS).
Enable fast fault ride-through (FRT) capability – the inverter must stay connected during voltage dips and inject reactive current per grid codes.

3. Design the Solar Inverter Control for Fault Support

Solar inverters typically are grid-following (GFL). However, they must be modified or supplemented with GFM functionality for a truly black-start-capable island. Options:

Use a hybrid approach: keep most solar as GFL but have a subset (e.g., 10% of capacity) operate as GFM.
Alternatively, rely entirely on the BESS for voltage and frequency formation while PV inverters follow the BESS’s signal.

Fortescue’s PV inverters were standard GFL but coordinated via fast communication (<5 ms latency) from the BESS control system.

4. Tune Fault Ride-Through and Protection Settings

A common trip condition for IBRs is overvoltage or undervoltage during a fault. Use these guidelines:

Set inverter voltage trip thresholds to stay connected for voltage sags down to 20% of nominal for up to 1 second (per IEEE 1547-2018).
Coordinate overcurrent protection: inverters have limited fault current (120%–150% of rated). Their protection should not trip faster than the transmission line relays.
Implement rate-of-change-of-frequency (RoCoF) ride-through up to at least 5 Hz/s.

5. Simulate the Transmission Fault Event

Use electromagnetic transient (EMT) software to replicate the scenario: a 3-phase or single-phase-to-ground fault on a remote transmission line, followed by breaker open and auto-reclose. In Fortescue’s case, the bushfire caused a phase-to-ground fault that persisted for ~1.5 seconds before line isolation. Monitor:

Frequency excursion (maximum deviation should stay within ±1 Hz for 60 Hz system).
Voltage recovery time (should not exceed 2 seconds to reach 90% of nominal).
Power oscillations (damping ratio > 0.1).

6. Validate with Hardware-in-the-Loop (HIL) Testing

Before deployment, verify the controls on a real-time simulator with actual inverter hardware. This step caught many issues in Fortescue’s development:

Test worst-case scenarios: zero voltage ride-through, loss of communication, abrupt load shedding.
Ensure the BESS can switch from charging to discharging within 20 ms.

7. Deploy with Redundant Communication

Fast and reliable communication between PV inverters and BESS is critical. Use dual fiber-optic rings with failover. Implement time-synchronized controls via IEEE 1588 PTP.

Common Mistakes

Underestimating the Need for Virtual Inertia

Many designs assume synchronous inertia is mandatory. Fortescue’s success shows that adequate virtual inertia from a BESS can handle RoCoF. Mistake: using too low H (e.g., <1 sec). Fix: model frequency response for the largest credible disturbance.

Neglecting PV Inverter Behavior During Faults

Standard GFL inverters may trip if voltage deviation exceeds their phase-locked loop (PLL) limits. Mistake: assuming all inverters have identical FRT curves. Fix: specify a unified FRT requirement for all procured inverters.

Poor Coordination of Protection Relays

Inverter fault currents are small, causing traditional overcurrent relays to misoperate. Mistake: using inverse-time OC curves designed for synchronous machines. Fix: implement voltage-restrained overcurrent (51V) or directional protection.

Ignoring Islanding Detection

When the transmission line opens, the local grid becomes islanded. Mistake: anti-islanding schemes in inverters may detect abnormal voltage and disconnect. Fix: modify anti-islanding settings to allow intentional island operation, or disable them in coordination with utility.

Overlooking Thermal Limits of Power Electronics

Sustained fault ride-through durations stress IGBTs. Mistake: assuming continuous operation at high currents. Fix: design inverter heat sinks for worst-case thermal loading during FRT (e.g., 1.5 p.u. current for 3 seconds).

Summary

Fortescue’s feat demonstrates that a 100% inverter-based grid can survive a transmission failure by using grid-forming battery controls, fault-ride-through capable solar inverters, and robust protection coordination. The core requirements are: (1) a BESS with fast grid-forming inverters providing virtual inertia and voltage support, (2) proper FRT settings across all IBRs, (3) careful tuning of protection schemes to accommodate low fault currents, and (4) high-speed communication for control coordination. By following the steps outlined here—from system modeling through HIL testing—you can replicate this milestone in your own green grid project.