Abstract
As grid-forming (GFM) technology becomes the core solution for high-renewable power systems, 300% rated current overload capability (≥10 seconds) has become a mandatory indicator for GFM power conversion systems (PCS) under latest Chinese standards GB/T 47968-2026 and T/CES 243-2023. Unlike current-source grid-following converters, GFM-PCS operates as a controlled voltage source and actively injects fault current to support grid voltage. This article explains the rationale behind the 3×/10s benchmarks, and presents the hardware design, control strategies and system coordination required to deliver this capability.
1. Introduction
Conventional grid-following converters limit output current to 1.1–1.5× rated value during faults and provide no voltage support, which is insufficient in weak grids with high renewable penetration, where voltage sags can cause cascaded generation tripping.
Grid-forming PCS emulates the inertial and voltage-support characteristics of synchronous generators. The 3×10s overload requirement aligns with the performance of synchronous condensers, enabling massive reactive power injection during short-circuit faults to rapidly restore PCC voltage and prevent system collapse.
Key requirements:
300% rated AC output current
Minimum duration: 10 seconds
Valid for symmetrical and asymmetrical faults
Voltage-source characteristics maintained throughout overload
2. Rationale for 3× Current & 10-Second Duration
The 3×/10s benchmarks are industry consensus derived from decades of power system engineering and power device physical limits, not pure mathematical calculation.
2.1 Why 300% Rated Current
Alignment with synchronous generators
The steady-state short-circuit current of synchronous generators and condensers, defined by synchronous reactance Xd, stabilizes at 2.5–3× rated current. GFM-PCS must match this level to seamlessly replace rotating equipment and work with existing grid protection schemes.
Reliable relay protection operation
Grid overcurrent protection settings are calibrated to system short-circuit current levels. Current below 3× would cause protection failure or mis-coordination between primary and backup relays, leading to false tripping or fault escalation.
Cost-performance balance
IGBT/SiC devices typically support 3–5× short-time overcurrent. Requiring more than 3× would force significant device derating and sharply increase cost, while less than 3× fails to meet grid support requirements. 3× is the optimal trade-off.
2.2 Why 10 Seconds
Covers full protection sequence
The 10-second window covers all fault clearance stages: primary protection (~50ms), breaker operation (~60ms), backup protection (3–5s) and breaker failure protection (additional 2–3s). Total worst-case clearance reaches 7–8s, and 10s ensures uninterrupted voltage support until fault removal.
Legacy standard inheritance
Traditional synchronous generator standards require 3× short-circuit withstand for 10 seconds. Adopting the same benchmark ensures full compatibility with existing grid protection logic without system-wide retuning.
Device thermal limit matching
Short-time overload of power semiconductors relies on their thermal capacitance. At 3× current, IGBT junction temperature reaches its protection threshold (150–175℃) in approximately 10 seconds. Longer duration causes permanent device damage, while shorter duration cannot cover backup protection timelines.
3. Hardware Implementation Pathways
3.1 Traditional Derating Design
Components are selected for 3× current capacity, and the PCS operates at 1/3 of hardware capability under steady state.
Semiconductors: Oversized IGBT/SiC modules with sufficient junction temperature margin
Magnetics: Inductors and transformers designed for 3× short-time current to avoid saturation
DC side: Enlarged DC-link capacitors; battery capable of 10-second high-rate discharge
Cooling: Liquid cooling with real-time junction temperature estimation and protection
3.2 Native Non-Derating Design (Next-Generation)
New 2025–2026 GFM-PCS platforms deliver 3×10s overload without power derating, enabled by:
High-accuracy electro-thermal modeling of power devices
Optimized switching and gate drive to reduce overload losses
Low-thermal-resistance SiC modules
Targeted thermal design for high-stress components
This approach improves cost-effectiveness and power density, and is now the dominant technical direction.
4. Control Algorithm Architecture
4.1 Voltage-Source Characteristic Preservation
GFM-PCS must not degrade into a current source during faults. Unlike grid-following converters that clamp current and lose phase lock, GFM control retains the outer voltage loop and regulates current via virtual impedance, maintaining voltage support capability throughout the fault.
4.2 Adaptive Virtual Impedance Control
Normal operation: nominal virtual impedance for proper power sharing
On voltage sag detection: virtual impedance dynamically reduced to allow current rise
Current is controlled to approach but not exceed the 3× limit, with smooth transient and no mode-switching shock
4.3 Active/Reactive Current Allocation
~1.1× rated current reserved for active power to maintain frequency stability
Remaining capacity up to 3× allocated to reactive power for PCC voltage support
Vector sum of active and reactive current strictly limited to 3× rated amplitude
4.4 Protection Sequencing
Normal operation: GFM control, 1.1× current limit
Fault detection: overload mode activated when PCC voltage drops below threshold
Overload phase: 3× current limit enabled, 10-second timer starts
Dual-limit protection: current and junction temperature monitored; either limit triggers immediate derating
Exit: smooth return to normal limit after 10s or fault clearance
4.5 Multi-Unit Parallel Coordination
Units share fault current autonomously via droop characteristics without high-speed communication. Circulating current suppression is maintained, delivering total plant current equal to 3× sum of all unit ratings.
5. Battery System Coordination
Cells and BMS must support 10-second high-rate discharge
BMS communicates real-time discharge capability to PCS based on SOC, temperature and cell voltage
PCS automatically reduces overload limit if battery capability is insufficient
6. Common Misconceptions
Myth: 300% overload means 300% continuous power.
Fact: It is strictly a 10-second short-time fault support capability; continuous operation is limited to ~1.1× rated current.
Myth: 3× overload is achieved by simply raising overcurrent protection thresholds.
Fact: This will destroy devices. 3× overload requires coordinated design of voltage-source control, adaptive virtual impedance, thermal modeling and timed protection.
Myth: Grid-following PCS can achieve 3× overload via firmware update.
Fact: Grid-following converters are current sources that inherently limit current during faults. They cannot actively build voltage or inject controlled short-circuit current.
References
GB/T 47968-2026 General Technical Specification for Grid-Forming Converter; T/CES 243-2023 Technical Specification for Grid-Forming Energy Storage System.
