Fault Ride-Through

The Voltage vs. Time characteristic curve defining the minimum voltage magnitude and maximum fault duration a generator must tolerate. Typically specified by the Transmission System Operator (TSO) in a Low Voltage Ride-Through (LVRT) curve. The unit must remain connected above the curve's knee point, often tolerating zero voltage at the point of common coupling for up to 150 milliseconds before a gradual voltage recovery is permitted.

During a voltage sag, the inverter must prioritize dynamic reactive power support to help stabilize the grid voltage. Modern grid codes require a fast-acting proportional control loop that injects additional reactive current (up to 100% of rated current) in response to the voltage deviation. This positive-sequence reactive current injection must activate within a specified response time, typically less than 40 milliseconds, to counteract the fault-induced voltage collapse.

Following fault clearance, the generation unit must restore active power output to its pre-fault level at a defined rate. An excessively fast ramp can cause post-fault frequency oscillations, while a slow ramp leads to generation deficit. The recovery rate is typically specified as a percentage of rated power per second (%Pn/s), ensuring a smooth transition back to normal operation without triggering subsequent instability.

Beyond voltage magnitude drops, faults cause sudden phase-angle jumps in the voltage waveform. FRT capability requires the inverter's Phase-Locked Loop (PLL) to remain stable and accurately track the grid angle during these abrupt phase shifts. A robust synchronization unit must ride through phase jumps of up to 30 degrees without losing synchronism, preventing erroneous tripping and ensuring correct active/reactive current injection alignment.

During unbalanced faults, a negative-sequence voltage component appears, causing a double-line-frequency ripple on the DC link and potential thermal stress. Advanced FRT control strategies inject a controlled amount of negative-sequence current to balance the phase voltages or mitigate active power oscillations. This prevents overcurrent tripping and ensures the inverter remains connected even under severe asymmetrical fault conditions.

Often coupled with FRT, this characteristic defines the generator's ability to remain connected during over-frequency and under-frequency excursions. The unit must operate continuously within a defined frequency band (e.g., 47.5 Hz to 52.0 Hz) for specified time durations. This prevents cascading generation loss during frequency events, which is critical for maintaining overall system inertia and primary frequency response.

The specific requirement for generation to remain connected during voltage sags typically down to 0% of nominal for defined durations. LVRT curves specify the voltage magnitude vs. time envelope below which disconnection is permitted. Key characteristics:

• Defined in grid codes like IEEE 1547-2018 and EU 2016/631
• Requires inverters to prioritize reactive current injection during the sag
• Critical for preventing cascading generation loss during transmission faults

Power electronic converters that synthesize a voltage waveform independently rather than following the grid. During fault ride-through, grid-forming inverters:

• Maintain voltage and frequency reference even when the main grid is weak
• Provide instantaneous fault current up to their thermal limits
• Enable stable operation in low short-circuit ratio environments
• Contrast with grid-following inverters that require a stable voltage reference to operate

A mandatory FRT function where inverters inject leading or lagging reactive power proportional to the voltage deviation. This supports grid voltage recovery:

• Typical requirement: 1-3% reactive current increase per 1% voltage drop
• Prioritized over active power during deep sags
• Helps stabilize transmission system voltage during and after fault clearance
• Coordinated with dynamic reactive devices like STATCOMs for large-scale support

The PLL is the control subsystem that synchronizes the inverter to the grid voltage angle. During faults, PLL performance is critical:

• Voltage phase jumps during faults can cause PLL transient errors
• Poor PLL tuning leads to incorrect current injection angles and potential instability
• Advanced PLL designs include dual second-order generalized integrators (DSOGI) for unbalanced fault ride-through
• PLL dynamics are a primary factor in converter-driven instability during weak grid conditions

Standardized procedures to verify FRT capability before grid connection. Testing involves:

• Subjecting the unit to predefined voltage sag profiles using a test container or simulator
• Measuring reactive current response time (typically < 20 ms)
• Verifying active power recovery ramp rate after fault clearance
• Confirming no undamped oscillations or protection trips during the sequence
• Standards include IEC 61400-21 for wind and IEEE 1547.1 for distributed resources

During asymmetrical faults (single-line-to-ground, line-to-line), the grid voltage contains negative sequence components. Advanced FRT control:

• Actively manages negative sequence currents to reduce torque ripple in wind turbines
• Prevents double-frequency power oscillations on the DC link capacitor
• Uses dual synchronous reference frame or resonant controllers
• Essential for compliance with unbalanced fault ride-through requirements in modern grid codes