Grid-Forming PCS Mandate — What Changes in Interlock Design

Starting with the conclusion. Starting with next year’s commissioning targets, PCS with Grid-Forming capability will be mandated for the first time on the 2nd Central Contract Market BESS (540 MW class). For a new commissioning project, choosing a grid-forming PCS has moved into the advantageous position, but for an operating site where permitting and grid-interconnection agreements have already been completed with an existing Grid-Following PCS, maintaining the current setup is more rational than forcing a retrofit. In both cases, a re-examination from the interlock design standpoint must come first.

1. Background — Why the Grid-Forming PCS Mandate Is Starting

According to a report by E2News, grid-forming inverters will be mandated for the first time starting with the 2nd Central Contract Market BESS (540 MW) entering commercial operation next year. The background is the decline in grid inertia driven by the expanding share of renewable energy. As synchronous generators are reduced, the range in which the PCS must actively form voltage and frequency to support the grid is growing.

PCS hardware trends are converging at the same time. SiC (silicon carbide) MOSFETs are increasingly replacing conventional silicon IGBTs, achieving conversion efficiencies above 99%. As switching frequency rises, EMI/noise countermeasures require separate, measurement-based re-validation (waveform analysis and field measurement required).

2. Grid-Forming vs. Grid-Following — Interlock Design Comparison Table

The row most important in practice in this table is ‘interlock design difficulty.’ The field’s response is that converting to grid-forming is not a simple firmware update, but a matter requiring full re-validation of the startup sequence, protection-relay setpoints, and even the EMO (Emergency Off) interconnection logic.

3. Pre-Conversion Checklist for Grid-Forming Transition

• [ ] Confirm whether the project is subject to the 2nd Central Contract Market mandate (based on commissioning date)
• [ ] Confirm whether the PCS manufacturer’s grid-forming mode simultaneously satisfies IEEE 2800 and the domestic grid-interconnection standard
• [ ] Confirm that the islanding-detection logic and the grid-forming autonomous-operation logic are interlocked as mutually exclusive
• [ ] Confirm that the EMO (Emergency Off) signal is directly hardwired through to the PCS gate trip — check that it is not substituted by software judgment alone
• [ ] For a PCS adopting SiC MOSFETs, perform a field measurement to check whether switching noise interferes with adjacent BMS communication (CAN/RS485)
• [ ] Confirm that protection-relay setpoints (overvoltage/overcurrent/frequency trip) were recalculated for grid-forming mode

4. Interlock Logic Example (Conceptual)

Below is a conceptual example of the safety-interlock judgment logic when starting up a grid-forming PCS. The safety interlock always confirms the hardwired path first, ahead of software judgment.

# Grid-Forming PCS Startup Interlock Logic (Conceptual Example)
# I/O MAP
# DI001 : EMO hardwired state (0=normal, 1=trip — SW override prohibited, hardwired interlock maintained)
# DI002 : Grid voltage detection signal (for grid-present judgment)
# DI003 : Islanding detection signal
# AI001 : PCS output voltage (V, RMS)
# AI002 : PCS output frequency (Hz)
# DO001 : PCS gate enable relay
# DO002 : Grid-forming mode select signal (0=grid-following, 1=grid-forming)

FREQ_MIN, FREQ_MAX = 59.3, 60.5  # Hz, example grid-interconnection reference (field spec confirmation required)
VOLT_MIN, VOLT_MAX = 342, 418     # V, example upper/lower limits for a 380V grid (field spec confirmation required)

def evaluate_gfm_interlock(emo_trip, island_detect, grid_present, volt, freq):
    # Safety interlock checked first — immediate trip on EMO hardwired signal, ahead of any SW judgment
    if emo_trip:
        return "DO001_GATE_DISABLE"  # Hardwired path maintained, not bypassed by software

    # Whether to hold grid-forming autonomous operation upon islanding detection follows policy settings (field spec confirmation required)
    if island_detect and not grid_present:
        # Grid-forming is held only at sites where autonomous operation is permitted; otherwise, stop
        return "DO002_GFM_HOLD_OR_TRIP"  # Branches according to per-site operating policy

    # Gate enabled after confirming voltage/frequency are within normal range
    if VOLT_MIN <= volt <= VOLT_MAX and FREQ_MIN <= freq <= FREQ_MAX:
        return "DO001_GATE_ENABLE"

    return "DO001_GATE_DISABLE"

This logic is for conceptual explanation only. Actual threshold values and islanding policy vary according to the regulations of the relevant grid operator and equipment manufacturer, so field measurement and regulatory cross-checking must always be performed before application.

5. Boundary Condition — Where This Judgment Flips

The point where this judgment flips is clear. For an operating site where grid-interconnection agreements and permitting have already been completed with an existing grid-following PCS, converting to grid-forming may incur interlock redesign and re-certification costs that exceed the resulting contribution to grid stability. For such a site, maintaining the current setup is rational, unless it falls under the mandate scope (new commissioning).

Conversely, for a new commissioning project — particularly one located in a region with weak grid inertia due to a high renewable-energy share — adopting a grid-forming PCS is advantageous by this single criterion. However, the final interlock logic and threshold values are governed first by the relevant AHJ and grid-operator regulations, and this article's classification is a reference guide only — it should not be taken as the basis for a definitive judgment.

References

• Grid-forming inverter mandate starting with next year's commercial-operation BESS (Korean) — E2News
• Guide to PCS Power Conversion System Use (2026 SiC MOSFET Trend) — BSL Battery
• US20220090578A1, System and method for providing grid-forming control of an inverter-based resource — Google Patents