Sunsynk and Deye Microgrid Frequency Droop Control Guide

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Overview: Frequency-Watt Control in Islanded Microgrids

In grid-forming microgrids and backup power installations, pairing existing grid-tied string inverters with a hybrid battery inverter allows facilities to utilize solar generation during outages and load-shedding. When the utility grid disconnects, the primary hybrid inverter forms the local AC voltage and frequency reference.

To prevent battery overcharging when solar generation exceeds instantaneous site demand, the hybrid inverter relies on frequency-watt droop control. Official technical documentation confirms that "the larger main Sunsynk unit controls the output of the connected inverter in string set-up by frequency shifting." As battery state-of-charge rises toward target thresholds, the master inverter increases local AC frequency above standard nominal 50.0Hz, prompting grid-tied string inverters to modulate their power output downward in accordance with standard overfrequency power reduction curves.

Microgrid frequency-watt regulation operates across three operational thresholds:

• Nominal Grid (50.0 Hz): Full 100% String Solar Output.
• Droop Modulation Begins: Linear Throttling Activated.
• Curtailment Cut-off (52.0 Hz): Complete Inverter Disconnection.

Sizing Ratios and Inverter Integration Rules

When integrating grid-tied string inverters or microinverters into an AC-coupled microgrid via the auxiliary/generator terminals of the hybrid inverter, strict sizing ratios must be maintained to prevent frequency instability and inverter tripping.

Managing F47 AC Overfrequency Faults

During aggressive solar throttling or when microgrid loads drop suddenly with fully charged batteries, the hybrid inverter rapidly elevates the AC bus frequency toward the upper disconnection boundary. This triggers protective responses on the downstream string inverter:

1. Expected Alarm State: When the downstream inverter senses the intentional frequency rise, an overfrequency alarm is logged. The manufacturer documentation explicitly states: "Therefore, when the smaller inverter detects a frequency shift outside the permitted range, an F47 AC_OverFreq_Fault is triggered. This alarm can therefore be expected in this set-up, and the unit is operating as designed."
2. Automatic Reconnection: Once site load increases or battery storage capacity becomes available, the hybrid inverter restores nominal 50.0Hz frequency. After standard reconnection delay timers expire, the string inverter automatically resumes generation.

DC Bus Stability and Voltage Discrepancy Protection

Stable microgrid operation depends on precise DC battery bus voltage tracking between the inverter power stage and the battery management system (BMS):

• Voltage Divergence Cut-off: "If there is a voltage discrepancy of more than 1.5 volts between the inverter battery terminals and the BMS voltage, the inverter will halt PV production until the issue is resolved."
• Thermal De-rating Limits: When cold ambient conditions cause battery cell temperatures to fall toward 0°C, charging current is restricted to prevent lithium plating, requiring ambient recovery above 12°C to restore full charge throughput.

What You Can Change Yourself, and What You Cannot

Plant managers and qualified system operators can configure basic operational parameters via local console interfaces or mobile monitoring portals:

• Basic Operational Adjustments: Users can monitor real-time microgrid power flow, state-of-charge percentage, and frequency levels on the main display.
• Safety and Electrical Work: Physical AC coupling interconnections, auxiliary breaker installations, and subboard modifications must be executed by a registered person in compliance with South African embedded generation standards. Working with live AC switchgear or DC high-voltage battery terminals carries severe electric shock and arc-flash hazards.

What the Published Sources Do Not Tell You

While manufacturer technical notes provide clear guidance on frequency shifting principles and sizing thresholds, certain commercial system parameters require specialized engineering review:

• Specific Droop Ramp Rates per Third-Party Brand: Detailed frequency-watt slope settings vary between string inverter manufacturers and must be configured directly within the string inverter grid-profile settings.
• Dynamic Response Latency: The exact response time required for frequency stabilization across mixed solar-diesel-battery hybrid microgrids is subject to total rotating inertia and localized system impedance.