Anti-Islanding

ALMM and inverter compliance: MNRE’s current ALMM page publishes solar PV module and cell lists, not an inverter list. Verify the inverter’s exact model documents and applicable scheme or DISCOM requirements separately.

What is anti-islanding

Anti-islanding is the protection function in a grid-tied solar inverter that detects a loss of the utility grid and rapidly disconnects the inverter’s AC output. It prevents the inverter from continuing to energise a section of the distribution network after the upstream feeder has been switched off, isolated by a protection device, or damaged. Our anti-islanding protection guide walks through the Indian compliance side of this in more depth.

The hazard it addresses is direct and physical. When a feeder is de-energised by the DISCOM for maintenance, fault clearance or a downed line, the lineworker assumes the section is dead. If a solar inverter on a customer’s roof continues to push power into that section, the line stays live. The lineman touches it and is electrocuted. Anti-islanding is the layer of protection that keeps this from happening.

In India, anti-islanding compliance, tested to IEC 62116 or IS 16169, is mandatory for every grid-connected solar inverter. The CEA Technical Standards for Connectivity to the Grid 2019 enshrine this requirement and DISCOMs verify it during commissioning.

Anti-islanding explained simply

Picture a section of road that the road department has closed for repairs. They have put up barricades. They expect the road to be empty. If a car somehow keeps driving into that section, the workers could be hit.

The same logic applies to a power line. When the grid is off, the utility lineworker expects it to be off. If a rooftop solar inverter keeps pushing power, the line stays live and unsafe. Anti-islanding is the rule that says: when the main grid disappears, your inverter must stop within seconds, automatically, every time.

There is no override switch. There is no “I am sure no one is working on the line” exception. The inverter trips on grid loss as a matter of design.

Why anti-islanding matters

Lineworker safety. The first and most important reason. India’s distribution network sees thousands of lineworker fatalities a year already. Solar-driven islanding would add to that number without anti-islanding.

Equipment safety on reclose. When the grid is restored, an islanded inverter is rarely in phase with the returning grid. The phase mismatch destroys motors, transformers and the inverter itself.

Power quality. An island has no voltage or frequency regulation beyond what the inverter provides. Voltage drift damages connected appliances.

Public safety. A downed line that should be dead but is fed from a rooftop solar inverter remains a public electrocution hazard.

How anti-islanding works

Modern inverters use multiple detection layers running in parallel.

1. The inverter continuously samples grid voltage, frequency and phase at its AC output terminals.
2. A passive layer trips on out-of-range voltage or frequency. Typical settings are voltage 0.85 to 1.10 pu and frequency 47.5 to 51.5 Hz.
3. An active layer injects a small periodic disturbance, often a slight frequency drift or reactive power pulse.
4. The control loop watches the grid’s response. A real grid corrects the disturbance through its inertia. A dead island does not.
5. If the disturbance grows unchecked, the active method declares islanding and trips the grid contactor.
6. The AC contactor opens, isolating the inverter from the grid.
7. The inverter logs the event with timestamp and waveform data.

The full cycle from grid loss to contactor open takes 100 to 500 ms in typical residential inverters, well inside the 2 second limit set by IEC 62116.

Technical deep dive

Detection methods

Non-detection zone

The NDZ is the region in load-power space where a passive method cannot distinguish island from connected operation. The classic NDZ is defined by:

• The local load consumes exactly the inverter’s output power.
• The load is resonant at the grid frequency.
• Voltage and frequency remain inside the inverter’s trip thresholds.

Active methods shrink the NDZ to near zero, at the cost of injecting a small disturbance into normal operation.

Multiple inverters

When several inverters share a feeder, their active methods can interfere. Frequency drift injected by one is corrected by another. The IEC 62116 test procedure includes a multi-inverter case to verify performance.

IEC 62116 test setup

The standard test connects the inverter through a resonant RLC load tuned to 50 Hz. The grid is disconnected by opening a switch. The inverter must disconnect within 2 seconds. The test is repeated across multiple combinations of inverter power, load quality factor and reactive power.

Real-world applications

Residential rooftop solar. Every grid-tied home solar inverter in India has anti-islanding. It is invisible until a grid outage, when the inverter shuts off automatically.

Commercial and industrial solar. C&I inverters with multiple MPPTs and three-phase output have the same requirement.

Utility-scale solar. Central inverters and string inverters in solar parks have anti-islanding, although the priority shifts toward fault ride-through and reactive power support.

Hybrid systems. The grid-tie side of a hybrid inverter has anti-islanding. The off-grid backup side forms an intentional island after the grid is confirmed lost.

Microgrids. Intentional islanding is allowed for microgrids designed to operate disconnected, with separate protection coordination.

Advantages of anti-islanding

Saves lives. The primary justification.

Prevents equipment damage during grid reconnection.

Protects the public from inadvertently energised downed lines.

Enables higher rooftop solar penetration. DISCOMs are willing to allow more rooftop solar when they trust the disconnect protection.

Standardised globally. The same inverter passes IEC 62116, IEEE 1547 and UL 1741 anti-islanding tests, simplifying global supply chains.

Limitations

Nuisance tripping during normal grid disturbances. Voltage dips, frequency excursions and momentary phase jumps can trigger unwanted trips. Fault ride-through requirements partly mitigate this.

Multi-inverter coordination challenges. Active methods from multiple inverters can cancel each other. Standards address this through test procedures, but real-world conflicts still appear.

Power quality cost. Active injection of small disturbances costs a fraction of a percent in efficiency and adds harmonics.

NDZ residuals. Passive methods alone leave a small NDZ. Active methods reduce but do not eliminate it.

Coordination with reclosers. Utility reclosers expect a fault to clear during dead time. An islanded inverter can confuse the recloser logic.

Common problems

Frequent unexpected trips. Investigate whether the grid actually went out of trip window. Often the grid did move briefly out of voltage or frequency bounds and the inverter did its job.

Inverter not reconnecting after grid restore. The standard reconnection delay is 60 to 300 seconds. The inverter holds off until it confirms steady grid voltage and frequency.

Anti-islanding test fails at commissioning. The inverter took longer than 2 seconds. Possible causes are firmware misconfiguration, undersized AC contactor, or a software bug. Update the firmware.

False trip during cloud passage. Cloud edges can cause inverter active power to swing, which can cross a passive detection threshold. Modern firmware handles this, older firmware may need an update.

Multiple inverters on the same feeder all tripping simultaneously. One inverter detects islanding, trips, the others see the drop and trip cascade. Normal and acceptable.

Inverter reconnecting too quickly. Some firmware reconnects in less than 60 seconds, against CEA expectations. Configure to 60 seconds minimum.

Best practices for anti-islanding

Use only inverters tested to IEC 62116 or IS 16169 in India. Verify on the certificate, not the brochure.

Verify anti-islanding test results at commissioning. Get the trip time in writing on the commissioning report.

Set reconnection delay to 300 seconds where DISCOM rules permit. This avoids cascade reconnect oscillations.

Update inverter firmware annually. Anti-islanding logic gets steady refinement.

Coordinate with DISCOM upstream protection settings. Some Indian DISCOMs require specific frequency and voltage trip windows narrower than IEC defaults.

Log anti-islanding events for at least 90 days. Patterns in trip events expose grid quality issues that may need DISCOM attention.

Avoid mixing inverter brands on small sites unless tested for multi-inverter compatibility.

Comparison: anti-islanding vs related protections

Indian market context

DISCOM commissioning protocols include a witnessed anti-islanding test. The test is typically performed by opening the customer’s main switch with the inverter running at moderate load and confirming the inverter trips within 2 seconds.

For large utility solar plants, anti-islanding is supplemented by upstream protection at the substation. CEA grid code, IEC 61400-27 and SECI PPA terms all reference anti-islanding compliance.

Common Indian inverter brands like Sungrow, Solis, Goodwe, Growatt, Solax, Deye, Huawei, Sineng, Luminous and Tata Power Solar ship with IEC 62116 certified anti-islanding as standard.

Standards and certifications

Common mistakes

Disabling anti-islanding to stop nuisance trips. Illegal, dangerous and voids warranty.

Setting trip windows wider than CEA allows. The inverter may pass site tests but fail DISCOM commissioning.

Mixing inverter brands without checking multi-inverter coordination. Persistent nuisance trips.

Ignoring reconnection delay. Inverters reconnecting in under 60 seconds risk damage from grid disturbances.

Skipping the anti-islanding test at commissioning. The first time the grid actually drops, the inverter behaviour is unknown.

Treating anti-islanding as overly conservative. Lineman fatalities from energised lines are a real and recurring issue, not a theoretical one.

Key takeaways

Anti-islanding is the inverter protection that keeps the grid safe for lineworkers when distributed solar is on the network. It is required in India under the CEA connectivity regulations, with testing to IEC 62116 or IS 16169. Modern inverters use both passive and active detection layers, with disconnect times well under 2 seconds. Disabling it is never acceptable. Proper commissioning, firmware updates and DISCOM coordination ensure the protection works without nuisance tripping.

Further reading

For how Anti-Islanding plays out in real projects, these guides go deeper:

• Anti-Islanding Protection in Solar Inverters
• Solar Inverter Certifications: 9 Standards to Verify
• India-Grid-Tuned Solar Inverters, Explained
• How to Choose Solar Inverters with Surge Protection

Sources

• IEC 62116:2014, Utility-interconnected photovoltaic inverters: Test procedure of islanding prevention measures, International Electrotechnical Commission.
• IS 16169, Indian Standard for anti-islanding test procedure for grid-connected PV inverters, Bureau of Indian Standards.
• IEC 61727:2004, Photovoltaic systems: Characteristics of the utility interface, International Electrotechnical Commission.
• CEA Technical Standards for Connectivity to the Grid (Amendment) Regulations 2019, Central Electricity Authority of India.
• IEEE Std 1547-2018, Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces, IEEE.
• MNRE Approved List of Models and Manufacturers (ALMM), Ministry of New and Renewable Energy, Government of India.
• UL 1741 SB:2021, Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources, Underwriters Laboratories.
• Ye Z., Kolwalkar A., Zhang Y., Du P., Walling R., Evaluation of anti-islanding schemes based on non-detection zone concept, IEEE Transactions on Power Electronics.
• NREL Technical Report TP-560-37408, Evaluation of Islanding Detection Methods for Photovoltaic Utility-Interactive Power Systems.