Fundamentals to the Formation of Islanding and Anti-Islanding Protection as Countermeasures

Either micro-grid or commercial power generation and distribution network of Solar and Wind energy faces the chances of instability like blackouts and grid outages and then they are still generating excessive power yet not transferred to the local utility grid.

This situation is referred to as islanding that can cause serious risk both to onsite technicians who are maintaining the facility as well as  for the public safety [1], as specified in standards like IEC 62116、UL 1741、IEEE 1547.

Implementing proper anti-islanding measures for in power grid has evolved correspondingly as standards and regulations enforce companies to do so, against which trend the verification of devices with such functions have been growing conspicuously. ActionPower’s proven solutions to offer dynamic and precise testing equipment including PV simulators and advanced grid emulators for solar energy generation industries have been trusted by Sungrow and GoodWe, you may find more information by visiting our page of photovoltaic testing solutions.

Understand What is Islanding Before Looking into Anti-Islanding Protection

A typical example of islanding happens with a solar energy generation system when solar panels feed a grid supply and which can be either a household solar installation or a large commercial solar farm and plant [2].

Despite that fact that islanding can sometimes be intentional [3], one of the islanding’s causes is when there are both battery storage and inverter are present in the solar panel system that the later converts the DC power generated from solar arrays into AC power for loads application. The islanding can happen when the battery is of excessive energy while the inverter continues let the system generate power independently to even there is grid outage or blackouts. The result would be that the affected area in the facility or installation is surrounded by lines that are not delivering power [4].

Supposing that in a community with solar energy system where substation breaker opens due to something as a fault happens so that it is disconnected from the local grid, meanwhile that PV inverters are still possibly looking for a voltage source in case that load by coincidence still matches the remaining generation from the PV, the feeder line then can be continued to be energized. It may happen that when electrical personnel who arrives to fix the issue will mistakenly perceive that the line has been disconnected from power, a result of which could be fatal electric shock.

How Does Anti-Islanding Protection Work?

Institute of Electrical and Electronics Engineers (IEEE) 1547 states in section 4 that an area electric power system must be de-energized within no more than 2 seconds when an islanding happens. [5]

Or say, the solar plant that energizes a specific part of the grid should detect the island once it happens and cease energizing the grid within 2 seconds to either ensure the electrical personnel’s safety as well as for the safety of devices along the interconnection against voltage and frequency runaway.

Anti-islanding is usually achieved through passive detection (monitoring grid conditions) or active detection (intentionally perturbing the system) while in real-world practice it is usually by the combination of both of them to detect when the utility grid goes offline and ceases the power export from the inverter.

Passive Anti-Islanding Protection

Passive anti-islanding protection works by continuously monitoring electrical parameters of the grid to detect signs of disconnection.
The core idea is to identify abnormal changes in system behavior—such as deviations in voltage, frequency, or phase—that result from
power imbalances once the utility grid is lost. During normal operation, the main grid provides a strong regulating force to keep
these parameters within acceptable limits. However, when the grid is disconnected and the local generation and load are not perfectly
matched, those parameters may shift significantly, indicating a possible islanding condition.

Specifically, passive methods rely on observing three key indicators:

• Voltage Magnitude: Protective thresholds are typically set at ±10%. For example, in a 220V system, the inverter will disconnect if the voltage drops below 198V or rises above 242V.

$$
V_{min} \leq V_{measured} \leq V_{max}
$$

$$
198\,\text{V} \leq V_{measured} \leq 242\,\text{V}
$$

• Frequency Deviation: Acceptable operating frequency range is usually 49.5Hz to 50.5Hz in a 50Hz grid.

$$
f_{min} \leq f_{measured} \leq f_{max}
$$

$$
49.5\,\text{Hz} \leq f_{measured} \leq 50.5\,\text{Hz}
$$

• Rate of Change of Frequency (RoCoF): This method detects the derivative of frequency over time:

$$
\text{RoCoF} = \frac{df}{dt}
$$

The greatest strength of passive protection lies in its simplicity and non-intrusive nature—it does not inject any signal into the grid,
thus preserving power quality. However, it suffers from a critical limitation known as the Non-Detection Zone (NDZ). This occurs when the power output of a distributed energy source closely matches the local load. In such cases, voltage and frequency may remain stable even after grid disconnection, making islanding difficult to detect.

Mathematically, NDZ is defined by the condition where both active and reactive power are balanced:

$$
P_{gen} \approx P_{load} \\
Q_{gen} \approx Q_{load}
$$

Under this condition:

$$
\Delta V \approx 0,\quad \Delta f \approx 0,\quad \frac{df}{dt} \approx 0
$$

Hence, the monitored parameters remain within their threshold limits, and the islanding event may go undetected. The size and shape of the NDZ depend on system characteristics and threshold settings, making it a crucial aspect of passive anti-islanding reliability assessment.

Active Anti-Islanding Protection

To overcome the limitations of passive methods, active anti-islanding protection introduces a proactive approach. These techniques involve injecting controlled disturbances into the grid to test the system’s response. The core principle is that during normal grid operation, disturbances are absorbed by the grid and have little effect. In an islanded state, however, these disturbances cause detectable anomalies that trigger protective actions.

Common active methods include:

• Active Frequency Drift (AFD): Periodically adjusts the inverter’s output frequency by adding a sinusoidal perturbation. Over time, in the absence of a strong grid, this causes frequency to drift outside acceptable limits.

The AFD method can be modeled as:

\[
f_{\text{inv}} = f_{\text{grid}} + k \cdot \sin(2\pi f_m t)
\]

Where:
\( f_{\text{inv}} \) is the inverter output frequency
\( f_{\text{grid}} \) is the nominal grid frequency
\( k \) is the modulation coefficient
\( f_m \) is the modulation frequency

• Harmonic Injection: Specific harmonic components are injected into the system and the resulting Total Harmonic Distortion (THD) in grid voltage is monitored to detect islanding.

Active methods offer significant advantages, such as minimizing or eliminating the NDZ and delivering faster response times, often detecting islanding within 1 second. However, they may introduce slight disturbances to power quality, especially in weak grid environments. Careful tuning is essential to balance detection reliability with system stability.

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Bibliography

• [1] Germany: Worker Injured on HelWin1 Offshore Grid Connection Project, April 18, 2013,  https://www.offshorewind.biz/2013/04/18/germany-worker-injured-on-helwin1-offshore-grid-connection-project/
• [2] Solar Anti-Islanding Protection: A Comprehensive Guide, By Strategy CyberOptik|November 30, 2023, https://suntegritysolar.com/uncategorized/solar-anti-islanding-protection-a-comprehensive-guide/
• [3] A review on intentional controlled islanding in smart power systems and generalized framework for ICI in microgrids, International Journal of Electrical Power & Energy Systems,  https://www.sciencedirect.com/science/article/abs/pii/S0142061519314474
• [4] How Does Anti-Islanding Work? | Grid-Connected Inverters, Windurance, July 27, 2020, https://blog.windurance.com/how-does-anti-islanding-work
• [5] Phase lock loop design and novel test schemes for islanding detection, Daniel Martin; Kuan-Hung Wu; Chien-Liang Chen; Jih-Sheng Lai, Date of Conference: 30 May 2011 – 03 June 2011, DOI: 10.1109/ICPE.2011.5944498, https://ieeexplore.ieee.org/document/5944498/