Island Operation in Power Systems

1. Island Operation

In recent years, the generation and integration of renewable energy sources (RES) such as wind farms, PV plants, and battery energy storage systems are increased in the power systems to meet the energy demand. Due to this integration of renewable energy sources, the power electronic converters are used for power generation in most of the renewables such as type-3 and type-4 wind turbine-based generators, PV inverters, and battery energy storage systems.

Consider the power system shown in Fig. 1. Suppose, If the electrical power grid is disconnected by breaker tripping due to intentional (i.e., maintenance or permanent fault) or unintentional (i.e., blackout due to disconnection from the utility), then the power grid is isolated from the converter-based sources and loads. In such a condition, the converter-based sources only supply to the loads called an island operation. The island operation is a standalone mode of operation of a generator (which is not connected to the electrical power grid) supplying to the loads.

2.   Problems with Island Operation

• If the utility cannot control the voltage and frequency on the island, there is a possibility of damaging the customer equipment (loads).

• The islanded system may be inadequately grounded by the RES interconnection.

• Reclosing into an island may result in re-tripping the line or damaging the RES equipment, or other connected equipment, because of out-of-phase reclosing.

• The protection system may not be coordinated because of significant changes in the short circuit current.

• The utility line workers and public safety cannot be guaranteed since the line remains energized because the utility does not have control.

• The increment of power mismatch between the power generation and loads can vary the system frequency and voltage beyond acceptable limits and can lead to instability.

• If there is a power mismatch between the RES generation and loads, the low frequency oscillation can be produced in the system voltage and current responses.

3. Islanding Detection Methods

There are two methods used for the detection of Island operation that are local and remote techniques.

3.1. Local Detection Methods

Local islanding detection methods use local measurements for island detection. It measures the system parameters such as voltage, frequency, active power, reactive power, phase angle, impedance, and harmonic distortion at the RES (locally) for island detection. Local islanding methods can be classified into two methods such as passive and active methods.

3.1.1. Passive Methods

In the passive methods, the system parameters are measured and compared with the predetermined threshold value to detect the island operation. The following system parameters can be measured.

• Over/under Voltage

• Over/under Frequency

• Voltage Phase Jump

• Voltage and current Harmonics

• Voltage unbalance

• Rate of change of power

• Rate of change of frequency

• Change of impedance

The passive methods can be implemented with low cost, and it will not affect the power quality. Therefore, it is a widely used technique. The passive methods have a non-zero detection zone (NDZ). Fig. 2 shows the non-zero detection zone (NDZ). Here, OF – Overfrequency, UF – Underfrequency, OV – Overvoltage, and UV – Undervoltage. The island operation will not be detected if the power mismatch between RES and loads happens inside this region. Since the NDZ region is large, this method fails to detect the island operation inside this region.

3.1.2. Active Methods

The active methods introduce perturbation or disturbances (i.e., inject the signal) into the frequency, voltage, and impedance parameters and observe the signals to detect the island operation.

• Impedance Measurement

• Slip-mode Frequency Shift

• Active and Sandia Frequency Shift

• Sandia Voltage Shift

• Negative sequence current injection

• Variation of active and reactive power

The active methods are more popular since it provides high accuracy in the measurement. However, the active methods can affect the power quality of the grid/system since it injects a perturbation signal, and it can lead to instability.

3.2. Remote Detection Methods

The remote islanding detection methods use advanced communication infrastructure between the grid and renewable energy generation to detect the island operation.

• Supervisory control and data acquisition (SCADA)

• Power line carrier communication (PLCC)

This method is more reliable than local techniques and it will not affect the power quality. However, it is more expensive to implement.

References

• Mohan Muniappan, “Analysis of low frequency oscillations in islanded operation of Microgrid” UPB Scientific Bulletin, Series C: Electrical Engineering and Computer Science, 2021.

• Walling, Reigh A., and Nicholas W. Miller. "Distributed generation islanding-implications on power system dynamic performance." IEEE Power Engineering Society Summer Meeting, 2002.

• Worku, Muhammed Y., et al. "Islanding detection methods for microgrids: A comprehensive review." Mathematics, 2021.

• Trujillo, César, et al. "Local and remote techniques for islanding detection in distributed generators." Distributed generation, 2010.

• Pvps, I. E. A. "Evaluation of islanding detection methods for photovoltaic utility-interactive power systems." Report IEA PVPS T5-09 180, 2002.