Islanding Modelling and Detection for a Grid Connected Renewable Energy System

This thesis, written by Lujain Arif Altowairqi under the direction of his thesis supervisor and approved by his/her thesis committee, was submitted and accepted by the Dean of Graduate Studies and Research on .., in partial fulfillment of the requirements for the degree MASTER'S/MASTER'S in Energy. Finally, I would like to thank my parents who have helped me with their valuable suggestions and guidance which have been helpful in various stages of completing my thesis. Islanding is one of the problems associated with grid-connected PV systems that affect system voltage and frequency.

I declare that this thesis entitled “Islanding Modeling and Detection for Grid-Connected Renewable Energy System” is based on my original work except for quotations and citations duly acknowledged.

O VERVIEW
P ROBLEM S TATEMENT
O BJECTIVE
I MPORTANCE AND M OTIVATION
E NVIRONMENTAL AND S OCIAL I MPACT

Due to the previously mentioned reasons, it is better to design a robust, fast and efficient protection system for systems equipped with renewable energy sources. With the penetration of renewable energy sources, especially those connected to the grid, the development of the protection system is essential. The aim of this study is to design a new protection system for islanding detection in a low voltage system for those systems equipped with renewable energy sources and connected to the grid.

Islanding detection will help design an accurate protection system for those systems equipped with renewable energy sources.

B ACKGROUND

SA systems suffer from the high cost of batteries and have a storage limit that leads to the disposal of additional energy generated. This type of systems is not connected to the grid, therefore requires batteries to store the electricity produced in the period of off-peak demand, additional batteries require high costs [5]. In a grid-connected (GC) system, local electrical needs are met, and excess energy is connected to the grid, a surplus being used when needed.

The system may exist only to feed the grid, or it may be used to meet local demand and the excess may be fed into the grid.

L ITERATURE R EVIEW

Where ∆P is the power mismatch on the DG side, H is the moment of inertia for the DG system, and G is the rated generating capacity of the DG system. The ROCOF relay monitors the voltage waveform and operates if the ROCOF is greater than the set value for a certain period. Continuous monitoring of the source impedance will give an impression of whether the system is islanded or not.

Voltage imbalance: When the island layout takes place, the DG must take responsibility for the island's loads. The change in the third harmonic of the DG's voltage also gives a good picture of when the DG is islanded [21]. Islanding can be detected if the level of the reactive power flow at the set value is not maintained.

Impedance measurement method: The basic philosophy is the same as the passive technique that differs by islanding the system impedance. Sliding Mode Frequency Shift (SMS) Algorithm: [26] Uses positive feedback that changes the phase angle of the inverter current according to the PCC frequency deviation. An SMS curve is designed so that its slope is greater than that of the load phase in the unstable region.

The AFDPF problem is that the phase angle of the RLC parallel load depends on the operating frequency, which can contribute to insulation failure. Tav′ is the average of the previous four voltage periods. Uav is the average of Tav′.

Figure 2: A power line signaling scheme [6]

T HE H YPOTHESIS
T HE P ROPOSED S YSTEM
I SLANDING M ODELLING
C ONTROL OF THE S OLAR S YSTEM O UTPUT
S IGNAL A NALYSIS
S IMULATION
I SLANDING D ETECTION M ETHODS

The above block diagram in Figure 6 shows the proposed system, and the following subsections represent the modeling of each component of the studied system. As long as the light reaches the surface of the PV cell, the resulting electric current continues. The switching duty cycle, k, is defined as the ratio between on duration and the switching period.

Discontinuous conduction mode: The current IL of the inductor does not flow continuously under DCM. The fundamental function of the inverter is to convert the DC performance of the solar panels into an AC variable. AC converters must make the grid connection to the grid-tied PV system by inverting the direct current coming from the PV array into a sinusoidal waveform that is synchronized with the power grid. The DC-AC converter is also used to stabilize the DC bus voltage to a specified amount as the PV output voltage varies with MPPT (max. power point tracking) temperature, irradiance and effect.

The upstream converters are shown to give an idea of the control and DC–. The array is attached to several PV modules in series on the DC side. This is an indirect MPPT, probably due to its behavior matching the output I–V characteristic of the DC–DC converter and the I–V characteristic of the PV array when paired.

The reliability of the PV I–V array without control interference then serves as a fundamental feature for a DC–DC converter. Where, ψ(t) represents the mother or fundamental wavelet, 𝑎 is the scale factor and functions to translate the function to x(t); it allows compression or expansion of ψ(t), and τ is the variable to adjust the time scale of the probe function ψ. Mother wavelets are best suited for predicting local signal behavior such as irregularities and spots.

The fundamental frequency of 60 Hz is used and the maximum frequency reaches 1000 Hz. The analysis is done on a signal cycle that is at 0.16 sec, when the circuit breaker opens to separate the PV system from the utility grid simulating the islanding phenomenon.

Figure 6: The block diagram of the system under study

D ISCRETE F OURIER T RANSFORM B ASED I SLANDING D ETECTION

In Figure 21, the total harmonic distortion is 2.97% before islanding, and when compared to the analysis of the same case after islanding in Figure 22, which shows that the total harmonic distortion has increased due to islanding by 33, 78%. . The figure is divided into two plots, the first plot shows the selected voltage signal. In this case for a simulation time of 0.2 seconds, startup with the system connected to the grid and islanding occurs after 0.16 seconds from startup.

In the case of inductive loads, the total harmonic distortion was 12.8% before the occurrence of the island as shown in Figure 23 and it did not change after the occurrence of the island as shown in Figure 24.

Figure 21: Simulation results before the islanding with capacitive load

W AVELET T RANSFORM B ASED I SLANDING D ETECTION

In this case, the wavelet transform analysis method was tested with a full-load capacitive type. Table 2 presents a summary of the full load results of the haar mother wavelet analysis technique before all details were set to zero. The results of haar mother wavelet technique with 25% loading under different details are summarized in Table 3.

The following Table 4 shows the results of its parent wavelet analysis method under 50% of full load with various details. The following table 5 summarizes the results of the hair mother wavelet analysis at 75% of full load before and after the onset of islet formation with various details. In this case, the wavelet transform analysis method was tested with the full-load inductive type.

The following Table 8 shows the results of the haar mother wavelet analysis process with specific information under 50% of full load.

Figure 25: Full load haar mother wavelet capacitive case.

The wavelet transform analysis method has confirmed its effectiveness in instantly detecting the disconnection of the PV system from the power grid. Whereas the Fourier method may either experience some delay in detecting the occurrence of islanding or fail to detect the occurrence of islanding at all. Wavelet transform allows the use of long time intervals in low-frequency information and shorter regions in high-frequency information, therefore the wavelet method is faster than the Fourier method in detecting the occurrence of islanding.

The wavelet transform method will be part of any monitoring system to detect the occurrence of the islanding in the fastest time to reduce the negative impact of islanding on the loads. The following flowchart in Figure 33 explains the signal analysis mechanism using the Wavelet transform method. After checking all cases, the time required to detect the island is only the duration of one cycle, which means fast detection and fast response from the relay system.

If the values of the details amplitude are greater than zero or certain thresholds, the decision is island detection and there will be action by a protection system. If not, it means normal operation and the program returns to take another voltage sample. The previous software program will be part of a reinterpretation algorithm when designing a fast and efficient protection system for those systems equipped with renewable resources and containing classic high and low voltage relays and high and low frequency relays.

C ONCLUSIONS

F UTURE WORK

Fielding, “Protection against loss of utility grid feed for a distributed storage and generation unit,” IEEE Transaction on Power Delivery, vol. Palladino, “A power line signal-based technique for anti-islanding protection of distributed generators - Part ii: field test results,” IEEE Tran. Huang, “A detection algorithm for islanding prevention of consumer-owned distributed storage and generation units,” IEEE Trans.

Kim, “An islanding detection method for distributed generation using voltage unbalance and total current harmonic distortion,” IEEE Tran. Hwang, “Islanding detection method of distributed generation units connected to power distribution system,” in Proc. Bonn, "Determining the relative effectiveness of islanding detection methods using phase criteria and non-detecting areas," IEEE Transaction on Energy Conversion, vol.

Rohatgi, "Analysis and Performance Assessment of the Active Frequency Drift Method for Island Prevention", IEEE Tran. Island detection in an integrated distributed generation power system using phase space technique and probabilistic neural network. In Proceedings of the 2013 IEEE International Conference on in Industrial Technology (ICIT), Cape Town, South Africa, February 25–28, 2013; p.p.

Rahim, 'A review of the islanding detection methods in grid-connected PV inverters', Renewable and Sustainable Energy Reviews, vol. A paper accepted in the International Conference on Biofuels, Biodiesel Engineering and Technology (ICBBET 2020.