An Active Islanding Protection Scheme for a Grid Connected Solar PV System

Summary: Since the Past few decades the world is moving towards electrical energy, almost everything is being run by Electrical Energy across the globe. Specially in times of industrialization the energy demand rate is increasing more than ever. The U.S. Energy Administration stated in I

Project Title

An Active Islanding Protection Scheme for a Grid Connected Solar PV System

Project Area of Specialization

Electrical/Electronic Engineering

Project Summary

Summary:

Since the Past few decades the world is moving towards electrical energy, almost everything is being run by Electrical Energy across the globe. Specially in times of industrialization the energy demand rate is increasing more than ever. The U.S. Energy Administration stated in International Energy Outlook 2009 that the rate of generation of global electricity will be increased to 23.2trillion kWh in 2015, and for the next five years it will increase to 31.8 trillion kWh. The conventional electricity generation methods such as burning fossil fuels like coal, gas, oil is not only unable to meet up the sudden increase in demand, but also create a havoc in the environment. Fossil fuel emits toxic gas like carbon dioxide which is harmful for our health as well as the environment. The only viable, economic and environmental friendly method which can fill this gap is renewable energy. Among all forms of Renewable Energy such as solar energy, wind energy, biomass energy, tidal energy, the popularity of solar energy is increasing. The renewable electrical energy market has experienced an extraordinary increase in scope in recent years. Solar photovoltaic in 2016 was the main catalyst in its popularity. The solar panels are boosting the capacity of renewable resources across the globe. In the long run the low costs of solar energy is an attractive aspect compared to the conventional methods such as fossil fuels. According to the International Energy Agency, Two-thirds of overall electricity additions in 2016 were from renewable sources of energy. Solar is leading in additions compared to wind and hydropower.

The statistics of net additions and retirements in electricity capacity are shown in figure 1.1.

Figure 1.1. Grid-tied inverter in a typical micro grid scenario.

The photovoltaic cells can generate electric power of approximately 100 W per unit square meter under fine weather conditions, the PV power conditioner adopts the maximum power point tracking (MPPT) technique to utilize the PV array efficiently. The PV voltage and PV current are required to calculate the PV output power for the MPPT operation.

To inject power into the grid a Fly back inverter will be used. The Fly back inverter is a device that converters the DC into AC with the use of Fly back transformer. For this inverter topology, an advantage is that no Dc-Dc converter is required as the dc voltage needed. It is simple, has less components, therefore reduced cost and has another major advantage of galvanic isolation which is provided by the transformer. Islanding is undesirable because it leads to safety issues since a portion of the system remains energized while it is not expected to be,so the lives of the personal is at risk and has a few other major consequences. To detect islanding an active islanding protection scheme is used. It ensures the generators disconnects electrical supply to the grid in the event of disconnection of grid from the power system.

Project Objectives

The objective of this project is to design a grid tied inverter that is able to work in very high frequencies. Normally in electrical engineering with the increase in frequency, the size of the wire needed is bigger, along with this the resistance increases as well. In the case of the transformers it is the opposite, with the increase in frequency, the core size decreases, so with the increase in frequency, the smaller the transformer will be. After the design of fly back inverter, the next step is to synchronize it with the grid, so that the current from inverter is injected into the grid without any disturbance. The main goal after the grid tied inverter is to detect islanding.

Islanding is the condition in which a distributed generator continues to power a location even though electrical grid power is no longer present. An island is formed when one or more distributor generator units and an aggregate of local loads are disconnected (islanded) from the main grid and remain operational as an islanded entity. Islanding is either due to

• Preplanned events, e.g. maintenance outage. (intentional)
• Accidental events, e.g. faults and their subsequent switching actions. (un-intentional)

Figure 2.5. Grid and Distributed Generator providing power to load (normal operating state)

Figure 2.6. Islanded Condition (Grid Disconnected)

Islanding is undesirable because it leads to safety issues since a portion of the system remains energized while it is not expected to be, so the lives of the personal is at risk. It also has a few other consequences:

• loss of control over voltage and frequency
• excessive transient stresses upon reconnection to the grid
• uncoordinated protection
• in-adequate grounding.

Therefore, islanding must be detected and the islanded Distributive generator units must be disconnected from the rest of the system.

 For this reason, instead of the conventional H-bridge inverters, a Fly-Back inverter is designed. The inverter will convert the DC power that is produced from the photovoltaic cells into AC, and inject it into the grid.  

For the injection of power from the inverter to the grid, the phase of inverter current and the grid voltage should be locked. To execute this before the hardware, Psim is used for the implementation of software. After the synchronization of the inverter output with the grid. An islanding detection scheme will be designed. The conventional methods fail to support active islanding systems.

Project Implementation Method

Flyback Inverter Topology

Working of a flyback inverter can be explained in 3 modes. Mode I is defined for the situation where Sm is at on-state with all other switches are off. Mode II is defined for the duration where Sacp is at on-state with all the rest in off, iModes I and II are switched alternately at high switching frequency during the positive half cycle. The envelope of the peak current through the primary winding of the flyback transformer is modulated by the pulse width modulated (PWM) gate pulse of Sm to a sinusoidal form and is in phase with the ac utility grid line voltage. Mode III is defined for the duration where Sacn is at on-state with all other switched in off state, implying the stored energies in tertiary transformer winding and low pass capacitor being released to the ac utility grid line and giving the negative polarity. Mode I and III are switched alternately at high switching frequency during the negative half cycle. The current flowing through Sm is controlled in the same manner in the negative half cycles as it is controlled in the positive half cycles. 

The perturb and observe method is used for Maximum Power Point Tracking:

Implementation of islanding detection:
Active islanding detection schemes make a perturbation into the PV inverter output current by injecting an active signal. Due to the perturbation, the power balance between the PV generated power and local load power can be broken. For this current injection we had to control the inverter current that is entering the grid. This inverter current was controlled by converting phase current to Alpha-beta frame and then to DQ frame. In the DQ frame we can easily change the magnitude and phase of the inverter current and then convert back from DQ frame to Alpha-Beta Frame to generate V-reference that is used for PWM to produce a sinusoidal waveform of current.
Current-Disturbance Injection:
The current perturbation method is based on injecting a disturbance signal into the system through quadrature axis current controllers of the interface voltage-sourced converter. Signal injection through the q-axis controller causes a frequency deviation at the point of common coupling PCC, under islanded conditions. 

 Block Diagram of overall controller with I-Disturbance
The injection of current disturbance through the q-axis current controller culminates the frequency variation on post islanding due to the non-fundamental frequency components, which are forced to flow into the load. The voltage components cause the waveform to deviate from its pre-islanding fixed frequency of 50 Hz, whereas its amplitude is not affected. The islanding detection strategy is based on this frequency deviation. The frequency deviation reaches value up the standard limits and by the frequency deviation detector,t is possible to evaluate the islanding detection time. When the value of frequency is detected outside limits (49.5-50.5),the system will detect islanding.

Benefits of the Project

Benefits:
•    The flyback inverter design has a built-in transformer which provides galvanic isolation from the grid.
•    In this project we are dealing with high frequencies which lessens the size of the transformer core.
•    The cost of flyback inverter is lower than that of H-bridge inverter because of the small size of the transformer core as compared to H-bridge inverter. We also need PI controllers in case of H-bridge inverters to stabilize the output current which is not the case in flyback inverter.
•    Islanding detection can be very beneficial in case of the safety of personnel. For instance, if grid is disconnected for some maintenance or fault clearance, there is possibility that current can flow towards grid from the inverter and that can be dangerous.
•     We are using active islanding detection scheme due to its high accuracy and low Non-Detection Zone (NDZ).

Technical Details of Final Deliverable

At the end of completion stage, we will have the microchip solar panel connected to the national grid through the high frequency fly-back converter with is also capable of detecting islanding condition by an active method in the national grid. The hardware of the project will be modelled on the PCB boards using Flyback transformer, Mosfets, Micro controller, diodes, circuit breaker, solar panel etc.

Final Deliverable of the Project

HW/SW integrated system

Core Industry

Energy

Other Industries

Core Technology

Others

Other Technologies

Sustainable Development Goals

Affordable and Clean Energy

Required Resources

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