The connection of a microgrid to the utility/grid requires consideration of many important factors. The interconnection is governed under various rules and regulations of the local electrical code. These rules are based on international standards with a focus on the protection of the power infrastructure, equipment and the people working on these systems. The interconnection must implement a protection scheme to ensure that the microgrid is a ‘good citizen’ [2.7] and follows all of the rules defined by the local distribution company.
2.2.1 Essential Components
A microgrid power system has two critical components for operation: the static switch and the micro-source [2.5].
2.2.1.1 Static Switch
The static switch controls the state of the grid connection to force the microgrid into autonomous island mode when the microgrid is separated from the main grid. This separation is typically performed when a disturbance is detected on the grid connection. The microgrid will reconnect to the grid on the re-closing of the static switch when the conditions causing the separation have been cleared. The grid re- connection can only be completed when the voltage, frequency and phase angles of the microgrid match those of the grid at the connection point.
2.2.1.2 Micro-Source
Each micro-source connected to the microgrid delivers power under the supervision of the system controller. The control mechanism will be different during island operation of the microgrid versus operation when connected to the grid. While islanded, the micro-sources must generate power to follow the load to ensure
adequate power delivery. The micro-sources will follow the command of the supervisory controller while connected to the grid, typically under a current control mode to deliver a specified amount of power.
2.2.2 Microgrid Control
A microgrid should be able to seamlessly isolate itself from the grid and quickly switch the operating mode. It is required that reactive and real power be independently controlled. The microgrid must be able to meet the dynamic needs of the loads without requiring information from the loads. Micro-sources must be independently controlled and allow sources to be added and removed when necessary [2.5]. Advanced control techniques are implemented in power electronics and are often required to integrate storage and renewable energy sources.
From a control perspective, grid connected operation and island operation are vastly different and require different strategies. In a grid connected operation, the grid provides a reference for both voltage and frequency. In an island operation, the microgrid is responsible for its own references as well as managing generation to meet load requirements. A common control strategy in island operation is to implement droop control to coordinate sources [2.8, 2.9] against load to form a balanced operation. The island controller also needs to consider the condition of black start (initial start-up without the grid being present) and the eventual connection to the grid [2.10, 2.11].
2.2.3 Standards and Guidelines
Since this research involves the construction of test equipment, special consideration must be given to the standards and guidelines that govern the interconnection with the grid. The research is being conducted at the University of Western Ontario located in London, Ontario so the jurisdictions that are covered include the province of Ontario and the country of Canada. The standards bodies have issued a few guidelines that small power producers are to follow to make a safe interconnection with the grid.
MicroPower Connect Interconnection Guideline: For inverter based
micro-distributed resource (DER) systems connected to 600 volt or less distribution system, Version 8, July 2003
Electrical Guidelines for Inverter-Based Micro-Generating Facility (10
KW and Smaller), ESA-SPEC-004, April 2010
The interconnection requirements can be organized into the following three categories: general specifications and requirements, safety and protection requirements, and power quality requirements. The main goal of the requirements is to ensure the safety of personnel, the public and the power system. These goals are reflected in the requirement for all interconnected microgrids to have compliance with national electric codes and standards [2.12]. The standards are listed in Appendix B.
2.2.4 Interconnection Protection
One of the biggest changes to the distribution system is the introduction of distributed generation (DG). One of the drivers behind this movement is the need to integrate renewable energy sources into the distribution system. Traditional protection schemes used is the distribution system need to be re-evaluated with the integration of DG associated with customer loads. The interconnection protection varies widely depending on factors such as: generator size, point of interconnection to the utility system (distribution or sub-transmission), type of generator (DC, synchronous, asynchronous) and interconnection transformer configuration. Newer DG systems are utilizing electronic power converters which results in special consideration for DG protection. The impact of DG on existing systems must be examined through detailed simulations and protection studies.
2.2.5 Safety
The project to interconnect the laboratory microgrid to the campus distribution system introduces new concerns with regards to safety. Safety for equipment must be considered both for the protection of the campus/building distribution system and the laboratory equipment. Safety for the people working on the laboratory and
campus distribution system must be considered in all aspects of the interconnection and bus designs. The reality of the laboratory back-feeding power into the building must be considered and all precautions taken to ensure that this is identified at the connection point and maintenance and research personnel are informed and trained on lock-out/tag-out procedures to be taken when working on the building and laboratory electric systems.