Microgrids – Old Concept on Steroids
Microgrids are gaining ground with each passing year in different parts of the world. In the United States, one of the main drivers that advanced the concept of microgrid was Hurricane Sandy. This hurricane, which hit the northeast part of the US in 2012, caused havoc with the transmission and distribution system. The advantages presented by smaller, self-sustaining grids started to look very attractive and there was a much greater interest in such small grids.
However, microgrid is not a new concept. If we look in the rear view mirror, we find that the original concept of a microgrid dates back to the 1800s. In 1882, Thomas Edison had built the first commercial direct current (DC) power plant. This plant, which was named the Manhattan Pearl Street station, was part of a small grid serving 82 customers with a distribution area of a few blocks. This plant also served as a combined heat and power source supplying both to customers. In addition, batteries at this plant served as power storage. This distribution was a standalone system with no external grid present. As years went by, alternating current (AC) started winning the race with DC (direct current) and the electrical grid as we know it today started taking shape. The energy sector started expanding and finally evolved into large electric utilities that generated, transmitted and distributed electric power. The result of this was a massive mesh of interconnected grids spanning the entire US. A similar evolution took place in Europe followed by Asia and other parts of the world.
The next big step in the electrical energy sector in the US occurred in 1978. In that year, a law called The Public Utility Regulatory Policies Act was enacted (PURPA). This act had its origins in the energy crisis of 1970s. The intent of PURPA was to encourage cogeneration and renewable sources that would promote competition and conservation. This resulted in several industrial plants installing steam/gas turbine generation. The local generators provided both electric power and steam that the plant could use. The generators operated in parallel with the grid. The plant could sell excess power to the host utility. In case of loss of the utility grid, the generators supplied the local plant load. Some plants supplied the local load and only upon loss of the local generator, would the host utility start to provide power to the plant in a seamless fashion. During under voltage conditions in the grid, the local generator provided reactive power to stabilize the grid voltage.
Later on, several independent power producers started building power plants to sell power to the electric utilities. PURPA had a clause that required the host utility to purchase power from these entities at reasonable prices. In several instances, host utilities offered attractive rates if the plant owners agreed to support grid stabilization by supplying MW and MVAR.
Differences and similarities between old cogeneration concepts and new microgrids
Local generation at the plant along with the plant distribution was in fact a microgrid. The generator supplied the local load and the plant had the option of switching over the load to the grid. This sounds very familiar with what we are seeing today with microgrids.
The in-house generation was also installed at parks, prison facilities, large telecommunication centers, etc. and the same applies to microgrids of today.
Microgrids consist of a diverse mix of distributed generation such as wind turbines, solar panels, fuel cells, and bio-mass fired. All of these sources if supported by a battery storage will enable power to be available around the clock. Microgrids have two things going for it ̶ the declining cost of Lithium-ion batteries and the increasing affordability of renewable energy.
There are some differences between what we see today in microgrids and the old cogeneration facilities:
- The generation at industrial plants, commercial facilities, parks, and jails consists of one or several generators located at one location. This is not the case with microgrids; generation is dispersed.
- Conventional generators did not create harmonics while the microgrids do generate harmonics. The issue of harmonics is resolved by applying appropriate filtering.
- The short circuit current provided by the generators at cogeneration facilities is adequate to accommodate effective relaying both at medium and low voltage buses. Renewable sources do not provide adequate short circuit currents.
- Microgrids do not possess adequate inertia to support system stability.
- Most of the generation in the microgrids are renewable sources but can also contain a conventional source.
- Microgrids with renewable sources are environment friendly and provide clean energy.
- In the case of microgrids the load is not necessarily one concentrated plant or facility. The load may consist of a mix of loads such as homes, schools and a hospital spread over a specific area.
Hence, we can see that the microgrids of today have several similarities with some exceptions. The advances made in the area of automation, communication and digitalization makes the microgrid quite different from the old cogeneration facilities.
Issues with new microgrids and possible solutions
There is no doubt that microgrids provide an array of benefits. However, microgrids do present some issues and challenges, which the industry is addressing.
The issues and possible solutions are:
- In situations where the microgrid operates in an isolated mode with no synchronous generator present starting of large motors will be difficult.
- Coordination between protective devices based on simple overcurrent relays is difficult due to inadequate short circuit current present both in terms of magnitude and time duration. This will be true for coordination between 480/277V as well.
- Differential relaying for both transformers and the lines, at several locations, will be required to provide a coordinated system with the required level of selectivity.
- Protective schemes will require adaptive relaying coupled with communication link between various protective devices.
- Application of protective devices that are compliant with IEC standard 61850 will be helpful. This means implementing protection based on GOOSE messaging which provides a good option.
- The 480V devices that consist of molded case circuit breakers and breakers with solid- state trip units. There is no formal coverage of 480V devices in IEC 61850. However, manufacturers have devised means of incorporating these devices into IEC 61850 based schemes.
- One of the drivers that pushed microgrids is the fact that during storms when the main grid is lost, the microgrids can keep supplying power to the assigned loads. However, this may not be true if the microgrid coverage area includes overhead distribution lines. Underground distribution will help alleviate this issue.
- There is a lot of research going on in the area of enhancing the converters applied at renewable generation. The issue of inertia, and lack of adequate short circuit both from magnitude and time duration perspective is being addressed.
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