by Alex Apostolov, Editor-in-Chief

It Is a Whole New World

Microgrids are the latest trend in the electric power industry that on one hand allows the improvement in the reliability of power supply and on the other presents a completely new set of challenges to the protection, automation and control community.

The definition of the United States Department of Energy (DOE) states that a microgrid is “A group of interconnected loads and distributed energy resources (DER) with clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid [and can] connect and disconnect from the grid to enable it to operate in both grid- connected or island mode.” 

It is clear that like any other part of the grid it contains both loads and generation that have to be balanced – a task that is common to energy management systems at all levels of the electric power system. But there are many differences between the grid that we know and the microgrids that we don’t know.

First of all, the transmission level of the electric power system operates in parallel all the time (with the exception of the very rare cases of wide area disturbances that may lead to unintentional islanding). Most of the time the major loads are far away from the large synchronous generation stations requiring the transmission system to transfer the power and the distribution system to bring it to the individual customers. In case of the separation of a part of the system in an island we can immediately detect locally the unbalance based on the changes of frequency and voltage and can use them as a tool to determine the required action to restore the balance.

In the case of a microgrid there are some significant differences, such as:

  • Loads and distributed energy resources (DERs) are within the same physical area of the distribution system
  • Most of the DERs are not synchronous machines
  • The voltage and frequency control is not based on the same principles as the interaction between synchronous generators and loads
  • The need to operate in an islanded mode can be much more frequent than in the transmission or sub-transmission systems
  • Determining the unbalance between load and generation in an island cannot be based on local measurements only

From the protection point of view there are significant differences as well. In the conventional grid we have developed in more than a century advanced protection algorithms that can detect an abnormal condition and determine a faulted component of the system. The fault current is a major factor used in different forms by various protection methods and algorithms.

And here we have another major difference in the case of microgrids. Many of the DERs are connected to the microgrid through inverters that convert the DC output of the DER to the AC required by the loads.

The fault contribution of the inverters is practically non-existent, which makes the fault detection and localization almost impossible with the conventional protection methods and tools.
Considering the brain power and innovative thinking of the global protection, automation and control community, I am sure that it will not take long and we will see a whole new range of protection and control solutions that will ensure the safe and secure operation of the microgrids when they are disconnected from the grid.  

“If I had an hour to solve a problem I’d spend 55 minutes thinking about the problem and 5 minutes thinking about the solution.”

Albert Einstein

Power. Flexible. Easergy.
BeijingSifang June 2016