PI: Osama Mohammed, Florida International University
The increased penetration levels of renewables and distributed energy resources lead to increased challenges in maintaining reliable control and operation of the grid. To overcome these problems, deep integration between intelligent measurement nodes, communication systems, IT technology, artificial intelligence, power electronics and physical power system components needs to be implemented to manage the modern smart grid resources. On one hand, this type of integration can dramatically improve grid performance and efficiency, but on the other, it can also introduce new types of vulnerability. The risk of vulnerability escalates when the level of integration between physical and cyber components of the power system increases.
The design and optimization of such complex systems requires coordination between the cyber and physical components in order to obtain the best performance while minimizing the risk of vulnerability. In other words, the physical power system must be designed as a security-aware system. A variety of distributed system architectures could be considered ideal for a given application purely from a power system perspective. However, determining which distributed system architecture would be the best choice from the cyber component aspect is significantly more challenging given the communication system topology required for control and management capabilities as well as the resiliency issues embedded in the cyber components of the system, such as the resiliency to cyber-attacks. The design of the cyber environment must correlate with the requirements and sensitivity of the physical component, for example taking into account such matters as the sensitivity of protection devices for communication delay.
Today’s Smart Grid constitutes several smaller interconnected microgrids. However, the integration of converter-interfaced distributed generation (DG) in microgrids has raised several issues such as the fact that fault currents in these systems in islanded mode are way less than those in grid connected microgrids. Therefore, microgrid protection schemes require a fast, reliable and robust communication system, with backup, to automatically adjust relay settings for the appropriate current levels according to the microgrid’s operation mode.
However, risks of communication link failures, cyber security threats and the high cost involved to avoid them are major challenges for the implementation of an economic adaptive protection scheme. This part of the work develops an adaptive protection scheme for AC microgrids which is capable of surviving communication failures. The contribution is the use of an energy storage system as the main contributor to fault currents in the microgrid’s islanded mode when the communication link fails to detect the shift to the islanded mode.
The design of an autonomous control algorithm for the energy storage’s AC/DC converter capable of operating when the microgrid is in both grid-connected and islanded mode. Utilizing a single mode of operation for the converter will eliminate the reliance on communicated control command signals to shift the controller between different modes. Also, the ability of the overall system to keep stable voltage and frequency levels during extreme cases such as the occurrence of a fault during a peak pulse load period.
The results of the proposed protection scheme showed that the energy storage -inverter system is able to contribute enough fault current for a sufficient duration to cause the system protection devices to clear the fault in the event of communication loss. The proposed method was investigated under different fault types and showed excellent results of the proposed protection scheme. In addition, it was demonstrated in a case study that, whenever possible, the temporary disconnection of the pulse load during the fault period will allow the utilization of smaller energy storage device capacity to feed fault currents and thus reduce the overall expenditures.
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