One of the problems with tetrapolar impedance measurements is the lack of spatial sensitivity within the measured volume. In this paper we compare the sensitivity of tetrapolar measurements and the focused impedance measurements (FIM) proposed by Rabbani et al (1999 Ann. New York Acad. Sci. 873 408-20), which give an improved sensitivity profile. Using a previously validated model of sensitivity based on Geselowitz's lead theory, the sensitivity of FIM using eight, six and four electrodes was investigated. All electrode configurations showed a maximum in the average sensitivity of a plane at a depth of one-third of the drive-receive electrode spacing. No difference was found in the sensitivity value of this maximum between electrode configurations having the same drive-receive electrode spacing. The six- and eight-electrode configurations showed negative sensitivity regions down to half of the drive-receive electrode spacing, whilst the four-electrode measurement showed negative sensitivity regions down to one-third of the drive-receive electrode spacing. The single peak in sensitivity beneath the centre of the electrode configuration became dominant at 0.56, 1.4 and 0.14 of the receive electrode spacing for the eight-, six- and four-electrode configurations respectively. Thus, the four-electrode FIM configuration gives a single peak closest to the surface.

The idea of microgrid, smart grid, and virtual power plant (VPP) is being developed to resolve the challenges of climate change in the 21st century, to ensure the use of renewable energy in the electrical grid. For the increasing demand for electricity, raising public consciousness about reducing carbon emission, the microgrid is established which is transformed into a virtual power plant (VPP) or a smart grid with the blessing of modern communication systems, intelligence technology, and smart devices. So, to keep these systems up-to-date and to ensure security, it is important to know the details about the uses and benefits of these systems in the developed world and also to improve control methods and automation, it is important to achieve the present essence of such systems. This paper is focused to contribute to this flourishing area of energy sustainability covering microgrid, smart grid, and virtual power plant by compiling and recapping their recent advancements, technical requirements, control problems, and solutions. The paper is mainly intended to address the role of control strategies applied to the microgrid, smart grid, and virtual power plant towards future energy generation, distribution, management, and security.INDEX TERMS Controllers, cyber security, microgrid, renewable energy, smart grid, system architectures, virtual power plant, world market.

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