# A Secure and Efficient Group Key Management Scheme for Clusters of String Inverters
###### Abstract  
A string inverter converts the low voltage direct current coming from the string of its Photovoltaic (PV) panels into alternating current to be exported to the grid. In today Smart Grid's context, PV plants feature clusters of cooperating smart string inverters that exchange information in a multicast fashion (typically) over the Internet Protocol (IP). However, IP multicast does not provide any mechanism to limit the access to multicast data to authorized subjects only. A security infringement may cause a cluster either into exporting no energy into the grid (zero energy attack) or more energy than the limit set (energy overflow attack). Both the attacks can lead to potential severe consequences. In this regard, we are the first addressing those issues. Particularly, we propose a Key Management Service (KMS) for group key generation and distribution. The KMS provides forward secrecy and periodic refresh. We implement a prototype on a cluster of Power-One Italy S.p.A. a member of FIMER Group smart string inverters and evaluate the performance. Experimental results indicate that the scheme scales up to clusters composed of 50 inverters with an efficiency of 90.5% in terms of latency for group key distribution and 99% in terms of memory overhead. key management; inverter; multicast; group communication; authentication; photovoltaic system +
Footnote †: journal: Applied Sciences  
_Article_  
A Secure and Efficient Group Key Management Scheme for Clusters of String Inverters  
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dynamically work with the grid to increase its resilience, reliability, safety and security [5; 6]. Smart solar inverters make autonomous decisions to keep the grid stable and reliable as more distributed energy resources come online. Instead of just feeding power into the grid, smart solar inverters have two-way communication with it [7; 8]. Smart solar inverters can perform specific grid-supportive functionalities related to voltage, frequency, communications and controls. There are many smart solar inverters available on the market. In this work we specifically focus on the so-called (smart) string inverters. A string inverter is a device powered by one or more strings (i.e., arrays) of PV panels. In general, the number of PV panels a string inverter can handle depends on several factors including the output current and voltage of the PV panel, the number of strings allowed by the inverter and the maximum and minimum voltage allowed per string for the inverter. Likewise, the number of PV panels in a plant depends on several factors, such as the amount of energy to be exported to the grid, the efficiency of the PV panels, the site available surface area, the sun exposure and the peak sunlight in that area. In this context, now more than ever, PV plants feature several string inverters, i.e., clusters composed of several inverters. Clusters of string inverters represent a central component in PV plants of any size, in both residential and Commercial and Industrial (C&I) applications. In Figure 1 we show an example of C&I application featuring a cluster of three PVS-100/120-TL FIMER smart string inverters installed between Geneva and Lausanne, Switzerland. Photo courtesy of KDISOLAR (Nantes, France) [9]. In modern-day Smart Grid's context, the group communication model proves to be a suitable and effective paradigm to design and implement applications and services. In particular, in PV plants the group communication naturally supports and promotes in-network processing. This is a fundamental technique for elaborating the wealth of data provided by the cluster of inverters. According to this model, inverters cooperate towards a given cluster application by exchanging multicast messages. A relevant example of PV systems application is export limiting. Distribution Network Operators (DNOs) set limits to the amount of power that a cluster of inverters can export to the grid at any time to guarantee grid stability.