A novel method of analysis is deployed in detail on the basis of nonlinear systems theory with aim to design suitable controllers with guaranteed stability for grid-connected photovoltaic (PV) systems driven by power converters. By this method, all the system nonlinearities are taken into account to allow a reliable analysis in a wide range of operation and to avoid instabilities that as shown in the paper can be occurred by inappropriate design. To this end, firstly a detailed accurate nonlinear dynamic model is presented for the PV system by including a cascade-mode control scheme. Then, taking into account the closed-loop system and incorporating the concept of input-to-state stability (ISS), a rigorous novel stability analysis is developed which achieves to prove asymptotic stability of the desired equilibrium point. A multi-step process is deployed in which the control loops operating in cascade-mode are involved. However, since the cascaded-mode control scheme and analysis is based on the time-scale separation principle, a systematic tuning method is conducted for the accurate gain selection of both innerloop and outer-loop controllers which is based on the construction of suitable Lyapunov functions. To further validate the overall analysis, experimental results are carried out.

The integration of distributed energy resources (DERs) in modern power systems has substantially changed the local control capabilities of the grid since the majority of DERs are connected through a controlled dc/ac inverter interface. Such long-distance located DER installations, usually represented by current regulated dc sources, can inject large amounts of power into the main ac grid at points where the strength of the ac connection is low. The efficient and stable performance of such a power scheme is related to the capability of the control applied to retain the power extraction close to the maximum and simultaneously to regulate the dc-side voltage as well as the ac-side voltage magnitude at the weak ac connection point. This is implemented by designing the controllers of the voltage source inverters (VSIs) in a manner that reliably satisfies the above tasks. To this end, decentralized cascaded control schemes, driven by novel, locally implemented phase locked loops (PLLs), suitable to work in weak ac connections, are proposed for the VSI performance regulation by using new fast inner-loop proportional-integral (PI) current controllers. A decisive innovation is proposed by inserting an extra damping term in the inner-loop controllers to guarantee stability and convergence to the desired equilibrium. This is analytically proven by a rigorous analysis based on the entire nonlinear system model, where advanced Lyapunov-based methods are deployed in detail. As a good transient response of the VSI interface is indeed critical for the energy and grid system management, the conducted simulation and experimental results confirm that the proposed scheme efficiently supports the ac- and dc-side voltages of the VSI under different varying conditions in the power production or any voltage changes of the main grid.

In this paper we examine and compare the efficiency of four European electricity markets (NordPool, Italian, Spanish and Greek) of different microstructure and level of maturity, by testing the weak form of the Efficient Market Hypothesis (EMH). To quantify the level of efficiency deviation of each market from the ‘ideal’ or ‘benchmark market of random walk’, we have constructed a Composite Electricity Market Efficiency Index (EMEI), inspired by similar works on other energy commodities. The proposed index consists of linear and nonlinear components each one measuring a different feature or dimension of the market efficiency such as its complexity, fractality, entropy, long-term memory or correlation, all connected to the associated benchmark values of the Random Walk Process (RWP). The key findings are that overall, all examined electricity markets are inefficient in respect to the weak form of EMH and the less inefficient market, as measured by the EMEI is the NordPool, closely followed by the Spanish market, with the Italian being the third. The most inefficient market is the Greek one. These results are in accordance with the predominant view about the maturity of these markets. This study contributes significantly on improving the research framework in developing consistent and robust tools for efficiency measurement, while the proposed index can be a valuable tool in designing improved guidelines towards enhancing the efficiency of electricity markets.

Energy Storage Systems installed at primary substations can be used by different participants of the power system for handling the emerging uncertainties caused by supply-side variations, demand-side flexibility and grid topology changes. This work presents a practical design for the monitoring system of the controller of the grid interactive energy storage system. This monitoring scheme takes advantage of synchrophasor measurements gathered by phasor measurement units of wide area measurement systems. The analysis of synchrophasor measurements provides real-time situational awareness over the status of the grid. Therefore, the integration of synchrophasor measurements into the control loop of GI-ESS will enable them to participate in power services of the flexibility market. Furthermore, the implementation of a basic power oscillation damping function as an example of power services using the proposed monitoring system is illustrated in this paper.

Congestion issues are becoming more prevalent as the share of variable renewable energy increases. Conventional methods to mitigate them, such as building new lines, require significant lead time and upfront investment costs. Thus, alternative approaches that deal with congestion problems faster and in a more efficient manner have gained significant interest. In this paper, a modular mobile power flow controller (MPFC) is proposed that can redirect power flow from congested lines to adjacent ones by modifying the line reactance, but also offers additional significant advantages: a) scalability due to the modular nature that allows the MPFC to be sized optimally for addressing specific system needs, b) redeployability due to the mobile deployment method that allows the MPFC to be rapidly deployed and easily removed and redeployed to another location, and c) replicability stemming from the minimal construction requirements for connecting the MPFC to the transmission grid. The whole process of planning, installing, operating and redeploying the MPFC on a 150kV transmission system is thoroughly documented and studied. Overall, this paper presents a novel technology that can offer a quicker and at a lower cost solution to congestion issues, while reducing the impact of new infrastructure on communities and the environment.