IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 19, NO. 4, NOVEMBER 2004 1733 The Coordinated Automatic Voltage Control of the Italian Transmission Grid—Part II: Control Apparatuses and Field Performance of the Consolidated Hierarchical System Sandro Corsi, Member, IEEE, Massimo Pozzi, Marino Sforna, and Giuseppe Dell’Olio Abstract—The hierarchical voltage control system, currently fully exploited in the Italian transmission grid and presented in Part I, improves voltage quality, network security and operation efficiency. This coordinated automatic system, operated by the Italian ISO (GRTN), is subdivided into three hierarchical control levels, requiring unconventional regulation apparatuses. They are the Voltage and Reactive Power Regulator (REPORT) at the power plant level, the Regional Voltage Regulator (RVR) at the regional dispatcher level, and the National Voltage Regulator (NVR) at the ISO control center. This survey paper provides a general description of the characteristics of these new automatic control apparatuses and of the interventions required on existing equipment for their installation, activation and operation. A brief presentation of the main dynamic performance of the hierarchical control system, from the inner to the outer control loops, is also given with reference to laboratory, commissioning and field tests. Index Terms—Automatic control, coordinated voltage control, multi-variable hierarchical control, optimal reactive power flow, reactive power, reactive power coordination, secondary and ter- tiary voltage regulation, stability margins, transmission losses, voltage, voltage ancillary service. I. INTRODUCTION T HE hierarchical voltage control scheme, presented in Part I, provides closed-loop real-time regulation of volt- ages at the main buses (pilot nodes) of the transmission network, through coordinated control of the reactive power resources associated with each pilot node (control area), mainly those of generators (control plants). The most significant levels of this hierarchical control realize the Secondary and Tertiary Voltage Regulations (SVR and TVR). SVR [1], [2] is a decentralized control scheme which automatically maintains the pilot node voltage at its set-point, through the adjustment of the reactive powers of local control generators and compensators: this area level control has a dominant time constant of 50 s. With a slower dynamics, SVR can also switch local shunt reactors/capacitor banks and control the transformer OLTC’s and SVC’s. Con- versely, TVR [3], [4] acts on the overall transmission network, Manuscript received April 19, 2004. Paper no. TPWRS-00185-2003. S. Corsi and M. Pozzi are with the Centro Elettrotecnico Sperimentale Ital- iano (CESI), 54-20134 Milan, Italy (e-mail: corsi.sandro@cesi.it; pozzi.mas- simo@cesi.it). M. Sforna and G. Dell’Olio are with the Gestore della Rete di Trasmissione Nazionale SpA (GRTN), 101-00138 Rome, Italy (e-mail: sforna.marino@ grtn.it, dell’olio.giuseppe@grtn.it). Digital Object Identifier 10.1109/TPWRS.2004.836262 with a dominant time constant of about 5 min, automatically updating all the pilot nodes voltage set-points. TVR aims to both minimize network losses and improve operation voltage security. The activities involved in practical application of the multi- level hierarchical control system in the Italian transmission grid are very advanced and are expected to be completed shortly. They include field commissioning of about 35 plant regulators (REPORT) [5], [6] and three regional regulators (RVR) [7], the majority of which (32 REPORTs and two RVRs) are already in operation. The Italian power plants involved are the largest hydraulic, thermal and combined-cycle plants, having in total more than 150 generators with an overall capability of about 20000 MVAR. The NVR control apparatus [8] is a combi- nation of a Tertiary Voltage Regulator (TVR), optimizing and controlling RVR voltage set-points in real-time, and a Losses Minimization Controller (LMC), forecasting by an Optimal Re- active Power Flow (ORPF) the network voltage, and reactive plan used by TVR as a reference input. NVR is under devel- opment for the ISO national control center and will be ready within 2004. The hierarchical voltage control scheme is very simple in comparison with other theoretical and unrealistic centralized schemes due to the small number of EHV controlled buses and telecommunication requirements. Notwithstanding the pursued objective of system complexity minimization, the effort to achieve an effective control system is still considerable, espe- cially for large transmission networks. This has been confirmed by past experience and existing applications. The main reasons for the involved effort are summarized below. – Unavailability on the market of the required control apparatuses. In the past the utilities involved had to specifically design and develop these apparatuses by themselves. Furthermore, fast telecommunication channels for control coordination and high-resolution transducers for pilot node voltage measurement are also required. – Significant impact of SVR and TVR application. This affects functional interaction and possible integration with the existing EMS (at the regional dispatcher and national control center), as well as interfacing with the AVR units and the power plant control room and operators. 0885-8950/04$20.00 © 2004 IEEE