CIRED Workshop - Rome, 11-12 June 2014 Paper 0046 Paper No 0046 Page 1 / 5 LOAD-FLOW MODELING OF A THREE-PHASE LOCAL VOLTAGE REGULATOR Arthur Barnes Vincent Martinelli Anthony Kam Gridco Systems – USA Gridco Systems – USA Gridco Systems – USA abarnes@gridcosystems.com vmartinelli@gridcosystems.com akam@gridcosystems.com Holger Wrede James Simonelli E.ON – Germany Gridco Systems – USA holger.wrede@eon.com jsimonelli@gridcosystems.com ABSTRACT Distributed power electronics are proposed as a solution for mitigating voltage regulation issues caused by photovoltaic generation. However, few commercial and open-source software packages for distribution system analysis support the modeling of such devices. A steady- state model for load-flow analysis of a three-phase voltage regulator is described. The model is multi-faceted and covers not just power flows and voltage regulation, but also design behaviours such as bypass. The model is implemented in Gridlab-D, but is suitable for implementation in other distribution analysis software packages. Results are demonstrated to be comparable to those obtained with an approximate model in a commercial package. INTRODUCTION Rapid reductions in the purchased cost of PV panels combined with attractive feed-in tariff programs are driving a rapid increase in the penetration of customer-owned PV installations. High penetration of PV on distribution feeders can cause voltage regulation problems in actual field installations, especially given the high variability in real power output from PV panels on a partly cloudy day [1]. Existing equipment, such as load tap-changers, operate in coarse increments and with slow time constants on the order of tens of seconds to minutes, preventing them from effectively regulating voltage. Coincident with the drop in cost of PV panels is the drop in cost of power electronics, which has now become a viable solution for low-voltage distribution system equipment. However, few tools for analysis and design of distribution feeders with such devices exist. This work presents the development of a steady-state model for a three-phase local voltage regulator (LVR) to be used in the distribution feeder. The model is multi-faceted and covers not just power flows and voltage regulation, but also design behaviours such as bypass. The model is implemented in the Gridlab-D simulation environment to assist distribution system engineers in deploying such devices. However, the modeling approach described can be applied to any extensible load flow analysis tool. MODEL & GRIDLAB-D IMPLEMENTATION The LVR topology we studied in this paper is based on a shunt-series UPFC architecture [2] (Fig. 1). This can be modeled with the series and shunt converters represented as voltage and current sources respectively (Fig. 2), which in turn can be modeled by its Norton equivalent (Fig. 3). Figure 1: UPFC architecture of an LVR Figure 2: static model of an LVR Figure 3: Norton equivalent of the static model We have implemented the Norton equivalent model in a custom build of Gridlab-D [3]. Gridlab- D is an open-source distribution system modeling tool developed by Pacific Northwest National Laboratory in Washington State, USA. The LVR is implemented as a new device (object) type, with its own controller logic. Some of the LVR controller pseudocode is shown in Table 1. Similar modeling technique can be applied to any extensible tool (e.g. CYMDIST via its COM interface or Python scripting module). Output Voltage − Injection XF Series Converter AC DC DC AC Shunt Converter Injected Voltage DC Bus − + Load + Load Current Input Voltage − + ↓ Input Current Source Voltage Source Input Bus Output Bus − + jX ↓ LVR Controller Output Current Source Input Current Source Reactive Line Input Bus Remote Voltage Sensing Bus Output Bus