Volt/var control with high solar PV penetration in distribution systems and its impact on the transmision grid

Singhal, Ankit
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With increasing distributed energy resources (DERs), both the distribution and transmission power systems are witnessing new challenges and opportunities. Voltage rise and voltage fluctuations are becoming major issues in the distribution network due to high solar PV penetration. Nevertheless, the volt/var control (VCC) capability of solar PV devices (inverter based DERs, in general) opens up various opportunities for both the distribution and transmission systems. In this work, we address the challenges in the distribution network and also explore the potential opportunities for the transmission system that can be provided by VCC capability of DERs in high PV penetration environment.

The first part of our work addresses the voltage challenges faced by the distribution system due to solar PV penetration by utilizing VVC capabilities of PV smart inverters. In this part, we focus on the slope sensitive local droop VVC recommended by the recent integration standards IEEE1547, rule21 and addresses their major challenges i.e. selection of appropriate parameters under changing conditions, issue of control being vulnerable to instability (or voltage oscillations) and bad set-point tracking performance i.e. high steady state error (SSE). This is achieved by proposing a local real-time adaptive VVC which has two major features i.e. a) it is able to ensure both low SSE and control stability simultaneously without compromising any of the objectives, and; b) it dynamically adapts its parameters to ensure good performance in a wide range of external disturbances such as sudden cloud cover, cloud intermittency and substation voltage change. Moreover, the adaptive control does not depend on the feeder topology information. The proposed control is implementation friendly as it fits well within the integration standard framework and depends only on the local bus information. The performance is compared with the existing droop VVC methods under several scenarios on a large unbalanced 3-phase feeder (IEEE 123 bus test system) with detailed secondary side modeling.

The second part of our work focus on investigating the impact of DER VVC on the bulk transmission grid.

We present a hypothesis that the multitudes of inverter-based DERs can be envisioned as geographically distributed reactive power (var) devices (mini-SVCs) that can offer enhanced var flexibility to a future grid as an ancillary service. To facilitate this vision, a systematic methodology is proposed to construct an aggregated var capability curve of a distribution system with DERs at the substation level, analogous to a conventional bulk generator. Since such capability curve will be contingent to the operating conditions and network constraints, an optimal power flow (OPF) based approach is proposed that takes curtailment flexibility, unbalanced nature of system and coupling with grid side voltage into account along with changing operating conditions. Further, the influence of several other factors such as revised integration standard 1547 on the capability curve is thoroughly investigated on an IEEE 37 bus distribution test system. In the last part of the work, we investigate the DERs' impact on the long-term voltage stability assessment on an integrated T-D system. Finally, a T-D cosimulation is employed to demonstrate how DER aggregated flexibility and var support can potentially enhance the transmission grid performance on an integrated T-D test system.

Distribution System, Smart Grid, Solar Energy, Solar PV, Transmission Grid