# Deriving DERs VAR-Capability Curve at TSO-DSO Interface to Provide Grid Services

## TL;DR

This paper presents a systematic OPF-based method to derive aggregated VAR capability curves of distribution systems with DERs, considering operational constraints and system conditions, to support grid services at TSO-DSO interface.

## Contribution

It introduces a novel methodology to derive VAR capability curves for DER-rich distribution systems, accounting for unbalanced conditions and inverter flexibility, validated through simulations.

## Key findings

- Capability curves are influenced by inverter headroom and system constraints.
- IEEE 1547 compliance affects the VAR capability.
- Validation confirms the effectiveness of the proposed method.

## Abstract

The multitudes of inverter-based distributed energy resources (DERs) can be envisioned as distributed reactive power (var) devices (\textit{mini-SVCs}) that can offer var flexibility at TSO-DSO interface. To facilitate this vision, a systematic methodology is proposed to derive 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 inverter headroom flexibility, unbalanced nature of system and coupling with grid side voltage into account along with changing operating conditions. Further, the influence of several factors such as compliance to IEEE 1547 on the capability curve is thoroughly investigated on an IEEE 37 bus and 123 bus distribution test system along with unbalanced DER proliferation. Validation with nonlinear analysis is presented along with demonstration of a scenario with T-D co-simulation.