PERFORMANCE EVALUATION AND SCALABILITY MODELING OF MODBUS TCP, RTU, AND HYBRID GATEWAY ARCHITECTURES FOR MULTI-INVERTER SOLAR FARM COMMUNICATION

Abstract

Modbus remains the dominant communication protocol for grid-tied photovoltaic (PV) inverters due to its simplicity and near-universal vendor support, yet its two principal transport variants Modbus RTU (serial) and Modbus TCP (Ethernet) exhibit markedly different scalability characteristics as the number of inverters in a solar farm grows. This paper presents a simulation-based performance study that quantifies polling latency, full-cycle response time, and throughput for three farm-communication topologies: pure Modbus TCP, pure Modbus RTU, and a hybrid RTU-to-TCP gateway architecture. A real Modbus TCP client-server pair was implemented and measured directly, while Modbus RTU timing was derived analytically from the official frame structure and simulated as a discrete-event queueing system that includes a stochastic per-device turnaround delay, validated against previously published benchmarks. Experiments were repeated across farm sizes of 10 to 500 inverters and baud rates of 9600 to 115200 bps, with ten trials per configuration. Results show that Modbus TCP sustains sub-200-millisecond full-farm polling cycles even at 500 inverters, whereas pure Modbus RTU at 9600 baud supports at most 10 inverters within a 1-second real-time refresh threshold. The hybrid gateway architecture is shown to close this gap only at higher baud rates: it offers negligible improvement at 9600 baud but restores full 500-inverter scalability at 115200 baud. These findings provide quantitative guidance for selecting and provisioning Modbus-based communication architectures in utility-scale PV plants.