Impact of Transformer Topology on Short-Circuit Analysis

This study analyzes short-circuit behaviors in distribution systems with inverter-based distributed generations (IBDGs), focusing on how interconnected transformer topologies affect zero-sequence fault currents. Unlike synchronous-based distributed generations (SBDGs), IBDGs have different short-circuit characteristics, making transformer topology and grounding critical for accurate fault analysis.

Key Concepts

Distributed Energy Resources (DERs): Renewable energy sources increasingly used in distribution systems.
Inverter-Based Distributed Generations (IBDGs): Power-electronics-based devices that convert direct current to alternating current, behaving as current sources.
Synchronous-Based Distributed Generations (SBDGs): Traditional DERs with rotating components, acting as voltage sources.
Zero-Sequence Fault Currents: Fault currents that flow through the zero-sequence network, influenced by transformer topology and grounding.

Transformer Topology and Zero-Sequence Networks

The interconnection transformer's topology significantly impacts zero-sequence fault current flow. Different transformer types either allow or block these currents:

D-Yg, Y-D-Yg, D-D-Yg: Block zero-sequence fault currents from DERs if not grounded at the grid side.
Yg-D: Allows zero-sequence fault currents to flow.

Variations in zero-sequence fault currents due to IBDGs can affect protective relay operations, especially with IBDGs' limited fault current contribution.

Fault Behavior Analysis: SBDG vs. IBDG

Using the superposition principle, the study compares fault analysis in systems with SBDGs versus IBDGs. IBDGs limit fault current magnitude due to their current-source behavior. A Norton equivalent resistance in the IBDG model further reduces fault current contributions.

Impact of Interconnection Transformer Topology

Different transformer topologies (D-Yg, Yg-D, Y-D-Yg, D-D-Yg) alter fault current characteristics. Grounding influences zero-sequence current contributions, affecting total fault current. The Yg-D transformer, grounded at the grid side, yields a larger total fault current compared to the ungrounded D-Yg transformer. Protective device settings should consider these effects for coordination and relaying.

Conclusions

IBDGs' fault current is generally smaller than SBDGs' due to their operation as current sources. Interconnection transformer topology is crucial due to its impact on negative- and zero-sequence fault currents. Grid codes must address transformer grounding design for DER systems, balancing fault current magnitude and transient overvoltage suppression with protective relay security.