DC Bus Overvoltage: 196 Spikes, One PCS Unit

At one of India’s earliest first-of-its-kind grid-scale BESS deployments, Ingro EMS logged 196 DC bus overvoltage events on a single PCS unit. Voltage spiking from a nominal 720–750V to as high as 881V. The safe operating ceiling: 840V.

We looked at the data and here is what the data actually showed.

The fault

One PCS unit at a grid-scale BESS deployment.

• DC bus voltage — spiking from 720V to 881V.
• Safe ceiling — 840V.
• Trip events — 196.
• Shutdowns in a single day — 57.

Each shutdown = node offline = lost grid service revenue.

One data point that ruled out the obvious

Every time voltage spiked — current dropped to zero. Simultaneously.

• Short circuit: Voltage ↓ | Current ↑↑↑
• What we saw: Voltage ↑↑↑ | Current = 0

These are physically opposite signatures.

This is not an isolated problem

NERC 2022 California BESS Disturbance Report: 622 MW of BESS capacity lost to DC bus overvoltage in a single incident.

Root cause? Unknown. Reason: none of the affected facilities could perform sequencing-level root cause analysis.

Ingro EMS could. That is what made the diagnosis possible.

After diagnostic testing — three most probable causes

1. DC bus capacitor degradation

Voltage-smoothing components degrade with age and heat. In Delhi’s conditions — faster than rated lifetimes suggest.

2. Precharge circuit failure

The startup sequence may have broken down. On most modern PCS platforms, a software-level compensatory fix exists. Corrective options constrained by OEM firmware on this platform.

3. AC contactor timing mismatch

Wear causes timing drift. Even milliseconds matter at these voltages. This unit: 2.5x more switching operations than its healthy peer.

A voltage spike is easy to see

DC bus overvoltage is not a fringe fault. It is a systemic risk — and at scale, the consequences are severe.

Knowing why it happened requires data most deployments don’t have.