(HT Switch gear Automation and remote start/stop based on voltage sensing).

Remote Operation and Sequential Closing of HT VCBs for Transformers Based on Power Availability reduced the shock loading and reduced the Breakdown in incomer line/H-pole and overall infrastructure. Detail briefing with calculation is as follows.

The purpose of this document is to define standardized operational and automation procedures for remote opening and closing of HT VCBs associated with transformers through sequential energization logic based on power availability.

This initiative aims to:

1. Improve system reliability and operational safety.
2. Reduce breakdown incidents and equipment stress.
3. Enable unmanned or remotely monitored operation.
4. Prevent simultaneous transformer energization.
5. Improve protection coordination and asset life.

This document applies to:

1. All HT receiving panels and transformer incomer VCBs.
2. Grid-connected transformer systems within the facility/Building/Mall/Retail/ Hotel etc
3. Automation logic implemented via Relay-based or PLC-based systems.
4. Maintenance and operation personnel responsible for HT infrastructure.

Background and Technical Justification

Simultaneous energization of multiple transformers from a single grid feeder introduces significant operational risks:

• Inrush Current Impact/starting current impact
  1. High magnetizing inrush current exceeding normal operational levels.
  2. Risk of nuisance tripping or delayed protection response.
  3. Mechanical and thermal stress on transformer windings and switching equipment.
  4. Potential damage to joints, cables, and auxiliary components.

Typical Inrush Current for a 2000 kVA Transformer

Step 1 — Calculate Full Load Current

Assume: Transformer rating = 2000 kVA

HT side voltage = 11 kV (example; may vary)

Full-load current: 𝐼=2000√3×11=𝑎𝑝𝑝𝑟𝑜𝑥.105𝐴

Normal HT current ≈ 100–110 A

Step 2 — Inrush Current Magnitude

Typical inrush current:

6 to 12 times rated current (can be higher depending on conditions)

So: Minimum inrush ≈ 600 A

Possible peak ≈ 1200 A or more

In extreme conditions:

Can reach 15–20 times rated current.

When Two 2000 kVA Transformers Energize Simultaneously

If both transformers are energized at the same time from one feeder:

Combined Inrush Current:

Transformer 1: ~1000 A (example peak)

Transformer 2: ~1000 A

Total instantaneous current may reach:

2000 A or more (very short duration but significant).

Important:

Inrush currents are not purely additive linearly due to phase angle differences.

However, system experiences very high transient current stress.

• Voltage Stability Issues
  1. Voltage dips or transient spikes due to sudden load changes.
  2. Reduced system stability.
  3. Contact arcing and overheating risks in breaker jaw contacts.
• Protection Coordination Challenges.
  1. Protection settings often adjusted to higher thresholds to prevent nuisance tripping.
  2. Compromised asset protection and delayed fault clearing.

Sequential transformer energization allows optimized protection coordination based on load and transformer impedance.

Expected Outcomes upon implementation of this Guideline

1. Reduced electrical stress on transformers.
2. Improved grid stability during restoration.
3. Enhanced protection performance.
4. Increased equipment life.
5. Reliable unmanned operational capability.