1. Project Statement
The objective of this analysis is to evaluate the voltage drop across the
electrical system during motor starting conditions and identify any critical voltage
sags that could lead to failure in motor operation. This study was specifically
focused on the performance of Bus 24 and Bus 25, each supplying power to large
induction motors during startup.
2. System Overview
- Source: Infinite Bus (U1 and U2) with 1000 MVA SC capacity
- Transformers: Multiple 6.5 MVA, 33/6.6 kV transformers (T1 to T4)
- Connected Load:
• Mtr6, Mtr14, Mtr15 — 1250 kW motors
• Several cables and lumped loads distributed across buses
- Nominal Voltage at Bus 24 and 25: 6.6 kV
3. Observations from Transient Stability Analysis
- Voltage observed on the Bus 24: 80.89% (19.11%) of nominal (i.e., ~5.34 kV)
- Voltage observed on the Bus 25: 80.89% (19.11%) of nominal (i.e., ~5.34 kV)
- These voltage levels were recorded during motor startup conditions.
4. Analysis and Impact
The measured voltage of 19.11% at both Bus 24 and Bus 25 represents a
voltage drop of approximately 19.11% from the nominal 6.6 kV supply.
•
Implications of High Voltage Drop:
- Motor Starting Issue: Large induction motors, especially those rated at 1250 kW,
require adequate voltage during starting. A voltage drop of more than 10–15% can:
• Severely increase starting current (6–8 times full load)
• Cause high torque slip and extended acceleration time
• Lead to motor stalling or failure to start
• Stress cables, transformers, and upstream components
• Trigger undervoltage protection if enabled
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- Bus Stability Concern: This significant voltage sag can cause relay misoperation,
frequency fluctuations, and system instability—especially under high inrush load
conditions.
5. Root Cause
- High Starting Load Demand: Simultaneous starting of multiple high-rating motors
(1250 kW) causes excessive inrush.
- Insufficient Dynamic Support: No evidence of dynamic VAR compensators or soft
starters observed in the system diagram to mitigate voltage dip.
- Cable/Transformer Impedance: Long cable runs, and transformer impedance
further contributes to voltage sag due to high current draw.
6. Recommendations
- Stagger Motor Starting: Introduce time delays between motor starts.
- Use Soft Starters or VFDs: Limit starting current to within system voltage dip
tolerance.
- Dynamic Reactive Compensation: Install capacitor banks or dynamic VAR
compensators (DYCO) to support voltage during transients.
- Check Tap Settings of Transformers: Optimize voltage regulation.
- System Reinforcement: If possible, increase transformer ratings or reduce feeder
impedance.
7. Conclusion
The voltage drop to 80.89% at Bus 24 and Bus 25 during motor starting is beyond
acceptable limits for reliable motor operation. This excessive voltage sag is the
primary reason the connected motors are unable to start, posing a critical issue for
system reliability and operation.
Without proper mitigation strategies, continuous operation under these conditions
may lead to system trips, equipment damage, and production losses.
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