Power Factor Lesson

Introduction

large-scale hydropower generation, power factor (PF) is more than a number. It is a control tool that affects excitation levels, generator temperature, and grid stability. Understanding PF from both machine and grid perspectives is essential for real-time operations.

At Bui Generating Station, the power factor isn’t just monitored but also actively managed. Through experience, we’ve learned that improper PF management can lead to thermal stress, grid imbalance, and inefficient operation. This article presents a field-based case study using actual data from 31st December 2021, highlighting the crucial relationship between Power Factor, Reactive Power, and Synchronous Condenser functionality.

Generator Design Parameters

Each of Bui’s synchronous generators is rated as follows:

• Type: SF133.34 - 40/9820

• Rated Capacity: 133.34 MW / 148.15 MVA

• Rated Voltage: 14.4 kV ±10%

• Rated Current: 5940 A

• Rated Power Factor: 0.9 (lagging)

• Rated Speed: 150 rpm

• Rated Frequency: 50 Hz

• Rated Excitation Voltage: 256 V

• Rated Excitation Current: 1536 A

• Short-Circuit Ratio: ≥1.0

• Rated Efficiency: 98.2%

These values define the generator’s safe operating envelope, especially under full-load and reactive demand conditions.

Grid Code Context: What Is Considered Normal?

According to the May 2023 National Electricity Grid Code, the National Interconnected Transmission System (NITS) is in a Normal State when:

• System Frequency is between 49.8 Hz and 50.2 Hz

• System Voltage stays within ±5% of nominal

For the 161 kV bus at Bui, this translates to a voltage window of 153–169 kV.

Exceeding these thresholds can trigger alarms, affect system coordination, or lead to forced VAR correction measures.

Field Case Study: 31st December 2021

Recorded Operating Data:

• Wicket Gate Opening: 83%

• Net Head: 77.6 m

• Active Power Output: 130 MW

• Reactive Power: –7 MVAr (leading)

• Power Factor: 0.99 (leading)

• Terminal Voltage: 14.71 kV

• System Voltage: 168 kV

• System Frequency: 50.28 Hz

• Field Current: 1150 A

• Field Voltage: 153 V

• Line Current: 5075 A

Despite operating below rated power, the generator was absorbing VARs, placing it in a leading power factor regime.

Why Was the Generator Absorbing Vars?

A key feature of Bui’s generator units is their ability to operate as synchronous condensers — a mode in which the generator is synchronized to the grid not primarily to deliver real power, but to regulate reactive power and support voltage stability.

In this mode, the generator typically absorbs approximately 2 MW of real power from the system to sustain rotor excitation and field winding operation, while delivering positive reactive power (MVAr) into the grid, especially useful during high system voltage conditions.

On 31st December 2021, the generator was:

• Supplying 130 MW

• Absorbing –7 MVAr

• Operating with a leading power factor of 0.99

• Responding to a system voltage of 168 kV (high, but still within the ±5% tolerance of the 161 kV nominal)

Although still delivering active power, the unit’s leading PF and negative MVAr output indicated its partial engagement in reactive support, mimicking synchronous condenser behavior. This helped absorb excess VARs from the grid and contributed to system voltage regulation at a time when the frequency (50.28 Hz) was already above nominal.

This behavior was not accidental. It demonstrates deliberate tuning based on real-time system needs, grid code awareness, and operational use of the generator’s dual-mode capabilities.

Impact on Machine and Grid

1. Machine-Side Impacts

• Field Current: Reached 1150 A, within safe range, but close to operational alert zone

• Stator End Heating: Leading PF operation risks increased heating at stator ends due to armature current distortion.

• Excitation Response: Generator adjusted to absorb VARs without exceeding field limits, thanks to tight PF control

2. Grid-Side Impacts

• Frequency at 50.28 Hz indicated over-generation

• High system voltage required VAR absorption, which our unit provided at the right moment

• Leading PF helped alleviate over-voltage stress on the grid without external capacitive compensation

Lessons Learned from Daily Dispatch

1. A PF of 0.99 Isn’t Always “Good.”

Unless you know the direction, a 0.99 PF could mean high excitation demand or risky stator stress. Always confirm if PF is lagging (supplying VARs) or leading (absorbing VARs).

2. Synchronous Condenser Mode Matters
   Generators that can act as synchronous condensers (like Bui’s) become dynamic VAR tools in grid control. Operators must recognize this and use it wisely, especially during off-peak or high-voltage periods.

3. Stay in Tune with the Grid

With frequency at 50.28 Hz and voltage near 168 kV, absorbing –7 MVAr helped offset systemic over-voltage. If the unit had continued at lagging PF, the instability could have worsened. 

4. Monitor Field Current Trends

Though the unit stayed below rated field current (1536 A), prolonged operation near 1150 A should trigger caution and load redistribution planning.

Conclusion

Power Factor is not just a machine metric, it’s a system lever. At Bui, our ability to read, interpret, and act on PF values in the context of field current, system conditions, and grid codes makes the difference between safe dispatch and excessive wear or instability.

When operating in synchronous condenser mode, the generator becomes a reactive power regulator, a vital tool in our national grid puzzle. Operators must treat PF not just as a number to be monitored, but as a real-time signal that speaks both to the generator and to GRIDCo.

“Safe and efficient power delivery begins with knowing what your machine is saying, and PF is often the first word.”

— David Kwaku Boadu