Key Considerations for Proper Generator Sizing
Choosing the right generator for your facility is not as straightforward as it may seem. Even experienced engineers face challenges due to limitations in sizing programs, uncertainties in load estimation, and technical complexities such as power factors, motor starting transients, harmonics, and UPS requirements. Below are the critical factors to evaluate when sizing a new generator.
Limitations of Generator Sizing Programs
Most generator sizing programs have not kept pace with modern generator technology. While these programs can analyze discrete loads and transient loads like motor starts, they often fail to:
- Support multiple load diversity factors.
- Accurately model non-linear loads that require harmonic analysis.
- Properly assess soft starters and UPS systems, as they treat all soft starters the same without accounting for different technologies.
Load Uncertainty and Its Impact
New Construction
Determining the exact power needs for a new facility can be difficult. Construction changes and evolving power demands may lead to under-sizing a generator. This can require adjustments in prioritizing essential versus optional loads during an emergency.
Load Growth Considerations
- Does your business plan anticipate future growth?
- Would an expandable generator solution be beneficial?
- Is paralleling multiple generators a viable option for long-term scalability?
Paralleling generators is often the best strategy, allowing businesses to invest in more capacity over time rather than purchasing an oversized generator upfront.
Existing Facilities
For existing facilities, understanding historical power demand is crucial. Analyzing past energy bills or using a power analyzer can provide valuable insights into the facility’s true power requirements before committing to a generator size.
Power Factor Considerations
Leading Power Factor
Generators are typically rated for a 0.8 power factor. If the power factor exceeds 0.8 (leading), voltage instability and over-voltage shutdowns can occur. Proper generator selection should account for power factor fluctuations to prevent operational disruptions.
Managing Motor Starting Transients
Starting motors can cause frequency dips that impact connected loads. UPS systems are particularly sensitive to these dips.
- A voltage dip should not exceed 15% to maintain system stability.
- Some generators may struggle to recover beyond 90% voltage after repeated dips.
- Oversizing the alternator can mitigate these issues and ensure reliable motor startup during an emergency.
Harmonics and Generator Sizing
Non-linear loads from computers, UPS systems, VFDs, and battery chargers can create harmonic distortions, leading to additional stress on the generator. These distortions:
- Cause harmonic currents to flow through the alternator’s impedance.
- Can only be accurately assessed through harmonic analysis rather than “rule of thumb” estimates.
Special Considerations for Power Electronics
IGBT Rectifiers
New Insulated Gate Bipolar Transistor (IGBT) rectifiers in drives and UPS systems improve power factor correction but can introduce electrical noise issues when operating alone. Connecting additional loads can help mitigate this issue.
Soft Starters
Soft starters are electronic devices that reduce voltage during motor startup. While they only generate harmonics during the starting phase, they can impact generator performance.
- Sequence starting can help manage harmonics.
- Enabling voltage ramping creates a smooth generator load transition and minimizes transients.
VFDs and Regeneration
Variable Frequency Drives (VFDs) act as rectifiers when connected to a generator, contributing to harmonic distortions. When dealing with regeneration (such as in crane or elevator applications):
- A braking resistor should dissipate excess energy.
- Other loads on the generator can help absorb excess power.
- Avoid relying solely on the generator for regenerative braking.
UPS Sizing Guidelines
The type of UPS used determines generator sizing requirements:
Passive Standby (Off-Line) UPS
- Generates harmonics, leading to potential voltage distortion.
- Loads should be added gradually to avoid issues.
- Generator kW rating should be UPS kW × 5.
Line-Interactive UPS
- Sensitive to frequency variations and generates significant harmonics.
- Load steps from the UPS itself are often a primary concern.
- Generator kW rating should be UPS kW × 5.
Double-Conversion UPS
- The industry standard for critical applications.
- No need for load stepping, but alternator size must consider harmonics.
- Additional considerations include battery recharge and HVAC requirements.
- Generator kW rating should be UPS kW × 2 to 2.5.
Final Thoughts
Proper generator sizing is essential to ensuring reliable power during emergencies. While sizing programs provide a starting point, they often lack the flexibility to account for complex factors like harmonics, load growth, and power factor variations. Conducting a thorough analysis, including historical power consumption and harmonics evaluation, can help select the most efficient and cost-effective generator for your facility. For expert guidance, consider consulting a professional engineer experienced in power system design.
