The Importance of Proper Generator Sizing for On-Site Engine/Generator Sets

Electric Generator Set | Source: Caterpillar


On-site engine/generator sets (gensets) are used in a variety of applications. They are becoming more popular for load management applications since the deregulation and privatization of the utility industry. Some gensets are used strictly as “back-up” for emergencies and some as the only power source. Although computer programs can be used to assist in proper generator sizing, an understanding of the formulas, calculations and non-numerical factors is beneficial to determine proper generator sizing. 

When selecting a diesel generator set for a given application, there are several important considerations that need to be taken into account. Some of these considerations include load types, harmonic content, generator considerations, and multi-engine installations.

Load Types

Load types refer to the type of loads that the generator set will be powering, such as resistive, inductive, or capacitive loads. The type of load can impact the size and type of generator set needed, as well as the overall efficiency and reliability of the system. All loads are different in their power quality needs. For example, a simple incandescent light bulb does not demand high quality power. The amount of light will drop proportionally to voltage, but frequency and voltage waveform free from distortion is not significant. Other loads are sensitive to those type of voltage variables. 

Harmonic Contents

Harmonic content is another important consideration in generator set selection, as it can impact the quality of the power supply. Harmonic distortion can cause issues such as overheating, voltage instability, and damage to sensitive equipment. Deviation from a pure, single sine wave can be expressed as additional sine waves of frequencies, which are a multiple of the generated frequency. These additional frequencies are called harmonics. Because three-phase generators are magnetically symmetrical, resulting in the cancellation of even harmonics, only odd harmonics are normally of any significance. For example, a 60 Hz generated waveform will contain the 60 Hz fundamental; a 180 Hz, 3rd harmonic; 300 Hz, 5th harmonic; 420 Hz, 7th harmonic and so on. A generator set with a low harmonic distortion level may be needed to ensure reliable and efficient operation. 

Figure 1. Linear and Non-Linear Loads

Generator considerations refer to the various factors that need to be taken into account when selecting the generator set itself, such as the power output, fuel type, and emissions requirements. Other factors that may need to be considered include the type of cooling system, the level of noise produced, and the overall size and weight of the generator set.

Generator Set Rating Considerations 

The rating definitions for diesel generator sets describe the conditions under which the generator set is designed to operate. The primary ratings are standby, prime, and continuous. Standby ratings are designed for use as backup power in emergency situations and have a limited number of operating hours per year. Prime ratings are designed for continuous operation at variable loads and have no limitations on operating hours. Continuous ratings are designed for continuous operation at a fixed load.

Accordingly, the load management, by which pertain to the process of balancing the electrical load on a generator set to ensure that it operates efficiently and reliably. Diesel generator sets are designed to operate at a specific load, and overloading the generator can lead to reduced efficiency, increased fuel consumption, and potential damage to the engine. There are various standards and regulations that apply to diesel generator sets, including emissions standards, safety standards, and performance standards. These standards ensure that generator sets are safe, reliable, and meet environmental requirements.

The site condition must also be considered in generator sizing. Site conditions such as altitude, temperature, and humidity can impact the performance of a diesel generator set. Higher altitudes, temperatures, and humidity levels can reduce the power output of the generator set and may require a higher rated generator set to meet the load demand.

Figure 2. Altitude Capability Data | Source: Caterpillar

Lastly, the transient response which refers to the ability of a generator set to respond to changes in load quickly and smoothly. The transient response time is the time it takes for the generator set to respond to a change in load and is an important factor in ensuring a stable power supply. There are various transient response standards that define the acceptable time for a generator set to respond to a change in load. These standards ensure that generator sets can respond to load changes quickly and efficiently and maintain a stable power supply. Examples of these standards include ISO 8528-5 and others. 

Figure 3. Mechanical Transients | Source: Caterpillar

Load analysis

When sizing a diesel generator set, load analysis is an important step to ensure that the generator set can supply the required power for the load demand. Load analysis involves examining the different types of loads that the generator set will be supplying power to and determining the appropriate size and type of generator set needed for each type of load. The types of loads that are typically analyzed in generator set sizing include lighting loads, motor loads, motor performance, motor starting techniques, and miscellaneous loads. 

  1. Lighting Loads - Lighting loads refer to the amount of power required for lighting systems, which is usually a small percentage of the total load. 
  2. Motor Loads - Motor loads, on the other hand, are typically the largest component of the total load, and include motors such as air conditioning units, pumps, and other machinery. 
  3. Motor Performance- Motor performance is also an important factor to consider in sizing the generator set, as it can impact the overall efficiency and reliability of the system. Factors such as the efficiency of the motor, the power factor, and the starting requirements of the motor can all impact the size and type of generator set needed.
  4. Motor starting techniques - this is another key consideration in sizing the generator set, as they can have a significant impact on the load demand during the starting process. Various techniques such as soft-start or variable frequency drives can help to reduce the inrush current during motor starting, which can allow for the use of a smaller generator set. 
  5. Miscellaneous loads - this include any other loads that may be present, such as electronic equipment or other power tools. These loads may require special consideration in terms of the type and size of generator set needed to ensure reliable and efficient operation. Overall, by analyzing the various types of loads and their requirements, an appropriate diesel generator set can be selected to meet the customer's power needs.

Customer Requirements

In order to select the appropriate diesel generator set for a customer's specific needs, it is important to consider various requirements that the customer may have. These requirements include the power demand, load steps, demand factor, and diversity factor. 

Figure 4. Sample Customer Load Profile

  • The power demand refers to the amount of power required by the customer's load and is an important factor in determining the size of the generator set needed. 
  • Load steps refer to changes in the load that occur over time and can impact the generator set's ability to respond to load changes quickly and efficiently. 
  • The demand and diversity factors are also important considerations in sizing the generator set, as they account for variations in the load demand and the number of loads being served.
  • Additionally, the starting requirements and performance are important factors to consider when selecting a generator set, as they impact the reliability and efficiency of the generator set in operation. 

By taking these customer requirements into account, a properly sized generator set can be selected that will meet the customer's power needs in a reliable and efficient manner.

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  • Title: Electric Power Applications, Engine and Generator Sizing
  • Source: Caterpillar

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