### Generators Sizing Calculations – Part Eight

 Subject of Previous Article Article Glossary of Generators – Part One Generators Sizing Calculations – Part One Glossary of Generators – Part Two Generators Sizing Calculations – Part Two First: Reasons for having on-site generators   Second: Applicable performance standards for generator sets   Third: Selection Factors Used For Generators Sizing Calculations Generator Power Ratings Application type Generators Sizing Calculations – Part Three Third: Selection Factors Used For Generators Sizing Calculations 3- Location Considerations, 4- Fuel Selection Considerations, 5- Site Considerations, Generators Sizing Calculations – Part Four Third: Selection Factors Used For Generators Sizing Calculations 6- Environmental Considerations, 7- System Voltage and Phase, Third: Selection Factors Used For Generators Sizing Calculations 8- Acceptable percent of voltage & frequency dip, 9- Acceptable duration of the voltage & frequency dip, Generators Sizing Calculations – Part Six Third: Selection Factors Used For Generators Sizing Calculations 10- Percent And Type Of Loads To Be Connected – Part One Generators Sizing Calculations – Part Seven

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Today, we will continue explaining other Selection factors used for Generators Sizing Calculations.

 Third: Selection Factors Used For Generators Sizing Calculations

 Here we will describe preliminary factors for selecting a generator for certain project, which will be as follows:   Generator Power Ratings, Application type, Location Considerations, Fuel Selection Considerations, Site Considerations, Environmental Considerations, System Voltage and Phase, Acceptable percent of voltage & frequency dip, Acceptable duration of the voltage & frequency dip, Percent and type of loads to be connected, Load step sequencing, Future needs.

 10- Percent And Type Of Loads To Be Connected – Part Two

 Introduction     Why analysis and categorization of generator loads is very important? Loads have different electrical characteristics. When developing a load analysis, it is helpful to analyze and categorize generator set loads into groups with common characteristics to assure proper consideration of their power demand because A generator set is a limited power source, sometimes referred to as a “limited bus”. The limited bus does not have the reserve capability of a utility grid. Note: There are no rigid standards for categorizing loads.

 Loads Information Used In Generator Sizing Calculations   So, the following loads requirements must be determined before Sizing a generator:   Knowledge of the customer’s loads, Knowledge of load management strategies, Knowledge of starting requirements.

 First: Knowledge Of The Customer’s Loads   A generator’s electrical loads can be classified into various categories according to various factors as follows:   According To Load Nature-1 According To Load Nature-2 According To Load /phase distribution According To Load Operation Time According To Load Importance According To Load Function   We explained the first five categories in article “Generators Sizing Calculations – Part Seven”.

The most important step in sizing a generator set is to identify every type and size of generator loads. It is necessary to segregate these loads in different application categories after gathering a reasonably accurate load schedules.

 Typical Electrical Loads Lighting loads –linear Incandescent lamps, Lighting loads – nonlinear Fluorescent lamps, High-intensity Discharge (HID) and arc. Heating Resistor Ovens, Convection Ovens, Dielectric Heating, Induction Heating, Arc Furnaces. Welding Resistance Welding, Arc Welding, Induction Welding. Critical loads Medical imaging loads, Data Centers/Computers, Communications Equipment. Motors DC Motors, Induction Motors, Synchronous Motors. Miscellaneous Adjustable /Variable Speed Drive (VSD), Rectifiers and solid-state controllers, Uninterruptible Power Supply (UPS), Battery charger loads. Regenerative loads Elevators, Cranes, Hoists.

 C- Welding Loads   Welders draw erratic fluctuating current. These current fluctuations produce voltage waveform distortion due to relatively high-load source impedance. Generator sets may require significant de-rating with welder loads.

 7- Regenerative Power   Some motor applications, such as elevators, cranes and hoists, depend on motors for braking. If a mechanical load causes the motor to turn faster than synchronous speed, the motor will act as a generator and feed power back into the system. The term “regenerative power” is sometimes used to describe the power produced by these loads. If no other loads are connected to absorb this energy, these loads will cause the generator to act as a motor, possibly causing engine over-speed which can lead to engine failure/shutdown. Regenerative power is usually not a problem when the utility is supplying power because it can be considered as an infinite power source with many loads. Only engine frictional horsepower can be relied on for braking. Exceeding frictional horsepower causes the generator set over-speed. In calculating the ability of a system to overcome regenerative power, it is conservatively recommended that only engine friction horsepower be considered. Engine friction horsepower at synchronous speeds is available from the engine manufacturer. Typically, a generator set will retard approximately 10% of its rating. When combinations of connected load and engine frictional horsepower are not sufficient to restrain regenerative energy, load banks may be added to protect the generator from being affected regeneration.

Second: Knowledge of Load Management Strategies

It is the deliberate control of loads on a generator and/or utility to have the lowest possible electrical costs.

Knowing the type of load management that is most economical to a facility can help determine the size of generator needed based on the load factor and application.

The Load Management strategy is used for the proper sizing of generator needed to operate within that strategy. However, there are two broad rating categories in terms of load management:
1. Isolated from a utility,
2. Paralleled with a utility.

 1- Isolated from the utility Type Under 500 Hours per Year Over 500 Hours per Year Output Output available with varying load for less than 6 hours per day Output available without varying load for over 500 hours per year and less than 6 hours per day Standard Fuel Stop Power in accordance with ISO 3046/1, AS2789, DIN6271 and BS5514 Prime Power in accordance with ISO 8528. Overload Power in accordance with ISO 3046/1, AS2789, DIN6271 and BS5514. Typical Load Factor 60% or Less 60% to 70% Typical Hours per Year Less than 500 hours More than 500 hours Typical Peak Demand 80% of rated kW with 100% of rating available for duration of an emergency outage 100% of prime plus 10% rating used occasionally Typical Application Interruptible utility rates, peak sharing Peak sharing or cogeneration

 2- Paralleled with the Utility Type Under 500 Hours per Year Over 500 Hours per Year Output Output available without varying load for under 500 hours per year Output available without varying load for unlimited time. Standard Continuous Power in accordance with ISO 8528, ISO 3046/1, AS2789, DIN6271 and BS5514. Typical Load Factor 60% to 70% 70% to 100% Typical Hours per Year Less than 500 hours No limit Typical Peak Demand 100% of prime rating used occasionally 100% of continuous rating used 100% of the time. Typical Application Peak sharing Base load, utility, peak sharing, cogeneration, parallel operation.

 Third: Knowledge of starting requirements   Starting Requirement Definition:   The time it takes to initiate a generator startup and when it is ready to accept load is defined as its starting requirement. Starting requirements will vary depending on the application. A typical starting requirement is 10 to 30 seconds.   Load Acceptance Definition:   Load Acceptance is the point at which breaker closure is initiated. This is considered to be 90% of rated frequency.

 Ten-Second Start Rule   Ten-Second Start Rule refers to the ability of a non-paralleled generator set to start, accelerate to rated speed and be ready to accept load within 10 seconds after receiving a signal to start. For 10-second starting, the following conditions must exist:   Cranking batteries must be adequately sized and fully charged. Combustion air must be a minimum of 21°C (70°F). A jacket water heater to maintain a minimum of 32°C (90°F) jacket water temperature. A readily available supply of clean fuel. The generator rotating inertia must not exceed that of the standard Generator.   Any variation in these conditions will affect the start time.   Note: There is a difference for battery versus ambient temperature sizing. Also, in cases where air starting is used, the air system must supply the required air volume and maintain a 100 psi (689.5 kPa) minimum pressure.   Ten-Second Start Rule for Natural Gas Engines   Special engine conditioning is required for a natural gas engine to start in 10 seconds. Ten-second starting is only possible on specific gas engines. Starting condition requirements for natural gas engines are the same as diesel engine starting requirements, except for one detail: The solenoid gas valve must be located as closely as possible to the carburetor or “A” regulator, depending on the fuel train consists. A maximum distance of 0.61 m (2 ft) is desired. A customer’s starting time requirements should be determined before sizing the generator. A quick starting application requires consideration of altitude, temperature and other factors that affect the engine starting to find the best solution.   Ten-Second Start Rule for Health Care Facilities as per NFPA 99   The standard NFPA 99 is written specifically for health care facilities. 3-4.1.1.8 states the following:   “The generator set(s) shall have sufficient capacity to pick up the load and meet the minimum frequency and voltage stability requirements of the emergency system within 10 seconds after loss of normal power.”   Customers requiring this capability can achieve it by using the correct generator system. Select an appropriate generator according to this standard for all health care facilities in the United States.

In the next article, we will continue explaining other Selection factors used for Generators Sizing Calculations. So, please keep following.

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