Generators: What to Consider While Selecting, Installing and Testing

There are several reasons for considering the provision of electrical power using generation sets:

The main reasons are;

1. Private Generation: the site or area that requires electricity is not able to be served by the grid, security reasons and economic reasons where power is generated for injection to the grid.
2. Standby generation: this is where there is power from the grid but there is a need for an alternative source of power to protect against failure or disruption.

Private Generation

Generators become attractive for supplying both base loads and peak loping purposes. Normally there are two or more sets used to produce power, especially for base load applications. This becomes ideal due to maintenance and reserve reasons where one set can be taken out of operation and the others will remain on-line. These types of generators are the ones being used by IPPs and small regional grids to generate power for sale.

Standby Generation

This is the most common utilization of generators in areas where it is necessary to maintain a secure supply for special needs such as offices, communication systems, high-risk systems, maintaining supply for safety systems, lifts, sewerage and water plants. The generators can be a single unit or multiple sets with programmed control to shift loads between the sets depending on demand.

In this article, we will focus on factors to consider while choosing a genset for standby purposes in small commercial and residential applications.

Type of Engine

The main power source, termed the prime mover, most used is an internal combustion engine powered by diesel as fuel. To obtain maximum power output from the engine, one must choose a turbocharged engine preferably with an intercooler (optional).
The declared power rating of the prime mover is the power available continuously at the output shaft. It is normal to accept 10% overload for a cumulative time of max 1 hour over a period of 12 hours.

Generator Rating

When considering standby power for your property, it is necessary to carry a comprehensive research on the loads to be catered for. Methods of starting, number of motors and existence of non-linear loads should also be investigated. This will ensure that the gensets will meet the load pattern and total power required.

Various load types can be categorized as follows:

1. Electric motors are used continuously, like those for fans, pumps, and industrial machinery.
2. Electric motors for intermittent tasks, such as lifts, refrigeration systems, cranes, and high-inertia start-up equipment.
3. Non-linear loads, encompassing discharge and fluorescent lighting, solid-state controlled devices (e.g., battery chargers), and variable speed motor drive power supplies.
4. Large linear loads, including electric heating, hot water systems, and tungsten lighting (unless dimmed with thyristor-controlled devices).
5. Industrial welding machinery.
6. Loads demanding stable voltage, frequency, and waveform adherence, are critical for mainframe computers, and medical diagnostic equipment (e.g., X-ray machines).

Most buildings have a mix of these load types. Proper generator selection depends on understanding their properties and characteristics. 

Selecting the right generator engine and alternator ratings for supporting both start-up and operational loads is crucial. An excessively large system is costly, takes up extra space, and requires additional distribution infrastructure. It may also operate inefficiently on its load curve and, in the case of a diesel prime mover under low loads, might face issues like over-oiling leading to failure.

Industrial and commercial installations normally include induction motors (i.e. machine drives, fan drives, compressors etc.). To avoid voltage dip problems associated with motor starting, it is essential to estimate the amount of current drawn by referring to the manufacturer’s sheets and using the value to find the maximum rated kVA (apparent power) of the motor. This will assist with the prudent calculation of the sum of all loads to be supported by the generator. The formula below will give you the total kVA (apparent power) of a motor.

Where S_a= rated kVA, V₁=output line volage, I₁=rated full load line current.

For generator sets powering motor drives with dynamic or regenerative braking, like cranes and lifts, they must handle the regenerated power. Using reverse power protection is vital to avoid ‘motoring,’ which could harm the alternator and the prime mover. Special starters can prevent feedback from regeneration.
Solid-state devices like radios, televisions, lighting, rectifiers/inverters, and computer power supplies don’t draw smooth sinusoidal currents from the power source. This can lead to distorted current waves, often with a high level of third harmonic (150 Hz) current, sometimes up to 80% of the total current. This third harmonic current can flow through the generator’s windings, potentially causing overheating in the windings and connecting cables. Regular over-current protection to safeguard the line conductors should be provided, but the neutral isn’t as well protected. When dealing with reasonably balanced 50 Hz loads, the neutral current tends to be low.
The connected load should not be lower than 30% or higher than 70% of the generator rating over a period greater than 1 hour in a 12-hour cycle.

Installation

Generators are housed in purpose-built structures or weatherproof enclosures with removable covers. The size of space required often increases with the generator rating. Allowance should be made for access and associated fittings or equipment. This equipment might be the starting system, changeovers, control panels, fuel storage, exhaust system and cooling system.

The table below gives the minimum recommended room dimensions for a rated single-generation set with a control panel and daily service tank.

Genset Rating (kVA)	Length (mm)	Width (mm)	Height (mm)
30	4000	2500	2500
50	4350	2500	2500
100	6150	3500	3000
150	6150	3500	3500
250	6800	4500	4000
300	7100	4500	4000
400	7500	4500	4000
700	9000	5000	4500
1000	9000	5000	4500
1500	9750	6000	4500
2000	12 000	6500	5000

Day Tanks and Bulk Storage

Typically, a day tank is used, either situated above the set or integrated into the base frame. Diesel fuel is often stored in bulk tanks, which can be above ground, buried, or underground. This setup allows for pumping fuel to the day tank and returning the excess to the bulk tank, controlled by level switches and solenoids. The day tank serves as a reliable source for the engine fuel pump.

Tanks may be buried or placed in containment pits to prevent spills, as required by local authorities. A filling point for delivery tankers, generally with hoses of 15 m to 30 m (or longer in special cases), should be within easy reach. This fill point typically includes the tanker connection, a level indicator, and a ‘tank full’ alarm as needed.

Typical dimensions for cylindrical bulk storage tanks;

Litres	Diameter (m)	Length (m)
5000	1.50	2.80
10 000	2.00	3.00
25 000	2.50	5.00
50 000	2.75	9.00

Testing & Commissioning

This is vital for the proper operation of gensets and should be well recorded. Tests are usually both works/factory tests (covers manufacture, assembly and performance of main components at the factory) and site tests (done on-site to prove operational performance of the entire installation). 

Works/Factory Tests

 Maximizing benefits from factory tests is crucial. Delaying certain tests until on-site commissioning can lead to program delays if issues arise. These are further grouped into two i.e. standard and type tests.

Standard test schedules include:

• Check plant build against the specification.
• Record reference and rating data.
• Prepare generating set for test with connections for exhaust, cooling system, lubricating oil, electrical power, control wiring, etc.
• Preliminary starting and running checks/adjustments.
• Performance tests (initial set-up checks).
• Load acceptance tests.
• Governing and voltage regulation (cold conditions).
• Load duration tests with a detailed examination of the set immediately following hot shutdown.
• Governing and voltage regulation (hot conditions).
• Transient switching tests.
• Insulation tests.
• A final re-run of the set to observe correct performance.
• Final check and clearance, including draindown, disconnections, and completion of test certificates.

Type tests: These may be a requirement in the specifications. To be performed, they must be agreed beforehand with the manufacturer. Examples include:

• Cold resistance of electrical windings
• Winding resistance and temperature rise following hot shutdown
• Transient performance test
• Vibration tests
• Noise tests
• Ability to withstand a short circuit

Site Tests

Before commissioning, it is essential to conduct comprehensive checks on the generator plant, encompassing fuel, cooling, exhaust, electrical, and fire safety systems. These checks, performed in both manual and automatic modes, primarily involve static assessments to ensure the installation’s correctness for subsequent testing.

• Check engine/alternator shaft alignment (if not skid-mounted or after transport).
• Verify generator output voltage, frequency, and phase rotation before further tests and final electrical system connection.
• Consider a load test using a suitable load bank due to the unavailability of the building’s actual load.
• Test duration: ≥4 hours (preferably 8) for sets over 100 kW, 1 hour for sets under 100 kW. Vary loads, and record instrument readings.
• Monitor parameters like water and exhaust temperature, oil level, temperature, pressure, voltage, frequency, and room temperature.
• Use an oscilloscope for precise voltage and frequency measurements as needed.
• Monitor noise and vibration if they may disturb nearby occupants.
• Commission multiple set installations, demonstrate synchronization, and, if necessary, parallel operation with the REC.
• Test all controls, emergency stops, protection devices, and interlocks during on-site commissioning.
• Record the time between initial start-up and load acceptance for the lead machine and assess load sharing for multiple sets.
• For generator sets starting large induction motors (e.g., chiller compressors), test compatibility before final acceptance, comparing works test data with motor manufacturer’s data, including starting device characteristics.

Operation & Maintenance

 After all site commissioning tests, the generator set supplier should offer:

• User training for the building owner/operator.
• Operating and maintenance instructions, along with detailed record drawings and documentation upon handover.
• Necessary special tools for maintenance, regardless of who handles the equipment.
• Recommended spare parts (included in the contract price).
• If needed, suggestions for an ongoing maintenance contract.