Generators - INACTIVE

Keith Lane, P.E., RCDD/NTS Specialist, LC, LEED AP
Vice President - Engineering
SASCO, Seattle

Commissioning is an important "last step" in specifying and installing gensets. A good commissioning plan is essential for providing instruction to an owner and maintenance personnel for long-term generator testing and maintenance. A comprehensive commissioning plan for a standby power system not only provides increased reliability of the generator system, but also establishes a baseline set of values that can be referenced during routine and ongoing maintenance.

What to look for

A generator system is comprised of electrical, mechanical, controls, annunciation and alarm, fuel piping, structural and architectural functions. The most common causes of generator failure, however, are in the fuel-supply system, typically, clogged fuel filters, poor quality diesel fuel or fuel pump failure. But the fuel system is not the only point of concern.

Batteries have to be functioning properly to instantly start the generator. Also, the jacket water heaters, which are the electrical heaters that maintain the jacket water at a high enough temperature to allow for easy starting of the generator, must remain functional, even in cold environments.

Another area of importance is in communication between the automatic transfer switch (ATS) and the generator. The ATS should communicate with the building automation system and motor controllers to ensure large motors and elevators operate and start within their prescribed sequences. This communication will help to make sure that the generator is not overloaded or that motor back EMF does not cause motor overcurrent protection to trip. Additional motor and other system loads added to the generator after the initial construction must be properly coordinated with the system to prevent an overload condition from taking down the standby system.

Critical systems that are required to transition loads within 10 seconds - per the National Electrical Code (NEC) Section 700 - require more intense coordination of the systems and final field commissioning. To ensure that the system generator starts, and that ATS transition within the 10 second, combustion air must be a minimum of 21 degrees C (70 degrees F), the jacket water heater must maintain a minimum of 32 degrees C (90 degrees F) water temperature and the batteries have to be fully charged.

As electrical distribution systems get more complex, the commissioning process becomes a higher priority. It is important to confirm that the entire emergency electrical distribution system functions appropriately as a whole. Many functions of the standby system control logic must be set properly during startup. Additionally, the engineer of record or the commissioning agent must verify these settings. For instance the automatic transfer time settings must be confirmed.

Most newer ATS have multiple time settings, including:

• Time delay start
• Time delay transfer
• Time delay retransfer
• Program transition
• Time delay cool down
• Exercise clock

Some of these settings can be standard factory settings, but many of the setting are going to be specific to a project. External parameters such as reliability and conditions of the utility service, size and number of motor loads in the electrical distribution system, and exercise requirements will all play a role in the actual settings of a specific generator set. In addition, generator-paralleling systems utilized in critical environments can have a far more complex operation that requires greater knowledge and expertise for proper long-term operation.

It is possible to have a single generator feed life-safety and non-life-safety loads. NEC Section 700.5 B allows an alternate power source to supply emergency, legally required standby and optional loads, where automatic selective load pickup and load shedding is implemented as needed to ensure adequate power to life-safety, legally required standby and optional loads - in that order. This can be achieved by providing a separate ATS for each of these three branches of loads. The engineer of record or the commissioning agent must verify that the load-shedding functions have been appropriately set and are functioning properly.

When determining the sequencing of ATS modules and the associated loads, the designer should rank the load sequence by the power required to start each piece of equipment. The first ATS to transition should be the one with the loads that require the largest starting power. The ATS with the loads that require the least starting power should be transitioned last. This protocol will ensure that the generator will have the largest power reserve for the largest step loads. During commissioning, it is important to verify that all designed sequences are actually occurring in the field start-up.

Annunciation and alarm

Other critical control functions in standby generators are the annunciation and alarm panels, the functionality of which must be verified during startup. More importantly, the maintenance personnel must have a prescribed protocol to follow in case of an alarm. A description of the system and the potential alarm conditions must be thoroughly described to the maintenance staff during the handoff training. These annunciations and alarms include but are not limited to:

In addition, if remote fuel fill is required, overfill prevention controls and alarms are essential. Typically these systems provide for continuous level indication and audio and visual alarms at 90% of full fill and system shut off at 95% of full fill (actual percentage settings can be field configured).

It is critical to confirm that the generator system control settings are properly set and are functioning not only at the startup of the project, but remain set through the life of the project. These settings may need to be tweaked as the load profile of the building or the reliability of the serving utility changes.

Generator calculations and load simulations will be required to ensure that future load does not exceed the capacity of the generator. In addition, care must be taken to ensure that new loads are placed on the appropriate ATS per load criteria (life safety, legally required standby or optional).

Future motor loads may need to be coordinated with any ATS program transition or transfer time delay to mitigate overcurrent protection tripping based on counter EMF causing out-of-phase inrush. Future harmonics-producing loads need to be monitored to verify that harmonic distortion limits that may cause excessive voltage distortion across the alternator are not exceeded.

In addition, building maintenance and operation staff must ensure that loads with internal static capacitors do not cause leading voltage power factor, as the excitation can cause havoc to a generator's alternator. And finally, as systems change, large motor, elevator, and HVAC loads need to be coordinated to minimize block loading that could cause excessive system voltage drop.

With today's complex systems, the handoff training session should cover the basic settings of the system, alarm conditions, protocols to deal with alarm conditions and methods and procedures involved in proper regular maintenance. Videotaping the training session has proven valuable for maintenance staff as both a regular refresher of the electrical distribution system for veteran staff and as a training tool for new hires.

Consulting the NFPA codes

NFPA 110 indicates the standards for monthly maintenance and operation of standby generator sets and provides recommendations that "generators should be exercised monthly at 30% of the nameplate rating or loaded to the minimum engine exhaust temperature recommended by the engine manufacturer."

Generator exercising can be programmed into the ATS to automatically start and run the standby system at predetermined times. If the load cannot tolerate a brief outage during transfer and retransfer between sources, a load bank can be used to isolate the generator from the electrical distribution system during testing. This load bank can take the form of a fixed in-place system or a temporary rolled up system.

Fixed systems can be configured to switch stepped amounts of resistive loads to simulate varying levels of real system load. A load bank will typically only simulate a totally resistive load. The resistive load bank does not represent inductive reactive loads that are typically found in electrical distribution systems. These systems can also be programmed to turn off the load bank resistive load in the case of an actual power outage.

Extended no-load testing can damage the generators engine and result in:

1. Wet stacking, which manifests itself in the accumulation of carbon particles, unburned fuel, lube oil, condensed water and acids in the exhaust system, due to incomplete combustion caused by low combustion temperatures; or
2. Carboning, which is the result of carbon particles deposited on top of the piston rings and in the injectors due to incomplete burning of fuel.

An improvement over a resistive load bank test would be a test of the entire emergency power system in a "real life" simulation of a power failure and a transition of the ATS. Planned, off-hour testing of the generator system will typically cause minimal impacts on the electrical and mechanical distribution systems. In addition, delayed transition or a programmed transition will eliminate the possibility of an out-of-phase transfer and will minimize the impact on motor loads and motor overcurrent protection.

Furthermore, a load bank test cannot simulate a real load profile that typically will include both reactive and harmonic components. If the automatic transfer switch is going to fail, it is far better to fail during a controlled test situation.

The ATS is a mechanical device with moving parts. Regular operation of the moving parts in an automatic transfer switch will help to facilitate long term proper operation. A UL 1008-rated ATS will be rated for a total of 3,000 to 6,000 operations. Regular monthly testing will take many years to exceed this number of operations.

Many jurisdictions are now requiring generators to be UL 1004-1 (Standard for Safety of Rotating Electric Machines) listed and labeled. This standard represents a national product safety requirement for generators. These requirements cover stationary and fixed generators rated 600 volts or less and are intended for installation and use in ordinary locations in accordance with the NEC. This requirement has only been enforced in the past few years but should add a level of reliability to generator systems.

The Uptime Institute (www.uptimeinstitute.org) indicates that 70% to 80% of the system outages for data centers and critical environments are based on human error. Proper commissioning, training and long term maintenance and operation programs can minimize outages based on human error.