Electrical System and Control Methods: Standby and Backup Systems
By Ed Butts, PE, CPI
With this column and the ones to follow throughout the year, we will venture into the realm of standby and backup systems used for supplemental, alternate, or emergency operation of commercial, industrial, and municipal water systems.
Figure 1. Vertical turbine standby pumping unit.
This column will introduce backup and standby systems, and later we will discuss internal combustion engines and generators, and eventually finish with sizing and engine room design techniques.
This diverse field of equipment is generally used to back up or supplement electrically driven pumps but can also be applied to drilling rig and test pump engines. They can include stationary or portable standby generators or direct-drive internal combustion engines (power units) that operate dedicated pumping units.
These pumps typically include vertical turbine pumps, submersible pumps, and end-suction and split case centrifugal pumps using internal combustion engines or generators to drive electric motors and combination drives (electrically and engine-driven units).
It should be noted that the following stated definitions, sizing recommendations, and calculated reliability are based on my personal experience and observations with similar systems and may vary with other regions and applications.
Defining Standby and Backup Service
There are six distinct types of supplemental and reserve systems within three groups: (1) manual or auto-start standby pumps, (2) manual or auto-start backup pumps, and (3) manual or auto-start backup generators.
Figure 2. Standby/backup engine-driven centrifugal pump.
In all three groups, internal combustion engines are typically used as the prime mover. Therefore, common features associated with possible engine failure such as battery failure or loss of the charge, starter solenoid and starter failure, fuel supply contamination or loss, and control failure apply to all methods.
Assuming a base reliability of 100%, these factors reduce the typical base reliability for all groups to approximately 97.5% to 98.75%. The variance in reliability depends on the type of engine and fuel (diesel, natural gas, or propane); the engine’s original and current quality; operating environment; fuel supply and integrity; and periodic exercise, service, and maintenance.
Standby pumps (Figure 1) are defined as pumping units that supplement or replace failed electrically or other engine-driven units. Backup pumps can be either electric motor- or engine-driven and are generally intended to operate behind and replace electrically driven units.
Standby and backup service is provided through the same unit (Figure 2) in some cases. The unit operates in a backup role behind electrically driven pumps during a pump failure or power outage but is also used to supplement flow during high demand periods such as fires.
The final group, backup generators, are used to provide an alternate power supply to electrically driven pumps. These units (Figure 3) are generally stationary and permanently placed at the site and are almost always intended to operate in an emergency or power failure role.
Figure 3. Examples of backup generators.
Standby units are generally applied to centrifugal or vertical turbine pumps driven directly from an engine or through a right-angle gearhead as shown in Figure 1. This type provides the highest reliability in terms of energy transference as the engine directly drives the pump or through the gearhead without the need for multiple conversions or transference of energy.
Where diesel-driven standby units are installed for pump failure replacement or reserve use, they generally serve as dedicated units for this demand and usually possess no secondary operational function. This characteristic often reduces the operating hours to maintain serviceability and superior reliability.
Properly exercised, maintained, and serviced—the standby power system typically provides high levels of predictable response, pump operation, and flow delivery into the water system at roughly 97% to 98.5% reliability.
The next step down in unit reliability is the backup pump. The backup engine, as the term implies, is designed to operate pumps that are normally driven by electrically powered motors using a separate power source.
Figure 4. Redi-Torq vertical turbine pump installation.
Although backup engines are generally permanently situated within a water plant or pump station as a stationary unit, there are exceptions where a portable engine, such as a tractor equipped with a Power-Take-Off (PTO), is used. This is more common in rural settings where adequate water storage exists and portable power sources, such as farm tractors, are readily available.
When tractors are used as a backup power source for vertical turbine pumps, adequate access must be provided to allow connection to the gearhead with a flexible driveshaft and the gearhead must be equipped with a gear ratio to increase speed to the pump from 540 or 1000 RPM PTO speed.
In cases where a backup engine operates an electrically driven vertical turbine pump, it requires a transition from the electric motor to the engine. This is conducted manually through a bolted connection to the pump through a combination right-angle gearhead or automatically through a special type of right-angle combination gearhead with a DC-powered electric solenoid clutch to transfer power from the engine. This second method is commonly referred to as a Redi-Torq drive (Figure 4), made by Johnson Gear but also available in similar configurations from other manufacturers.
The electric motor does not disengage from the system during emergency operation but simply goes along for the ride. However, the motor can be removed for service if necessary, with pump operation left to the engine.
The pump thrust is handled by either the gearhead or electric motor when left in place. Therefore, if the electric motor carries the thrust load and is locked up or fails to rotate due to a motor bearing failure, it is conceivable that this can also lock up the gearhead, engine, and pump.
Figure 5. 320 kW trailer-mounted portable generator.
In addition to this failure mode, a steady bearing within the gearhead is used to stabilize the pump shaft during normal electric motor operation. The constant rotation of this bearing creates another potential failure mode in the system, requiring frequent monitoring and replacement of the bearing to avoid an unexpected failure.
Finally, the failure potential of the DC clutch solenoid coil is relatively high, particularly if the coil is exposed to frequent operation resulting in numerous cycles of heating and cooling. This often leads to deterioration of the insulation and eventual shorting of the coil. These combined factors further reduce the system efficiency by approximately 1.75% up to 3.0%.
Onsite or portable backup engine-driven generator systems (gensets) use an alternate source of power generation at the facility site to provide electric power when the utility’s power source is not available. Portable or on-site electrical power generating systems are readily available in a wide variety of designs and sizes for specific uses and customer applications.
The backup generator’s power system is typically interconnected with the utility source through a transfer switch. To conserve financial resources and provide greater operating flexibility between sites, this type of system is often used with portable generators and usually consists of a trailer-mounted engine-driven generator (Figure 5) and a means to transfer power from the generator to power the load when an outage occurs using a heavy-duty cord and manual transfer switch.
The final method of providing supplemental or reserve production is using an auto-start and switchover generator. An auto-start generator and automatic transfer switch provides the lowest overall reliability to the water system as several transitions of energy must occur for the energy to arrive at the pump.
These additional transitions include the transfer of energy from the engine to the generator, transfer of energy from the generator to the automatic transfer switch and electrical switchgear, transfer of energy from the switchgear to the electric motor, and finally, the transfer of energy from the motor to the pump.
Each of these conversions of energy causes a progressive and cumulative decline in efficiency and reliability in addition to creating a series daisy chain where disruption or loss of any single link in the chain will interrupt the entire process. Due to the need to transition from the electric motor to the engine and use of more complex controls, the overall reliability of this method, including ATS, electrical switchgear, wiring, and motor, is approximately 95% to 96%.
Standby and Backup System Classifications
Standby and backup power systems are defined according to the following standards:
Emergency Standby: Typical usage of 50 hours per year, with a maximum of 200 hours per year and typical variable load factor of 70%.
Standby Power: Standby power-rated generators are the most common type of generator sets. Their primary application is to supply emergency power for a limited duration during a power outage or equipment failure.
With standby rated generators, there is no overload capability built into the units. It is important to note that standby rated generators under no circumstances should run in conjunction with a public utility source but should nonetheless be applied where public utility power is available.
The typical rating for a standby engine should be sized for a maximum of 80% average load factor and roughly 200 hours per year of operation. This includes less than 25 hours per year of running time at the standby rating. Standby power ratings should never be applied except in true emergency outage situations, but predetermined outages with the utility company under UL guidelines are not considered as emergency outages.
Manual load shifts for testing purposes can be performed with most automatic transfer switches. Maximum usage is 500 hours per year, with up to 300 hours continuous running with varying loads. No overload is available. Rating is equivalent to prime rating +10%. Load factor maximum is 70% of the standby rating.
Mission Critical Standby: In this application, the generator set can provide emergency backup power at the nameplate rating for the duration of an outage. The average load factor of a mission critical standby rated generator set should be no more than 85% of the nameplate rating with varying loads.
A mission critical standby generator set can run for a maximum of 500 hours per year. Typical peak demand is 100% of the rating for a maximum of 5% of the operating time. This rating is not used in utility paralleling applications.
Prime or Prime Plus 10%: The prime power rating is the maximum power accessible for an unlimited number of hours per year in a variable load setting. The variable load should not exceed 70% average of the prime power rating during any operational period of 250 hours. The prime power rating with a 10% overload is limited to one in 12 hours, but not to exceed 25 hours per year.
If the engine is running at 100% prime power, yearly hours should not exceed 500. In addition, overload situations should be avoided, but a 10% overload capability is available for a one-hour period within a 12-hour cycle of operation.
The 10% overload is available in accordance with ISO 3046-1 (2002). Life to overhaul of the engine is dependent on operation as outlined in ISO 8528 (2005) and time spent during operation above 100% load may affect the life to overhaul.
Continuous: Continuous power rating is used in applications where supplying power is at a constant 100% load for an unlimited number of hours each year. Continuous power-rated units are most widely used in applications where the power grid is unattainable. Such applications include remote mining, agriculture, and military operations. The units have unlimited hours of usage with the load factor at 100% of the published continuous power rating.
The applicable International Organization for Standardization (ISO) standards are listed below:
Prime Power Rating (PRP): Prime running power is the maximum power a generator set has during a variable power sequence for an unlimited number of hours under stated ambient conditions. Maintenance according to the manufacturer must be followed to reach these standards. Prime power-rated generators should be used in applications where the user does not purchase power from a public utility.
Continuous Operating Power (COP): Continuous operating power is the power a generator set can operate at a continuous load for an unlimited number of hours under stated ambient conditions. Maintenance according to the manufacturer must be followed to reach these standards.
Limited Running Time (LRT): Prime power is accessible for a limited number of hours in non-variable load situations. Limited prime power is intended for circumstances where power outages are expected, such as a planned utility power reduction. Engines in generator sets may operate up to 750 hours per year at power levels less than the maximum prime power rating. In these situations, it is important to never exceed the prime power rating. The end user should be aware that constant high load use will reduce the life of any engine. The engine should be rated for continuous power output for any application requiring more than 750 hours per year.
Application of Electrical Codes
As with most electrical related topics, the National Electrical Code (NEC) also applies to standby power systems, largely using the defined terms previously described. In general, NEC Article 700 applies to electrical systems or equipment that are required to protect human beings with egress or exiting during an emergency.
There are additional Articles under the auspices of 700 that further define the need and application of standby power; thus each Article will be referenced and cited in brackets [ ]. For example, NEC [Article 701] applies to the electrical systems or equipment needed to assist with emergency egress, such as emergency lighting. Unlike the previous service ratings, which are largely industry controlled, the NEC primarily uses just three defined terms:
- Emergency Power Systems, NEC Article 700
- Legally Required Standby Systems, NEC Article 701
- Optional Standby Systems, NEC Article 702.
Among these three systems, many of the requirements are the same, but legally required systems have more specific requirements than optional systems and emergency systems have more specific requirements than legally required systems.
With all three types of systems, the short circuit current rating of the transfer equipment must be field marked on the exterior of the transfer equipment. Transfer equipment must be automatic and installed to prevent the inadvertent interconnection of normal and alternate sources of supply [Articles 700.5, 701.5, 702.5].
Transfer equipment for emergency systems must supply only emergency loads. Part III of each Article then lists which power sources are permitted and what the requirements are for each source. One difference is Article 701 lists “connection ahead of service” [Article 701(E)] while Article 700 does not.
Because emergency standby systems protect life, the NEC gives priority to these systems over the other two, plus the legally required systems take priority over the optional systems regardless of other considerations. The design for all legally required and emergency power systems must each be made by an engineer or similarly qualified person and must also be documented and made available to those authorized to design, install, inspect, maintain, and operate the system. Table 1 shows the comparisons between each system type.
Diesel fuel has been used in standby and backup power systems for decades. Diesel-fueled engines are available in horsepower from 5 HP up to several hundred in both water- and air-cooled styles.
Among diesel-engine advantages includes its high thermal efficiency. This can yield a low capital cost in larger applications, typically 100 kW (75 HP) or more. Because diesel fuel must be stored on-site, the engines and generators can provide backup power in remote areas that do not have the benefit of a natural gas infrastructure.
Biofuels such as ethanol, methanol, and biodiesel offer an alternative fuel source and are rapidly becoming more popular and accessible. Biofuels are liquid fuels produced from renewable biological sources including plants, crops, and algae.
Gaseous fuels such as natural gas or liquid propane are gaining more acceptance with their use, particularly natural gas, limited to access to the fuel source.
Combining these fuels in unique ways provides additional fuel options. For example, dual-fuel generators run on either natural gas or liquid propane vapor fuel, depending on which fuel is available at the time. Bi-fueled generators run simultaneously on diesel and natural gas and use the benefits of each.
Gasoline is noticeably absent because it is generally a poor fuel choice for standby and backup power systems. It is extremely volatile and potentially explosive compared to diesel or gaseous fuel, making it problematic to store in quantity.
Gasoline also has a significantly lower thermal (BTU) value than diesel. Additionally, gasoline cannot be easily used in combination with gaseous fuel. Thus municipal, commercial, and industrial standby and backup power systems are rarely fueled by gasoline.
The information in Table 2 provides typical characteristics for various fuel types and can be used to compare fuels.
In April, we will continue this series with an overview on the fundamentals of internal combustion power units.
Until then, keep them pumping!
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at email@example.com.