Standby power supply & outages
The cost of a standby power system for a business can be pretty high, so it’s essential to weigh all of your options before committing to a piece of equipment. The direct and indirect costs must be clearly and unequivocally defined and suitably weighted by management. As always, a distinction must be made between emergency and standby power sources.
Strictly speaking, emergency systems supply circuits legally designated as being essential for safety to life and property. Standby power systems are used to keep a facility from losing production due to a power outage from a utility company.
Many commercial/industrial buildings rely on two independent utility services or one utility service plus on-site generating to assure AC power supply continuity.
Because of the growing complexity of electrical systems, a special attention must be given to power-supply reliability.
- Standby Power System in General
- Dual-Feeder Power Supply System
- Peak Power Shaving
- Advanced System Protection
- How to Choose a Generator?
- UPS Systems
1. Standby Power System in General
Figure 1 depicts a traditional standby power system with an engine and generator. In the event of a power outage, an automated transfer switch checks the AC voltage coming from the utility company line. The standby generator is started when an outage is detected for a predetermined amount of time (usually 1 to 10s); once the generator is up to speed, the load is transferred from the utility to the local generator.
The load is switched back on and the generator is turned off when the utility supply is restored.
The transfer device shown in Figure 1 is a contactor-type, break-before-make unit. This system protects a facility from prolonged utility company power failures.
Figure 1 – The classic standby power system using an engine-generator set
By replacing the simple transfer device shown with an automatic overlap (static) transfer switch, as shown in Figure 2, additional functionality can be gained. The overlap transfer switch permits the on-site generator to be synchronized with the load, making a clean switch from one energy source to another.
This functionality offers the following benefits:
- Switching back to the utility feed from the generator can be accomplished without interruption in service.
- The load can be cleanly switched from the utility to the generator in anticipation of utility line problems (such as an approaching severe storm).
- The load can be switched to and from the generator to accomplish load shedding objectives.
Figure 2 – The use of a static transfer switch to transfer the load from the utility company to the on-site generator
In some urban areas, usually in big cities, two utility power connections can be brought into a facility as a means of providing a source of standby power. As shown in Figure 3, two separate utility service connections (from separate power-distribution systems) are brought into the plant, and an automatic transfer switch changes the load to the backup line in the event of a main-line failure.
The dual-feeder system provides an advantage over the auxiliary diesel arrangement in that power transfer from main to standby can be made in a fraction of a second if a static transfer switch is used. Time delays are involved in the diesel generator system that limit its usefulness to power failures lasting more than several minutes.
The downside of the dual-feeder system is that it’s limited primarily to urban areas. Rural or mountainous regions generally are not equipped for dual redundant utility company operation. Even in urban areas, the cost of bringing a second power line into a facility can be extremely high, particularly if special lines must be installed for the feed.
Figure 3 – The dual-utility-feeder system of AC power loss protection. An automatic transfer switch changes the load from the main utility line to the standby line in the event of a power interruption
If two separate utility services are available at or near the site, redundant feeds generally will be less expensive than engine-driven generators of equivalent capacity. Figure 4 illustrates a dual-feeder system that utilizes both utility inputs simultaneously at the facility.
Notice that both AC lines feed loads during normal operation, and the “tie” circuit breaker is open. In the event of a loss of either line, the circuit-breaker switches reconfigure the load to place the entire facility on the single remaining AC feed.
Switching is performed automatically; manual control is provided in the event of a planned shutdown on one of the lines.
Figure 4 – A dual-utility-feeder system with interlocked circuit breakers
Figure 5 illustrates the use of a backup diesel generator for both standby power and peak power shaving applications. Commercial power customers often can realize substantial savings on utility company bills by reducing their energy demand during certain hours of the day. An automatic overlap transfer switch is used to change the load from the utility company system to the local diesel generator.
The changeover is accomplished by a static transfer switch that does not disturb the operation of load equipment.
The automatic overlap (static) transfer switch changes the load from the utility feed to the generator instantly so that no disruption of normal operation is encountered.
Figure 5 – The use of a diesel generator for standby power and peak power shaving applications
A more sophisticated power-control system is shown in Figure 6, where a dual feeder supply is coupled with a motor-generator set to provide clean, undisturbed AC power to the load. The M-G set (motor-generator set) will smooth over the transition from the main utility feed to the standby, often making a commercial power failure unnoticed by on-site personnel.
A conventional M-G set typically will give up to 0.5 s of power fail ride-through, more than enough to accomplish a transfer from one utility feed to the other. Switching circuits allow the M-G set to be bypassed, if necessary.
Figure 6 – A dual feeder standby power system using a motor-generator set to provide power fail ride-through and transient-disturbance protection.
This standby power system is further refined in the application illustrated in Figure 7, where a diesel generator has been added to the system. With the automatic overlap transfer switch shown at the generator output, this arrangement also can be used for peak demand power shaving.
An arrangement such as this would be used for critical loads that require a steady supply of clean AC.
Figure 7 – A premium power-supply backup and conditioning system using dual utility company feeds, a diesel generator, and a motor-generator set.
Figure 8 shows a simplified schematic diagram of a 220 kW UPS system utilizing dual utility company feed lines, a 750 kVA gas-engine generator, and five DC-driven motor-generator sets with a 20-min. battery supply at full load. The five M-G sets operate in parallel, and each is rated for 100 kW output.
Local gas storage capacity also is provided.
Figure 8 – Simplified installation diagram of a high-reliability power system incorporating dual utility feeds, a standby gas-engine generator, and five battery-backed DC M-G sets
Engine-generator sets are available for power levels ranging from less than 1 kVA to several thousand kVA or more. Machines also can be paralleled to provide greater capacity. Engine-generator sets typically are classified by the type of power plant used:
- Advantages: rugged and dependable, low fuel costs, low fire or explosion hazard.
- Disadvantages: somewhat more costly than other engines, heavier in smaller sizes.
Suggested video – How a diesel generator works (animation)
Natural and Liquefied Petroleum Gas Engine
- Advantages: quick starting after long shutdown periods, long life, low maintenance.
- Disadvantage: availability of natural gas during areawide power failure subject to question.
- Advantages: rapid starting, low initial cost.
- Disadvantages: greater hazard associated with storing and handling gasoline, generally shorter mean time between overhaul.
Gas Turbine Engine
- Advantages: smaller and lighter than piston engines of comparable horsepower, rooftop installations practical, rapid response to load changes.
- Disadvantages: longer time required to start and reach operating speed, sensitive to high input air temperature.
Suggested video – What is a Gas Turbine? (For beginners)
The type of power plant chosen usually is determined primarily by the environment in which the system will be operated and by the cost of ownership. For example, a standby generator located in an urban area office complex may be best suited to the use of an engine powered by natural gas, because of the problems inherent in storing large amounts of fuel.
It’s important to note that State or local building codes can place expensive restrictions on fuel-storage tanks and make the use of a gasoline- or diesel-powered engine impractical. The use of propane usually is restricted to rural areas. The availability of propane during periods of bad weather (when most power failures occur) also must be considered.
An engine-driven standby generator typically incorporates automatic starting controls, a battery charger, and automatic transfer switch. Control circuits monitor the utility supply and start the engine when there is a failure or a sustained voltage drop on the AC supply. The switch transfers the load as soon as the generator reaches operating voltage and frequency.
See Figure 9.
Figure 9 – Typical configuration of an engine-generator set
Upon restoration of the utility supply, the switch returns the load and initiates engine shutdown. The automatic transfer switch must meet demanding requirements, including:
- Carrying the full rated current continuously,
- Withstanding fault currents without contact separation,
- Handling high inrush currents, and
- Withstanding many interruptions at full load without damage.
The nature of most power outages requires a sophisticated monitoring system for the engine-generator set. Most power failures occur during periods of bad weather. Most standby generators are unattended.
More often than not, the standby system will start, run, and shut down without any human intervention or supervision. For reliable operation, the monitoring system must check the status of the machine continually to ensure that all parameters are within normal limits.
The transfer of motor loads may require special consideration, depending upon the size and type of motors used at a plant. If the residual voltage of the motor is out of phase with the power source to which the motor is being transferred, serious damage can result to the motor.
Excessive current draw also may trip overcurrent protective devices. Motors above 50 hp with relatively high load inertia in relation to torque requirements, such as flywheels and fans, may require special controls. Restart time delays are a common solution.
Automatic starting and synchronizing controls are used for multiple-engine-generator installations. The output of two or three smaller units can be combined to feed the load. This capability offers additional protection for the facility in the event of a failure in any one machine. As the load at the facility increases, additional engine-generator systems can be installed on the standby power bus.
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Generators for standby power applications can be induction or synchronous machines. Most engine-generator systems in use today are of the synchronous type because of the versatility, reliability, and capability of operating independently that this approach provides. Most modern synchronous generators are of the revolving field alternator design.
Essentially, this means that the armature windings are held stationary and the field is rotated. Therefore, generated power can be taken directly from the stationary armature windings. Revolving armature alternators are less popular because the generated output power must be derived via slip rings and brushes.
The exact value of the AC voltage produced by a synchronous machine is controlled by varying the current in the DC field windings, whereas frequency is controlled by the speed of rotation. Power output is controlled by the torque applied to the generator shaft by the driving engine. In this manner, the synchronous generator offers precise control over the power it can produce.
Practically all modern synchronous generators use a brushless exciter. The exciter is a small AC generator on the main shaft; the AC voltage produced is rectified by a three-phase rotating rectifier assembly also on the shaft. The DC voltage thus obtained is applied to the main generator field, which is also on the main shaft.
A voltage regulator is provided to control the exciter field current, and in this manner, the field voltage can be precisely controlled, resulting in a stable output voltage.
There are many types of governors; however, for auxiliary power applications, the isochronous governor is normally selected. The isochronous governor controls the speed of the engine so that it remains constant from no-load to full load, assuring a constant AC power output frequency from the generator.
A modern system consists of two primary components: an electronic speed control and an actuator that adjusts the speed of the engine. The electronic speed control senses the speed of the machine and provides a feedback signal to the mechanical/hydraulic actuator, which in turn positions the engine throttle or fuel control to maintain accurate engine rpm.
The National Electrical Code provides guidance for safe and proper installation of on-site engine-generator systems. Local codes may vary and must be reviewed during early design stages.
Suggested reading – How not to select the wrong generator set (genset) for your application
An uninterruptible power system is an elegant solution to power outage concerns. The output of the UPS inverter can be a sine wave or pseudosine wave. When shopping for a UPS system, consider the following:
- Power reserve capacity for future growth of the facility.
- Inverter current surge capability (if the system will be driving inductive loads, such as motors).
- Output voltage and frequency stability over time and with varying loads.
- Required battery supply voltage and current. Battery costs vary greatly, depending upon the type of units needed.
- Type of UPS system (forward-transfer type or reverse-transfer type) required by the particular application.Some sensitive loads may not tolerate even brief interruptions of the AC power source.
- Inverter efficiency at typical load levels. Some inverters have good efficiency ratings when loaded at 90% of capacity, but poor efficiency when lightly loaded.
- Size and environmental requirements of the UPS system. High-power UPS equipment requires a large amount of space for the inverter/control equipment and batteries. Battery banks often require special ventilation and ambient temperature control.
Suggested video – How to Perform a UPS Transfer in a Critical Data Center
Batteries are the lifeblood of most UPS systems. Important characteristics include the following:
- Charge capacity: How long the battery will operate the UPS?
- Charging characteristics
Additional features that add to the utility of the battery include:
- Built-in status/temperature/charge indicator or data output port
- Built-in overtemperature/overcurrent protection with auto-reset capabilities
- Environmental friendliness
Failure to follow the proper procedures could have serious consequences.
Suggested video – Battery Energy Storage Systems
Research has brought about a number of different battery chemistries, each offering distinct advantages. Today’s most common and promising rechargeable chemistries include the following:
Nickel cadmium (NiCd) – used for portable radios, cellular phones, video cameras, laptop computers, and power tools. NiCds have good load characteristics, are economically priced, and are simple to use.
Lithium ion (Li-Ion) – typically used for video cameras and laptop computers. This battery has replaced some NiCds for high energy-density applications.
Sealed lead-acid (SLA) – used for uninterruptible power systems (UPS), video cameras, and other demanding applications where the energy-to-weight ratio is not critical and low battery cost is desirable.
Lithium polymer (Li-Polymer) – when commercially available, this battery will have the highest energy density and lowest self-discharge of common battery types, but its load characteristics will likely only suit low-current applications.
Reusable alkaline – used for light-duty applications. Because of its low self-discharge, this battery is suitable for portable entertainment devices and other noncritical appliances that are used occasionally.
No single battery offers all the answers; rather, each chemistry is based on a number of compromises. A battery, of course, is only as good as its charger. Common attributes for the current generation of charging systems include quick-charge capability and automatic battery condition analysis and subsequent intelligent charging.
Suggested reading – Sizing of 110V DC charger for MV panels – Auxiliary power supply in power substations
The following terms are commonly used to specify and characterize batteries:
- Energy density: The storage capacity of a battery measured in watt-hours per kilogram (Wh/kg).
- Cycle life: The typical number of charge-discharge cycles for a given battery before the capacity
decreases from the nominal 100% to approximately 80%, depending upon the application.
- Fast-charge time: The time required to fully charge an empty battery.
- Self-discharge: The discharge rate when the battery is not in use.
- Cell voltage: The output voltage of the basic battery element. The cell voltage multiplied by the
number of cells provides the battery terminal voltage.
- Load current: The maximum recommended current the battery can provide.
- Current rate: The C-rate is a unit by which charge and discharge times are scaled. If discharged at 1C, a 100 Ah battery provides a current of 100 A; if discharged at 0.5C, the available current is 50 A.
- Exercise requirement: This parameter indicates the frequency that the battery needs to be exercised to achieve maximum service life.
The lead-acid battery is a commonly used chemistry. The flooded version is found in automobiles and large UPS battery banks. Most smaller, portable systems use the sealed version, also referred to as gel-cell or SLA.
The lead-acid chemistry is commonly used when high power is required, weight is not critical, and cost must be kept low. The typical current range of a medium-sized SLA device is 2 Ah to 50 Ah. Because of its minimal maintenance requirements and predictable storage characteristics, the SLA has found wide acceptance in the UPS industry, especially for point-of-application systems.
Unlike the common NiCd, the SLA prefers a shallow discharge. A full discharge reduces the number of times the battery can be recharged, similar to a mechanical device that wears down when placed under stress.
In fact, each discharge-charge cycle reduces (slightly) the storage capacity of the battery. This weardown characteristic also applies to other chemistries, including the NiMH.
The charge algorithm of the SLA differs from that of other batteries in that a voltage-limit rather than current-limit is used. Typically, a multistage charger applies three charge stages consisting of a constant-current charge, topping-charge, and float-charge. See Figure 10.
During the constant-current stage, the battery charges to 70% in about 5 hours; the remaining 30% is completed by the topping-charge. The slow topping-charge, lasting another 5 hours, is essential for the performance of the battery.
Figure 10 – The charge states of an SLA battery
If not provided, the SLA eventually loses the ability to accept a full charge, and the storage capacity of the battery is reduced. The third stage is the float-charge that compensates for self-discharge after the battery has been fully charged.
During the constant-current charge, the SLA battery is charged at a high current, limited by the charger itself. After the voltage limit is reached, the topping charge begins and the current starts to gradually decrease. Full-charge is reached when the current drops to a preset level or reaches a low-end plateau.
If a faster charge is required and the room temperature remains below 30°C, 2.40 or 2.45 V/cell can be used.
Suggested course – Battery Fundamentals: Principles, Terminology, Operations, Design and Hazards
Source: AC Power Systems by J.C. Whitaker