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Discussion on the Application of Emergency Power Supplies (EPS) in Building Power Distribution and Supply Design
Release time:
2019-02-20
As building heights continue to rise, the use of large-scale power-consuming equipment within buildings is also increasing steadily—ranging from elevators and high-capacity lighting systems to fire protection and water supply systems. The growing number of power-consuming devices has led building power distribution systems to increasingly adopt Class I and Class II load designs. Elevators, water supply systems, lighting, and other such facilities directly impact people’s work and daily life—for example, emergency power supply in hospitals—and these types of electrical loads must be reliably guaranteed. Given the characteristics of building power distribution systems—such as numerous power users, extremely high fire safety requirements, and substantial electrical loads—the design must ensure high reliability, continuity, safety, and cost-effectiveness. Properly designing building power distribution systems...
As building heights continue to rise, the use of large-scale power-consuming equipment within buildings is also increasing steadily. Such equipment includes elevators, high-capacity lighting systems, fire protection systems, and water supply systems, among others. The growing number of power-consuming devices has led building power distribution systems to increasingly adopt Class I and Class II load designs. Elevators, water supply systems, lighting, and other such facilities directly affect people’s work and daily life—for example, emergency power supply in hospitals—and these loads must be reliably guaranteed. Given the characteristics of building power distribution systems—such as numerous power users, extremely high fire safety requirements, and substantial electrical loads—the system must exhibit high reliability, continuity, safety, and cost-effectiveness. Properly and rationally utilizing emergency power sources in building power distribution systems can ensure stable and uninterrupted power supply.
Working Principle of Emergency Power Supply EPS
An UPS is an uninterruptible power supply that incorporates an energy storage device. An EPS power supply is a specialized variant of the UPS, designed primarily to provide emergency power for critical loads such as emergency and accident lighting, fire protection systems, and other first-class load equipment. It plays a crucial role in building power distribution and electrical design by ensuring reliable emergency lighting and fire safety.
EPS operating principle: When the main power supply is normal, the main power input flows through the output switching device to supply power to the load. At the same time, the main power flowing through the charger can also charge the battery. If the main power fails or the voltage exceeds the acceptable range, the controller immediately responds by automatically starting the inverter. The switching device instantly switches the power supply mode to inverter power, providing emergency power to the load equipment. Once the main power is restored, the EPS emergency power supply immediately stops providing power.
UPS uninterruptible power supplies and EPS emergency power supplies share some common features while also having certain differences. Both are designed to ensure stable, uninterrupted power supply and distribution for buildings. The differences are summarized in Table 1:
As can be easily seen from the table above, EPS employs the mature inverter technology used in UPS circuits; both systems are capable of continuing to supply AC power to the load in the event of a main power failure.
The switching time of a UPS is very short (<10 milliseconds), whereas that of an EPS is within 4 seconds. The UPS has stringent requirements for the operating environment and is designed to ensure uninterrupted power supply and high-quality power delivery. In contrast, the EPS can adapt to various environments and can operate without being confined to indoor spaces; its primary purpose is to prevent major accidents. The EPS is particularly well-suited for providing backup power to critical first- and second-level loads such as emergency lighting, fire-service elevators, and fire-fighting pumps. The EPS emergency power system is especially effective in situations involving unexpected power outages or voltage surges, offering higher efficiency than a UPS. It also features a lower price, longer service life than a UPS, energy savings, and relatively lower noise levels. On the other hand, UPS systems are more widely used—and often indispensable—in certain specialized settings (such as real-time computer data processing and collection systems where even the slightest power interruption is unacceptable).
EPS types
In building power supply and distribution design, EPS emergency power supplies are primarily classified according to the loads they serve. The common classifications fall into three categories:
Emergency lighting EPS, inductive loads (such as elevators, smoke exhaust fans, rolling shutters, and pumps), hybrid three-phase EPS series, and variable-frequency EPS (for powering electric motors).
EPS Applications in Building Power Supply and Distribution
1. EPS for emergency and accident lighting applications
As a backup power source for emergency or accident lighting, single-phase systems are generally preferred. They provide centralized power supply primarily for emergency lighting applications and are commonly used in entertainment venues, office spaces, and other similar settings. The EPS system for emergency or accident lighting must meet the following requirements regarding continuous operating duration and switching time:
1. For areas with standard emergency lighting, the EPS power interruption time should be less than 5 seconds; for high-risk and critical areas, the EPS power interruption time should be less than 0.25 seconds.
2. The EPS must be equipped with a sufficient battery pack to ensure continuous emergency lighting for more than 60 minutes in the event of a main power failure.
3. Prioritize the use of main power supply. When the main power voltage is within a certain range (187–242V), the inverter remains in sleep mode.
4. In the EPS system, the maximum charging time T for the battery shall be ≤24 hours, and the maximum continuous overcharge current I shall be ≤0.05C5A.
In addition to the basic application requirements for emergency lighting EPS systems mentioned above, it is also necessary to appropriately select the output power of the EPS based on the specific type of emergency lighting fixtures being used. For conventional emergency lighting fixtures, the active power is indicated and labeled in kilowatts (kW); however, the output power of the EPS inverter is specified in apparent power (kVA), with a lagging power factor of cos = 0.8. Therefore, the final selected EPS capacity should be: S = P / 0.8. For emergency lighting fixtures that use high-pressure gas, it is advisable to choose an EPS with a switchover time of less than twenty milliseconds. This is because if the power supply to a high-pressure gas lamp is interrupted for longer than 20 milliseconds, the discharge arc inside the gas lamp may extinguish or be disrupted, and it could take several minutes before the lamp can be reignited.
2. EPS Applications for Inductive and Hybrid Loads
Hybrid-load EPS systems are generally suitable for locations with relatively complex load characteristics. They are typically designed for three-phase power supplies. Hybrid-load EPS systems require a switching time of less than twelve milliseconds. Inductive and hybrid-load EPS systems are commonly used in high-rise buildings, large hotels, hospitals, and major shopping malls.
The current drawn by the motor at the moment of startup can cause a surge in output power. When supplying power to EPS systems or hybrid loads with inductive electromechanical loads, the EPS output power PEPS must be greater than or equal to the total calculated load power P and should also meet the maximum power requirements for starting the largest motor (or motor group). During variable-frequency start-up, the EPS output power PEPS must satisfy PEPS ≥ P; during star-delta start-up, the starting capacity Pstart must be at least 3 times the rated power P; and when the motor is directly started, the required starting capacity Pdirect start must be at least 5 times the rated power P. η represents the overall efficiency of the total load and is typically taken as 0.85.
3. EPS with variable-frequency starting specifically designed for motors
Variable-frequency-start EPS systems are widely used for high-power motor loads and find practical application in critical environments such as elevators and fire-fighting water pumps. They can prevent equipment damage caused by the high-current surges during motor startup. Unlike other EPS systems, this type typically features a single output channel. When the three-phase power supply at the input is normal, the rectifier converts the AC power into DC, which then charges the battery pack and supplies power to the inverter. If the three-phase input power fails or becomes abnormal, the system immediately switches over to using the battery pack to provide DC power to the variable-frequency drive, thereby maintaining an emergency power supply.
When the motor load needs to operate, a start signal (such as a run signal or remote control signal) is sent to the frequency converter, which immediately outputs power. The frequency converter adjusts the output frequency from 0 to 50 Hz, providing variable-frequency starting for the motor. Once the frequency reaches 50 Hz, the motor maintains normal operation.
Closing remarks
The application of EPS emergency power supplies in building power distribution systems has become an indispensable trend. As an emergency power device, the EPS boasts advantages such as stable performance, noiselessness, energy efficiency, no inherent fire hazards, cost-effectiveness and durability, and ease of environmental adaptation. It meets the requirements for the layout of power distribution systems for fire-fighting emergency equipment, enables fire-fighting interlock and computerized monitoring, and provides a solid guarantee for powering emergency equipment and facilities.
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