Rotary UPS

24.2.5 Rotary UPS

deals with current-based distortions. It mitigates load cur- rent harmonics and improves input power factor. At the same Rotary UPS systems use the stored kinetic energy in the elec- time, it charges the battery pack. Series converter deals with trical machines to provide power to the load when a power voltage-based distortions. It cancels input voltage harmonics outage occurs. There are different configurations for rotary and compensates voltage sags and swells. Most of the power UPS systems. The simplest topology consists of an AC motor is supplied directly from the AC line to the load. Only a small and an AC generator, which are mechanically coupled. A fly- percentage of the input power is absorbed by parallel converter. wheel is also used on the shaft of the machines to store more This power is used to compensate the differences between kinetic energy in the system. In normal operation, the input input and reference voltages and to charge the battery pack. AC line provides power to the AC motor and this AC motor On the other hand, when the input voltage shuts down, the drives the AC generator. The configuration of this system is static switch separates the source and the load and the system shown in Fig. 24.10a. In backup mode, the kinetic energy goes to backup mode. In this situation, the parallel inverter acts stored in the motor, flywheel, and generator is converted as a DC/AC inverter and supplies power to the load. Since a to electric power and supplies the load. This simple topol- large portion of the power flows without any conversion from ogy is designed to provide short time backup power to the

632 A. Nasiri V sa

Non-linear Load

C dc P1a Battery

A universal UPS topology based on two three-leg bi-directional converters.

Static Switch Flywheel

AC Motor

AC Generator

Utility AC

Power

G Load

(a) Static Switch

Flywheel

AC Motor

DC Machine

AC Generator

Utility AC Power

MG

G Load

Battery (b)

Static Switch Utility AC Normally On Load

Power

Static Switch Normally Off

Diesel/ Natural

Gas Standby

E G AC Generator

Engine

(c)

FIGURE 24.10 Different configuration of rotary UPS systems: (a) motor–generator set; (b) rotary UPS with battery backup; and (c) rotary UPS with standby diesel/natural gas engine.

load (typically less than 2 s) in case of power interruption. the DC machine. The DC machine drives the AC generator, In another configuration of rotary UPS system which is shown which supplies the load. During the backup mode of oper- in Fig. 24.10b, an AC motor, a DC machine, an AC genera- ation, the battery bank supplies the DC machine, which, in tor, and a battery bank are used. During the normal mode of turn, drives the AC generator and the AC generator supplies operation, the AC line supplies the AC motor, which drives the load. This system can provide long time backup power to

24 Uninterruptible Power Supplies 633 the load depending on the capacity of the battery set. These two

24.2.6 Hybrid Static/Rotary UPS

rotary UPS systems are much more reliable than the static UPS systems and provide complete electrical isolation between the Hybrid static/rotary UPS systems combine the main features load and input AC line. Yet, they require more maintenance of both static and rotary UPS systems. They have low out- and have much bigger size and weight. Therefore, they are put impedance, high reliability, excellent frequency stability, usually used for high power applications [14, 15]. The config- and low maintenance requirements [6]. Typical configurations uration of a standby rotary UPS system is shown in Fig. 24.10c. of hybrid static/rotary UPS are depicted in Fig. 24.11. They This system does not provide electrical isolation between the are usually used in high power applications. In the system load and input AC. There is also a transition delay for switch- shown in Fig. 24.11a, during normal operation, the input AC ing from main AC to backup AC generator. However, it can power feeds the AC motor. The power is provided to the load provide power to the load as long as needed.

from the AC generator, which is driven by the AC motor. In

Static Switch

AC Motor

AC Generator

Utility AC

Power

G Load

Fast Switch

Bi-directional AC/DC Converter

Battery

(a)

Static Switch

AC Motor

AC Generator

Utility AC

AC/DC

DC/AC

G Load

Battery

(b)

Static Switch

Utility AC

Series Inductor

Load Power

Main Breaker

Gen. Breaker Flywheel

Diesel Engine

MG

AC/DC

DC/AC

Rectifier

Inverter Inverter

Clutch

AC Machine

Breaker

(c)

FIGURE 24.11 Three configurations of hybrid rotary-static UPS systems: (a) motor–generator set with battery backup; (b) motor–generator set with power conditioning at input side; and (c) battery-less hybrid UPS system.

634 A. Nasiri case of low input power quality or power interruption, the

bidirectional AC/DC converter acts as an inverter and feeds the AC motor from battery pack. Configuration of a hybrid UPS system with power conditioning at the input is shown in Fig. 24.11b. Figure 24.11c shows the configuration of a more complicated hybrid UPS system. This system has three opera- tion modes. In normal operation, the load is directly supplied by the main AC input and the AC motor is rotated at no- load. In the case of short power interruption, main breaker and generator breaker are opened and the inverter breaker is closed. The DC/AC inverter provides power to the load from the kinetic energy stored in the AC machine. If power is not restored in the short-term, the diesel engine is turned on, which provides power to the load through the AC gen- erator. In this mode, the main breaker and inverter breaker remain open. One of the advantages of this topology is opera- tion without a battery set to minimize cost, space, and required maintenance. The second advantage is avoiding double power conversion in long-term power interruption.