Power distortions such as power interruptions, voltage sags and swells, voltage spikes, and voltage harmonics can cause severe impacts on sensitive loads in the electrical systems. Uninterruptible power supply (UPS) systems are used to provide uninterrupted, reliable, and high quality power for these sensitive loads. Applications of UPS systems include medical facilities, life supporting systems, data storage and computer systems, emergency equipment, telecommunications, industrial processing, and on-line management systems [1–3]. The UPS systems are especially required in places where power outages and fluctuations occur frequently. A UPS provides a backup power circuitry to supply vital systems when a power outage occurs. In situations where short time power fluctuations or disturbed voltage occur, a UPS provides constant power to keep the important loads running. During extended power failures, a UPS provides backup power to keep the systems running long enough so that they can be gracefully powered down.

Most of the UPS systems also suppress line transients and harmonic disturbances. Generally, an ideal UPS should be able to simultaneously deliver uninterrupted power and provide the necessary power conditioning for the particular power application. Therefore, an ideal UPS should have the following features: regulated sinusoidal output voltage with low total harmonic distortion (THD) independent from the changes in the input voltage or in the load, on-line operation that means zero transition time from normal to back-up mode and vice versa, low THD sinusoidal input current and unity power factor, high reliability, high efficiency, low EMI and acoustic noise, electric isolation, low maintenance, low cost, weight, and size. Obviously, there is not a single configuration that can provide all of these features. Different configurations of UPS systems emphasize on some of the features mentioned above. Classifications of UPS systems are described in Section 24.2.

24.2 Classifications

24.2.1 Standby UPS

This configuration of UPS system is also known as “off-line UPS” or “line-preferred UPS” [4, 5]. Figure 24.1 shows the configuration of a typical standby UPS system. It consists of an AC/DC converter, a battery bank, a DC/AC inverter, and a static switch. A passive low pass filter may also be used at the output of the UPS or inverter to remove the switching frequency from the output voltage. The static switch is on during the normal mode of operation. Therefore, load is supplied from the AC line directly without any power conditioning. At the same time, the AC/DC rectifier charges the battery set. This converter is rated at a much lower power rating than the power demand of the load. When a power outage occurs or the primary power is out of a given preset tolerance, the static switch is opened and the DC/AC inverter provides power to the load from the battery set for the duration of the preset backup time or till the AC line is back again. This inverter is rated at 100% of the load power demand. It is connected in parallel to the load and stays standby during the normal mode of operation. The transition time from the AC line to DC/AC inverter is usually about one quarter of the line cycle, which is enough for most of the applications such as personal computers. The main advantages of this topology are simple design, low cost, and small size. On the other hand, lack of real isolation of the load from the AC line, no output voltage regulation, long switching time, poor performance with non-linear loads, and no line conditioning are the main disadvantages of this configuration.

FIGURE 24.1 Configuration of a typical standby UPS system.

Different configurations of AC/DC rectifiers such as linear or switching may be used in this system. To reduce the cost, a simple diode-bridge rectifier with a capacitor at the front end is used. A full-bridge or half-bridge full controlled converter is also used to charge the battery bank. Two typical topologies for a single-phase UPS system are shown in Fig. 24.2. The full controlled topologies can provide power factor correction (PFC) to meet the corresponding standards. To optimize the charging process, the charging cycle is divided into “constant current” and “constant voltage” modes. In the constant current mode, the converter injects a constant current into the battery till the battery is charged up to about 95% of its capacity. After this mode, the constant voltage mode starts that applies a constant voltage on the battery. In this mode, the input current of the battery declines exponentially until it is fully charged.

FIGURE 24.2 Two simple topologies of AC/DC rectifier: (a) full-bridge diode rectifier and (b) full-bridge full controlled topology.

The purpose of the DC/AC inverter is to provide high quality AC power to the load when the static switch is opened. A full- or half-bridge topology is used for this inverter. Figure 24.3 shows two simple single-phase topologies for the DC/AC inverter.

FIGURE 24.3 Two simple single-phase topologies for the DC/AC inverter: (a) full-bridge and (b) half-bridge.

In some topologies of standby UPS systems, an isolating transformer is used at the output stage of the UPS. This topology is called ferroresonant standby UPS system. The transformer also acts as a low pass filter that cancels out switching frequency from the output voltage of the DC/AC inverter. On the other hand, the transformer stores electromagnetic energy in the core and acts as a buffer when a power outage occurs. For a short time, the transformer provides power to the load and protects sensitive equipment from being affected during the transfer time from the input AC to the UPS. Figure 24.4 shows the configuration of a ferroresonant standby UPS system. Since the transformer is bulky and expensive, this configuration is more appropriate for high power applications.

FIGURE 24.4 Typical configuration of ferroresonant standby UPS system.

24.2.2 On-line UPS System

Similar to standby UPS systems, on-line UPS systems also consist of a rectifier/charger, a battery set, an inverter, and a static switch (bypass). Other names for this configuration are “true UPS,” “inverter preferred UPS,” and “double-conversion UPS” [6, 7]. Figure 24.5 shows the block diagram of a typical online UPS. The rectifier/charger continuously supplies power to the DC bus. The power rating of this converter must be designed appropriately to supply power to the load and charge the battery bank at the same time. The batteries are rated in order to supply full power to the load during the backup time. The duration of this time varies in different applications. The inverter is rated at 100% of the load power since it must supply the load during the normal mode of operation as well as during the backup time. It is connected in series with the load; hence, there is no transfer time associated with the transition from normal mode to stored energy mode. This is the main advantage of on-line UPS systems. The static switch provides redundancy of the power source in the case of UPS malfunction or overloading. The AC line and load voltages must be in phase in order to use the static switch. This can be achieved easily by a phase-locked loop control. During the normal mode of operation, the power to the load is continuously supplied via the rectifier/charger and inverter. In fact, a double conversion from AC to DC and then from DC to AC takes place. This configuration of the UPS allows good power conditioning. The AC/DC converter charges the battery set and also supplies power to the load via the inverter. Therefore, it has the highest power rating in this topology, thereby increasing the cost. When the AC input voltage is outside the preset tolerance, the inverter and battery maintain continuity of power to the load. The duration of this mode is the duration of preset UPS backup time or till the AC line returns within the preset tolerance.

FIGURE 24.5 Block diagram of an on-line UPS system.

The main advantages of on-line UPS are very wide tolerance to the input voltage variation and very precise regulations of output voltage. In addition, there is no transfer time during the transition from normal to stored energy modes. It is also possible to regulate or change the output frequency [8]. The main disadvantages of this topology are low power factor, high THD at the input, and low efficiency. The input current is distorted by the rectifier unless an extra PFC circuit is added; but, this adds to the cost of the UPS system [9].

As mentioned for the standby UPS system, different topologies are employed for the AC/DC rectifier and DC/AC inverter. Unlike standby UPS system, in this system, these converters provide power to the load continuously. Therefore, more care should be given to the quality of the input current and output voltage as well as the efficiency of the system. Figure 24.6 shows the configuration of a three-phase on-line UPS system. The proper switching method such as PWM is employed for the AC/DC rectifier to minimize the input current harmonics and provide regulated DC bus voltage. A low pass filter at the output of the system removes the switching frequency from the output voltage.

FIGURE 24.6 Configuration of a three-phase on-line UPS system.

24.2.3 Line-interactive UPS

Line-interactive UPS systems consist of a static switch, a series inductor, a bi-directional converter, and a battery bank. An optional passive filter can be added at the output of the bi-directional converter or at the input side of the load. A line-interactive UPS can operate either as an on-line UPS or as an off-line UPS. For an off-line line-interactive UPS, the series inductor is not required. However, most of the line-interactive UPS systems operate on-line in order to either improve the power factor of the load or regulate the output voltage for the load. When the AC line is within the preset tolerance, it feeds the load directly. The AC/DC converter is connected in parallel with the load and charges the battery. This converter may also be used to improve the power factor of the system and compensate the load current harmonics. [10, 11]. Typical configuration of a line-interactive UPS is shown in Fig. 24.7.

FIGURE 24.7 A typical configuration of a line-interactive UPS system.

When a power outage occurs or input voltage falls outside the preset tolerance, the system goes to bypass mode. In this mode, the bi-directional converter operates as a DC/AC inverter and supplies power to the load from the battery set. The static switch disconnects the AC line in order to prevent back feed from the inverter. The main advantages of the line-interactive UPS systems are simple design and, as a result, high reliability and lower cost compared to the on-line UPS systems. They also have good harmonic suppression for the input current. Since this is a single stage conversion topology, the efficiency is higher than on-line UPS system. The main disadvantage is the lack of effective isolation of the load from the AC line. Employing a transformer in the output can eliminate this; but, it will add to the cost, size, and weight of the UPS system. Furthermore, the output voltage conditioning is not good because the inverter is not connected in series with the load. In addition, since the AC line supplies the load directly during the normal mode of operation, there is no possibility for regulation of the output frequency.

24.2.4 Universal UPS

This type of UPS is also called “series-parallel” or “delta conversion.” Its topology is derived from unified power quality conditioner (UPQC) topology and combines the advantages of both on-line and line-interactive UPS systems [12, 13]. It can achieve unity power factor, precise regulation of the output voltage, and high efficiency simultaneously. Its configuration is shown in Fig. 24.8. It consists of two bi-directional converters connected to a common battery set, static switch, and a series transformer. The series bi-directional converter is rated at about 20% of the output power of the UPS system and it is connected via a transformer in series with the AC line. The second bi-directional converter is the usual inverter for a line-interactive UPS connected in parallel to the load and rated at 100% of the output power.

FIGURE 24.8 Block diagram of a universal UPS system.

When the input voltage is in the acceptable range, the system is in the bypass mode. In this mode, parallel converter deals with current-based distortions. It mitigates load current harmonics and improves input power factor. At the same time, it charges the battery pack. Series converter deals with voltage-based distortions. It cancels input voltage harmonics and compensates voltage sags and swells. Most of the power is supplied directly from the AC line to the load. Only a small percentage of the input power is absorbed by parallel converter. This power is used to compensate the differences between input and reference voltages and to charge the battery pack. On the other hand, when the input voltage shuts down, the static switch separates the source and the load and the system goes to backup mode. In this situation, the parallel inverter acts as a DC/AC inverter and supplies power to the load. Since a large portion of the power flows without any conversion from the AC line to the load, the efficiency is higher than that of an on-line UPS system. Having eliminated the main drawback of double-conversion UPS systems, the universal UPS topology appears to be a strong competitor of on-line UPS systems in many applications. Figure 24.9 shows the topology of a three-phase universal UPS system.

FIGURE 24.9 A universal UPS topology based on two three-leg bi-directional converters.

24.2.5 Rotary UPS

Rotary UPS systems use the stored kinetic energy in the electrical machines to provide power to the load when a power outage occurs. There are different configurations for rotary UPS systems. The simplest topology consists of an AC motor and an AC generator, which are mechanically coupled. A flywheel is also used on the shaft of the machines to store more kinetic energy in the system. In normal operation, the input AC line provides power to the AC motor and this AC motor drives the AC generator. The configuration of this system is shown in Fig. 24.10a. In backup mode, the kinetic energy stored in the motor, flywheel, and generator is converted to electric power and supplies the load. This simple topology is designed to provide short time backup power to the load (typically less than 2 s) in case of power interruption. In another configuration of rotary UPS system which is shown in Fig. 24.10b, an AC motor, a DC machine, an AC generator, and a battery bank are used. During the normal mode of operation, the AC line supplies the AC motor, which drives the DC machine. The DC machine drives the AC generator, which supplies the load. During the backup mode of operation, the battery bank supplies the DC machine, which, in turn, drives the AC generator and the AC generator supplies the load. This system can provide long time backup power to the load depending on the capacity of the battery set. These two rotary UPS systems are much more reliable than the static UPS systems and provide complete electrical isolation between the load and input AC line. Yet, they require more maintenance and have much bigger size and weight. Therefore, they are usually used for high power applications [14, 15]. The configuration of a standby rotary UPS system is shown in Fig. 24.10c. This system does not provide electrical isolation between the load and input AC. There is also a transition delay for switching from main AC to backup AC generator. However, it can provide power to the load as long as needed.

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.

24.2.6 Hybrid Static/Rotary UPS

Hybrid static/rotary UPS systems combine the main features of both static and rotary UPS systems. They have low output impedance, high reliability, excellent frequency stability, and low maintenance requirements [6]. Typical configurations of hybrid static/rotary UPS are depicted in Fig. 24.11. They are usually used in high power applications. In the system shown in Fig. 24.11a, during normal operation, the input AC power feeds the AC motor. The power is provided to the load from the AC generator, which is driven by the AC motor. In 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 operation 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 generator. In this mode, the main breaker and inverter breaker remain open. One of the advantages of this topology is operation without a battery set to minimize cost, space, and required maintenance. The second advantage is avoiding double power conversion in long-term power interruption.

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.

24.2.7 Comparison of UPS Configurations

Table 24.1 below provides the comparison between characteristics of different types of UPS systems.

TABLE 24.1 Performance comparison of different configurations of UPS systems

24.3 Performance Evaluation

There are four criteria for evaluating the performance of a UPS system: quality of output voltage, input PFC and current harmonic cancellation, transition time, and efficiency. The quality of output voltage is the most important factor. The output voltage of a UPS system should be sinusoidal with low THD in different loading conditions even with non-linear loads. The control system should have small transient responses to provide appropriate line conditioning in different loading profiles. Typically, rotary UPS systems, which employ an AC generator at the load side, have better output voltage quality than static UPS systems. In these systems, there is no converter switching frequency present at the output voltage. Among the static UPS systems, on-line UPS configuration provides better output voltage quality. In this system, output voltage is provided by a DC/AC inverter regardless of input voltage quality. Usually, a pulse width modulation (PWM) method is used to regulate the output voltage. This kind of UPS should be designed to have minimum switching frequency at the output and provide pure sinusoidal voltage at different loading conditions. Followed by on-line UPS system are universal and line-interactive configurations. In universal topology, during normal mode of operation, the series converter provides voltage conditioning and regulates output voltage. In the backup mode, the parallel converter provides the load with sinusoidal voltage. In the line-interactive topology, during normal operation mode, input voltage directly supplies the load and no voltage conditioning is provided. In the backup mode, the DC/AC inverter provides the load with sinusoidal voltage.

The second criterion is transition time from normal mode of operation to stored energy mode. On-line rotary and static UPS systems have superior performance in this regard. The output voltage is always provided by the output generator or output DC/AC inverter and there is no transition time between operation modes of the systems. However, some of the rotary and hybrid configurations shown in Figs. 24.10 and 24.11 can only provide power to the load for a limited time. This time is determined by the amount of kinetic energy stored in the mechanical system. The transfer time in universal and line-interactive topologies depends on the time necessary for converting the power flow from the battery bank through the inverter to the load. Improved performance is achieved by choosing the DC bus capacitor voltage at the battery side to be slightly higher than the floating voltage of the batteries. Therefore, when the AC line fails, it is not necessary to sense the failure because the DC bus voltage will immediately fall under the floating voltage of the batteries and the power flow will naturally turn to the load. For off-line UPS systems, the transfer time is the longest. It depends upon the speed of sensing the failure of the AC line and starting the inverter.

The next important factor is the input power factor and the ability of the system to provide conditioning for load power. Universal UPS system has better performance followed by line-interactive and on-line UPS in this regard. During normal mode of operation, the parallel converter acts as an active filter and compensates reactive current and current harmonics generated by the load. In the line-interactive system, the bi-directional AC/DC converter performs this task. In an on-line UPS system, an additional system must be added to improve PFC and mitigate current harmonics.

The last criterion for performance evaluation is efficiency. To emphasize this factor, it should be noted that losses in UPS systems represent about 5–12% of all the energy consumed in data centers. Efficiency in rotary and hybrid configurations depends on the topology of the system but typically for low power application due to mechanical loss in the motor and generator, the efficiency is not very high. Among the static UPS systems, on-line UPS system has the poorest efficiency due to double conversion. Line-interactive and universal topologies provide higher efficiencies since most of the power directly flows from the input AC to the load during normal operation.

24.4 Applications

The UPS systems have wide applications in a variety of industries. Their common applications range from small power rating for personal computer systems to medium power rating for medical facilities, life supporting systems, data storage, and emergency equipment and high power rating for telecommunications, industrial processing, and on-line management systems. Different considerations should be taken into account for these applications. For emergency systems and lighting, the UPS should support the system for at least 90 minutes. Except for emergency systems, the UPS is designed to provide backup power to sensitive loads for 15–20 minutes. After this time, if the power is not restored, the system will be gracefully shut down. If a longer backup period is considered, a larger battery with higher cost and space is required. For process equipment and high power applications, some UPS systems are designed to provide enough time for the secondary power sources such as diesel generators to start up.

For industrial applications, it should be noted that UPS systems add to the complexity of the electrical system. They also add installation and ongoing maintenance costs. They may also add non-linearity to the system, decrease the efficiency, and deteriorate the input PFC mechanism. The power rating of the UPS should be appropriately selected considering the existing load and future extensions. For many applications, input voltage surges and spikes cause more damage than power outages. For these systems, another device instead of UPS can be utilized. Load characteristics should also be considered in UPS selection. For motor loads, the inrush current, which is sometimes 2.5 times of the rated current, should be considered. A good UPS for the motor loads is the one with higher transient overloads. For non-linear loads such as switching power supplies, the input current is not sinusoidal. Therefore, the instantaneous current is higher than the RMS current. This high instantaneous current should be considered in UPS selection.

For a power distribution network, two different approaches are taken to support sensitive loads. In a distributed approach, which is more suitable for highly proliferated loads such as medical equipment, data processing, and telecommunications many separate UPS units operate in parallel to supply critical loads. UPS units are placed flexibly in the system to form a critical load network. A typical on-line distributed UPS system is shown in Fig. 24.12. High flexibility and redundancy are the main advantages of distributed systems. Individual load increase can be supported by adding more UPS systems. Consideration for future extension can also be delayed until the loads are added. On the other side, this method has some disadvantages. The load sharing between different UPS units is a difficult task. Complicated digital control methods and communication between units are required to perform optimal load sharing. The second disadvantage is that the monitoring of the whole system is difficult and requires specially trained staff.

FIGURE 24.12 Typical configuration of a distributed UPS network.

The other method to support distributed loads is to use a large UPS unit to supply all the critical loads in a centralized approach. This approach is more desirable for industrial and utility applications. The advantage of this method is easier maintenance and troubleshooting. The disadvantages on the other side are lack of redundancy and high installation cost. In addition, consideration for system expansion should be taken into account when the original UPS unit is selected.

24.5 Control Techniques

The main task of the control system in a UPS unit is to minimize the output voltage total harmonic distortion in different loading profiles. In addition, it should provide the proper mechanism to recharge the battery set and maintain high input power factor and low total input current harmonic distortion. Other factors considered for a good control technique are nearly zero steady-state inverter output voltage error, good voltage regulation, robustness, fast transient response, and protection of the inverter against overload under linear/non-linear loads.

The most common switching technique is Sinusoidal PWM. This method can be utilized for both single-phase and three-phase systems. The advantage of this method is low output voltage harmonic and robustness. This strategy uses a single feedback loop to provide well-regulated output voltage with low THD. The feedback control can be continuous or discontinuous. Analog techniques are used in continuous approach. The sinusoidal PWM (SPWM) can be of natural sampling type, average type, or instantaneous type [17, 18].

In natural sampling type, the peak value of the output voltage is detected and compared with a reference voltage in order to obtain the error, which is used to control the reference to the modulator. The average approach is basically the same; but, the sensed voltage is converted to an average value and after that, is compared with a reference signal. These approaches control only the amplitude of the output voltage and are good only at high frequencies. In an instantaneous voltage feedback SPWM control, the output voltage is continuously compared with the reference signal improving the dynamic performance of the UPS inverter.

A typical block diagram of a three-phase DC/AC inverter for UPS systems and SPWM switching control technique is shown in Fig. 24.13. The disadvantage of this method is lack of flexibility for non-linear loads. Other programmed PWM techniques such as selective harmonic elimination, minimum THD, minimum loss, minimum current ripple, and reduced acoustic noise may be used for the inverter.

FIGURE 24.13 (a) Configuration of a three-phase DC/AC inverter for UPS systems and (b) simple voltage controller using PWM technique.

Better performance even with non-linear and step-changing loads can be achieved by multiple control loop strategies [19]. As shown in Fig. 24.14, there are two control loops: an outer and an inner. The outer control loop uses the output voltage as a feedback signal, which is compared with a reference signal. The error is compensated by a Pi-integrator to achieve stable output voltage under steady-state operation. This error is also used as a reference signal for the inner current regulator loop, which uses the inductor or the capacitor output filter current as the feedback signal. The minor current loop ensures fast dynamic responses enabling good performance with non-linear or step-changing loads. The basic current regulators employed as minor current loop are: hysteresis regulators, sinusoidal PWM regulator, and predictive regulators. In a typical hysteresis regulator, the reference signal is compared with the feedback signal. The sign and predetermined amplitude of the error determine the output of the modulator. The duration between two successive levels is determined by the slope of the reference signal. The output voltage tracks the reference signal within the upper and lower boundary levels. This hysteresis control has fast transient response; but, the switching frequency varies widely [20].

FIGURE 24.14 Typical current and voltage control loops for UPS inverter.

In SPWM control technique, the output voltage feedback is compared with a sine reference signal and the error voltage is compensated by a PI-regulator to produce the current reference. The current through the inductor or the capacitor is sensed and compared with the reference signal. After being compensated by a PI-regulator, the error signal is compared with a triangular waveform to generate SPWM signal for switching control. The SPWM current control has a constant switching frequency and also provides fast dynamic responses. In predictive current control method, the switching instants are determined by suitable error boundaries. When the current vector touches the boundary line, the next switching state vector is determined by prediction and optimization in order to minimize the error. Predictive current control requires a good knowledge of the load parameters. All these current regulators are typically used as an inner loop to regulate the current in the filter inductor. The current reference for the current regulator is obtained by summing together the error in an outer voltage loop with the actual load current to yield the rated output voltage.

With the increase of speed and reliability of digital processors and a decrease in their cost, digital processors have been facing an enormous growth of popularity in control applications in the past few years. Many digital and discrete control techniques such as dead-beat control [21], dissipativity-based control [22], sliding-mode control [23], space vector-based control [24], and multiple-feedback loop [25] have been developed using digital signal processors (DSP).

In this section, fundamental analysis of a dead-beat control method is explained for the three-phase UPS configuration shown in Fig. 24.13a. The state space equations of one phase of this system in the continuous time-domain are as follows.

(24.1)

(24.2)

Considering V_a and i_LF as state variables, the state space equation of the system is as follows:

(24.3)

These continuous time-domain state space equations are converted to the discontinuous time domain with a sampling period of T_s [26].

(24.4)

Where ω₀ is the angular resonance frequency of L_F and C_F. The sampling frequency of the system is always considered much higher than the resonance frequency of L_F and C_F. With this assumption, Eq. (24.4) is simplified to Eq. (24.5). This conversion is valid for almost f_s ≥ 20f₀.

(24.5)

The current equation according to Eq. (24.5) is given by:

(24.6)

Alternatively, this equation can be achieved by converting Eq. (24.2) from a differential equation to a difference equation. The same suggestion of f_s ≥ 20f₀ has to be made for this conversion as well. If V_a and i*_LF are considered constant over the next switching period, the output voltage of the inverter, which corrects the error of i_LF after two sampling periods, is described by:

(24.7)

A linear estimation of V_a(k+1) can be achieved from previous values:

(24.8)

By substituting Eqs. (24.8) and (24.10) in Eq. (24.9) and updating reference current for i_LF in every two sampling periods, the dead-beat digital control for series converter is described by:

(24.9)

Equation (24.9) ensures that the current error between i_LF and i*_LF at time k + 2 goes to zero with a delay of two sampling periods. Avoiding interaction between voltage and current control loops, load voltage, V_a, is sampled at half of the current sampling frequency. The voltage equation according to Eq. (24.5) is as follows.

(24.10)

(24.11)

As current control is suggested to be dead-beat with a delay of two sampling periods, capacitor current at time k and (k + 1) are given by:

(24.12)

Substituting Eq. (24.12) in Eq. (24.11) and updating the reference current at each of the two sampling periods, V_a(k + 2) is given by:

(24.13)

The current of i*_CF at time k which corrects the voltage error of V_a at time k + 4 is as follows.

(24.14)

A block diagram of the implementation of voltage and current control of the inverter is shown in Fig. 24.15. Block diagram of the current and voltage controller for the inverter is also shown in Fig. 24.16. Voltage regulator is a pure dead-beat controller with a delay of two sampling periods including the consumed time for calculation. G₁ is the time delay needed for calculations and analog to digital conversions. G₂ is the time delay caused by the PWM inverter and G₃ is the transfer function of the low pass filter. Current regulator is also considered as a pure delay. The output voltage of the inverter follows its reference with four sampling periods of delay. In practice, the dynamics of the current regulator is not a pure delay and shows some deviation from the dead-beat controller.

FIGURE 24.15 Implementation of the current and voltage control for the inverter shown in Figure 24.13a.

FIGURE 24.16 Block diagram of the current and voltage controller for the inverter shown in Figure 24.13a.

24.6 Energy Storage Devices

In this section, three dominant energy storage devices for the existing and future UPS systems are described. These energy storage devices are battery, flywheel, and fuel cell.

24.6.1 Battery

Battery is the energy storage component of current static UPS systems. It determines the capacity and run-time of the UPS.

For small units, it is the size of battery that determines the size of the UPS. Different types of batteries are used in UPS systems but the most commonly used types are lead-acid, nickel–cadmium, and lithium ion. The lead acid batteries used in this application are the same as the ones used in the cars. However, there is one small difference. Car batteries generate electricity by the reaction of sulfuric acid on lead plates that are drowned under the liquid. These types of battery cells are not suitable for UPS applications because there is a chance of acid spillage from them. In addition, during the charging process, they release hydrogen that is explosive and dangerous in a closed environment. Lead acid batteries used in UPS systems are a special kind called sealed or valve-regulated. The nickel–cadmium batteries are another popular type of batteries used in UPS systems. They usually provide higher energy and power density compared to lead-acid batteries. The nominal voltage of nickel–cadmium cells is 1.2 V, which is smaller than 1.5 V of lead-acid batteries. However, the cell voltage variation throughout different charge levels is less than lead-acid batteries. These batteries also have less series resistance and can provide higher surge currents. Lithium-ion batteries have much higher energy density. This kind of battery can be molded into different shapes. They have a nominal voltage of 4.2 V. The main disadvantage of lithium-ion battery is that they lose their capacity from the time of manufacturing regardless of their charge level and conditions of use. Table 24.2 shows a comparison between different kinds of batteries for UPS application.

TABLE 24.2 A comparison between different types of batteries for UPS systems

The traditional method of charging batteries is to apply constant current and constant voltage in two consecutive periods. Constant current is applied at the beginning of a typical full-charge cycle, when the battery voltage is low. When the battery voltage rises to a specified limit, the charger switches to constant voltage and continues in that mode until the charging current declines to nearly zero. At that time, the battery is fully charged. During the constant-voltage phase, the current drops exponentially due to the sum of battery resistance and any resistance in series with the battery (much like charging a capacitor through a resistor). Because current drops exponentially, a complete, full charge takes a long time.

24.6.2 Flywheel

Flywheel is simply a mechanical mass that is placed on the shaft of a motor-generator set and stores mechanical energy in the form of kinetic energy. When the electrical power is required, this kinetic energy is converted to electricity by the generator coupled with the flywheel. Flywheels are the oldest type of energy storage devices. The advantages of flywheel energy storage systems are high efficiency, high energy and power density, and long life. On the other hand, flywheels are more expensive and require more space than batteries and fuel cells. There are also some safety concerns about flywheels rotating at high speeds.

24.6.3 Fuel Cell

Due to high efficiency and low emissions, fuel cell systems have been gaining popularity in recent years. A fuel cell uses hydrogen as fuel and produces electricity, heat, and water from the reaction between hydrogen and oxygen. Each cell consists of an electrolyte and two electrodes as anode and cathode. Figure 24.17 shows the configuration of a typical fuel cell system. There are different kinds of fuel cell system depending on the types of electrolyte and hydrogen sources. Some fuel cell systems have an on-board fuel reformer and generate hydrogen from natural gas, methanol, and other hydrocarbons. Recent technology development in this field has made fuel cells a more reliable and cost-effective alternative for batteries. Fuel cells currently have a variety of applications in automotive, electric utility, and portable power industries. Table 24.3 provides a comparison between the most popular types of fuel cells.

FIGURE 24.17 Configuration of a typical fuel cell system.

TABLE 24.3 A comparison between different types of fuel cell system

Table of Contents for
24. Uninterruptible Power Supplies

24