The following points highlight the major technologies available for improving voltage sag performance. The technologies are: 1. Ferroresonant Transformers 2. Magnetic Synthesizers 3. Active Series Compensators 4. On-Line UPS 5. Standby UPS 6. Hybrid UPS 7. Motor-Generator Sets 8. Flywheel Energy Storage Systems and a Few Others.
1. Ferroresonant Transformers:
Ferroresonant transformers, also called constant voltage transformers (CVTs), can handle most voltage sag conditions. CVTs are especially attractive for constant, low power loads. Variable loads, especially with high inrush currents, present more of a problem for CVTs because of the tuned circuit on the output.
Ferroresonant transformers are basically 1:1 transformers which are excited high on their saturation curves, thereby providing an output voltage which is not significantly affected by input voltage variations.
Figure 2.13 shows the voltage sag ride-through improvement of a process controller fed from a 120 VA ferroresonant transformer. With the CVT, the process controller can ride through a voltage sag down to 30 percent of nominal, as opposed to 82 percent without one. Notice how the ride-through capability is held constant at a certain level. The reason for this is the small power requirement of the process controller, only 15 VA.
Ferroresonant transformers should be sized significantly larger than the load.
Figure 2.14 shows the allowable voltage sag as a percentage of nominal voltage (that will result in at least 90 percent voltage on the CVT output) versus ferroresonant transformer loading, as specified by one manufacturer. At 25 percent of loading, the allowable voltage sag is 30 percent of nominal, which means that the CVT will output over 90 percent normal voltage as long as the input voltage is above 30 percent.
This is important since the plant voltage rarely falls below 30 percent of nominal during voltage sag conditions. As the loading is increased, the corresponding ride-through capability is reduced, and when the ferroresonant transformer is overloaded (e.g., 150 percent loading), the voltage will collapse to zero.
2. Magnetic Synthesizers:
Magnetic synthesizers use a similar operating principle to CVTs except they are three- phase devices and take advantage of the three-phase magnetics to provide improved voltage sag support and regulation for three-phase loads. They are applicable over a size range from about 15 to 200 kVA and are typically applied for process loads of larger computer systems where voltage sags or steady-state voltage variations are important issues.
Energy transfer and line isolation are accomplished through the use of non-linear chokes. This eliminates problems such as line noise. The ac output waveforms are built by combining distinct voltage pulses from saturated transformers. The waveform energy is stored in the saturated transformers and capacitors as current and voltage. This energy storage enables the output of a clean waveform with little harmonic distortion. Finally, three-phase power is supplied through a zigzag transformer.
Figure 2.16 shows a magnetic synthesizer’s voltage sag ride-through capability as compared to the CBEMA curve, as specified by one manufacturer.
3. Active Series Compensators:
Advances in power electronic technologies and new topologies for these devices have resulted in new options for providing voltage sag ride-through support to critical loads. One of the important new options is a device that can boost the voltage by injecting a voltage in series with the remaining voltage during a voltage sag condition. These are referred to as active series compensation devices. They are available in size ranges from small single-phase devices (1 to 5 kVA) to very large devices that can be applied on the medium’ voltage systems (2 MVA and larger).
A one-line diagram illustrating the power electronics that are used to achieve the compensation is shown in Fig. 2.17. When a disturbance to the input voltage is detected, a fast switch opens and the power is supplied through the series connected electronics. This circuit adds or subtracts a voltage signal to the input voltage so that the output voltage remains within a specified tolerance during the disturbance.
The switch is very fast so that the disturbance seen by the load is less than a quarter cycle in duration. This is fast enough to avoid problems with almost all sensitive loads. The circuit can provide voltage boosting of about 50 percent, which is sufficient for almost all voltage sag conditions.
4. On-Line UPS:
Figure 2.18 shows a typical configuration of an on-line UPS. In this design, the load is always fed through the UPS. The incoming ac power is rectified into dc power, which charges a bank of batteries. This dc power is then inverted back into ac power, to feed the load. If the incoming ac power fails, the inverter is fed from the batteries and continues to supply the load. In addition to providing ride-through for power outages, an on-line UPS provides very high isolation of the critical load from all power line disturbances. However, the on-line operation increases the losses and may be unnecessary for protection of many loads.
5. Standby UPS:
A standby power supply is sometimes termed off-line UPS since the normal line power is used to power the equipment until a disturbance is detected and a switch transfers the load to the battery backed inverter. The transfer time from the normal source to the battery-backed inverter is important. The CBEMA curve shows that 8 ms is the lower limit on interruption through for power-conscious manufacturers. Therefore a transfer time of 4 ms would ensure continuity of operation for the critical load.
A standby power supply does not typically provide any transient protection or voltage regulation as does an on-line UPS. This is the most common configuration for commodity UPS units available at retail stores for protection of small computer loads. UPS specifications include kilo-volt-ampere capacity, dynamic and static voltage regulation, harmonic distortion of the input current and output voltage, surge protection, and noise attenuation. The specifications should indicate, or the supplier should furnish, the test conditions under which the specifications are valid.
6. Hybrid UPS:
Similar in design to the standby UPS, the hybrid UPS utilizes a voltage regulator on the UPS output to provide regulation to the load and momentary ride-through when the transfers from normal to UPS supply is made.
7. Motor-Generator Sets:
Motor-generator (M-G) sets come in a wide variety of sizes and configurations. This is a mature technology that is still useful for isolating critical loads from sags and interruptions on the power system. The concept is very simple.
A motor powered by the line, drives a generator that powers the load. Flywheels on the same shaft provide greater inertia to increase ride-through time. When the line suffers a disturbance, the inertia of the machines and the flywheels maintains the power supply for several seconds. This arrangement may also be used to separate sensitive loads from other classes of disturbances such as harmonic distortion and switching transients.
While simple in concept, M-G sets have disadvantages for some types of loads:
1. There are losses associated with the machines, although they are not necessarily larger than those in other technologies.
2. Noise and maintenance may be issues with some installations.
3. The frequency and voltage drop during interruptions as the machine slows. This may not work well with some loads.
Another type of M-G set uses a special synchronous generator called a written- pole motor that can produce a constant 60-Hz frequency as the machine slows. It is able to supply a constant output by continually changing the polarity of the rotor’s field poles. Thus, each revolution can have a different number of poles than the last one. Constant output is maintained as long as the rotor is spinning at speeds between 3150 and 3600 revolutions per minute (rpm).
Flywheel inertia allows the generator rotor to keep rotating at speeds above 3150 rpm once power shuts off. The rotor weight typically generates enough inertia to keep it spinning fast enough to produce 60 Hz for 15 s under full load. Another means of compensating for the frequency and voltage drop while energy is being extracted is to rectify the output of the generator and feed it back into an inverter. This allows more energy to be extracted, but also introduces losses and cost.
8. Flywheel Energy Storage Systems:
Motor generator sets are only one means to exploit the energy stored in flywheels. A modern flywheel energy system uses high-speed flywheels and power electronics to achieve sag and interruption ride-through from 10 s to 2 min.
While M-G sets typically operate in the open and are subject to aerodynamic friction losses, these flywheels operate in a vacuum and employ magnetic bearings to substantially reduce standby losses. Designs with steel rotors may spin at approximately 10,000 rpm, while those with composite rotors may spin at much higher speeds. Since the amount of energy stored is proportional to the square of the speed, a great amount of energy can be stored in a small space.
The rotor serves as a one-piece storage device, motor, and generator. To store energy, the rotor is spun up to speed as a motor. When energy is needed, the rotor and armature act as a generator. As the rotor slows when energy is extracted, the control system automatically increases the field to compensate for the decreased voltage. The high-speed flywheel energy storage module would be used in place of the battery in any of the UPS concepts.
9. Superconducting Magnetic Energy Storage (SMES) Devices:
An SMES device can be used to alleviate voltage sags and brief interruptions. The energy storage in an SMES-based system is provided by the electric energy stored in the current flowing in a superconducting magnet. Since the coil is lossless, the energy can be released almost instantaneously. Through voltage regulator and inverter banks, this energy can be injected into the protected electrical system in less than 1 cycle to compensate for the missing voltage during a voltage sag event.
The SMES-based system has several advantages over battery-based UPS systems:
1. SMES-based systems have a much smaller footprint than batteries for the same energy storage and power delivery capability.
2. The stored energy can be delivered to the protected system more quickly.
3. The SMES system has virtually unlimited discharge and charge duty cycles. The discharge and recharge cycles can be performed thousands of times without any degradation to the superconducting magnet.
The recharge cycle is typically less than 90 s from full discharge. Figure 2.23 shows the functional block diagram of a common system. It consists of a superconducting magnet, voltage regulators, capacitor banks, a dc-to-dc converter, dc breakers, inverter modules, sensing and control equipment, and a series injection transformer. The superconducting magnet is constructed of a niobium titanium (NbTi) conductor and is cooled to approximately 4.2 kelvin (K) by liquid helium. The cryogenic refrigeration system is based on a two-stage recondenser. The magnet electrical leads use high-temperature superconductor (HTS) connections to the voltage regulator and controls. The magnet might typically store about 3 megajoules (MJ).
In the example system shown, energy released from the SMES passes through a current-to-voltage converter to charge a 14-microfarad (mF) dc capacitor bank to 2500 Vdc. The voltage regulator keeps the dc voltage at its nominal value and also provides protection control to the SMES. The dc-to-dc converter reduces the dc voltage down to 750 Vdc.
The inverter subsystem module consists of six single-phase inverter bridges. Two IGBT inverter bridges rated 450 amperes (A) rms are paralleled in each phase to provide a total rating of 900 Aper phase. The switching scheme for the inverter is based on the pulse-width modulation (PWM) approach where the carrier signal is a sine-triangle with a frequency of 4 kHz.
A typical SMES system can protect loads of up to 8 MVA for voltage sags as low as 0.25 pu. It can provide up to 10 s of voltage sag ride through depending on load size. Figure 2.24 shows an example where the grid voltage experiences a voltage sag of 0.6 pu for approximately 7 cycles. The voltage at the protected load remains virtually unchanged at its pre-fault value.
10. Static Transfer Switches and Fast Transfer Switches:
There are a number of alternatives for protection of an entire facility that may be sensitive to voltage sags. Another alternative that can be applied at either the low voltage level or the medium voltage level is the automatic transfer switch. Automatic transfer switches can be of various technologies, ranging from conventional breakers to static switches. Conventional transfer switches will switch from the primary supply to a backup supply in seconds. Fast transfer switches that use vacuum breaker technology are available that can transfer in about 2 electrical cycles.
This can be fast enough to protect many sensitive loads. Static switches use power electronic switches to accomplish the transfer within about a quarter of an electrical cycle. The most important consideration in the effectiveness of a transfer switch for protection of sensitive loads is that it requires two independent supplies to the facility.
For instance, if both supplies come from the same substation bus, then they will both be exposed to the same voltage sags when there is a fault condition somewhere in the supply system. If a significant percentage of the events affecting the facility are caused by faults on the transmission system, the transfer switch might have little benefit for protection of the equipment in the facility.