Uninterruptible power supplies provide backup power, protecting equipment from damage in the event of grid power failure. An uninterruptible power supply (UPS) is a type of device that powers equipment, nearly instantaneously, in the event of grid power failure, protecting the equipment from damage. Input and Output nr of Phases vary depending on the place of use.
UPS’es provide backup power to the system in the event that the current supplying your computer system is reduced or exhausted as a result of a voltage drop or a power outage, as well as during fluctuations in electrical current, either short-term and mild or long-term and high increases. UPS’es ensure that the system continues to function normally in the event of a power outage, giving you the time, you need to save the work you are working on and shut down your system in a healthy manner.
The power supplies of personal computers (PCs) used in homes are fed from single-phase UPS’es with output power ranging from 500 VA to 1000 VA, depending on the computer and peripheral equipment used (monitor, printer, faxmodem, etc.). Most of the time, line-interactive UPSs are sufficient for PC use. In advanced architecture (e.g. server systems) on-line UPS systems are widely used.
Line-interactive UPSes apply the mains energy directly and without any voltage / frequency correction to the computer during normal operation. Therefore, voltage / frequency fluctuations and distortions that may occur in the mains voltage are applied to the computer as it is, without any regulation. The mains voltage used in the home and office environment, where the energy quality problems encountered in industrial applications are not in question, is not foreseen to be applied directly to the power supplies of computers, and this situation does not cause a problem in the computer due to the voltage waveform. In the event of a mains failure, Line-interactive UPSs convert the direct voltage electrical energy from the battery into square wave alternating voltage and apply it to the computer. Undoubtedly, the quality of the square wave or zero-level square wave output voltage applied to the computer by the UPS during a power failure is lower than the sinusoidal waveform of the mains, depending on the computer model, but the power supplies of computers can tolerate this relatively short (7-10 minutes) operation.
On-Line UPSes are used at higher powers and in more mission-critical applications. They can be single-phase and three-phase. Today, they are widely used to supply critical loads such as automation systems, ATM machines, communication systems and medical devices. Unlike the line-interactive structure, the electrical power requirement of the information processors, which are considered as critical loads, is met through the UPS unit instead of the mains during normal operation, and the UPS isolates the load in a way that protects it from all the negativities that may occur in the mains voltage (voltage / frequency variations, harmonics, EMI, Flicker, etc.). Online UPSs, which offer not only uninterrupted but also high quality and regulated clean electrical power, provide the power protection required by today’s computing technologies with their advanced power and control structures.
The power conversion that takes place in both a one-phase and a three-phase UPS can be briefly summarized as follows:
The alternating voltage of the mains (220 V / 380 V, 50Hz) is first rectified to meet the supply requirements of the inverter power stage and to provide the charging voltage for the batteries in systems without a separate battery charging circuit. The rectification is performed by a rectifier located at the input of the UPS. In models where the battery charger is separate, only a second power conversion unit, the inverter, is fed with the rectified voltage.
The inverter converts the rectified voltage again, but this time into a much more stable and regulated alternating voltage. This voltage at the output of the UPS deviates from its nominal value by as little as 1%. The same can be said for the frequency. As a result, the output of the UPS is much better-quality electrical energy in terms of voltage and frequency characteristics than the grid energy at the UPS input. The cost, complexity, weight, dimensions, etc. of on-line systems are the price of their advantages. In most UPSs, there is also a structure called a bypass switch, which is placed to prevent critical loads from being left without supply in the event of a failure in the inverter power layer. When the mains voltage and the UPS output voltage are fully synchronized, which is provided by the UPS, in the event of a fault, the load current is transferred to the bypass source without interruption. The same mains voltage is often applied to the bypass switch and the UPS unit.
Let’s examine the UPS selection criteria in order:
1- UPS selection according to apparent power: The number and power of the device (computer, printer, fax, etc.) to be connected to the UPS output determines the power of the UPS to be selected. This power is the apparent power and is expressed in kVA. Apparent power is given by the relation Apparent power (VA) = Voltage (V)*Current (A), and its value is determined directly by the magnitudes measured by the measuring devices measuring the voltage/current RMS value. However, the actual power consumed in loads consuming electrical energy is active power, its unit is Watt and the relation between apparent power and active power = Apparent power * Power factor is valid.
1- UPS selection according to apparent power: The number and power of the device (computer, printer, fax, etc.) to be connected to the UPS output determines the power of the UPS to be selected. This power is the apparent power and is expressed in kVA. Apparent power is given by the relation Apparent power (VA) = Voltage (V)*Current (A), and its value is determined directly by the magnitudes measured by the measuring devices measuring the voltage/current RMS value. However, the actual power consumed in loads consuming electrical energy is active power, its unit is Watt and the relation between apparent power and active power = Apparent power * Power factor is valid.
2- UPS selection according to the number of phases: Unless otherwise specified, the number of phases of the UPS is given for the output phases. Generally, UPSs above 10 kVA are three-phase and are used in businesses where a three-phase electrical distribution system is installed. For home and small office applications, one phase UPS is sufficient. It is essential that the one-phase UPSs fed from the three-phase electrical distribution system are as evenly distributed as the one-phase loads to be connected to the output of the three-phase UPSs are evenly distributed. UPSs/critical loads unbalanced to draw very different currents from the phase conductors increase the currents flowing through the neutral line. Excessive neutral currents can cause increased power losses, fluctuations in monitor screens, and in some cases even hard disk failures, necessitating additional precautions. This type of negativity is not an issue when using a one-phase UPS fed from a one-phase electrical system.
As a result, UPSs to be used only at home should be one-phase; where three-phase electrical installations are installed, one-phase or three-phase UPSs should be used, depending on the user’s preference. In three-phase systems, proper sizing of phase and neutral conductors and correct load distribution will minimize possible problems that may be encountered in the future.
3- UPS selection according to battery duration: The main purpose of the UPS is to continue to supply critical loads in the event of a power failure for a period of time determined by the battery capacity depending on the load current drawn. Batteries are at least as important system components as the UPS itself and their selection and maintenance require special attention. The feeding time from the battery is proportional to the battery capacity used, in other words, the capacity (Ah) of the batteries to be considered for long power outages must be high. This requires you to use a UPS with high power. However, there are also non-standard UPSs with low output power but high battery runtime. These models usually have higher prices, higher current ratings of the charging circuits or use additional charging circuits.
Standard UPS for a PC only commonly use 12V/7Ah batteries and have a battery run time of about 7-10 minutes. How much we can utilize a UPS during a power outage depends on the size of the battery in the UPS and how much energy the device we are using consumes.
Let’s assume that our monitor and computer need a total of 350 watts of power. Let the battery of the UPS we use be 12 Volt and 10 Ampere Hour. The current drawn by 350 watts of energy at 12Volt is approximately 350/12 = 30 Amps. If the battery used in the UPS was 30 Ampere hours, our computer would be able to work for 1 hour without power failure. But since the battery used is a 10 Ampere hour battery, this type of UPS will last us about 20 minutes. Of course, over time, this value may drop to 10 minutes as the batteries will wear out.
To avoid problems with the battery in the UPS, you should make sure that the battery connections are firm, the battery charging voltage is correct and the battery ambient temperature is appropriate (average temperature 20° degrees). The battery of a UPS that has not been used for a long period of time will discharge through its own internal resistance and may not be recharged depending on the condition of the battery, the make/model, and the duration of use. It is recommended to leave the UPS running (connected to the mains) even when the computer is not in use. When it is desired to increase the battery supply time, batteries must be added to the existing battery bank. The batteries to be added should be connected in parallel with the existing group and should be the same number as the batteries used in the existing group. If possible, the same brand and model battery should be used. Make sure that UPS can provide the required charging current when the battery is added. It is not foreseen to use different manufactured batteries together even if they are of the same brand.
4- UPS selection according to monitoring features: Advances in UPS technology are reflected in the user interface. Analog or numerical indicators on the front panel provide the user with information about UPS’s operating status and alarms. The overload alarm on almost all UPSs indicates that the UPS is overloaded, the battery alarm indicates that the battery supply time has decreased or there is a problem with the battery. Users should pay attention to these alarms and take necessary precautions by consulting the user manual or the vendor. Some UPSs are available with monitoring software. By installing this software on your computer and establishing a communication link between the serial communication port of the computer and the RS232 port of your UPS, you can continuously monitor the entire operating status of your UPS on your computer screen. SNMP software is also available for advanced UPS models. SNMP kits allow you to monitor your UPS over the internet or network, but with a delay. In very critical applications, monitoring features are important because they shorten service time during fault detection and troubleshooting.
5- UPS selection according to mains voltage variation range: Apart from the above-mentioned, the range of effective value and frequency of the mains voltage to be supplied by the UPS is another issue affecting the UPS selection. If the mains voltage varies over a very wide range, the UPS to be used must be able to adapt to this situation. When the mains (input) voltage goes too high or too low above or below its nominal value, the critical load will be fed from the battery and after a while the battery will be completely discharged. In the event of a second power failure before the battery is recharged, UPS will not be able to provide the necessary power protection. To avoid this situation, you should make sure that the mains voltage you use and its variation range, the technical data of your UPS supports this situation, you can use a voltage regulator to regulate the voltage variation when necessary.
Electromechanical or electronic regulators designed to protect the loads connected to it against voltage fluctuations (voltage drop or rise) occurring in the city network.
In general, there are two types of Servo Type and Static Type in the market. Both types have an electronic control card with microprocessors.
1) Servo Type Voltage Regulator
This type of Voltage Regulators basically has a Variac winding, and an output transformer controlled by a motor in voltage changes. The microprocessor control card monitors the city network and adjusts the voltage at the output to the desired value by controlling the motor according to the need.
2) Static Type Voltage Regulator
This type of Voltage Regulators consists of a transformer with stepped output values and Thyristors whose steps are managed by a microprocessor control card. It adjusts the output voltage to the desired value by controlling the thyristors according to the change in the city network.
Harmonics is the distortion or fluctuations of current / voltage values. Harmonics can disrupt electronic cards, make noise in machines, reduce efficiency, frequently blow fuses and circuit breakers, and are an undesirable factor for these reasons. In new generation Ups, the negative effects of harmonic oscillations on the input power factor are minimized. For this purpose, 12-pulse rectifiers and input harmonic filters have come to the fore, especially in new generation UPSs with large powers, and the use of PFC (Power Factor Correction) circuits has become widespread.
It is a system that can compensate with capacitors to correct the voltage – current imbalance of inductive loads such as computers, motors, lighting devices consisting of coils. PFC system is designed and applied as active or passive. In passive PFC system, the load is balanced by using capacitive (capacitor) or inductive (coil) circuit elements according to the type of load. Active PFC system consists of a special circuit. This circuit enters between the power grid and the power supply and provides the voltage required by the power supply through itself. Active PFC systems are much more successful in correcting the power factor and can increase this value up to 0.99, which is very close to the ideal value of 1
The Static Bypass switch ensures uninterrupted supply of the critical load from the bypass source (most often mains) in the event of failure of the UPS inverter. For a smooth transfer of load to the bypass source, the UPS inverter must be in constant synchronization (monitoring the bypass source) with the bypass source. The bypass supply voltage must remain within the operating range of the critical load, otherwise no-load transfer is allowed.
In UPS systems, UPS devices of the same power and type are connected in parallel to each other for capacity increase and backup purposes. In a parallel connected system, UPSs must work with each other and with the grid. Parallel connected UPSs share the load current equally.
The UPS manufacturer’s recommendations for installation and commissioning must be strictly followed. Ambient temperature, humidity, air flow, flammable and explosive free environment are the most important considerations for installation.