There may be disagreements among AC- and DC-based UPS designers, but the telecom and IT professionals are sure that they need solutions that can address physical space constraints, fast-track construction realities, long critical system lead times, and bus scalability. Traditionally, DC-based UPS systems have been the mainstay of telcos to support their telecom switches. The enterprises protect their IT infrastructure using AC-based UPS systems.

TECHNOLOGY

n Telecom Power Systems: Both power plant and battery technologies have made rapid advances in the recent past. Switch Mode Power Systems (SMPS) have become the norm, as have Valve Regulated Lead acid (VRLA) batteries. Both these technologies make for a far more compact power plant design, modular approach, and low maintenance. Power plants are made up of rectifier modules, controllers, input AC distribution, load distribution, and battery connection modules.

DC versus AC: Telecom equipment works with DC power. The voltage levels are generally 48V or in a few cases 24V. A typical rectifier converts the AC utility power to 48 V DC, charging a bank of batteries usually designed for two to eight hours of critical protection from power outage. Since a battery bank is floated across the DC bus, to provide battery backup, the working voltage is in reality the float voltage of the battery. For VRLA batteries, the float voltage is typically 2.25V/cell or 54V for a 48V system and 27V for a 24V system. The positive in a 48V system is grounded, whereas in a 24V system, the negative is grounded. This difference is due to the practice adopted by the pioneering telecom equipment manufacturers, which is continuing today. In contrast to AC systems, DC systems do not require converting the source power from AC/DC and DC/AC.

Basic DC Technology: Three basic technologies comprise large DC power plants: silicon-controlled rectifier (SCR) phase, controlled ferroresonant rectifiers, and high-frequency switch-mode rectifiers. The SCR phase control and controlled ferroresonant technologies have been around for more than three decades and are widely accepted. Single-phase switch-mode rectifiers have been around since the Eighties and have not been a problem.

n New Look Power Plants: Though the need for power is constant, the way telecom carriers obtain it is not. New types of power plants, with the intelligence and with additional features that support the increased demand on telco operations and supporting the data transported, are coming up. The key here is remote management. Critical features in today’s power systems automate routine tasks, allowing the plant to monitor self-health and even notify the provider to perform preventive action maintenance. Tasks such as load sharing, generator control, and data logging are becoming standard issues in many of today’s premier power systems.

Load Sharing: Today, rectifiers still are used in power systems to change standard AC power into a flat DC voltage, making power plants less dangerous. In today’s systems, the current that delivers power to network equipment and the backup battery charge current is measured at actual value and is evenly divided across the rectifiers installed in the system. Further, rectifiers can communicate directly with each other in the event that a control unit is removed for maintenance or for a software feature upgrade.

Generator control: Power systems can monitor the AC main input, which is particularly helpful during power outages. By integrating the monitor system with an auto-transfer switch, batteries and a generator set, the power system can control fuel usage during failures.

Data Logging: Data collection is a valuable tool for service providers. The intelligence built into power plants to support data logging can be used by technicians to pinpoint potential system failures and proactively plan for maintenance. Data logs can help determine the source of potential power problems within an equipment site.

n Power for Data Communication: When it comes to data communication equipment, one has a choice of AC or DC powered systems. AC powered systems are used because of the availability of uninterrupted AC power source, but in application where reliability is the paramount consideration, as in data centers, DC powered equipment are preferred.

Single Conversion: Under normal operating conditions, AC power from the utility passes straight through the UPS to the critical load. A charger or ‘4-quadrant converter’ converts AC power to DC to charge the battery. The inverter is used to convert the DC power from the battery to create AC power to support the load when the utility fails. Normally, the inverter is operating in the standby mode, keeping the batteries charged. Should the utility power go out of specification, the inverter powers the load, drawing energy from the battery. At any point in time, power is only being converted once (AC to DC, or DC to AC).

The Line-Interactive UPS: It resembles the offline product, but inserts a transformer or inductor in series between the utility power source and the load. This inline inductor enables the UPS inverter to ‘interact’ with incoming power and provide a measure of power conditioning to the load. This ‘buck-and-boost’ circuitry helps with high and low input voltage conditions.

Delta Conversion Topology: Delta conversion UPS consists of input stage called the delta conversion stage or active filter made of delta inverter and delta transformer. The delta transformer not only bucks or boost input voltage, but also has many other functions such as input current control to reduce harmonics, improvement of power factor, and control input power flow using power transformation method. On the output side of Delta conversion UPS is a main inverter which is similar to any inverter used in double conversion stage which continuously monitors the output voltage and keeps it regulated to +/- 1 percent of nominal voltage setting (380V/400V/415V). In case of the mains’ failure, it supplies the 100 percent power to the load from battery without any break in the output voltage waveform. The UPS has input mains’ static switch used to isolate the UPS from mains under the battery operation to avoid any floating voltage to appear at main. The UPS also has the internal bypass static switch to transfer the load to mains in case of overload above specified limit or inverter failure like any other double conversion UPS.

Double Conversion Topology: As the name indicates, this technology uses two conversion stages—the rectifier at input and the inverter at out put. In double conversion topology, input AC power is first converted to DC power using a rectifier. This DC power is then converted to AC power using an inverter to supply the load. This topology has been in use since last two decades by the manufacturers. The double conversion online topology is believed to give 100 percent protection against the typical power problems.

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