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Line Interactive vs. Online Double Conversion

A line interactive UPS and a double conversion UPS are two different types of uninterruptible power supplies (UPS) used in different situations. A line interactive UPS provides good protection from power outages, voltage sags and spikes, AVR boost/trim, while a double conversion UPS provides protection against broader range power issues and provides consistent clean power to connected devices.

A line-interactive UPS typically monitors the power coming in from the utility and actively adjusts the voltage coming into the device before sending the voltage to the protected equipment, thereby protecting it from surges or brownouts. In the event of a power outage, it seamlessly switches from “line” power to battery power, providing protection from power outages. There is a transfer time of about 8ms, but it’s not noticeable by most devices. Line interactive UPSs are the most common power protection systems used by data centers and businesses.

A double-conversion UPS operates by converting incoming AC power to DC through a rectifier and an inverter, so that your critical electronic devices will always get clean, stable, and reliable power. They are completely disconnected from the AC input. There is no transfer time with a double conversion UPS. In contrast to a line interactive UPS, a double conversion UPS will offer continuous power to devices, with no interruption or change in output voltage. The output frequency is also more stable and the UPS provides complete power protection for protected devices including medical equipment, test equipment and other power sensitive devices. This level of protection is ideal for very sensitive devices where any kind of interruption would be costly.

In conclusion, the main difference between line-interactive and double-conversion UPS systems is the type of protection they provide against different types of power issues and the level of clean power they can deliver to devices. While line-interactive UPS systems provide good protection from surges and sags, double-conversion systems offer a clearer and more stable source of power consistently. There is no transfer time with a double conversion online UPS.

Examples of line interactive UPS systems:

APC Smart-UPS 750
APC Smart-UPS 1000VA
APC Smart-UPS 1500VA

Examples of double conversion online UPS systems:

APC Smart-UPS SRT 1000VA
APC Smart-UPS SRT 1500VA
APC Smart-UPS SRT 2200VA
APC Smart-UPS SRT 3000VA

Why is generator power considered dirty?

Generators can produce what’s often referred to as “dirty” power. This term is used to describe power that is inconsistent with standard voltage and frequency levels. Generators, particularly during their start-up phase, can produce power with voltage and frequency fluctuations outside the range accepted by many electronic devices. This can be due to several factors, such as load changes, fuel type, and quality of the generator itself. These fluctuations, if severe, can lead to operational issues or even damage sensitive electronic devices, including the UPS that is plugged into them.

Standby, or offline, UPS systems will have trouble recognizing the “dirty” power from a generator as a valid power source. This can cause the UPS to remain on battery power even when the generator is running, leading to a premature draining of the UPS battery. Standby UPSs are designed to switch to battery power when they detect an interruption or significant deviation in input power, and the irregularities in generator power can trigger this switch. The UPS may also repeatedly switch back and forth between line and battery power, it will struggle to decide whether the generator power is acceptable, leading to wear and tear on the UPS and its connected equipment. For these reasons, when using a generator as a backup power source, use a more robust type of UPS, such as a line-interactive or double-conversion UPS, which are better equipped to handle the irregularities of generator power. Remember to set the power quality to low (if the UPS has the setting), that will help avoid unnecessary switches from utility power to battery power by the UPS.