In the power protection playing field, the enemy offensive team often could include the following players: unreliable grid, adverse conditions, industrial machinery disturbances, external phenomena, and distributed architecture. In those cases, you need to make sure your defense team is prepared to tackle all of them and stop them from scoring.

Even though there are rumors that transformer-based UPS are obsolete, nowadays they are still one of the most valuable players in critical facilities where downtime is simply not acceptable. In this article, we will discuss the reasons why transformer-based UPS are still a strong defense.

How it works

The standard transformer-based UPS has two alternating (AC) power inputs from a single connection to the mains power supply: the rectifier and the bypass. In case the AC power fails, the inverter receives the power supply from the battery, otherwise, if a fault occurs, the load is automatically transferred to bypass – if available.

With galvanic isolation between the two AC power sources – the rectifier and the bypass – the load is protected from electrical faults (or fault migration) on either AC power source and receives a continuous AC supply.

With other UPS typologies, if the installation of an additional isolation transformer, either on their output or input, is required, this can lead to higher initial investments, installation costs, and Total Costs of Ownership (TCO).

Moreover, with a transformer-based UPS, the rectifier AC supply only requires a delta three-phase, three-wire AC supply (with no neutral).

This is because the inverter output is referenced to the incoming neutral on the secondary side of the output isolation transformer.

The bypass AC supply must be a three-phase, four-wire (with neutral). This configuration allows the transformer-based UPS to receive the power supply from two separate grounding arrangements and prevents input earth or rectifier/battery faults from disrupting bypass operation.

The output isolation transformer allows the UPS to be connected to two separate inputs (for the rectifier and bypass) and even two independent power sources. This can further increase UPS system resilience and improve the availability of the load.

Any DC voltage present on the inverter output AC waveform (due to a fault) can lead to damages on the connected IT equipment or to the activation of their internal protection systems. This is a rare but potentially detrimental problem for connected loads.

However, the inherent galvanic isolation of the output isolation transformer provides dual protection because DC voltages cannot pass through the transformer.

In fact, connected UPS loads and the UPS battery bank are protected from the potential damage that can result from internal UPS DC faults, which could affect the output supply to connected loads, or the potential for DC voltage disruption of the AC inverter output waveform, following a component failure.

Transformer-based UPS provides galvanic isolation between two AC power sources, the rectifier and bypass, and the recreation of a fixed TN-S system (grounding system), often without reducing operating efficiency.

When there is a bypass isolation transformer upstream the bypass connection, full power does not pass through except when the load is transferred from the inverter output to the bypass and back. The only efficiency losses during normal operation are related to the bypass being available.

In some cases, to achieve this level of galvanic isolation with a transformer-free UPS, an additional isolation transformer must be installed at the input or output.

This device will work with full power running through under all operating conditions and this could decrease the overall system efficiency by two to three percent on average (up to 1.5 percent at best).

Furthermore, since the load cannot be directly connected to the bypass supply, the transformer can introduce a single point of failure into the power protection system design.

Other strengths resulting from the embedded isolation transformer include, higher dynamic response and output voltage distortion (on load) and overall higher short circuit current, and sometimes, even higher output power from the UPS or smaller overall footprint (transformer-based UPS with its built-in transformer).

When it comes to dodging that dreaded downtime, it may well be time to get on the (transformer-based) defensive – Getty Images

In it for the long haul

Transformer-based UPSs are more suited for installations with long cable runs and several protection levels.

In a transformer-based UPS, the short circuit current value set by the inverter is greater if the downstream short circuit is phase-neutral, as opposed to phase-phase. This higher short circuit current provides improved protection downstream of the UPS, even when the UPS is operating without a mains or bypass supply.

Although all UPS are designed to protect loads from power quality problems, the transformer-based UPS offers a higher degree of protection thanks to the galvanic isolation of the isolation transformer.

The isolation transformer is more suited to tackling mains disturbances on the supply phases and provides a more robust design.

Industrial devices have specific power demands and waveforms. If the transformer is not included in the UPS, even with back feed protection installed, these loads can disrupt the power supply and force a transfer to the bypass.

Therefore, you will require an output isolation transformer within the UPS to power this type of load; and the only concern will be making sure the UPS is sized correctly according to the application.

You know a transformer-based UPS will be a great addition to your team because you won’t get any surprises: you have seen it play; you have seen what it is capable of in terms of efficiency, operating costs, active power, and footprint.

Having to add an external isolation transformer basically means hiring a second player to protect what the first cannot. As a result, you may end up with higher operating costs, a greater initial investment, the largest footprint, and less efficiency. Also, you may introduce new single points of failure.

Therefore, to win the championship fair and square, you need the robust MVP that will protect your facility, whilst requiring no second player.

When it comes to dodging that dreaded downtime, it may well be time to get on the (transformer-based) defensive.