Riello Whitepaper: Advantages of a Transformer-based UPS System

Riello UPS MasterThe Master HP (MHT series) uninterruptible power supplies are transformer-based UPS systems with discreet unit sizes from 100-600kVA that can be operated as standalone systems or in parallel to support MVA-sized critical Loads. The Mean Time Between Failure (MTBF) of this UPS design can be in excess of 250,000 hours (BSEN62040-3 – VFI topology and online mode) and it can be more suited to some critical sites and extreme power environments, providing a greater degree of power protection than a transformerless UPS system.

The Master HP UPS design incorporates a built-in Output Isolation Transformer, as standard. The transformer is located between the UPS Inverter output stage and Static Switch. It provides Galvanic Isolation, a functional separation (or barrier) between (a) the input supply (to the UPS Rectifier/Power Factor Correction (PFC) Converter) and (b) the UPS Inverter output to the Static Switch. Within the transformer, energy flows through the primary and secondary windings. There is no direct current path through which potentially damaging power quality problems can flow; either to the Load from an upstream distribution fault or downstream from the load back into the mains power supply. This type of transformer provides Master HP UPS with higher Levels of resilience and reliability than transformerless UPS designs, protecting both connected loads and the upstream distribution system. The same level of protection cannot be achieved by a transformerless UPS in its standard configuration.

The advantages resulting from a transformer-based UPS, increase when two or more systems are installed in a parallel N+X architecture, with or without a Decentralised or Centralised Bypass. For a single transformer-based UPS system these can include:

  1. Galvanic isolation between two power supply paths (Rectifier and Static Switch Inputs).
  1. The ability to supply a Master UPS from two independent mains power supplies.
  1. Dual Load protection from potentially Load damaging DC (direct current)
  1. The ability to provide Galvanic isolation between the mains power supply and the Load, and to recreate a fixed TN-S system downstream of the UPS without reducing operating efficiency.
  1. Provision for a higher Phase-Neutral inverter short circuit current than a Phase-Phase short circuit
  1. Superior power protection, compared to some transformerless UPS systems, from power quality problems such as spikes, electrical noise and transient surge voltages.
  1. Greater robustness with respect to back feed protection compared to some transformerless UPS systems.

Galvanic Isolation

With an AC power supply present (L1), the load is supplied via the Static Switch from the secondary winding of the Output Isolation Transformer. The Rectifier/PFC Converter is connected to the AC power source (L1) and is used to generate the direct current (DC), which in turn charges the Battery and powers the Inverter.

When the primary AC power supply fails (L1), the Inverter is supplied directly from the Battery. If a fault condition occurs the load is automatically transferred to the Bypass (L2), if available, through the Static Switch. With Galvanic isolation between the two AC power sources (L1and L2) – the Rectifier and the Bypass – the load is protected from electrical faults on (or fault migration) either AC power source and is presented with a continuous AC supply.

Transformerless UPS do not include an Isolation Transformer as part of their standard design – hence the name. To achieve the same level of AC supply separation and fault tolerance, transformerless UPS have to be installed with an additional Isolation Transformer; on either their output or input sides. This can lead to a more expensive initial capital spend and a less energy efficient UPS installation with higher Total Costs of Ownership (TCO)

A transformer-based UPS supply (L1) 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 (L1) must be a three-phase, four-wire (with neutral). The configuration provides the option to supply a transformer-based UPS from two separate earthing arrangements and prevents input earth or Rectifier/Battery faults from disrupting Bypass operation.

In contrast, a transformerless UPS requires a neutral from (i) the AC supply as part of Rectifier/Booster circuit and mid-point battery set configuration and (ii) the bypass supply.

Independent Mains Power Supplies

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 (TR1 and TR 2). This can further increase UPS system resilience and improve load power availability.

Dual Load Protection From DC Voltage

The inherent Galvanic isolation of the Output Isolation Transformer provides two-way protection. Connected UPS loads and the UPS Battery Set are protected from the potential damage that can result from:

  • Internal UPS DC faults, which could affect the output supply to connected load.
  • The potential for DC voltage disruption off the AC Inverter output waveform, following a component failure.

The UPS output is an AC waveform generated from a DC source, from within the UPS itself. Potential DC sources include the Rectifier/PFC Converter and Battery Set (and/or DC Flywheel or Super Capacitor Bank).

Any DC component present on the Inverter output AC waveform (due to a fault condition) can lead to load damaged or activation of their internal protection systems. This is a rare but potentially damaging problem for connected loads. Its severity is limited by active monitoring of the UPS Inverter waveform for DC voltage and IGBT (Insulated Gate Bipolar Transistor) failure within the UPS itself. In either scenario, a transformerless UPS will instantaneously transfer the load via the Static Switch to the Bypass supply. In this scenario the transformerless UPS has a higher potential for let-through voltage than a transformer-based UPS because DC voltage cannot pass through the transformer itself.

The Provision Of Galvanic isolation and Recreation of a Fixed TN-S System, Earthing System Without Reducing Operating Efficiency

The Inverter Isolation transformer provides the opportunity for full Galvanic isolation between the mains and the load via the installation of a Bypass Isolation Transformer (TR) upstream of the bypass line.

With the Bypass Isolation Transformer upstream of the bypass connection, full power does not pass through except when the load is switched from Inverter output to the Bypass and back. The only additional efficiency losses (during normal system operation) are those related to the bypass supply being available – but not under load. To achieve the same design configuration using a transformerless UPS requires the installation of an additional Isolation Transformer (at the input or output stages) with full power running through and under all operating conditions.

The addition of a separate Isolation Transformer to a transformerless UPS design introduces two further issues:

  • the overall system efficiency can reduce by 2-3% on average and up to 5% best case.
  • the transformer can introduce a single-point-of-failure in to the UPS design as the load cannot be directly connected to the Bypass supply

A comparison between the two systems shows how the overall AC/AC efficiency of a transformer-based Master HP UPS with an Isolation Transformer on the inverter output is greater than that of a transformerless UPS with an additional Isolation Transformer (even when no-load losses of the transformer upstream of the bypass line are taken into consideration).

Other issues resulting from the addition of an external Isolation Transformer to a transformerless UPS installation include:

  • poorer dynamic response and output voltage distortion (on load) than a trans former-based UPS with a built-in Output Isolation
  • an overall lower short circuit current than the design figure because the external Isolation Transformer reduces amplitude towards the load
  • reduced output power from the UPS, due to additional transformer losses
  • a larger overall footprint than that of a transformer-based UPS with its built-in transformer

Provision For A Higher Phase-Neutral inverter Short Circuit Current than a Phase-Phase Short Circuit Current

In a transformerless UPS the short circuit current set by the Inverter usually has the same value whether the downstream short circuit is Phase-Phase or Phase-Neutral. 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 between Phase-Phase. This higher short circuit current provides improved discrimination downstream of the UPS, even when the UPS is operating without a mains or bypass supply.

In its standard configuration, a transformer-based UPS is therefore more suited to installations with long cable runs and several discrimination (protection) levels.

Superior Power Protection When Presented With Power Quality Problems

Both transformerless and transformer based UPS are designed to protect their loads from power quality problems. A Transformer based UPS, like the master HP provides a higher degree of power protection and especially when installed in one of the configurations described in sections 1, 2 and 4. This is predominantly due to the Galvanic isolation of the Isolation transformer described above, which is more suited to tackling mains borne disturbances on the supply phases and provides for a more robust design. To match the power protection and electrical performance of the Master HP design, a transformerless UPS must be installed with an external Isolation Transformer. The downsides of this include a more capital intensive purchase and a lower overall operating efficiency.

Greater Robustness With Respect To Back Feed Protection

In a transformer-based UPS, the Output Isolation Transformer allows the UPS to power loads, such as motors (with four-quadrant drive systems) and industrial devices without disruption; even when the loads are installed with the type of back feed protection that can disrupt transformerless UPS system operation and force them into bypass. This requires the transformer-based UPS to be sized correctly to cope with the power demands and waveforms.

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