Wednesday, November 2, 2011

Delta Conversion Online UPS is a modern innovation that eliminates the disadvantages of the traditional double conversion online UPS. The enhancements are based on having a delta inverter and allowing bidirectional power flow. Today, this design is the only major uninterruptible power supply technology protected by patents and is unlikely to be offered from a broad range of UPS suppliers. Also, delta conversion online units are available in the market from 5 kVA to 1 MVA.

Similarities and Differences

A. Components

The delta conversion online UPS differs physically from the conventional design because of two main components – the delta inverter and delta transformer.

Delta Inverter

The delta conversion online UPS has a so-called delta inverter, which is a current-controlled IGBT Pulse Width Modulator (PWM) inverter. It performs two major functions:

1. Input power and current regulation

Delta inverter acts like a variable current source in the secondary circuit of the transformer, such that it works like a load without power dissipation (except for switching losses). This active front end draws power that is sinusoidal and in-phase with the source voltage. Consequently, current magnitude, wave shape and power factor can be controlled. Thus, with a delta conversion online UPS, the following can be achieved:

  • Reduced heating and system wear in the electrical power system.
  • Minimized harmonics.
  • Optimal conditions for utility and generator systems.

2. Charge the battery of the UPS

Delta inverter converts AC voltage to the suitable DC charging voltage.

Delta Transformer

The delta transformers in a delta conversion online UPS are actually three single-phase isolation transformers with one for each phase. Usually, these transformers have a voltage ratio of 1:5 (primary:secondary). It also follows that the current ratio is normally 5:1.

B. Power Flow

In a delta conversion online UPS, both the main and delta inverters are bidirectional devices, such that they have the capability to transform AC power to DC power, and at the same time DC power to AC power.
Delta Conversion Online UPS
Delta Conversion Online UPS Block Diagram
On the other hand, the double conversion online UPS has only one power path. This is because the rectifier/charger and the main inverter are unidirectional power conversion devices. In such a case, first, the rectifier changes real AC power (watts) to DC. Then, the main inverter transforms back DC power to AC, which is supplied to the loads.
Double Conversion Online UPS
Double Conversion Online UPS Block Diagram
In other words, both designs have similar fundamental functions, but the delta conversion devices can perform improved functionality.

The delta conversion online UPS always has the inverter supplying the load during normal conditions - which is the basic concept of an online UPS. However, the delta inverter also contributes power to the main inverter output. As a result, it can deliver power to the loads with much greater efficiency than the double conversion. Yet, under conditions of AC power disturbances, the delta conversion online UPS functions just like the conventional design.

C. Benefits

The delta conversion online UPS provides output characteristics that are similar to the double conversion online design. Nonetheless, the former can control both input and output power, which makes possible to gain the following benefits:

  • Significant reduction in energy losses
  • Makes the UPS compatible with all generator sets
  • Reduces the need for wiring and generator oversizing
  • Power Factor Correction

To sum up, the delta conversion online UPS is more efficient since it will directly deliver power only the difference (delta) between the input and the output. Under normal conditions, it bypasses the steps wherein AC power is converted to DC (for battery charging), and transformed back to AC energizing the loads.


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About Me

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.