UPS design with active series-parallel power-line conditioning



The double conversion uninterruptible power supply (UPS) is the most common version of online UPSs  in use today, but has a number of drawbacks. The line-interactive UPS system with active series-parallel power-line conditioning capabilities offers a solution to a number of the problems.

The double conversion UPS (DCUPS) , shown in diagrammatic form in Fig. 1 , is the most common form of high reliability UPS in use today.  The reliability and smoothness  of operation of the DCUPs  comes from the fact that the load is always supplied from the inverter, reducing problems of phase shift and voltage steps when the grid power fails.

Fig. 1: The double conversion UPS.

One of the biggest problems with double conversion UPS is the conversion process itself. Incoming AC is converted to DC and then back to AC before being fed to the load. The rectifier has to be sized to carry the full load of the UPS output as well as the full battery charging load. The double conversion process results in a relatively low efficiency, compared to the line interactive UPS, as losses occur in both stages of conversion.

The rectifier makes use of multipulse rectification, and if not fitted with harmonic filters, produces harmonics which can cause problems with the grid. The addition of harmonic filters, or the use of higher pulse rectifiers, increases the cost of the system. The majority of converter circuits today present nonlinear characteristics which cause current distortion, which  can cause problems in transmission and distribution lines.

Harmonics, low power factor and high energy waste from traditional double conversion UPSs are becoming of increasing concern to operators of such systems. A line-interactive, online UPS technology that eliminates harmonic input current distortion, provides unity controlled power factor and saves energy is available on the market.

The UPS system allows output voltage conditioning, such as voltage harmonic suppression and sag or swell compensation. Additionally, the input current is also conditioned, allowing current harmonic suppression and reactive power compensation. In other words, the UPS system acts as a true unified power quality conditioner.

Unified power quality conditioners ( UPQC)

Unified power quality conditioners (UPQC), also known as universal active filters, are among the most suitable devices to improve power quality. A combination of series and shunt active filters form the basis of all UPQCs. The series active filter suppresses and isolates voltage-based distortions such as voltage harmonics, voltage unbalance, voltage flicker, and voltage sag and swell. The shunt active filter cancels current-based distortions such as current harmonics, load unbalance, and neutral current. At the same time, it compensates the reactive current of the load.

Fig. 2: UPQC filter circuit [2].

The series active filter works as a sinusoidal current source in phase with the input voltage, drawing sinusoidal and balanced input currents with low total harmonic distortion from the utility. The parallel active filter works as a sinusoidal voltage source in phase with the input voltage, providing regulated and sinusoidal output voltages with low harmonic distortion. The two converters are interconnected by means of a DC Bus. Fig. 2 gives an example of such a UPQC.

Line interactive series parallal converter (LISP) UPS

In the UPS configuration, a battery pack is used as the power source on the DC bus. This battery supplies the load when the input voltage is out of the specific range or shuts down. When the AC supply is within the pre-set tolerance, most of the power is supplied directly from the AC line to the load. Only a small portion of the total power, usually up to 15%, flows through the series and parallel converters. This power is needed to compensate for any differences between the input and the output voltages and to improve the power factor of the system. At the same time, the parallel inverter draws the fundamental current to charge the battery.

On the other hand, when the input voltage shuts down, the static switch separates the source and the load. In this operating mode, the parallel converter acts as a DC/AC converter (called an inverter) and supplies the load from the battery pack. Since an important portion of the power flows from the AC line to the load without any conversion, the efficiency is higher than that of a double conversion on line UPS system. Having eliminated the main drawback of double-conversion UPS systems, the series-parallel UPS topology appears to be a strong competitor in many UPS applications [3].

The delta converter UPS 

The delta converter UPS [4], manufactured by APC, is the only known commercial example of a LISPC UPS on the market. The UPS consists of:

  • A series bidirectional converter, called the delta converter , consisting of a transformer in series with the grid feed, the secondary of which is connected to the converter.
  • A parallel bidirectional converter , the output of which is connected in parallel with the load.

The two converters are connected together through a DC bus, to which the battery is also connected (Fig. 3).

This UPS design is a new technology introduced to eliminate the drawbacks of the double conversion on-line design and is available in sizes ranging from 5  to 1600  kVA. Similar to the double conversion on-line design, the delta conversion on-line UPS always has the inverter supplying the load voltage. However, the additional delta converter also contributes power to the inverter output. Under conditions of AC failure or disturbance, this design exhibits behaviour identical to the double conversion on-line.

Fig. 3: The delta conversion UPS  (APC).

The double conversion on-line UPS converts the power to the battery and back again whereas the delta converter moves components of the power from input to output. The delta converter acts to control the input power characteristics. This active front-end draws power in a sinusoidal manner, minimising harmonics reflected onto the utility, ensuring optimal utility and generator system compatibility.

The second function of the delta converter is to control input current to regulate the charging of the battery system. The delta conversion on-line UPS provides the same output characteristics as the double conversion on-line design, but the input characteristics are often different. Delta conversion on-line designs provide dynamically power factor corrected input, without the inefficient use of the filter banks associated with traditional solutions. The input power control also makes the UPS compatible with all generator sets and reduces the need for wiring and generator oversizing.

In UPS standby mode, the series active power filter acts as a sinusoidal current source and the parallel active power filter acts as a sinusoidal voltage source. A 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 which is sinusoidal and in-phase with the source voltage. Consequently, current magnitude, wave shape and power factor can be controlled

The output voltages are controlled to have constant values and low total harmonic distortion (THD) and the source currents are controlled to be sinusoidal and balanced with low THD. Both input currents and output voltages are simultaneously controlled to be in phase with respect the input voltages. Therefore, an effective power factor correction is carried out [4].

Under mains fail operations the UPS  behaves in a similar manner to the double conversion UPS, with energy  drawn from the battery and supplied to the load via the main inverter. The UPS will also disconnect from the grid and there will be no path for current from the series converter.

References

[1] SA da Silva: “A line-interactive UPS system implementation with series-parallel active power-line conditioning for three-phase, four-wire systems”, International Journal of Electrical Power & Energy Systems,  July 2004.
[2] D Shimoda: “Single phase UPS with FACTS capabilities by using p-q theory” IECON, 2012.
[3] A Nasiri: “Series-parallel active filter/uninterruptible power supply system”, Electric Power Components and Systems,  2004.
[4] APC: “Understanding Delta Conversion Online”, APC Application notes 39 to 45.

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