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Reliability And Efficiency of Inverter and Modified UPS

Reliability And Efficiency of The Electrical System

The reliability and efficiency of the electrical system supplied to the hospital is an important feature. Because it uses equipment with high sensitivity and accuracy. Therefore, this point plays an important role in this regard.

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As we have seen previously, the elimination of the ripple on the direct current side allows the inverter to work in optimal conditions. This must work with a carrier at a high frequency as in figures 2 and 3. So the first voltage harmonic, which finds before the output filter, has a very high value, so this filtering is indispensable to guarantee an optimal output towards the load, that is, with negligible THDv. APC ups price in Pakistan.

Simulation of the carrier and modulator of an inverter that works in PWM.

UPS inverter waveform modulated with sinusoidal carrier, as shown.

By fixing the duty cycle of the carrier based on the variability of the RMS value of the output voltage, the voltage will be stable on the load.

It is opportune to remember that a PWM inverter with the ratio mf, between the carrier and modulating frequency m f = fp/f1, the harmonic content is of the type [5]: m f ±2 = 31.8%, m f ±4 = 1.85%, 2m f ±1 = 18.1%, 2m f ±3 = 21.2%, 2m f ±5 = 3.3%, etc.

If the carrier frequency is 3Khz, the first harmonic voltage found in the bandwidth is 2.88Khz.

Modified UPS

The implementation of the UPS used is a little different due to the characteristics and needs of the analyzed hospital. The designed scheme shown in the following figure:

Real scheme with the possibility of two power supplies to guarantee the use of external plants.

The most important difference with the classic scheme is that the transformer has only one secondary and two primary ones, which allows two possible voltages to use for the input power supply, that is, 208 Veff and 480 Veff.

UPS Solution Power Continuity and Compatibility

This solution choses to have a greater guarantee that, in the event of an absence of power for a long time, the unit fed from a Generator Set that is available at any of the voltage values. A condition that increases the possibility of UPS input power continuity. But the compatibility of the relationship between the apparent power of the same plant with the apparent power of the UPS must consider as described.  APC ups price in Pakistan.

However, it is of fundamental importance to limit the THDV between acceptable ranges (THDV ≤ 8%), when the UPS powered by a Power Plant. If the UPS fed with a transformer, the damages generate by a THDV.

They may be evident for a longer or shorter time, but with a power plant supply voltage regulation instability could see and the generator could shut down.

IGBTs Of the Inverter

In the scheme, the 30° phase shift achieves with the inductor located before one of the two converters and the voltage fixes with the transformer installed before the second converter. It is important to say that due to electrical loads it was necessary to oversize the IGBTs (Insulated Gate Bipolar Transistor) of the inverter, so that they work well for more than 60 seconds. at 150% of its rated power.

This is since in imaging machines, the peak current can also be 10 to 15 times the value of the rated current for times between 5 and 6ms.

Load Factors of Different Equipment

Also in this operating condition, the UPS voltage does not have to distort and for this reason an output auto-transformer with a very low Vcc (≈1%) has used. The inductance to filter the waveform of Fig. 3 is the equivalent reactance of the transformer that is before the Switches (SW); this, together with the capacitors, constitute an LC filter that sees a load with PF≈1, so it is almost resistive. APC ups price in Pakistan.

UPS Output Filter

The transfer function of this filter given by:

where ω f 0 is the natural frequency of LC and G the equivalent conductance of the load. Depending on the high load and the equivalent resistance of the load, the module of the transfer function can reduce, with a good approximation, as follows:

A value is set for |H| at first very small (which is equivalent to low ω0), in the bandwidth to reduces, that is, starting from (mf – 2); then if L is known (due to the short-circuit power of the autotransformer) and the value of |H|, the capacity of the filter can also be known. If the frequency of the carrier is f = 3Khz (50° arm.), if for example we make f0 = 8° harmonic, the result is |H| ≈ 0.03, that is, the first harmonic of the inverter reduces from 31.8% to 0.8%; as shown in the graph of

Transfer Function of The Inverter Output Filter.

shows the waveform at the inverter output, which will still filtered by the inductance of the line up to the load (≈ 65μH); in this operating situation the THDv is ≤1%

Sinusoidal waveform after the filter at the UPS output.


You need a power supply that ensures full continuity and can deal with possible breakdowns and interference to ensure the quality of electricity needed. This describes some of the key issues that characterize the design and installation of standby power systems. Consider efficiency (UPS). Current and future needs.

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