Hiperglucemia durante la nutrición parenteral como factor predictor de mortalidad intrahospitalaria

The modern rotary type of UPS operation is understood by reviewing the four topics below: startup mode, normal operation mode, discharge mode and recharge mode.

Startup Mode

The UPS output is energized on bypass as soon as power is applied from the source to the system input. The UPS continues the startup procedure automatically when the front panel controls are placed into the “Online” position. Internal UPS

system checks are performed then the input contactor is closed. The static disconnect switch is turned on and the conduction angle is rapidly increased from zero to an angle that causes the DC bus voltage between the utility converter and the flywheel converter to reach approximately 650V through the rectifying action of the freewheel- ing diodes in the utility converter. As soon as this level of DC voltage is reached, the static disconnect turns on fully. The next steps involved the utility converter IGBTs to start firing, which allows the converter to act as a rectifier, a regulating voltage source and an active harmonic filter. As the IGBTs begin to operate, the DC bus is increased to a normal operating voltage of approximately 800V, and the output bus is transferred from bypass to the output of the power electronics module. The transfer from bypass is completed when the output contactor is closed and the bypass contactor opened in a make-before- break manner.

The firing of the SCRs in the static disconnect switch is now changed so that each SCR in each phase is only turned on during the half-cycle, which permits real power to flow from the utility supply to the UPS. This firing pattern at the static disconnect switch prevents power from the flywheel from feeding backward into the utility supply and ensures that all of the flywheel energy is available to support the load.

Figure 1.4-18. Typical-High Speed Modern Rotary UPS It = Ir + Ic + Ig

Id = Output Current Ih = Harmonic Current Ix = Reactive Load Current Ir = Real Load Current Source Field Coil Driver Integrated Motor/Flywheel/ and Generator ac dc dc ac Ih Ix

Flywheel Converter Utility Converter Ic

Ig Filter Inductor

Inverter Fuse

Line Inductor Output

Contactor Input Contactor Static Disconnect Switch Bypass Contactor Static Bypass Option

Load Output Transformer

Id = Ih + Ix + Ir It = Input Current

Ir = Real Load Current Ic = Charging Current Ig = Voltage Regulation Current

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Sheet 01

Uninterruptible Power Systems (UPS)

083

Immediately after the output is trans- ferred from bypass to the power elec- tronic module, the flywheel field is excited, which also provides magnetic lift to unload the flywheel bearings. The flywheel inverter is turned on and gradually increases frequency at a constant rate to accelerate the flywheel to approximately 60 rpm. Once the flywheel reaches 60 rpm, the flywheel inverter controls the acceleration to keep currents below the maximum charging and the maximum input settings. Once the flywheel reaches 4000 rpm, the UPS is fully functional and capable of supporting the load during a power quality event. flywheel acceleration continues until the Flywheel reaches “full charge” at 7700 rpm. The total time to complete startup is less than 5 minutes. Normal Operation Mode

Once the UPS is started and the flywheel is operating at greater than 4000 rpm, the UPS is in the normal operating mode where it is regulating output voltage and supplying reactive and harmonic currents required by the load. At the same time it cancels the effect of load current harmonics on the UPS output voltage.

Input current consists of three compo- nents: real load current, charging current, and voltage regulation current. Real current is current that is in phase with the supply voltage and supplies real power to the load. Real current flowing through the line inductor causes a slight phase shift of the current lagging the voltage by 10 degrees and ensures that the UPS can quickly transfer to bypass without causing unacceptable switching transients. The second component is charging current required by the flywheel to keep the rotating mass fully charged at rated rpm, or to recharge the rotating mass after a discharge. The power to main- tain full charge is low at 2 kW and is accomplished by the IGBTs of the flywheel converter gating to provide small pulses of motoring current to the flywheel. This current can be much higher if fast recharge times are selected. The final component of input current is the voltage regulation current, which is usually a reactive current that circulates between the input and the utility converter to regulate the output voltage. Leading reactive current

causes a voltage boost across the line inductor, and a lagging current causes a bucking voltage. By controlling the utility converter to maintain nominal output voltage, just enough reactive current flows through the line inductor to make up the difference between the input voltage and the output voltage. The load current consists of three components: the harmonic current required by the load, the reactive load current, and the real current, which does the work. The utility converter supplies both the harmonic and reactive currents. Because these currents supply no net power to the load, the flywheel supplies no energy for these currents. They circulate between the utility converter and the load. The power stage controls analyze the harmonic current requirements of the load and set the firing angle of the inverter IGBTs to make the utility converter a very low impedance source to any harmonic currents. Thus, nonlinear load currents are supplied almost entirely from the utility converter with little effect on the quality of the UPS output voltage waveform and with almost no transmission of load harmonics currents to the input of the UPS. Discharge Mode

The UPS senses the deviation of the voltage or frequency beyond programmed tolerances and quickly disconnects the supply source by turning off the static disconnect switch and opening the input contactor. The disconnect occurs in less than one-half cycle. Then the utility converter starts delivering power from the DC bus to the load, and the flywheel converter changes the firing point of its IGBTs to deliver power to the DC bus. The UPS maintains a clean output voltage within 3% or nominal voltage to the load when input power is lost. Recharge Mode

When input power is restored to acceptable limits, the UPS synchronizes the output and input voltages, closes the input contactor and turns on the static disconnect switch. The utility converter then transfers power from the flywheel to the input source by linearly increasing the real input current. The transfer time is program- mable from 1 to 15 seconds. As soon

as the load power is completely transferred to the input source, the util- ity converter and flywheel converter start to recharge the flywheel and return to normal operation mode. The flywheel recharge power is program- mable between a slow and fast rate, and using the fast rate results in an increase of UPS input current over nominal levels. Recharging the flywheel is accomplished by controlling the utility and flywheel converter in a similar manner as is used to maintain full charge in the normal operation mode, however the IGBT gating points are changed to increase current into the flywheel.

High-Speed Rotary Advantages

■ Addresses all power quality problems

■ Battery systems are not required or used

■ No battery maintenance required ■ Unlimited discharge cycles

■ 150-second recharge time available ■ Wide range of operating tempera-

tures can be accommodated (–20° to 40°C)

■ Small compact size and less floor space required (500 kW systems takes 20 sq ft)

■ N+1 reliability available up to 900 kVA maximum

■ No disposal issues

High-Speed Rotary Disadvantages

■ Flywheel does not have deep reserve capacity—rides through for up to 13 seconds at 100% load ■ Some enhanced flywheel systems

may extend the ride through to 30 seconds at 100% load

■ Mechanical flywheel maintenance required every 2–3 years, and oil changes required every year ■ Recharge fast rates require the

input to be sized for 125% of nominal current

■ Flywheels failures in field not understood

■ Requires vacuum pumps for high-speed flywheels

■ Limited number of vendors and experience

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Sheet 01

Uninterruptible Power Systems (UPS)

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Static UPS Systems

In document Prevalencia de hiperglucemia y diabetes en pacientes no críticos con nutrición parenteral en España: estudio multicéntrico (página 155-161)