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Home / Technical Articles / The facts about modern rotary and static uninterruptible power systems (UPS)

Power quality problems and UPS…

This technical article covers the brief explanation of the power quality issues with the reference to the past as well as two main types of modern UPS systems – rotary and static uninterruptible power systems, their characteristics, advantages and disadvantages.

The facts about modern rotary and static uninterruptible power systems (UPS)
The facts about modern rotary and static uninterruptible power systems (UPS)

The advent and evolution of solid-state semiconductors has resulted in a proliferation of electronic computational devices that we come in contact with on a daily basis. These machines all rely on a narrow range of nominal AC power in order to work properly. Indeed, many other types of equipment also require that the ac electrical power source be at or close to nominal voltage and frequency.

Disturbances of the power translate into failed processes, lost data, decreased efficiency and lost revenue.

The normal power source supplied by the local utility or provider is typically not stable enough over time to continuously serve these loads without interruption. It is possible that a facility outside a major metropolitan area served by the utility grid will experience outages of some nature 15–20 times in one year.

Certain outages are caused by the weather, and others by the failure of the utility supply system due to equipment failures or construction interruptions. Some outages are only several cycles in duration, while others may be for hours at a time.

In a broader sense, other problems exist in the area of power quality, and many of those issues also contribute to the failure of the supply to provide that narrow range of power to these sensitive loads.

Power quality problems take the form of any of the following:

  1. Power failure,
  2. Power sag,
  3. Power surge,
  4. Undervoltage,
  5. Overvoltage,
  6. Line noise,
  7. Frequency variations,
  8. Switching transients and
  9. Harmonic distortion.

Regardless of the reason for outages and power quality problems, the sensitive loads can not function normally without a backup power source. Additionally, in many cases, the loads must be isolated from the instabilities of the utility supply and power quality problems and given clean reliable power on a continuous basis, or be able to switch over to reliable clean electrical power quickly.

Uninterruptible power supply (UPS) systems have evolved to serve the needs of sensitive equipment and can supply a stable source of electrical power, or switch to backup to allow for an orderly shutdown of the loads without appreciable loss of data or process.

UPS systems have evolved along the lines of rotary types and static types of systems, and they come in many configurations, including hybrid designs having characteristics of both types.

The discussion that follows attempts to describe, compare and contrast the two types of UPS systems, and give basic guidance on selection criteria. This discussion will focus on the medium, large and very large UPS systems required by users who need more than 10 kVA of clean reliable power.

Contents:

  1. Power Ratings of UPS Systems
  2. Rotary UPS Systems
    1. Typical Rotary Configurations
    2. High-Speed Rotary Concept of Operation
      1. Startup Mode
      2. Normal Operation Mode
      3. Discharge Mode
      4. Recharge Mode
  3. Static UPS Systems
    1. Typical Static UPS Configurations
      1. Double Conversion Concept of Operation
      2. Standby UPS Concept of Operation
      3. Static Line Interactive UPS Concept of Operation

1. Power Ratings of UPS Systems

  1. Small UPS: Typically 300 VA to 10 kVA, and sometimes as high as 18 kVA
  2. Medium UPS: 10–60 kVA
  3. Large UPS: 100–200 kVA units, and higher when units are paralleled
  4. Very Large UPS: 200–2 MW units, and higher when units are paralleled

Each of these categories is arbitrary because manufacturers have many different UPS offerings for the same application.

Question to ask about UPS choice…

The choice of UPS type and the configuration of UPS modules for a given application depends upon many factors, including:

  1. How many power quality problems the UPS is expected to solve?
  2. How much future capacity is to be purchased now for future loads?
  3. The nature of the sensitive loads and load wiring?
  4. Which type of UPS system is favored, rotary or static?
  5. Choices of battery or DC storage technology considered?
  6. A host of other application issues?

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2. Rotary UPS Systems

Typical Ratings: 300–3 MW maximum.


2.1 Typical Rotary Configurations

Rotary UPS systems are among the oldest working systems developed to protect sensitive loads. Many of these systems are complicated engine-generator sets coupled with high inertial flywheels operated at relatively low rotational speeds. These legacy types of hybrid UPS systems are not the focus of this discussion, because only one or two vendors continue to offer them.

See Figure 1 for the modern high speed Rotary UPS systems discussed in this section. These types of modern rotary UPS systems are advanced, integrated designs using scalable configurations of high-speed flywheel, motor and generator in one compact UPS package.

The new rotary technologies have the potential to replace battery backup systems, or at least reduce the battery content for certain applications.

The appeal of rotary systems is the avoidance of the purchase, maintenance and facility space required by DC battery based backup systems.

HiTec Dynamic UPS PowerPRO Series Diesel Rotary UPS
HiTec Dynamic UPS PowerPRO Series Diesel Rotary UPS

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2.2 High-Speed Rotary Concept of Operation

The modern rotary type of UPS operation is understood by reviewing the four topics below:

  1. Startup mode,
  2. Normal operation mode,
  3. Discharge mode and
  4. Recharge mode.

2.2.1 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 650 V through the rectifying action of the freewheeling diodes in the utility converter.

As soon as this level of DC voltage is reached, the static disconnect turns on fully.

The next step involves 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 800 V, and the output bus is transferred from bypass to the output of the power electronics module.

Typical-High Speed Modern Rotary UPS
Figure 1 – Typical-High Speed Modern Rotary UPS (click to expand scheme)

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 Silicon Controlled Rectifiers (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.

Immediately after the output is transferred from bypass to the power electronic 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.

The point that 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.

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2.2.2 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 components:

  1. Real load current,
  2. Charging current, and
  3. 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 maintain 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 he 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:

  1. The harmonic current required by the load,
  2. The reactive load current, and
  3. 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.

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2.2.3 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.

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2.2.4 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 programmable from 1 to 15 seconds.

As soon as the load power is completely transferred to the input source, the utility converter and flywheel converter start to recharge the flywheel and return to normal operation mode. The flywheel recharge power is programmable between a slow and fast rate. 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.

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High-Speed Rotary Advantages

  1. Addresses all power quality problems.
  2. Battery systems are not required or used.
  3. No battery maintenance required.
  4. Unlimited discharge cycles.
  5. 150 seconds recharge time available.
  6. Wide range of operating temperatures can be accommodated (–20 ° to 40 °C).
  7. Small compact size and less floor space required (500 kW systems takes 20 sq ft).
  8. N+1 reliability available up to 900 kVA maximum.
  9. No disposal issues.

High-Speed Rotary Disadvantages

  1. Flywheel does not have deep reserve capacity – rides through for up to 13 seconds at 100% load.
  2. Some enhanced flywheel systems may extend the ride through to 30 seconds at 100% load.
  3. Mechanical flywheel maintenance required every 2–3 years, and oil changes required every year.
  4. Recharge fast rates require the input to be sized for 125% of nominal current.
  5. Flywheels failures in field not understood.
  6. Requires vacuum pumps for high-speed flywheels.
  7. Limited number of vendors and experience.

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3. Static UPS Systems

Typical Ratings: 20 kW to 1 MVA / 1 MW, and higher when multiple units are paralleled.


3.1 Typical Static UPS Configurations

Static UPS systems modules are available in three basic types of configurations known as:

  1. Double conversion,
  2. Standby, and
  3. Line interactive.

The lower power ratings are likely to be one of the first two types of configurations, e.g., standby or line interactive. Most medium or large static UPS installations use the double conversion technology in one or multiple module configurations, i.e., or multiple UPS units in parallel.

Special UPS high-efficiency operating modes like Economic mode can provide efficiency improvements to over 99%, equating to less than 1% losses through the UPS. These modes depend on the system operating with the static switch closed and power conversion sections suspended (not off).

Modern UPSs can instantly revert to traditional double conversion operation within 2 ms on detection of any power anomaly.

Figure 1 illustrates the one-line diagram of a simple single Double Conversion UPS module. Brief explanations appear for the standby and line interactive UPS systems after the text explaining the Double Conversion static UPS type of system.

Typical Static UPS, Double Conversion Type with Battery Backup
Figure 1 – Typical Static UPS, Double Conversion Type with Battery Backup (click to expand scheme)

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3.1.1 Double Conversion Concept of Operation

The basic operation of the Double Conversion UPS is:


Step #1

Normal power is connected to the UPS input through the facility electrical distribution system. This usually involves two input circuits that can either come from the same source or from separate sources such as utility and site generation.


Step #2

The Rectifier/Charger function converts the normal AC power to DC power to charge the battery and power the inverter. The load is isolated from the normal input source.


Step #3

The battery stores DC energy for use when input power to the UPS fails. The amount of power available from the DC battery system and time to discharge voltage is a function of the type of battery selected and the ampere-hour sized used.

Battery systems should be sized for no less than 5 minutes of clean power usage from a fully charged state, and, in many cases, are sized to provide more time on battery power.

Step #4

The DC link connects the output of the rectifier/charger to the input of the inverter and to the battery. Typically the rectifier/charger is sized slightly higher than 100% of UPS output because it must power the inverter and supply charger power to the battery.


Step #5

The bypass circuit provides a path for unregulated normal power to be routed around the major electronic sub-assemblies of the UPS to the load so that the load can continue to operate during maintenance, or if the UPS electronics fails.

The bypass static switch can switch to conducting mode in lees than 1 millisecond. When the UPS recognizes a requirement to transfer to the bypass mode, it simultaneously turns the static switch ON, the output breaker to OPEN, and the bypass breaker to CLOSE. The output breaker opens and the bypass breaker closes in about 50 milliseconds.

The restoration of normal conditions at the UPS results in the automatic restoration of the UPS module powering the load through the rectifier/charger and inverter with load isolation from power quality problems, and the opening of the bypass circuit.


Static Double Conversion Advantages

  1. Addresses all power quality problems.
  2. Suitable for applications from 5 kVA to over 2500 kVA.
  3. Simple battery systems are sized for application.
  4. Long battery backup times and long life batteries are available.
  5. Higher reliability is available using redundant UPS modules.

Static Double Conversion Disadvantages

  1. Battery systems, battery maintenance and battery replacement are required.
  2. Large space requirement for battery systems (higher life takes more space, e.g., 500 kW takes 80–200 sq ft depending upon the type of battery used, VRLA 10 year, VRLA 20 year or flooded).
  3. Limited discharge cycles of battery system.
  4. Narrow temperature range for application.
  5. Efficiencies are in the 90–97%.
  6. Bypass mode places load at risk unless bypass has UPS backup.
  7. Redundancy of UPS modules results in higher costs.
  8. Output faults are cleared by the bypass circuit.
  9. Output rating of the UPS is 150%.
  10. Battery disposal and safety issues exist.

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3.1.2 Standby UPS Concept of Operation

The basic operation of the Standby UPS is:


Step #1

The Standby UPS topology is similar to the double conversion type, but the operation of the UPS is different in significant ways. Normal power is connected to the UPS input through the facility electrical distribution system.

This usually involves two input circuits that can come from one or two sources such as utility and site generation. See Figure 2 for details.

Typical Static UPS, Standby Type with Battery Backup
Figure 2 – Typical Static UPS, Standby Type with Battery Backup (click to expand scheme)
Step #1

The rectifier/charger function converts the normal AC power to DC power to charge the battery only, and does not simultaneously power the inverter. The load is connected to the bypass source through the bypass static switch.

The inverter is in the standby mode ready to serve the load from battery power if the input power source fails.


Step #3

The battery stores DC energy for use by the inverter when input power to the UPS fails. The amount of power available from the DC battery system and time to discharge voltage is a function of the type of battery selected and the ampere-hour sized used.

Battery systems should be sized for the anticipated outage.


Step #4

The DC link connects the output of the rectifier/charger to the input of the inverter and to the battery. Typically the rectifier/charger is sized only to supply charger power to the battery, and is rated far lower than in the double conversion UPS.


Step #5

The bypass circuit provides a direct connection of bypass source to the load. The load operates from unregulated power.

The bypass static switch can switch to non-conducting mode in less than 8 milliseconds. When the UPS recognizes the loss of normal input power, it transfers to battery/inverter mode by simultaneously turning the Inverter ON and the static switch OFF.

Static Standby UPS Advantages

  1. Lower costs than double conversion.
  2. Rectifier and charger are economically sized.
  3. Efficient design.
  4. Batteries are sized for the application.

Static Standby UPS Disadvantages

  1. Impractical over 2 kVA.
  2. Little to no isolation of load from power quality disturbances.
  3. Standby power is from battery alone.
  4. Battery systems, battery maintenance and battery replacement are required.
  5. Limited discharge cycles of battery system.
  6. Narrow temperature range for application.
  7. Output faults are cleared by the bypass circuit.
  8. Battery disposal and safety issues exist.

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3.1.3 Static Line Interactive UPS Concept of Operation

The basic operation of the Line Interactive UPS is:


Step #1

The Line Interactive type of UPS has a different topology than the static double conversion and standby systems. The normal input power is connected to the load in parallel with a battery and bi-directional inverter/charger assembly. The input source usually terminates at a line inductor and the output of the inductor is connected to the load in parallel with the battery and inverter/charger circuit.

See Figure 3 for more details.

Typical Static UPS, Line Interactive Type with Battery Backup
Figure 1.1-89. – Typical Static UPS, Line Interactive Type with Battery Backup (click to expand scheme)
Step #2

The traditional rectifier circuit is eliminated and this results in a smaller footprint and weight reduction. However, line conditioning is compromised.


Step #3

When the input power fails, the battery/inverter charger circuit reverses power and supplies the load with regulated power.


Static Line Interactive UPS Advantages

  1. Slight improvement of power conditioning over standby UPS systems.
  2. Small footprints and weights.
  3. Efficient design.
  4. Batteries are sized for the application.

Static Line Interactive UPS Disadvantages

  1. Impractical over 10 kVA.
  2. Not as good conditioning as double conversion.
  3. Standby power is from battery alone.
  4. Battery systems, battery maintenance and battery replacement are required.
  5. Limited discharge cycles for the battery system.
  6. Narrow temperature range for application.
  7. Battery disposal and safety issues exist.

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Source: Power system design basics – Eaton

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

4 Comments


  1. Richard Haigh
    Sep 05, 2019

    Too many things very wrong with this article. No consideration to D-UPS and how it works actually describing a hybrid static/rotary system operation which is not a true dynamic system… oh dear – try again C-


  2. Aamir Shahzad
    Sep 05, 2019

    What will happen if UPS output load side have short circuit or overload


  3. Nitin
    Sep 04, 2019

    Good article on ups system, send more information on ups.


  4. Ian Bitterlin
    Sep 04, 2019

    This is very misleading in many parts and technically ‘wrong’ is several of the concepts. Whoever wrote it does not understand the difference between static and rotary UPS.

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