Uninterruptible Power Supply UPS Design Notes

Main keywords for this article are Uninterruptible Power Supply UPS Design Notes, USP Working Principle and Block Diagram, UPS Modes of Operation, UPS Components, UPS Selection Criteria. 

Uninterruptible Power Supply UPS Design Notes, USP Working Principle and Block Diagram

UPS References

American National Standards Association / Institute for Electrical and Electronics Engineers (ANSI/IEEE)
ANSI/IEEE C62.41 Surge Voltages in Low-Voltage AC Power Circuit
IEEE 450 Maintenance, Testing, and Replacement of Large Load Storage Batteries for Generating Stations and Substations
National Electrical Manufacturer’s Association (NEMA)
250 Enclosures for Electrical Equipment (1000 Volts Maximum)
National Fire Protection Association (NFPA)
70 National Electrical Code
Underwriters’ Laboratories (UL)
1778 Uninterruptible Power Supply Equipment

UPS Definitions

Bypass Source (Standby Power). The power intended to replace the normal alternating current (ac) input in case of normal power failure. The bypass source is often derived from a separate source or back-up diesel generator.
Inverter. An electronic converter that converts direct current (dc) to alternating current (ac).
Maintenance Bypass. A maintenance bypass circuit external to the UPS cabinet in a matching cabinet with switching mechanism, circuit breakers, transformers, and metering. This circuit shall totally isolate the UPS module from the electrical system for testing, maintenance, and repair.
Normal Power (Prime Power). The ac power source expected to serve power normally to the UPS input.
On-line Configuration. A UPS design where power normally flows through the inverter section so that no switching is required to sustain out-put power to the critical load when the normal ac power input fails.
Recharge Time. The time required by the charging and energy storage means (battery) to provide a second rated ‟reserve time‟ after providing a specified amount of ‟reserve time.‟
Reserve Time (Battery Time, Stored Energy Time). The minimum amount of time that the UPS provides continuity of load power (under specified service conditions, starting with a fully charged energy-storage means), in the absence of ac input power.

Retransfer. UPS switching action to bring critical load back to the inverter from the bypass source.
Static Switch. A fast-action, electronic, power-rated switch that is used to connect, interrupt, isolate, or transfer power flow within the UPS.
Transfer. An action taken by the UPS controls to transfer (switch) the critical load from the inverter to the static bypass line.
Walk-in. A controlled power increase by the UPS from the ac input into the UPS. This feature is necessary when utility power is restored to minimize the inrush of power that could trip protective devices.

General UPS Design Points

Audible Noise. Noise generated by UPS under any condition of normal operation shall not exceed 65 dBa, measured at one meter from the surface of the UPS.
Power Failure. When utility ac power fails or is restored, no interruption in power to the critical load being fed by the UPS shall occur within the reserve battery time.
Power Stability. The UPS shall seek the most reliable and stable power for the critical load from inverter, primary utility feed, or bypass utility feed.
Inverter Protection. The inverter shall be self-protecting against any magnitude of connected output load, overload, or fault.
Power Isolation. The packaging and wiring of the system shall effectively isolate all UPS power conversion components for safe removal or maintenance purposes when the system is de-energized or (when the option has been included) in the maintenance bypass mode.
Lock-out Provisions. Proper lock-out features shall be provided for all incoming power sources.
Maintenance Bypass Option. When selected, the optional maintenance bypass section shall ensure that the UPS section shall be secured in an electrically safe condition (totally de-energized), while continuous power is delivered to the critical load via the bypass source.
Reserve Time. Reserve time shall be in accordance with UPS Application Data Sheet, , specified in minutes at full load, 0.8 power factor, with ambient temperature between 20 to 40°C.
Recharge Time. The battery shall be able to be recharged to 95% capacity within ten times of the discharge time.

USP Working Principle and Block Diagram

Uninterruptible Power Supply UPS Design Notes

1. UPS Input and Output Voltage Requirement

Input Voltage

UPS Under 10 kVA. The primary input power supply shall be single-phase or  three-phase as required.
UPS 10 kVA and Larger. Normal input power supply shall be three-phase, 480 V ac plus ground.
Bypass ac source shall originate at different buses in the electrical system.

These buses may have different degrees of reliability and stand by power backup. If separate sources are not available, separate breaker from the same source can be utilized to energize UPS.

Output Voltage Requirement

UPS under 10 kVA shall provide 120 V, 1 phase 3 wire system, 60 Hz.
UPS 10 kVA to 125 kVA shall have an output voltage levels to 120/208 Y or 240/480 Y three-phase 4 wire and ground.
Input/output voltage shall be in accordance with UPS Application Data Sheet, Appendix A.

2. UPS Modes of Operation

The UPS shall be designed to operate as an on-line reverse transfer system in the  following modes.

Normal. The critical ac load is continuously supplied by the UPS inverter. The rectifier/ charger derives power from a utility ac source and supplies dc power to the inverter while simultaneously float-charging a power reserve battery.

Emergency. Upon failure of utility ac power, the critical ac load is supplied by the inverter, which obtains power from the battery without any switching. No interruption in power to the critical load shall be possible upon utility ac source failure/restoration.

Recharge. Upon restoration of utility ac power, after a utility ac power outage, the rectifier/charger shall automatically restart, walk in, and gradually assume the inverter and battery recharge loads.

Bypass. If the UPS is to be taken out of service for maintenance or repair, or if the inverter overload capacity is exceeded, the static transfer switch shall perform a reverse transfer of the load from the inverter to the bypass source with no interruption in power to the critical ac load. 

3. UPS Performance Requirements

The ac input to the UPS shall conform to the following:

(i) Voltage Configuration For Standard Units: Single-phase or threephase, three-wire plus ground with neutral point grounded.

(ii) Voltage Range: +10 to -15% of nominal with no battery contribution (continuous capability).
(iii) Frequency: Nominal frequency ± 5%.
(iv) Power Factor: 0.90 lagging minimum at nominal input voltage and full output load.

(v) Inrush Current: 800% of full-load current of full-load input current.
(vi) Current Limit: 125% maximum of nominal ac input current. 100% of nominal for optional generator operation.
(vii) Input Current Walk-in: 15 seconds to full-rated input current maximum.
(viii) Total Harmonic Distortion (THD): 5% THD maximum at full load at nominal input voltage and frequency. 3% on nonlinear load at rated capacity.
(ix) Surge Protection: Sustains input surges without damage in accordance with the criteria listed in ANSI/IEEE C62.41.

The ac Output from UPS Inverter. The ac output to the UPS inverter shall conform to the following:

(i) Voltage Configuration: single-phase or three-phase, four-wire, plus ground. All components and conductors in the neutral circuit shall be fully rated (same ampacity as the phase circuits).
(ii) Voltage Regulation: Specified output voltage ± 1% three-phase RMS average for a balanced three-phase load for the combined variation effects of input voltage, connected load, battery voltage, ambient temperature, and load power factor.
(iii) Frequency: nominal ± 0.1%.
(iv) Frequency Slew Rate: 1.0 hertz per second maximum; field-selectable from 0.1 to 1.0 hertz per second.
(v) Phase Displacement: ± 1 degree for balanced load; ± 3 degrees for 50%  unbalanced load.
(vi) Bypass Line Sync Range: ± 0.5 hertz; field selectable ± 0.5, 1.0, 3.0, 5.0 hertz.
(vii) Voltage Distortion: 5% THD for linear loads. 3% maximum for any single harmonic distortion.
(viii) Load Power Factor Range: 0.9 leading to 0.5 lagging.
(ix) Output Power Rating: Rated kVA at 0.8 lagging power factor.
(x) Overload Capability: 125% for 10 minutes (without bypass source). 150% for 30 seconds (without bypass source).
(xi) Inverter Output Voltage Adjustment: ± 5% manual adjustment.
(xii) Voltage Transient Response:
20% load step ± 4%
30% load step ± 5%
50% load step ± 8%
loss or return of ac input power ± 1%
manual transfer of 100% load ± 4%.
(xiii) Transient Recovery Time: To within 1% of output voltage within 50 milliseconds.
(xiv)Voltage Unbalance:
20% load step ± 4%
30% load step ± 5%
50% load step ± 8%.
(xv) Static switch transfer time shall be under 4 milliseconds.

4. UPS Components

Input Transformer

An input transformer or auto-transformer shall be included for the following reasons:
(i) Input voltage selection and matching
(ii) Harmonic reduction or buffering
(iii) Fault current limitation.

The input transformer shall be factory installed inside the UPS module cabinet. A wide range of input voltages shall be served by the same transformer through tap selection.

Rectifier / Charger

  • General. The term rectifier/charger shall denote the solid-state equipment and  controls necessary to convert incoming ac power to regulated dc power for input to the inverter and for battery charging. The rectifier/charger shall be a phase-controlled, solid-state SCR Type with constant voltage and current limiting control circuitry.
  • Input Filter. When selected on the data sheet, the rectifier/charger shall include an input filter to reduce reflected input current distortion to 5% THD at full load, with nominal input voltage. The input filter shall maintain the input power factor at 0.90 lagging or better from full load to half load with nominal input voltage.
  • The ac input Current Limiting. The rectifier/charger unit shall be provided with ac input current limiting, whereby the maximum input current shall be limited to 125% of the full input current rating. The rectifier/charger shall operate at a reduced current-limit mode whenever the critical load is powered from the UPS static bypass circuit so that the maximum UPS input current will not exceed 125% of full-load input current. In addition, the rectifier/charger input current limit shall be automatically reduced to 25% of full-load input current whenever the critical load is powered from the UPS internal maintenance bypass circuit.
  • An optional second circuit shall limit input current to 100% of rated full-load current when activated by a customer-supplied contact closure to signal a customer function, for example, generator operation.
  • Input Current Walk-in. The rectifier/charger shall contain a timed walk-in circuit that causes the unit to gradually assume the load over a 20-second time interval after input voltage is applied. Walk-in time shall be field selectable for 5 or 20 seconds.
  • Fuse Failure Protection. Power semiconductors in the rectifier/charger shall be fused with fast acting fuses so that the loss of any one power semiconductor shall not cause cascading failures.  
  • DC Filter. The rectifier/charger shall have an output filter to minimize ripple voltage into the battery. Under no conditions shall ripple voltage into the battery exceed 2% RMS. The filter shall be adequate to ensure that the dc output of the rectifier/charger meets the input requirements of the inverter. The inverter shall be able to operate from the rectifier/charger with the battery disconnected.
  • Automatic Restart. Upon restoration of utility ac power after a utility ac power outage and prior to a UPS automatic end-of-discharge shutdown, the rectifier/charger shall automatically restart, walk in, and gradually assume the inverter and battery recharge loads.
  • Battery Charger. In addition to supplying power for the inverter load, the rectifier/charger shall be capable of producing battery-charging current sufficient to replace 95% of the battery discharge power within ten times the discharge time. After the battery is recharged, the rectifier/charger shall maintain the battery at full charge until the next emergency operation.
  • Overvoltage Protection. The UPS shall have dc overvoltage protection so that if dc voltage rises to the preset limit, the UPS shuts down automatically and initiates an uninterrupted load transfer to the static bypass line.  

{Main keywords for this article are Uninterruptible Power Supply UPS Design Notes, USP Working Principle and Block Diagram, UPS Modes of Operation, UPS Components, UPS Selection Criteria. }

Inverter and Static Switch

  • Inverter. The term inverter shall denote the solid-state equipment and controls  to convert dc power from the rectifier/charger or battery to regulated ac power for supporting the critical load. The inverter shall be a Silicon Control Rectifiers (SCRs) phase-controlled, pulse-width modulated (PWM) design capable of providing the specified ac output.
  • The inverter frequency shall be independent of any voltage transients.
  • The output frequency of the inverter shall be controlled by an oscillator that can operate as an independent unit or as a slave for synchronized operation from the standby source. The inverter oscillator shall track the synchronizing source within ± 2 electrical degrees. If the external synchronizing source deviates from the preset 60.0 Hz frequency by ± 0.6 Hz, the oscillator shall automatically revert to free-operation and at the same time the transfer from the inverter source to the external standby ac source via the static switch shall be automatically blocked and prevented.
  • Static Transfer Switch. A static transfer switch and bypass circuit shall be provided as an integral part of the UPS. The static switch shall be a naturally commutated, high-speed, static (SCR Type) device rated to conduct full-load current continuously. The static switch no-break maximum transfer time, and sensing and transfer initiation (forward and reverse), shall not exceed 4 milliseconds (1/4 cycle). The static switch shall function as follows.
  • Uninterrupted Transfer. The transfer control logic shall automatically turn on the static switch, transferring the critical ac load to the
    bypass source, after the logic senses any of the following conditions:
    (i) Inverter overload capacity exceeded
    (ii) Critical ac load over voltage or under voltage
    (iii) Battery reserve time period expired
    (iv) UPS fault condition.
  • The transfer control logic shall inhibit an automatic transfer of the critical load to the bypass source if any of the following conditions is present:
    (i) Inverter/bypass voltage difference exceeds preset limits
    (ii) Bypass frequency out of limits
    (iii) Bypass out-of-synchronization range with inverter output.
  • Uninterrupted Retransfer. Retransfer of the critical ac load from the bypass source to the inverter output shall be automatically initiated, unless it is inhibited by manual control. The transfer control logic shall inhibit an automatic retransfer of the critical load to the inverter if any of the following conditions is present:
    (i) Bypass out of synchronization range with inverter output
    (ii) Inverter voltage difference exceeds preset limits
    (iii) Overload condition exists in excess of inverter full-load rating
    (iv) UPS fault condition.

UPS Overload Capability

The inverter shall be capable of supplying current and voltage for over-loads  up to 150% of full-load current for 30 seconds without transferring the load to the bypass. The inverter shall be capable of supplying lesser overloads (100- 150%) for longer times (typically 125% for 10 minutes). A status indicator and audible alarm shall indicate overload operation. The UPS shall transfer the load to bypass when overload capacity is exceeded. The inverter shall be self protecting against any magnitude of connected output overload or short circuit. 

The static switch shall have overload ratings that match or exceed the inverter overload ratings. In addition, the static switch shall have a short-time rating of 2000% rated load for two cycles, to enable transfers to the bypass source for load inrush or fault-clearing events.

Manual Bypass Switch

A manual bypass switch shall be provided to isolate the static switch and inverter for maintenance. The switch shall be a full-load rated, three position rotary marked: Normal; Bypass; and Bypass-Isolate. In the normal position, the switch shall connect the load to SCRs on the inverter side and the bypass line side. In the bypass position, it shall be possible to isolate the static switch SCRs, but still control their switching action for diagnostic purposes. Also, in this bypass position, the sync circuit of the inverter will also be operative to allow visual checking of the sync circuit. In the bypass-isolate position, all electronic circuitry of the static switch shall be disconnected. The contacts shall be make-before-break in both directions. Means shall be provided to lock the operating mechanism in any position with a 1/4 inch diameter shackle.

UPS Display and Controls

Monitoring and Control. The UPS shall be provided with a microprocessor-based unit status display and controls section designed for convenient and reliable user operation. Illuminated visual indicators shall be of the long-life, light-emitting diode (LED) type. All of the operator controls and monitors shall be located on the front of the UPS cabinet.
The UPS shall also include the following additional features:
(i) Real-time clock (time and date)
(ii) Alarm history with time and date stamp
(iii) Battery back-up memory.

Metering. The following parameters shall be displayed with the accuracy of 1%
(i) Input frequency, voltage, current
(ii) Battery voltage
(iii) Battery charge/discharge current
(iv) Output ac voltage line-to-line and line-to-neutral for each phase
(v) Output ac current for each phase
(vi) Output frequency
(vii) Percent of rated load being supplied by the UPS
(viii) Battery time left during battery operation
(ix) Power factor output.

UPS Alarms

Messages. An audible alarm shall be provided and activated by any of the  following alarm conditions. 0 to 45 seconds to override transient condition. The following alarm messages shall be displayed:

(i) Input power supply failure

(ii) Input phase rotation incorrect
(iii) Incorrect input frequency
(iv) Charger in reduced-current mode
(v) Battery charger problem
(vi) Battery failed test
(vii) Low battery warning (adjustable from 1 to 99 m(viii) Low battery shutdown
(ix) The dc bus overvoltage
(x) Bypass frequency out of range
(xi) Load transferred to bypass
(xii) Static switch failure
(xiii) UPS output not synchronized to input power
(xiv) Input power single-phased
(xv) Input voltage sensor failed
(xvi) Inverter leg over current in X-phase
(xvii)Output under voltage
(xviii)Output over voltage
(xix) Output over current
(xx) System output overloaded
(xxi) Load transferred to bypass due to overload
(xxii)Overload shutdown
(xxiii)Control error
(xxiv)Critical power supply failure
(xxv)Load transferred due to internal protection
(xxvi)External shutdown
(xxvii)Fan failure
(xxviii)Over-temperature shutdown impending
(xxix)Over-temperature shutdown
(xxx)The dc ground fault.

Remote contacts. If selected, in accordance with UPS Application Data Sheet, Appendix A, an optional common alarm contact shall be provided for connection to the purchaser‟s remote alarm system. Contact shall be capable of being connected as closed in normal operating condition and open to alarm. Contact shall be rated for 1 ampere at 120 volts ac or dc minimum. The contact shall be wired to a terminal block in Maintenance Bypass Cabinet (MBC), when specified, for easy identification and connection. The alarm function shall include summary of all standard alarms plus the position of the rotary MBC switch when in ‟maintenance‟ position.

Status Messages. The following UPS status messages shall be displayed:
(i) Normal operation
(ii) Load on maintenance bypass
(iii) Bypass source available
(iv) Load on static bypass

(v) System shutdown
(vi) Load on battery.

Controls. UPS start-up, shutdown, and maintenance bypass operations shall be accomplished by a single rotary control switch. An advisory display and menu-driven user prompts shall be provided to guide the operator through system operation without the use of additional manuals. Pushbuttons shall be provided to display the status of the UPS and to test and reset visual and audible alarms.

5. UPS Construction and Mounting

The UPS unit (input isolation transformer, rectifier/ charger with input filter, inverter, static transfer switch, and bypass switch), shall be housed in free standing NEMA enclosures with the following features:
(i) Cabinet doors/covers shall require a tool for gaining access.
(ii) Casters and leveling feet shall be provided for ease of installation.
(iii) Front access only shall be required for expedient servicing, adjustments, and installation.
(iv) The UPS cabinet shall be structurally adequate and have provisions for hoisting, jacking, and forklift handling.
(v) The UPS cabinet shall be cleaned, primed, and painted with the manufacturer‟s standard color. The UPS shall be constructed of replaceable sub assemblies. Printed circuit assemblies shall be plug-in. Like assemblies and like components shall be interchangeable.

Cooling. Cooling of the UPS shall be by forced air. Redundant fans shall be used. Low velocity fans shall be used to minimize audible noise output. Fan power shall be provided by the UPS output. The thermal design, along with all thermal and ambient sensors, shall be coordinated with the protective devices before excessive component or internal cabinet temperatures are exceeded.

Grounding. The ac output neutral shall be electrically isolated from the UPS chassis. Provisions for local bonding of the neutral to the ground terminal shall be made.
The incoming line section shall have a single equipment ground terminal common to all cabinets. A grounding connection shall be provided by either a lug for a No. 4 AWG stranded copper cable, or a 0.9 cm, 40.6 X 2.5-cm (3/8in., 16- X 1.00-in.) grounding stud. The grounding connection shall be located near the cable entry and exit points and be readily accessible for connection of purchaser‟s bare ground cables.

All cabinets shall be bonded together to provide a common electrical connection to the incoming line cabinet (UPS or optional MBC). This can be accomplished by bolting the clean, bare metal frame members or by a common ground bus between the cabinets that is bolted to each cabinet frame. Care shall be taken to remove any paint or surface contamination from the frame at the bonding points.

Wiring. Wiring practices, materials, and coding shall be in accordance with NFPA 70, or local codes applicable to the installation region.

All bolted connections of bus bars, lugs, and cables shall be in accordance with NFPA 70 and other applicable standards. All electrical power connections are to be torqued to the required value and marked with a visual indicator.

Ample wiring space shall be provided for purchaser‟s incoming and out-going cables to enter or leave from the top or bottom of the appropriate cabinet, in separate conduits to provide electrical and mechanical isolation from each other.

Terminal fastening devices, bus assembly fastenings, and all wiring fastenings in all parts of the unit shall be adequate for the continuous-rated current and the specified percent overload for the specified time period. They shall also be braced for carrying maximum fault current of the protective device rating without distress or need of servicing.

Line terminals shall be isolated by location or guarded by a suitable barrier or line shield to prevent accidental contact with the equipment energized during maintenance or checkout.

6. Batteries, Rack, and Accessories

UPS Batteries

  • Batteries shall be lead calcium or nickel cadmium sealed maintenance free type (3 cells per container) as described in SES E12-S01. They can be open stationary cells with transparent containers. Battery shall be designed as following.
  • Float Voltage: 2.20 to 2.25 V/Cell
    Equalize Voltage: 2.3 to 2.38 V/Cell
    Min. Voltage: 1.75 V/Cell
  • Batteries shall be sized to provide continuous operation at maximum rated load and shall be specified on UPS Application Data Sheet.
  • The float voltage of the UPS system shall be compatible with the battery manufacturer‟s recommendation to support the stated life requirements of the battery.
  • All battery interconnections shall be lead-plated copper with acid-resistant hardware.
  • The transparent container material of the battery shall be polystyrene. This transparent container shall have electrolyte level lines marked on all four sides and shall have explosion resistant vent caps and flame arrestors.
  • The batteries shall deliver 100 percent of the specified capacity.
  • The battery case or posts shall be stamped with a date code identifying the month and year of its manufacture.

UPS Rack

  • When a battery rack is provided, it shall be 2-tier or 2-step/tier depending on space limitations. When a battery cabinet is provided for the sealed batteries, it shall have movable shelves that will slide easily with the batteries mounted on them, so that battery inspection can be accomplished.  
  • The rack shall be made of steel, painted with two coats of acid-resistant paint, and shall be of adequate length to suit the batteries. 

UPS Accessories

Battery and battery rack accessories shall be with:
(i) Intercell connections to allow a minimum of one-half inch between enclosures
(ii) Terminal lugs and hardware and connector bolt wrench
(iii) Thermometer and wall-mounted holder
(iv) Cell number set 1-60 or 1-120 as appropriate
(v) Flame arrestor for each cell for the stationary open batteries
(vi) Oxidation retardant grease for battery posts.

Power Status Diagram

A mimic panel shall be provided to depict a single-line diagram of the UPS. Indicating  lights shall be integrated within the single-line diagram to illustrate the status of the UPS. Two digital displays on the diagram shall indicate whether UPS input or output power or both are present. The diagram shall be color-coded with the positions of the rotary control switch for visual confirmation of the UPS operating mode.

7. UPS Documentation

(i) System configuration with single-line diagrams
(ii) Functional relationship of equipment, weights, dimensions, and heat dissipation
(iii) Descriptions of equipment to be furnished, and deviations from this specification
(iv) Size and weight of shipping units to be handled by the installing contractor
(v) Detailed layouts of customer power and control connections
(vi) Detailed installation drawings with all terminal locations.

(iii) Spare parts list.

8. UPS Unit Start-Up and Site Testing

Site testing shall be provided by the manufacturer‟s field service personnel, if required. Site testing shall consist of a complete test of the UPS system and the associated accessories supplied by the manufacturer. A full-load power and a partial battery discharge tests shall be provided as a standard start-up procedure. This test shall not disturb user wiring and completed prior to operation of the site critical load from the UPS output. The test results shall be documented, signed, and dated for future reference.

Typical UPS Configurations

There are five main typical UPS system design configurations that distribute power to the critical loads. The selection of the appropriate configuration for a particular application is determined by the availability needs, risk tolerance, types of loads, budgets, and existing infrastructure. Refer Figures 1 to 5 for these typical UPS configurations.

UPS Selection Criteria

{Main keywords for this article are Uninterruptible Power Supply UPS Design Notes, USP Working Principle and Block Diagram, UPS Modes of Operation, UPS Components, UPS Selection Criteria. }



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