Theoretical review
Contents
Supplying sensitive loads
...
2
Types of electrical disturbances ...2
Main disturbances in low-voltage electrical power ...3
UPSs
...
4
The UPS solution ...4
UPS applications ...5
Types of UPSs
...
7
Static or rotary UPS ...7
Types of static UPSs ...9
UPS components and operation
...
16
Components of a UPS ...16
Main characteristics of UPS components ...19
Summary diagram for main characteristics ...24
UPS operating modes ...25
UPS configurations ...26
Technology
...
28
Transformerless UPSs ...28
Electromagnetic compatibility (EMC)
...
34
Electromagnetic disturbances ...34
EMC standards and recommendations ...35
UPS standards
...
36
Scope and observance of standards ...36
Main standards governing UPSs ...36
Energy storage
...
39
Possible technologies ...39
Batteries ...39
Flywheels ...43
UPS / generator-set combination
...
46
Use of a generator ...46
UPS / generator-set combination ...46
Transient load conditions
...
48
Review of inrush currents ...48
Harmonics
...
49
Harmonics ...49
Characteristic harmonic values ...51
Non-linear loads and PWM technology
...
54
Non-linear load performance of UPSs using PWM technology ...54
Comparison of different sources ...57
Free-frequency chopping ...58
Supplying sensitive loads
Power distribution systems, both public and private, theoretically supply electrical equipment with a sinusoidal voltage of fixed amplitude and frequency (e.g. 400 volts rms, 50 Hz, on low-voltage systems).
In real-life conditions however, utilities indicate the degree of fluctuation around the rated values. Standard EN 50160 defines the normal fluctuations in the LV supply voltage on European distribution systems as follows:
•
Voltage +10% to -15% (average rms values over 10-minute intervals), of which 95% must be in the +10% range each week.•
Frequency +4 to 6% over one year with ±1% for 99.5% of the time (synchronous connections in an interconnected system).Practically speaking, however, in addition to the indicated fluctuations, the voltage sine-wave is always distorted to some degree by various disturbances that occur on the system.
ÎSee White Paper WP 18 “The Seven Types of Power Problems”
Origins of disturbances
Utility power
Utility power can be disturbed or even cut by the following phenomena:
•
Atmospheric phenomena affecting overhead lines or buried cables: - lightning which can produce a sudden voltage surge in the system, - frost which can accumulate on overhead lines and cause them to break,•
Accidents:- a branch falling on a line, which may produce a short-circuit or break the line, - cutting of a cable, for example during trench digging or other construction work, - a fault on the utility power system,
•
Phase unbalance,•
Switching of protection or control devices in the utility power system, for load shedding or maintenance purposes.User equipment
Some equipment can disturb the utility power system, e.g.:
•
Industrial equipment:- motors, which can cause voltage drops due to inrush currents when starting,
- equipment such as arc furnaces and welding machines, which can cause voltage drops and high-frequency interference,
•
Power electronics equipment (switch-mode power supplies, variable speed drives, electronic ballasts, etc.), which often cause harmonics,•
Building facilities such as lifts which provoke inrush currents or fluorescent lighting which causes harmonics.Types of disturbances
Disturbances that are due to the above causes are summed up in the following table, according to the definitions contained in standards EN 50160 and ANSI 1100-1992.
Types of electrical
disturbances
Supplying sensitive loads
(Cont.)
Disturbances Characteristics Main causes Main consequences
Power outages
Micro-outages Total absence of voltage ≤ 10 ms. Atmospheric conditions, switching, faults, work on the utility.
Faulty operation and loss of data (computer systems) or interrupted production (continuous
processes).
Outages Total absence of voltage for more
than one period:
- short outage: ≤ 3 minutes (70% of outages last less than 1 s) - long outage: > 3 minutes
Atmospheric conditions, switching, faults, incidents, line breaks, work on the utility.
Depending on the duration, shutdown of machines and risks for people (e.g. lifts), loss of data (computer systems) or interrupted production (continuous processes).
Voltage variations
Voltage sags Reduction in the rms value of voltage to less than 90% of the rated value (but greater than 0%), with return to a value greater than 90% within 10 ms to 1 minute.
Atmospheric phenomena, load fluctuations, short-circuit on a neighbouring circuit.
Shutdown of machines, malfunctions, damage to equipment and loss of data.
Overvoltage Temporary increase to more than
10% over the rated voltage, for a duration of 10 ms to a few seconds.
- Quality of utility generators and transmission systems.
- Interaction between generators and load fluctuations on the utility power system.
- Switching on the utility power system. - Stopping of high-power loads (e.g. motors, capacitor banks).
- For computer systems: corruption of data, processing errors, system shutdown, stress on components.
- Temperature rise and premature aging of equipment.
Undervoltage Drop in voltage lasting from a few
minutes to days. Peak in consumption, when the utility cannot meet demand and must reduce its voltage to limit power.
Shutdown of computer systems. Corruption or loss of data. Temperature rise.
Premature ageing of equipment.
Voltage spike Sudden, major jump in voltage (e.g. 6 kV).
Close lightning strikes, static discharges.
Processing errors, corruption of data, system shutdown. Damage to computers, electronic boards.
Voltage unbalance (in three-phase systems)
Condition where the rms value of the phase voltages or the unbalances between phases are not equal.
- Induction furnaces.
- Unbalanced single-phase loads. - Temperature rise. - Disconnection of a phase.
Frequency variations
Frequency fluctuations Instability in the frequency. Typically +5%, - 6% (average for ten-second time intervals).
- Regulation of generators. - Irregular operation of generators. - Unstable frequency source.
These variations exceed the tolerances of certain instruments and computer hardware (often ± 1%) and can therefore result in the loss or corruption of data.
Flicker Flicker in lighting systems due to a drop in voltage and frequency (< 35 Hz).
Welding machines, motors, arc furnaces, X-ray machines, lasers, capacitor banks.
Physiological disturbances.
Other disturbances
HF transients Sudden, major and very short
jump in voltage.
Similar to a voltage spike.
Atmospheric phenomena (lightning) and switching.
Destruction of equipment, accelerated aging, breakdown of components or insulators.
Short duration < 1 µs
Amplitude < 1 to 2 kV at frequencies of several tens of MHz.
Starting of small inductive loads, repeated opening and closing of low-voltage relays and contactors. Medium duration > 1 µs and ≤ 100 µs
Peak value 8 to 10 times higher than the rated value up to several MHz.
Faults (lightning) or high-voltage switching transmitted to the low-voltage by electromagnetic coupling. Long duration > 100 µs
Peak value 5 to 6 times higher than the rated value up to several hundred MHz.
Stopping of inductive loads or high-voltage faults transmitted to the low-voltage system by electromagnetic coupling.
Harmonic distortion Distortion of the current and voltage sine-waves due to the harmonic currents drawn by non-linear loads. The effect of harmonics above the 25th order is negligible.
Electric machines with magnetic cores (motors, off-load
transformers, etc.), switch-mode power supplies, arc furnaces, variable speed drives.
Oversizing of equipment, temperature rise, resonance phenomena with capacitors, destruction of equipment (transformers).
Electromagnetic compatibility (EMC) Electromagnetic or electrostatic conducted or radiated disturbances. The goal is to ensure low emission and high immunity levels.
Switching of electronic components (transistors, thyristors, diodes), electrostatic discharges.
Malfunctions of sensitive electronic devices.
UPSs
Modern economic activities are increasingly dependent on digital technologies which are very sensitive to electrical disturbances.
As a result, many applications require a backed up supply of power to protect against the risk of disturbances in utility power:
•
Industrial processes and their control/monitoring systems - risks of production losses,•
Airports and hospitals - risks for the safety of people,•
Information and communication technologies related to the internet - risks of processing shutdowns with very high hourly downtime costs due to the interruption in the exchange of vital data, required by global companies.UPSs
A UPS (uninterruptible power system) is used to supply sensitive applications with secure power.
A UPS is an electric device positioned between the utility and the sensitive loads that supplies voltage offering:
•
High quality: the output sine-wave is free of any and all disturbances in utilitypower and within strict amplitude and frequency tolerances,
•
High availability: the continuous supply of voltage, within the specified tolerances, is ensured by a backup supply of power. The backup supply is generally a battery that, if necessary, steps in without a break in the supply to replace utility power and provide the backup time required by the application.These characteristics make UPSs the ideal power supply for all sensitive
applications because they ensure power quality and availability, whatever the state of utility power.
Components of a UPS
A UPS generally comprises the main components listed below.
Rectifier/charger
It draws utility power and produces a DC current to supply the inverter and charge or recharge the battery.
Inverter
It completely regenerates a high-quality voltage output sine-wave:
•
Free of all utility-power disturbances, notably micro-outages,•
Within tolerances compatible with the requirements of sensitive electronic devices (e.g. tolerances in amplitude ± 0.5% and frequency ± 1%, compared to ± 10% and ± 5% in utility power systems, which correspond to improvement factors of 20 and 5, respectively.Note. The term inverter is sometimes used to designate a UPS, when in reality it is only a part of the UPS.
Battery
The battery provides sufficient operating backup time (6 minutes to a number of hours) by stepping in to replace utility power as needed.
Static bypass
The static bypass ensures no-break transfer of the load from the inverter to direct utility power and back. No-break transfer is carried out by a device implementing SCRs (sometimes called a static switch).
The static bypass makes it possible to continue supplying the load even if an internal fault occurs or during maintenance on the rectifier/charger and inverter modules. It can also serve for transfers to call on the full power available upstream in the event of overloads (e.g. short circuits) exceeding UPS capacity.
During operation on the static bypass, the load is supplied directly by utility power and is no longer protected (operation in downgraded mode).
Maintenance bypass
This bypass may be used to supply the load directly with utility power, without calling on the inverter or the static switch. Transfer to the maintenance bypass is user initiated with switches. By actuating the necessary switches, it is the means to isolate the static bypass and the inverter for maintenance, while continuing to supply the load in downgraded mode.
UPSs
(Cont.)
Fig. 5.1. The UPS solution.
UPSs are used for a wide range of applications requiring electrical power that is available at all times and not affected by disturbances on the utility power system. The table below presents a number of applications.
For each, it indicates the sensitivity of the application to disturbances and the type of UPS that is suitable for protection.
The applications requiring this type of installation are:
•
Computer systems,•
Telecommunications,•
Industry and instruments,•
Other applications.The required UPS typologies are presented on page 9, "Types of static UPSs". They include static UPSs implementing the following typologies:
•
Passive standby,•
Interaction with the distribution system,•
Double conversion. HV/LV transformer HV systemNon-sensitive loads
Normal utility power (disturbances and system tolerances) Static bypass Maintenance bypass Inverter Battery Rectifier/ charger Sensitive loads UPS Reliable power (no disturbances, within
strict tolerances and available due to battery backup power)
UPSs
(Cont.)
UPS applications
Application Protected devices Protection required against UPS type
(see p. 8) Micro-outages Outages Voltage variations Frequency variations Other Computer systems
Data centres - Large bays for rack-mounted servers
- Internet data centres ***** ***** ***** ***** ***** Double conversion
Company networks - Sets of computers with terminals and peripheral devices (tape storage units, disk drives, etc.)
***** ***** ***** ***** ***** Double conversion
Small networks and servers
- Networks made up of PCs or workstations, server networks (WAN, LAN)
**** **** *** *** ** Interaction with the distribution system
Stand-alone computers - PCs, workstations
- Peripheral devices: printers, plotters, voice mail
** ** * * ** Passive standby
Telecommunications
Telecommunications - Digital PABXs ***** ***** ***** ***** ***** Double conversion
Industry and instruments
Industrial processes - Process control - PLCs
- Numerical control systems - Control systems
- Robot control/monitoring systems - Automatic machines
*** ***** *** *** **** Double conversion
Medical and laboratories - Instrumentation
- Scanners (60 Hz) **** ***** **** **** *** Double conversion
Industrial equipment - Machine-tools - Welding robots - Plastic-injection presses
- Precise regulation devices (textile, paper, etc.)
- Heating equipment for manufacture of semi-conductors, glass, pure materials
*** **** *** *** *** Double conversion
Lighting systems - Public buildings (elevators, safety equipment)
- Tunnels
- Runway lighting in airports
** **** *** *** ** Double conversion
Interaction with the distribution system
Other applications
Special frequencies - Frequency conversion
- Power supplies for aircraft (400 Hz) **** **** **** ***** *** Double conversion * low sensitivity to disturbances.
Types of UPSs
Static or rotary UPS solutions
There are two main types of UPSs (figure 5.2 and details in ÎWhite Paper WP 92 - "Comparison of Static and Rotary UPS") which basically differ in the way the UPS inverter function is implemented.
Static solution
These UPSs use only electronic components to perform the inverter function. A "static-inverter function" is obtained.
Rotary solution
These UPSs use rotary machines to perform the inverter function. A "rotary-inverter function" is obtained.
These UPSs in fact combine a motor and a generator with a highly simplified static inverter.
The inverter filters out utility-power disturbances and regulates only the frequency of its output voltage (generally in "square-wave" form) which supplies a regulated motor/generator set that is sometimes combined with a flywheel.
The motor/generator set generates an output voltage sine-wave, taking the inverter output frequency as the reference.
Fig. 5.2. Static and rotary UPSs.
Comparison
Rotary solution
The arguments often put forward in favour of this solution are as follows:
•
High generator short-circuit current on the order of 10 In (ten times the rated current) that makes setting of protection devices easier,•
150% overload capacity (of the rated current) over a longer period (two minutes instead of one),•
Downstream installation galvanically isolated from upstream AC source due to the motor/generator set,•
Internal impedance providing high tolerance to the non-linear loads frequently encountered with the switch-mode power supplies used by computer systems.Static or rotary UPS
See WP 92
Î
Types of UPSs
(Cont.)
Static solution
Compared to the advantages of rotary solutions
The static UPSs from APC by Schneider Electric offer the advantages listed below.
•
Operation in current-limiting mode (e.g. up to 2.33 In for MGE Galaxy 5000) with discrimination ensured for circuits rated up to In/2.These features, which are more than sufficient in practice, prevent the disadvantages of rotary systems:
- overheating of cables,
- the effects of an excessive short-circuit current and the corresponding voltage drop on sensitive devices, during the time taken by protective devices to clear the fault.
•
150% overload capacity (of the rated current) for one minute.The two-minute overload capacity is of no practical use because most overloads are very short (less than one second, e.g. in-rush currents of motors, transformers and power electronics).
•
Galvanic isolation, when required, by means of an isolating transformer.•
Double-conversion operation which completely isolates the load from utility power and regenerates the output voltage with precise regulation of the voltage amplitude and the frequency.•
Very low internal impedance for higher performance with non-linear loads due to the use of power-transistor technologies.Other advantages
)
Static solutions provide many other advantages as well, due to power-transistor technology combined with a PWM chopping technique.•
Simplified overall design, with a reduction in the number of parts and connections, and in the number of possible causes of failure.•
Capacity to react instantaneously to utility-power amplitude and frequency fluctuations by means of microprocessor-controlled switching regulation based on digital sampling techniques. The voltage amplitude returns to regulated conditions (± 0.5% or ± 1% depending on the model) in less than 10 milliseconds for load step changes up to 100%. Within the indicated time interval, such a load step change produces a load voltage variation of less than for example ± 2% for MGE Galaxy PW and Galaxy 5000.•
High, constant efficiency whatever the percent load, which is a major advantage for redundant UPS units with low percent loads. A static UPS unit with a 50% load maintains high efficiency (94%), whereas the efficiency of a rotary UPS drops to the 88-90% range (typical value), which directly impacts on operating costs.•
Redundant configurations providing high availability in the framework of ultra-reliable supply systems (e.g. for data centres).•
Possible integration in redundant architectures with separate functions that facilitate maintenance by isolating parts of the installation.Rotary systems integrate the UPS, the backup power and the generator as a single component, thus making it impossible to separate the functions.
•
No single points of failure. Rotary systems incorporating flywheels depend on the capacity of the motor to start quickly (typically in less than 12 seconds). This means the motor must be in perfect condition and rigorously maintained. If it does not start, there is no time to shut down the critical loads in an orderly manner.)
Consider also the following non-negligible advantages:•
reduced dimensions and weight,•
no wear on rotating parts, hence easier and faster maintenance. For example, rotary systems require checks on the alignment of the rotating parts and thereplacement of the bearings after 2 to 6 years is a major operation (lifting equipment, heating and cooling of the bearings during the replacement).
Conclusion
Given the advantages presented above, static UPSs are used in the vast majority of cases, and for high-power applications in particular.
)
In the following pages, the term uninterruptible power supply (UPS) is taken to mean the static solution.Types of UPSs
(Cont.)
Standards
UPSs
Due to the vast increase in the number of sensitive loads, the term "UPS" now includes devices ranging from a few hundred VA for desktop computers up to several MVA for data centres and telecommunications sites.
At the same time, different typologies have been developed and the names used for the products on the market are not always clear (or even misleading) for end users. That is why the IEC (International Electrotechnical Commission) established standards governing the types of UPSs and the techniques used to measure their performance levels, and those criteria were adopted by Cenelec (European standardisation commission).
Standard IEC 62040-3 and its European equivalent EN 62040-3 define three standard types (topologies) of UPS and their performance levels.
UPS technologies include:
●
Passive standby,●
Line interactive,●
Double conversion.AC input power
These definitions concern UPS operation with respect to the power source including the distribution system upstream of the UPS.
The standards define the following terms:
•
Primary power: power normally continuously available which is usually supplied by an electrical utility company, but sometimes by the user's own generation,•
Standby power: power intended to replace the primary power in the event ofprimary-power failure,
Practically speaking, a UPS has one or two inputs:
•
Normal AC input (or Mains 1), supplied by primary power,•
Bypass AC input (or Mains 2), supplied by standby power (generally speaking viaa separate cable from the same main low-voltage switchboard (MLVS).
UPS operating in passive-standby mode
)
The UPS is installed in parallel to the utility and backs it up. The battery ischarged by a charger that is separate from the inverter.
Operating principle
•
Normal mode- The inverter operates in passive standby mode.
- The load is supplied by utility power via a filter which eliminates certain disturbances and provides some degree of voltage regulation.
- The standards do not mention this filter and speak simply of a "UPS switch". They also indicate that "additional devices may be incorporated to provide power
conditioning, e.g. ferroresonant transformer or automatic tap-changing transformer".
•
Battery backup mode- When the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a very short transfer time (generally less than 10 ms). The standards do not stipulate a time, but do indicate that "the load [is] transferred to the inverter directly or via the UPS switch (which may be electronic or
electromechanical)".
- The UPS continues to operate on battery power until the end of battery backup time or utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).
Types of UPSs
(Cont.)
Fig. 5.3. UPS operating in passive-standby mode.
Advantages
•
Simple diagram.•
Reduced cost.Disadvantages
•
No real isolation of the load with respect to the upstream distribution system.•
Transfer time. It operates without a real static switch, so a certain time is required to transfer the load to the inverter. This time is acceptable for certain individual applications, but incompatible with the performance required by more sophisticated, sensitive systems (large computer centres, telephone exchanges, etc.).•
No regulation of the output frequency, which is simply that of the utility power.Usage
This configuration is in fact a compromise between an acceptable level of protection against disturbances and cost.
The mentioned disadvantages mean that, practically speaking, this type of UPS can be used only for low power ratings (< 2 kVA) and cannot be used as a frequency
converter.
UPS operating in line-interactive mode
)
The inverter is connected in parallel with the AC input in a standbyconfiguration, and also charges the battery. It thus interacts (reversible operation) with the AC-input source.
Operating principle
•
Normal modeThe load is supplied with conditioned power via a parallel connection of the AC input and the inverter. As long as the utility power is within tolerances, the inverter regulates fluctuations in the input voltage. Otherwise (reversible operation), it charges the battery. The output frequency depends on the AC-input frequency.
•
Battery backup mode- When the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load. The power switch (e.g. static switch) also disconnects the AC input to prevent power from the inverter from flowing upstream.
- The UPS continues to operate on battery power until the end of battery backup time or utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).
Types of UPSs
(Cont.)
•
Bypass modeThis type of UPS may be equipped with a bypass. If one of the UPS functions fails, the load can be transferred to the bypass AC input via the maintenance bypass.
Fig. 5.4. UPS operating in line-interactive mode.
Advantages
•
The cost can be less than that for a double-conversion UPS with an equivalent power rating because the inverter does not operate continuously.Disadvantages
•
No real isolation of the load with respect to the upstream distribution system, thus: - sensitivity to variations in the utility voltage and frequent demands placed on the inverter,- influence of downstream non-linear loads on the upstream input voltage.
•
No regulation of the output frequency, which is simply that of the utility power.•
Mediocre conditioning of the output voltage because the inverter is not installed in series with the AC input. The standard speaks of "conditioned power" given the parallel connection of the AC input and the inverter. Conditioning is, however, limited by the sensitivity to upstream and downstream voltage fluctuations and the reversible operating mode of the inverter.•
Efficiency depends on:- the type of load. With non-linear loads, the current drawn comprises harmonics that alter the fundamental. The harmonic currents are supplied by the reversible inverter which regulates the voltage and efficiency is sharply reduced.
- the percent load. The power required to charge the battery becomes increasingly significant as the percent load decreases.
•
A single point of failure exists due to the absence of a static bypass, i.e. if a malfunction occurs, the UPS shuts down.Usage
This configuration is not well suited to regulation of sensitive loads in the medium to high-power range because frequency regulation is not possible. For this reason, it is rarely used other than for low power ratings.
Types of UPSs
(Cont.)
Double-conversion UPSs
)
The inverter is connected in series between the AC input and the application. The power supplied to the load continuously flows through the inverter.Operating principle
•
Normal modeDuring normal operation, all the power supplied to the load passes through the rectifier/charger and inverter which together perform a double conversion (AC-DC-AC), hence the name. The voltage is continuously regenerated and regulated.
•
Battery backup mode- When the AC-input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load.
- The UPS continues to operate on battery power until the end of battery backup time or utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).
•
Bypass modeThis type of UPS comprises a static bypass (sometimes called a static switch) that ensures no-break transfer of the load from the inverter to direct utility power and back.
The load is transferred to the static bypass in the event of the following: - UPS failure,
- load-current transients (inrush or fault currents), - overloads,
- end of battery backup time.
The presence of a static bypass assumes that the input and output frequencies are identical, which means it cannot be used as a frequency converter. If the voltage levels are not the same, a bypass transformer is required.
The UPS is synchronised with the bypass AC input to ensure no-break transfers from the inverter to the bypass line.
Note. Another bypass line, often called the maintenance bypass, is available for maintenance purposes. It is closed by a manual switch.
Types of UPSs
(Cont.)
Advantages
•
Complete regeneration of the output power, whether it comes from the utility or the battery.•
Total isolation of the load from the distribution system and its disturbances.•
Very wide input-voltage range, yet precise regulation of the output voltage.•
Independence of the input and output frequencies, thus ensuring an output frequency within strict tolerances. Capacity to operate as a frequency converter (if planned as such), by disabling the static switch.•
Much higher performance levels under steady-state and transient conditions.•
Instantaneous shift to battery backup mode if utility power fails.•
No-break transfer to a bypass line (bypass mode).•
Manual bypass (generally standard) to facilitate maintenance.Disadvantages
•
Higher price, but compensated by the many advantages.Usage
This configuration is the most complete in terms of load protection, regulation possibilities and performance levels. It notably ensures independence of the output voltage and frequency with respect to the input voltage and frequency.
Its many advantages mean that it is virtually the only configuration used for medium and high power ratings (from 10 kVA upwards).
Conclusion
Double-conversion UPSs represent the vast majority of the medium to high-power systems sold (95% starting from a few kVA and 98% for 10 kVA and higher).
This is due to their numerous strong points in meeting the needs of sensitive loads at these power ratings and is largely the result of the inverter positioned in series with the AC input.
What is more, they have very few weak points except their high cost that is
required to offer a level of performance that is often indispensable given the critical nature of the protected loads. A further weak point is slightly higher losses (a few percent).
In the power ranges under consideration, the other technologies are marginal, in spite of a significantly lower cost.
They have the disadvantages listed below.
•
No voltage regulation for passive-standby UPSs.•
No frequency regulation for passive-standby UPSs and line-interactive UPSs.•
Mediocre isolation (often a surge arrestor) from the AC input due to the parallel configuration of the inverter.Conclusion
)
For low power ratings (< 2 kVA), the three standardised technologiescoexist.
It is the cost effectiveness of the protection functions with respect to the requirements of the loads and the risks run (for people, production, etc.) that determines selection of one of the three typologies.
Types of UPSs
(Cont.)
The delta conversion on-line UPSs
This UPS design, illustrated in Figure 5.6, is a newer, 10 year old technology introduced to eliminate the drawbacks of the double conversion on-line design and is available in sizes ranging from 5 kVA to 1.6 MW. 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 disturbances, this design exhibits behavior identical to the double conversion on-line.
DELTA CONVERTER BATTERY MAIN INVERTER AC DC DC AC STATIC BYPASS SWITCH DELTA TRANSFORMER
Figure 5.6: Delta conversion on-line UPS
A simple way to understand the energy efficiency of the delta conversion topology is to consider the energy required to deliver a package from the 4th floor to the 5th floor of a building as shown in Figure 5.7. Delta conversion technology saves energy by carrying the package only the difference (delta) between the starting and ending points. 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 the output.
X
4th Floor 5th FloorDOUBLE CONVERSION DELTA CONVERSION
X
4th Floor
5th Floor
Types of UPSs
(Cont.)
In the delta conversion on-line design, the delta converter acts with dual purposes.
The first is to control the input power characteristics. This active front end draws
power in a sinusoidal manner, minimizing harmonics reflected onto the utility. This
ensures optimal utility and generator system compatibility, reducing heating and
system wear in the power distribution system. The second function of the delta
converter is to control input current in order to regulate charging of the battery
system.
The delta conversion on-line UPS provides the same output characteristics as the
double conversion on-line design. However, the input characteristics are often
different. Delta conversion on-line designs provide dynamically-controlled, power
factor corrected input, without the inefficient use of filter banks associated with
traditional solutions. The most important benefit is a significant reduction in energy
losses. The input power control also makes the UPS compatible with all generator
sets and reduces the need for wiring and generator over sizing. Delta conversion
on-line technology is the only core UPS technology today protected by patents and is
therefore not likely to be available from a broad range of UPS suppliers.
During steady state conditions the delta converter allows the UPS to deliver power to
the load with much greater efficiency than the double conversion design.
UPS components and operation
The information presented below concerns double-conversion UPSs, the
technology most commonly used by APC by Schneider Electric for power ratings greater than 10 kVA.
General diagram of a UPS
The various items in the diagram below have been assigned numbers that correspond to the sections on the following pages.
Fig. 5.6. Components of a UPS.
Power sources and UPS inputs
Practically speaking, a UPS has one or two inputs:
•
Normal AC input (or Mains 1), supplied by primary power,•
Bypass AC input (or Mains 2), supplied by standby power (generally speaking viaa separate cable from the same main low-voltage switchboard (MLVS). Î AC sources, see p. 9.
UPS connection to both the primary and standby-power sources (UPS inputs supplied by two separate circuits from the MLVS) is recommended because overall system reliability is increased. However, if two separate circuits from the MLVS are not available, it is possible to have both AC inputs (normal and bypass) supplied by primary power (second cable).
Management of transfers between the two input lines is organised as follows.
•
The UPS synchronises the inverter output voltage with that of the bypass line as long as the latter is within tolerances. It is thus possible, if necessary, for the static switch to transfer the load to the bypass AC input, without a break (because the two voltages are synchronised and in phase) or disturbances (because the standby power is within tolerances) for the load.•
When standby power is not within tolerances, the inverter desynchronises and transfer is disabled. It can, however, by carried out manually.UPS components and operation
(Cont.)
Components of a UPS
Rectifier/charger (1)
Transforms the AC power from the primary-power source into DC voltage and current used to:
•
Supply the inverter,•
Charge and float charge the battery.Inverter (2)
Using the DC power supplied by the:
•
Rectifier during normal operation,•
Battery during autonomous operation,the inverter completely regenerates a sinusoidal output signal, within strict amplitude and frequency tolerances.
Battery (3)
Makes the UPS autonomous with respect to the utility in the event of:
•
A utility outage,•
Utility-power characteristics outside specified tolerances for the UPS.Battery backup times range from 6 to 30 minutes as standard and can be extended on request. Depending on the duration of the backup time, the battery is housed in the UPS cabinet or in a separate cabinet.
Static bypass (4)
A static switch is used to transfer the load from the inverter to the bypass without any interruption* in the supply of power to the load (no break because the transfer is performed by electronic rather than mechanical components). The switch is possible when the frequencies upstream and downstream of the UPS are identical.
Transfer takes place automatically for any of the following reasons:
•
Voluntary shutdown of the UPS,•
An overload exceeding the limiting capacity of the inverter (this transfer can be disabled),•
An internal fault.It can also be carried out manually.
* No-break transfer is possible when the voltages at the inverter output and on the bypass AC input are synchronised. The UPS maintains synchronisation as long as the standby power is within tolerances.
Manual bypass (5)
A manual switch is used to transfer the load to the bypass for maintenance
purposes. The switch is possible when the frequencies upstream and downstream of the UPS are identical.
The shift to manual-bypass mode is carried out using manual switches.
Manual switches (6, 7, 8)
These devices isolate the rectifier/charger and inverter modules and/or the bypass line for servicing or maintenance.
Battery circuit breaker (9)
The battery circuit breaker protects the battery against excessive discharge, and the rectifier/charger and inverter against a battery short-circuit.
Upstream isolating transformer (10)
(optional equipment)
Provides UPS input/output isolation when the downstream installation is supplied via the bypass.
It is particularly useful when the upstream and downstream system earthing arrangements are different. May be installed in the UPS cabinet in the MGE Galaxy PW range.
Voltage-matching transformer (11)
(optional equipment)
UPS components and operation
(Cont.)
Filters (12)
(optional equipment)
•
Upstream of the rectifier/charger, when it is of the thyristor-based Graetz bridge type (the case for MGE Galaxy PW and 9000 UPSs), a harmonic filter (see “Key factors in UPS installation” p. 24) reduces the current harmonics resulting from the switching of the rectifier thyristors. This reduces the voltage distortion on theupstream busbars resulting from the flow of harmonic currents (the level required is generally <5%). What is more, these UPSs from APC by Schneider Electric are equipped with an oversized neutral conductor installed as standard to overcome the consequences of third-order harmonics and their multiples which flow in the neutral conductor.
•
All the other UPSs of the MGE Galaxy and Symmetra ranges are equipped with a PFC-type rectifier that eliminates the need for a filter (see “Key factors in UPS installation” p. 24).•
Downstream, UPSs implementing new PWM-chopping techniques may be directly connected to non-linear loads. This technique makes it possible for UPSs from APC by Schneider Electric to maintain the THDU below 3%.Built-in communication (13) (14)
In addition to the need for a user-friendly human/machine interface for effective monitoring of UPS operation, it is today increasingly important for UPSs to
communicate with their electrical and computing environment (supervision systems, building management systems (BMS), computer management systems, etc.).
UPSs from APC by Schneider Electric are designed with built-in capacity for total communication and include:
•
A user-friendly human/machine interface (HMI) with an advanced graphic display and mimic panel. The interface is built up around self-monitoring and self-diagnostic systems that continuously indicate the status of the various UPS components, in particular the batteries.For example, for the MGE Galaxy ranges:
- the Digibat system continuously monitors the status of the battery with full battery management features,
- the B2000 or Cellwatch battery-monitoring system immediately detects and locates battery faults and provides predictive monitoring.
For the Symmetra ranges:
- The rack-mountable (1U) APC battery management system, accessible via a web browser, combines battery monitoring and testing with individual boost charging for peak battery performance.
•
A large selection of communication cards compatible with market standards: - Network Management Card (Ethernet)- Modbus – Jbus card (RS232 & RS485) - Relay card (dry contacts) for indications - Teleservice modem card
These cards can be used to implement supervision, notification, controlled shutdown and Teleservice functions.
Î Human-machine interface and Communication: see “Key factors in UPS
installation”p. 49.
Upstream and/or downstream distribution and protection devices (15) (16)
(optional equipment)
The UPS can be supplied with the following equipment:
•
Upstream LV circuit-breakers for the AC inputs (normal and bypass),•
Upstream LV switchboard with circuit-breaker protection for the AC inputs (normal and bypass),•
Downstream LV switchboard with circuit-breaker protection for the different outgoing circuits.APC by Schneider Electric can offer a selection of UPSs and protection devices that are perfectly coordinated in terms of ratings and performance.
Complete solutions
APC by Schneider Electric can provide complete solutions comprising all the components listed above, including air-conditioning solutions for data centers, in conjunction with Schneider Electric. For users, the result is a single partner and an installation that offers optimum performance and reliability.
UPS components and operation
(Cont.)
These characteristics are based on the main technical specifications presented in the
IEC 62040-3 / EN 62040-3 standards on UPS performance requirements.
Certain terms used here differ from the common jargon and a number of new features have not yet been assimilated by manufacturers. New terms or characteristics used by the standard are indicated between parentheses and preceded by an asterisk.
For example, the title of a section "input current during battery float charging", a commonly used term, is followed by (*rated input current), the term used in the standard.
Note that a number of numerical values are indicated as examples. They are, for the most part, drawn from the technical characteristics of the
corresponding UPSs, indicated in chapter 4, or indicated simply for the purposes of the example.
AC input power
Number of phases and system earthing arrangement
The AC-input supply (primary power) is three-phase + neutral. Single-phase inputs are not used for the power levels dealt with here.
The system earthing arrangement is generally imposed by standards (IT, TT, TNS or TNC).
Normal AC input
The normal AC input is supplied with utility power for the rectifier/charger, within the specified tolerances.
•
Example: 400 V rms ± 15% at a frequency of 50 or 60 Hz ± 5%, three-phase.Bypass AC input
The bypass AC input is supplied with standby power. Practically speaking, this a cable connected to a utility feeder in the MLVS other than the one supplying the normal AC input.
In general, it supplies voltage with the same characteristics as that of the primary power.
•
Example: 400 V rms ± 15% at a frequency of 50 or 60 Hz ± 5%, and a short-circuit current Isc2 = 12.5 kA. The short-circuit current is important information for the downstream protection devices in the event of operation via the static or maintenance bypass.Supply of separate primary and standby power is recommended because it increases overall system reliability, but is not mandatory. However, if two separate circuits from the MLVS are not available, it is possible to have both AC inputs (normal and bypass) supplied by primary power (second cable).
Rectifier/charger
Floating voltage
This is the voltage supplied by the rectifier/charger which keeps the battery fully charged.
It depends on the batteries used and the manufacturer's recommendations.
Input current during battery float charging (* rated input current)
This is the current, under normal operating conditions, required to supply the inverter at its rated power while float charging the battery.
)Example: for a 100 kVA MGE Galaxy PW with a battery backup time of 10 minutes, this current is I input float = 166 A while float charging the battery.
Input current during battery charging
This corresponds to the current required to supply the inverter at its rated power while charging the battery. It is consequently higher than the previous current and is used to size the charger input cables.
)Example: for the same UPS as above, the input current is I input float = 182 A, i.e. higher than above because it is necessary to charge the battery.
Main characteristics of UPS
components
UPS components and operation
(Cont.)
Maximum input current
This is the input current with the UPS operating under worst-case conditions of permitted overload, with the battery discharged. It is higher than the above input current during battery charging (due to the overload current) but is limited in time (as is the overload).
)Example: for the same UPS as above, the MGE Galaxy PW can accept a 25% overload for ten minutes and a 50% overload for one minute. In the worst-case situation with the battery charging, the input current can reach:
I input max. = 182 A x 1.25 = 227.5 A for ten minutes, I input max. = 182 A x 1.5 = 273 A for one minute.
Beyond the above limits, the UPS initiates no-break transfer of the load to the bypass line and automatically transfers back when the overload has ended or been cleared by the corresponding protection devices.
Battery (* energy storage means)
Type
A battery is characterised by its type (vented or sealed lead acid, or nickel/cadmium) and how it is installed. APC by Schneider Electric proposes sealed lead-acid batteries mounted in cabinets.
Service life
This is defined as the operating period, under normal usage conditions, for which the battery supplies at least 50% of the initial backup time.
)For example, MGE Galaxy PW is supplied as standard with sealed lead-acid batteries with a service life of ten years or more. This type of battery, rated for 30 minutes of backup time, will contractually supply only 15 minutes at the end of the specified service life.
It may supply more if it has been used under optimum conditions (notably concerning the temperature). However, it is contractually guaranteed not to supply less, unless used improperly.
Operating modes
The battery may be:
•
Charging. It draws a charge current (I1 charge) supplied by the rectifier/charger.•
Float charging. The battery draws a low, so-called floating current (I1 floating),supplied by the rectifier/charger, which maintains its charge by compensating for open-circuit losses.
•
Discharging. The battery supplies the inverter until its shutdown voltage is reached.When this voltage, set by the battery manufacturer, is reached, the battery is automatically disconnected (UPSs from APC by Schneider Electric) to avoid damage by deep discharge.
Rated voltage
This is the DC output voltage that the battery supplies to the inverter.
)Example: 450 V DC for the MGE Galaxy PW range.
Capacity
Battery capacity is expressed in ampere/hours.
)Example: for a 100 kVA MGE Galaxy PW equipped with a battery offering ten minutes of backup time and a service life of five years, the capacity is 85 A/h.
Number of cells
Number of single battery cells making up the entire battery string.
)Example: the battery of a 100 kVA MGE Galaxy PW comprises, for a given type of battery, 33 cells providing 13.6 V each, for a backup time of ten minutes.
Floating voltage
This is the DC voltage used to maintain the battery charge, supplied by the rectifier/charger.
)Example: for a MGE Galaxy PW, the floating voltage is between 423 and 463 V DC.
UPS components and operation
(Cont.)
Backup time (* stored energy time)
This is the time, specified at the beginning of the battery service life, that the battery can supply the inverter operating at full rated load, in the absence of the AC-input supply.
)Example: MGE Galaxy PW offers as standard backup times of 8, 10, 15, 20, 30 and 60 minutes.
This time depends on the UPS percent load.
•
For a UPS operating at full rated load (100% of rated power), the end of the battery backup time is reached when the battery voltage drops to the shutdown voltage specified by the manufacturer. This provokes automatic shutdown of UPSs from APC by Schneider Electric.•
For a UPS operating at a lower percent load (e.g. 75%), the actual backup time may be longer. However, it always ends when the battery shutdown voltage is reached.Recharge time (* rated restored energy time)
This is the time required by the battery to recover 80% of its backup time (90% of its capacity), starting from the battery shutdown voltage. The rectifier/charger supplies the power.
)Example: for a MGE Galaxy 5500 UPS, the recharge time is eight to ten hours, depending on the battery and the backup time. Note that the probability of the battery being called on to supply power twice within such a short period is low. This means the indicated recharge time is representative of actual performance.
Maximum battery current (Ib)
When discharging, the battery supplies the inverter with a current Ib which reaches its maximum value at the end of discharging. This value determines battery protection and cable dimensions.
)Example: for a 100 kVA MGE Galaxy 5500, this current is Ib max = 257 A.
Inverter
Rated power (Sn)
(* rated output apparent power)
This is the maximum apparent power Sn (kVA) that the inverter can deliver to a linear load at a power factor of 0.8, during normal operation under steady-state conditions.
The standards also define this parameter for operation on battery power. Theoretically speaking, it is the same if the battery is correctly sized.
)Example: a MGE Galaxy 5500 with a rated power (Sn) of 100 kVA.
Active output power (Pa)
(* rated output active power for linear or reference non-linear load)
This is the active power Pa (kW) corresponding to the apparent output power Sn (kVA), under the measurement conditions mentioned above. This value may also be indicated for a standardised reference non-linear load.
)Example: the previous UPS, a MGE Galaxy 5500 with a rated power of 100 kVA supplies an active power of Pa = Sn x 0.8 = 80 kW.
Rated current (In)
This is the current corresponding to the rated power.
)Example: again for a 100 kVA MGE Galaxy 5500 UPS and an output voltage of 400 V, this current is:
In Sn Un = 3 = 100000 400 1732x , = 144.3 A
UPS components and operation
(Cont.)
Apparent load power (Su) and percent load
This is the apparent power Sn (kVA) actually supplied by the inverter to the load, under the selected operating conditions.
This value is a fraction of the rated power, depending on the percent load. .Su ≤ Sn. and .Tc = Percent load (%) = Su / Sn..
)Example: for the UPS mentioned above, if the inverter supplies 3/4 of its rated power (75% load), it delivers an apparent power of 75 kVA, which under standard operating conditions (PF = 0.8) corresponds to an active load power of
Pa = Su x PF = 75 x 0.8 = 60 kW.
Load current (Iu)
This is the current corresponding to the load power, that is, to the percent load in question. It is calculated from Pu as for the rated current, where the voltage is the rated voltage Un (value regulated by the inverter).
)Example: for the UPS mentioned above (75% load) Iu Su Un = 3 = 75000 400 1732x , = 108.2 A which is the same as:
.Iu = In x Tc. = 144.3 x 0.75 = 108.2 A
Efficiency (
η
)
This is the ratio of active power Pu (kW) supplied by the UPS to the load to the power Pin (kW) that it draws at its input, either by the rectifier or from the battery. .η= Pu / Pin.
For most UPSs, efficiency is optimum at full rated load and drops sharply with lower percent loads. Due to their low output impedance and no-load losses, the efficiency of MGE Galaxy UPSs is virtually stable for loads from 25 to 100%. The MGE Galaxy range offers efficiency greater than 90% starting at 25% load, up to 93% at full rated load, as well as an ECO mode which increases efficiency by 4%, i.e. up to 97%. Practically speaking, for MGE Galaxy UPSs, an efficiency value of 0.93 can be used for all input-power calculations for loads from 30 to 100%.
)Example: for a 100 kVA MGE Galaxy at 75% load, 0.93 efficiency corresponds to a UPS active input power of
Pin = Pu / η= 60/0.93 = 64.5 kW.
Output voltage Un
Number of phases
The output can be three-phase (3ph-3ph UPS) or single-phase (3ph-1ph UPS), depending on the situation. Note that the upstream and downstream system earthing arrangements may be different.
Rated output voltage
In general, it is the same as that of the AC input. However, a voltage-matching transformer may be installed.
Static characteristics
These are the tolerances (maximum permissible variations) for the amplitude and frequency of the output voltage under steady-state conditions. Stricter than those applying to utility power, they are measured for normal operation on AC-input power and for operation in battery backup mode.
•
Output voltage variationThe amplitude tolerance is expressed as a percentage of the nominal rms value and may be adjustable.
)Example: for a MGE Galaxy, the voltage 400 V rms ± 1% may be adjusted to ± 3%.
The standards also stipulate a rated peak output voltage and the tolerance with respect to the rated value.
•
Output frequency variationThe tolerance is expressed as a percentage of the rated frequency.
)Example: for a MGE Galaxy, 50 or 60 Hz ± 0.1% during normal operation on primary power and ± 0.5% in battery backup mode.
UPS components and operation
(Cont.)
Frequency synchronisation with primary power
The inverter supplies an output voltage within the above tolerances, regardless of the disturbances affecting the upstream power.
To that end, the UPS:
•
Monitors the voltage parameters (amplitude, frequency, phase) for the primary-power source to determine whether they are within specified tolerances,•
Reacts to any drift in parameters so as to:- readjust the inverter (phase and frequency) to the standby power, as long as the drift remains within tolerances, in view of load transfer, if necessary,
- transfer the load to battery power as soon as the drift goes outside tolerances. The new IGBT and PWM chopping technologies used in UPSs from APC by Schneider Electric allow an excellent adaptation to these variations.
)Example: for MGE Galaxy UPSs, the maximum variation in frequency corresponding to the tolerance is 50 Hz x 0.5% = 0.25 Hz.
Frequency synchronisation with bypass AC power is possible from 0.25 to 2 Hz, in 0.25 Hz steps. Practically speaking, this signifies that frequency variations may be monitored at dF/dt = 0.25 Hz/s and readjustment carried out within 0.25 to 1 second.
Dynamic characteristics
These are the tolerances under transient load conditions.
MGE Galaxy UPSs are capable of withstanding the following conditions.
•
Load unbalanceFor unbalance in the load voltage (phase-to-neutral or phase-to-phase) of: - 30%, the output voltage variation is less than 0.1%,
- 100% (one phase at Pn and the others at 0), the output voltage does not vary more than 0.2%.
•
Load step changes (voltage transients)For load steps from 0 to 100% or from 100 to 0% of the rated load, the voltage does not vary more than:
± 2% on utility power;
+ 2% to -4 % on battery power.
Overload and short-circuit capacity
•
Overloads- 1.1 In for 2 hours, - 1.5 In for 1 minute,
with no change in the output tolerances.
•
Short-circuitsBeyond 1.65 In, MGE Galaxy inverters operate in current-limiting mode up to 2.33 In for 1 second, corresponding to:
I peak max. = √2 x 1.65 In = 2.33 In.
Beyond this value, the inverter transfers the load to standby power or performs a static shutdown (self-protection feature).
Total output-voltage distortion
UPSs must guarantee performance levels for all types of loads, including non-linear loads.
)Example: MGE Galaxy UPSs limit the voltage total harmonic distortion (THDU) in output power to the following levels for:
•
100% linear loads: - THDU ph/ph < 1.5 %, - THDU ph/N < 2%,•
100% non-linear loads: - THDU ph/ph < 2 %, - THDU ph/N < 3%.MGE Galaxy UPSs operate in compliance with the specified characteristics for all types of loads.
General note. The standard specifies certain of the previously mentioned
performance levels for output power during normal operation and operation on battery power. In general, they are identical.
UPS components and operation
(Cont.)
Summary diagram for main
characteristics
Fig. 5.7. Diagram showing the main characteristics (see the list below).
Normal AC input
●
Voltage Un + 10% to - 15%
●
Frequency f + 4% to - 6%
Bypass AC input
●
Voltage Un + 10% to - 15%
●
Frequency f + 4% to - 6%
●
Short-circuit current Isc2 (withstand capacity of the static bypass)
Rectifier/charger
●
Floating voltage
●
Input currents
- rated (battery float charging) - maximum (battery charging)
Battery
●
Backup time: standard 5, 6, 8, 10, 15, 20, 30, 60 minutes, longer times on request)
●
Service life: 10 years or longer
●
Maximum current Ib max.
Inverter
●
Apparent output power:
- rated: Sn (kVA)- load power: Su (kVA) = Sn x Tc%
●
UPS percent load Tc% = Su / Sn
●
Active output power:
- rated: Pn (kW) = Sn (kVA) x 0.8
- load power: Pu (kW) = Su (kVA) x PF = Sn x Tc% x PF = Un Iu PF
●
Efficiency:
η Pu / Pn = 93% (97% in ECO mode).●
Static characteristics (output-voltage tolerances under steady-state conditions)
- amplitude: Un ± 1% adjustable to ± 3%- frequency: f ± 1% during normal operation, f ± 0.5% in battery backup mode - inverter output voltage synchronised (frequency and phase) with that of the standby power as long as the latter is within tolerances.
●
Dynamic characteristics (tolerances under transient conditions)
- maximum voltage and frequency variations for load step changes from 0% to 100% or 100% to 0%: Un ± 2%, f ± 0.5%
●
Output voltage distortion
- 100% non-linear loads THDU < 2%
●
Overload and short circuit capacity:
- overloads: 1.5 In for 1 minute- short-circuits: current limiting to 2.33 In for 1 second
Load
●
Load current (Iu)
UPS components and operation
(Cont.)
Normal mode
(on utility power, see fig. 5.8 on left-hand side)The UPS draws the AC utility power required to operate via the rectifier/charger which provides DC current.
Part of the utility power drawn is used to charge or float charge the battery:
•
I1 floating, if the battery is already fully charged,•
I1 charge if the battery is not fully charged (i.e. charging following a recent discharge).The remaining current is supplied to the inverter with generates an output-voltage sine-wave within the specified amplitude and frequency tolerances.
Battery backup mode (on battery power, see fig. 5.8 on right-hand side)
The battery steps in to replace primary power and supplies the power required by the inverter for the load, with the same tolerances as in normal mode.
This takes place through immediate transfer (the battery is parallel connected) in the event of:
•
Normal AC-input failure (utility-power outage),•
Normal AC input outside tolerances (degradation of utility-power voltage).Normal mode. Battery backup mode. Fig. 5.8. Normal mode and battery backup mode.
Bypass mode
(on static-bypass line, see fig. 5.9 on left-hand side)A static switch (SS) ensures no-break transfer of the load to the bypass AC input for direct supply of the load by standby power.
Transfer is automatic in the event of:
•
An overload downstream of the UPS exceeding its overload capacity,•
An internal fault in the rectifier/charger and inverter modules.Transfer always takes place for internal faults, but otherwise is possible only if the voltage of the standby power is within tolerances and in phase with the inverter. To that end:
•
The UPS synchronises the inverter output voltage with that of the bypass line as long as the latter is within tolerances. Transfer is then possible:- without a break in the supply of power. Because the voltages are in phase, the SCRs on the two channels of the static switch have zero voltage at the same time, - without disturbing the load. The load is transferred to a bypass line that is within tolerances.
•
When standby power is not within tolerances, the inverter desynchronises and operates autonomously with its own frequency. Transfer is disabled.It can, however, by carried out manually.
Note 1. This function greatly increases reliability due to the very small probability of a downstream overload and a standby-power failure occurring at the same time. Note 2. To ensure correct operation of the bypass line, discrimination must be ensured between the protection device upstream of the bypass AC input (on the MLVS outgoer) and those on the UPS outgoing circuits (see information on discrimination below).
UPS components and operation
(Cont.)
Maintenance mode
(on maintenance bypass, see fig. 5.9 right-hand side)Maintenance is possible without interrupting load operation. The load is supplied with standby power via the maintenance bypass. Transfer to the maintenance bypass is carried out using manual switches.
The rectifier/charger, inverter and static switch are shut down and isolated from power sources. The battery is isolated by its protection circuit breaker.
Bypass mode (static bypass). Maintenance mode (maintenance bypass). Fig. 5.9. Bypass mode and maintenance mode.
Parallel UPS with redundancy
Chapter two is entirely devoted to a presentation of the various configurations. Below is some additional information on parallel connection for redundancy.
It concerns MGETM GalaxyTM UPSs in particular. The modular SymmetraTM UPSs also use parallel connection.
Î Configurations, see “Selection of the UPS configuration”
Types of parallel configurations
There are two types of parallel configurations.
•
Integrated parallel UPS unitsThis upgradeable configuration can be started using a single UPS unit with an integrated static bypass and manual maintenance bypass. For configurations with more than two UPS units, a common maintenance bypass is housed in an external cubicle (see fig. 5.10).
•
Parallel UPS units with a centralised static-switch cubicle (SSC)The static-switch cubicle comprises an automatic bypass and a maintenance bypass that are common for a number of UPS units without a bypass (see fig. 5.11). This configuration, less upgradeable than the previous due to the rating of the bypass, offers greater reliability (SSC and UPS units are independent).
•
Modular UPSsUPSs of the SymmetraTM range are made up of dedicated and redundant modules (power, intelligence, battery and bypass).
Modular design with plug-in power modules improves dependability, in particular maintainability and availability, as well the upgradeability of the installation.
Redundancy
Redundancy in parallel configurations can be N+1, N+2, etc.
This means that N UPS units are required to supply the load, but N+1 or N+2 are installed and they all share the load.
See the example below.
UPS components and operation
(Cont.)
Example
•
Consider a critical load with a 100 kVA rating.•
2+1 redundancy- 2 UPS units must be capable of fully supplying the load if redundancy is lost. - Each UPS unit must therefore have a 50 kVA rating.
- 3 UPS units normally share the 100 kVA load, i.e. each supplies 33.3 kVA. - The 3 UPS units normally operate at a percent load of 33.3 / 50 = 66.6%. - Integrated parallel UPS units are each equipped with a static bypass. Transfer is managed such that the three UPS units transfer to the bypass simultaneously, if necessary.
Fig. 5.10. Integrated parallel UPS units with common maintenance bypass and 2+1 redundancy. Operation with all units OK (redundancy available).
•
Loss of redundancy- One UPS unit shuts down, the two remaining units operate at 100%. - The faulty UPS unit can be serviced due to the maintenance bypass.
Fig. 5.11. Integrated parallel UPS units with common maintenance bypass and 2+1
