Technical Bulletin 1

Types and Configurations Of UPS Systems


There are many UPS designs on the market today and choosing which one will give the required level of clean and reliable power to a critical load can be confusing. Even the more esoteric configurations fall into one of the two main categories of UPS, namely On-line and Off-line. Both On-Line and Off-Line designs provide battery backup when a blackout (total power failure) occurs but differ in the range and extent of other benefits they provide.

An important new development, that should be considered, are the new fault tolerant UPS systems offered by PK Electronics.

UPS Building Blocks

All uninterruptible power supplies use the same basic building blocks.

The input section is how AC voltage is supplied to the UPS and may be an attached or detachable power cord or hardwired terminals. Small UPS have a combined Input and Bypass point. Larger UPS have separate Input and Bypass terminals.
   
After the input section will be a filter section. Filter here is used in a general sense and includes surge protection, EMC filtering and electrical noise filtering. A filter has a frequency response and does not attenuate all frequencies the same amount.
   
All UPS configurations have an inverter. Low cost UPS have an inverter that has a quasi square-wave output and higher powered UPS inverters use a more expensive inverter to provide a sine-wave output.
   
A battery is needed to keep the UPS running when the power fails or sags. Low power UPS use internal sealed lead-acid, maintenance free batteries. Large UPS systems often use wet-cell batteries. 10 to 15 minutes is a common back up time for a small UPS.
   

 

A charging circuit is needed to recharge the battery after a power failure and to keep it fully charged while not being used.

   
In an On-line UPS, a mechanical or static switch is used as part of an automatic bypass circuit. In an Off-Line UPS a mechanical switch is used, when the power fails, to switch from mains power to inverter power.
A double conversion UPS (True On-Line) has a rectifier as well as an inverter. The rectifier is one stage of conversion (AC to DC) and the inverter is the other stage of conversion (DC back to AC). An Off-Line UPS has a single stage of conversion (inverter).
   
The output section is where the critical load is attached to, or plugged into, the UPS. Single or multiple power outlets might be used that are country specific or generic (IEC 320). A UPS with a higher power rating will have hardwired output terminals.
   
The ability of the UPS to communicate has become very important so that shutdown, control and monitoring software can be used. The use of RS-232, TCP/IP and SNMP is now very common in UPS products. Contact closures to indicate Battery Operation and Low Battery are normally provided as well.
   

 

Most UPS systems operate automatically, have an audible alarm and have a simple control and status panel. Larger UPS systems have metering and more sophisticated controls.

   
A Standby UPS with a crude voltage regulator (V.R.) is called a Line-Interactive UPS. The V.R. is used to boost a low voltage during a brown out situation. A better V.R. is commonly used as an addition to a UPS in situations where wide voltage changes often occur on the incoming mains.
   
An isolation transformer (XFMR) is normally not needed in an installation but is necessary in some UPS designs. A transformer adds weight, size and cost to a UPS. Most UPS companies have an optional transformer for cases where galvanic isolation is required.

Arranging The Building Blocks To Make A UPS

Most UPS configurations use only these basic building blocks. Each configuration has its advantages and disadvantages such as lower cost, better noise filtering, higher efficiency, power conditioning, etc. There are two main categories for UPS equipment: On-Line and Off-Line. Most of the more esoteric UPS designs fall into the Off-Line category.

 

Definitions

On-Line: If 100% of the load current is normally supplied by the inverter, the UPS is an On-Line type.
Off-Line: If the load current normally comes from the mains directly, no matter what the marketing literature says, the UPS is an Off-Line type.

The difference between an On-Line and an Off-Line UPS can be best illustrated by diagrams of the power flow during normal and back-up operation. Note in Figure 1 that the power flow in a Standby type Off-Line UPS is from the input, through the filter and transfer switch to the output and isn’t much different from connecting the load equipment directly to the utility mains.

 

Figure 1.

When the UPS switches to battery backup, the power flow is from the inverter with the battery supplying the energy as illustrated in Figure 2.

Figure 2.

When the power fails, a switch-over from mains to inverter is necessary. For most loads this 5 to 10 millisecond switch-over (1/4 to 1/2 cycle) is not a problem; but, some loads cannot accept the dip in voltage caused by even such a small loss of voltage.

 

During normal operation most Standby UPS have a quasi square-wave output as shown in Figure 3. Most loads, including computers will operate from a square-wave voltage but special and older equipment might require a sine-wave voltage from the inverter and may not operate properly from a UPS with a quasi square-wave output.

Figure 3.

 

During a brownout situation, when the input voltage is low for minutes or hours or days, a Standby UPS will switch to inverter and drain the battery.

 

Compare the operation of a Standby UPS with Figures 4 - 7 for a true On-line UPS.

Figure 4.

In an On-line UPS, the normal power flow is from the input through the filter, rectifier, inverter, switch and output. The inverter supplies the load with power all the times and at the same time conditions the power delivered to the load. (Compare with the Standby UPS where the utility is always connected to the load and which sees any power perturbations to some extent.) When the input power fails, the inverter draws power from the battery as shown in Figure 5. Note that the switch does not operate when going to battery backup.

Figure 5.

An On-Line UPS has a sine-wave (See Figure 6) inverter and the wave shape does not change, as it does in a Standby UPS, when switching from normal mode to battery backup mode. All loads that operate from the sine-wave provided by the utility power company will operate from a properly rated UPS with a sine-wave output.

 

Figure 6.

 

 

An On-Line UPS has a third mode of operation (that a Standby UPS does not have). Figure 7 illustrates the Bypass Mode and would be used if the UPS fails, for maintenance or if the output voltage sags as a result of starting a load with a high inrush current.

Figure 7.

In Bypass Mode the switch has activated and if the UPS is OK, the switch will automatically return the load to inverter power. Filtering and surge suppression are present, in Bypass Mode, to the same extent provided by a Standby UPS in Normal Mode.

 

Line-Interactive UPS

An important improvement to the Standby UPS was the introduction of crude voltage regulation by placing a tap changing transformer in the normal power path. Figure 8 shows the resulting Line-Interactive UPS. The voltage regulator helps during brownout conditions, but has the disadvantage of using battery power each time a tap changes. A Line-Interactive UPS will cost more than an equivalently rated Standby UPS but normally offers, in addition to better brownout protection, a sine-wave output and improved, communications. (Sometimes called "Smart" communications.)

Figure 8.

The operation of a Line-Interactive UPS in Backup Mode is identical to a Standby UPS. The inverter starts, the switch activates and the battery is used.

 

Figure 9 illustrates the operation of the simple voltage regulator (V.R.) used in a Line-Interactive UPS. In low power designs the transformer has only two taps. For larger power ratings, two relays giving four different voltages may be used. Notice how the output voltage tracks the input voltage until a step change in the output voltage occurs when a tap changes. Most loads will not be affected by these step changes in voltage.

Figure 9.

 

Other Off-LIne Designs

Two other common UPS topologies on the market, which are essentially Off-Line in operation, are Ferro-resonant and Triport.

Ferro-resonant designs use a special transformer in the output that is tuned to either 50Hz or 60Hz depending on the frequency of the local utility power supply. The special transformer regulates the output voltage and can be thought of as a voltage stabilizer. An additional winding on the transformer is used for the inverter connection. When the power fails, the inverter starts and supplies the load. Figure 10 shows a Ferro-resonant UPS in the Normal Mode.

Figure 10.

A Triport UPS is the "real" Line-Interactive UPS in that it continuously interacts with the mains utility. In Figure 11 note that there is an inductor between the UPS output and the mains input. This inductor is what separates the Triport from other UPS topologies.

Figure 11.

 

The name Triport comes from the fact that the inverter, mains and load form the three ports. There is a voltage drop across the inductor in Normal Mode and the inverter is needed to regulate the load voltage to prevent a voltage sag. The inverter also takes a small amount of power and charges the battery. If power were taken from the battery to run the inverter, a discharge would occur over time and the battery would not be available when the power fails so all power is taken from the mains input. When the power fails, the switch opens and the inverter supplies the load. Triport UPS designs are sometimes marketed as "single conversion on-line" UPS but they really are an Off-Line UPS. A Triport UPS does not function well when operated from a generator or a source that has an unstable frequency and often will not charge the battery properly.

 

Fault Tolerant, Modular UPS

PK Electronics is unique in offering a line of fault tolerant, redundant, modular UPS that offer very high mission reliability at a cost on par with other UPS designs. Both True On-Line and Line-Interactive UPS are available.

 

Figure 12.

Figure 12 is a simple diagram showing multiple UPS Modules and a cabinet for input and output connections. Each Module is a complete UPS using the latest techniques in fault tolerant design to provide mission reliability that is unsurpassed in the industry.

 

In a fault tolerant UPS, at least one module is kept in reserve. If this module fails, it is isolated and the UPS continues to operate normally. A lamp on the faulty module indicates which module needs to be changed. Because a module can be easily and safely changed (hot-swap capability) while the UPS is running, a technician is not needed. To repair the UPS, a module is removed from the cabinet and a replacement module is reinstalled.

 

To repair a traditional UPS one of two methods is used. If the UPS is small, light and low cost, it is replaced when it fails. It is not economically feasible to replace larger UPS equipment so they are repaired. When a failure occurs, the UPS will not do its job (i.e. provide battery backup, power conditioning, etc.) and a qualified service technician must attend to the problem on site. During the time the UPS is not working, the load is at risk from a mains problem or failure. These problems are overcome by the PK Electronics fault tolerant UPS design.

 

Number Of Phases

Most UPS products are configured to give a single phase input and single phase output because they are used for small power requirements. Larger power requirements are met by using three phase power. Two common configurations are three phase (input and output) and three phase input and single phase output. Table 1 summaries the type of UPS normally found for one and three phase applications.

Table 1. Single and Three Phase UPS Applications
No. Of Input Phases

One

Three

Three

One
No. Of Output Phases

One

Three

One

Three
Fault Tolerant        
  • Line-Interactive (400 to 4400VA)

?

?

?

?
  • True On-Line (1 to 500kVA)

?

?

?

?
On-Line        
  • True On-Line (1to 500kVA)

?

?

?

?
Off-Line        
  • Standby (150 to 600VA)

?

?

?

?
  • Line Interactive (250 to 5000VA)

?

?

?

?
  • Triport (1 to 5kVA)

?

?

?

?
  • Ferro-resonant (500VA to 10kVA)

?

?

?

?

For network file servers, workstation and small computer rooms, a single phase UPS will be adequate and the user will have a choice of many UPS types. If a three phase UPS is needed, a user will most likely have to select a true On-Line UPS. In all cases, especially for very critical applications, a fault tolerant UPS should be considered.

 

How The Various UPS Designs Compare

Each UPS design represents a tradeoff between performance and cost. On-Line designs offer better performance at a higher cost. Fault tolerant systems have higher mission reliability than non-redundant systems. Table 2 gives a summary of the expected features and benefits of each design.

Table 2. UPS Feature Comparisons
UPS Designs

Filtering

Reliability

Bypass

Service
Fault Tolerant        
  • Line-Interactive (400 to 4400VA)

Low

Very High

N/A

Easy
  • True On-Line (1 to 500kVA)

High

Very High

Optional

Easy
Off-Line        
  • Standby (150 to 600VA)

Low

Good

N/A

Replace
  • Line Interactive (250 to 5000VA)

Low

Good

N/A

Technician
  • Triport (1 to 5kVA)

Medium

Good

N/A

Technician
  • Ferro-resonant (500VA to 10kVA)

Medium

Good

N/A

Technician
On-Line        
  • True On-Line (1to 500kVA)

High

Good

Yes

Technician
Notes:
  1. The Filtering column is a composite of the ability of the UPS to filter noise and power problems and includes surge suppression.
  2. Compared to fault tolerant UPS, the reliability of other UPS equipment is good but not state of the art.
  3. N/A = Not Applicable. Only On-Line UPS use an automatic bypass. Off-Line UPS can be thought of as always being on bypass except when they are in battery backup mode. Bypass is option on a fault tolerant On-Line UPS, in most cases, because the reliability is so high that an automatic bypass is not justified.
  4. In some cases a UPS requiring a technician for repair will, for economic reasons, be replaced.


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