Chapter N

Characteristics of particular sources and loads


Types of static UPSs

From Electrical Installation Guide

Types of static UPSs are defined by standard IEC 62040.

The standard distinguishes three operating modes:

  • Passive standby (also called off-line)
  • Line interactive
  • Double conversion (also called on-line)

These definitions concern UPS operation with respect to the power source including the distribution system upstream of the UPS.

Standard IEC 62040 defines 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 of primary-power failure
  • Bypass power: power supplied via the bypass

Practically speaking, a UPS is equipped with two AC inputs, which are called the normal AC input and bypass AC input in this guide.

  • The normal AC input, noted as mains input 1, is supplied by the primary power, i.e. by a cable connected to a feeder on the upstream utility or private distribution system
  • The bypass AC input, noted as mains input 2, is generally supplied by standby power, i.e. by a cable connected to an upstream feeder other than the one supplying the normal AC input, backed up by an alternate source (e.g. by an engine-generator set or another UPS, etc.)

When standby power is not available, the bypass AC input is supplied with primary power (second cable parallel to the one connected to the normal AC input).

The bypass AC input is used to supply the bypass line(s) of the UPS, if they exist. Consequently, the bypass line(s) is supplied with primary or standby power, depending on the availability of a standby-power source.

UPS operating in passive-standby (off-line) mode

Operating principle

The inverter is connected in parallel with the AC input in a standby (see Fig. N16).

  • Normal mode
The load is supplied by utility power via a filter which eliminates certain disturbances and provides some degree of voltage regulation (the standard speaks of “additional devices…to provide power conditioning”). The inverter operates in passive standby mode.
  • 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 (< 10 ms) transfer time.
The UPS continues to operate on battery power until the end of battery backup time or the utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).

Usage

This configuration is in fact a compromise between an acceptable level of protection against disturbances and cost. It can be used only with low power ratings (< 2 kVA).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 centers, telephone exchanges, etc.).

What is more, the frequency is not regulated and there is no bypass.

Note: In normal mode, the power supplying the load does not flow through the inverter, which explains why this type of UPS is sometimes called “Off-line”. This term is misleading, however, because it also suggests “not supplied by utility power”, when in fact the load is supplied by the utility via the AC input during normal operation. That is why standard IEC 62040 recommends the term “passive standby”.

Fig. N16 – UPS operating in passive standby mode

UPS operating in line-interactive mode

Operating principle

The inverter is connected in parallel with the AC input in a standby configuration, but also charges the battery. It thus interacts (reversible operation) with the AC input source (see Fig. N17).

  • Normal mode
The load is supplied with conditioned power via a parallel connection of the AC input and the inverter. The inverter operates to provide output-voltage conditioning and/or charge 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 following a transfer without interruption using a static switch which 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 the utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).
  • Bypass mode
This 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 (supplied with utility or standby power, depending on the installation).

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.

Fig. N17 – UPS operating in line-interactive mode

UPS operating in double-conversion (on-line) mode

Operating principle

The inverter is connected in series between the AC input and the application.

  • Normal mode
During 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.
  • 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 transfer without interruption using a static switch. 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 mode
This type of UPS is generally equipped with a static bypass, sometimes referred to as a static switch (see Fig. N18).
The load can be transferred without interruption to the bypass AC input (supplied with utility or standby power, depending on the installation), in the event of the following:
  • UPS failure
  • Load-current transients (inrush or fault currents)
  • Load peaks

However, the presence of a bypass assumes that the input and output frequencies are identical and if the voltage levels are not the same, a bypass transformer is required.

For certain loads, the UPS must be synchronized with the bypass power to ensure load-supply continuity. What is more, when the UPS is in bypass mode, a disturbance on the AC input source may be transmitted directly to the load because the inverter no longer steps in.

Note: Another bypass line, often called the maintenance bypass, is available for maintenance purposes. It is closed by a manual switch.

Usage

In this configuration, the time required to transfer the load to the inverter is negligible due to the static switch.

Also, the output voltage and frequency do not depend on the input voltage and frequency conditions. This means that the UPS, when designed for this purpose, can operate as a frequency converter.

Practically speaking, this is the main configuration used for medium and high power ratings (from 10 kVA upwards).The rest of this chapter will consider only this configuration.

Note: This type of UPS is often called “on-line”, meaning that the load is continuously supplied by the inverter, regardless of the conditions on the AC input source. This term is misleading, however, because it also suggests “supplied by utility power”, when in fact the load is supplied by power that has been reconstituted by the double-conversion system. That is why standard IEC 62040 recommends the term “double conversion”.

Fig. N18 – UPS operating in double-conversion (on-line) mode
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