What Is Inverter Bypass Mode? (Great Feature)

Some inverters and, more specifically, Uninterruptible Power Supply (UPS) inverters must run continuously to supply alternating current of stable voltage and current strength to the connected AC loads. Such an inverter system cannot remain operational indefinitely and will break down or require maintenance from time to time.

Inverter Bypass Mode allows the AC grid power to bypass the inverter and directly power the connected AC loads if the inverter breaks down or requires maintenance. An automatic transfer of power from the grid AC will bypass the inverter when it experiences an internal failure.

Installing a bypass switch or inverter with a bypass mode function is recommended in grid-tied situations. In an off-grid solar-powered system, a bypass switch or bypass mode will be redundant as the inverter will function for as long as the battery bank is sufficiently charged.

The types of bypass switches available are:

  • Static bypass switch
  • External maintenance bypass switch

Let’s look at the differences between these two types of bypass switches and how to bypass mode on inverters works.

Inverter with bypass mode

How Does A Static Bypass On A UPS Inverter Work?

An Uninterruptible Power Supply (UPS Inverter) is linked to a battery bank from where it draws direct current (DC) and converts it to alternating current (AC), which is stable and available for as long as the battery charge remains sufficient.

The battery bank is charged from the grid AC power supply via a battery charger. The function of the UPS is to feed the AC loads with the power of a constant voltage and operating frequency and to ensure that the power supply to the connected loads is not suddenly disrupted.

UPS Inverters also fail eventually due to heat degradation of the circuitry and components of the inverter. UPS failure is negated by installing a static bypass switch either in the UPS or as a separate switch.

Voltage Detection

If the UPS inverter fails, the static bypass switch will detect the voltage drop and instantaneously switch to the grid AC power supply. This will ensure continuity of power to the connected AC loads and allow time for the UPS Inverter to be replaced or repaired.

A static bypass switch senses the continuity of the AC supply of the UPS and will switch to AC-grid power when it detects an interruption in supply from the UPS. This ensures that the connected AC loads can continue running without any disruption.

The static bypass switch must not be confused with the maintenance bypass switch, which is engaged manually when the UPS Inverter has to be maintained or replaced.

See also: What Is A Solar Inverter? (Explained With Examples)

Why Does An Inverter need To Have A Bypass Mode?

Grid-tied inverters need to have a static bypass switch as part of the inverter assembly to enable the AC-loads to keep running on grid power in the event of a failure of the inverter or backup battery.

There is no backup to the power stored in the battery bank in an off-grid power supply system. The battery bank is charged via solar panels, wind power generators, or a backup generator.

Still, if the battery bank is depleted or the inverter breaks down, there is no redundancy power available to maintain the power supply to the AC loads.

In a grid-tied system, the power grid is the backup power source if the inverter becomes defective or the battery is depleted beyond the maximum depth of discharge.

The inverter can switch over to bypass mode when it detects that the inverter fails and routes grid AC directly to the distribution panel and the AC loads.

Repair or Replace

This bypass mode operation will allow the inverter to be repaired or replaced or for the battery charge to be replenished to a point where the inverter can function again. The battery technology determines the depth of discharge possible.

Lead-acid batteries should not be discharged lower than 50% of maximum charge capacity as this will cause damage to the battery chemistry and significantly reduce the number of recharge cycles of the battery.

Modern Lithium-Ion batteries can continue to operate to a much lower depth of discharge level without any detrimental effect on the battery chemistry.

They can be fast-charged back to operational levels and last as much as ten times longer than conventional lead-acid batteries.

Planning Your Inverter Installation Features

The principle of building redundancy into your power supply system ensures that power supply continuity can be maintained when the primary power supply system fails. For a grid-tied Uninterrupted Power Supply Inverter system, it is imperative that the inverter is fitted with a static bypass switch.

UPS systems are designed to filter grid AC and ensure that connected AC loads do not suffer from sudden power loss. The UPS batteries are charged via a rectifier circuit to maintain the battery state of charge within the operational range.

The UPS inverter draws direct current from the UPS batteries and converts it to AC power of uniform voltage and sufficient current to power the connected AC loads. If the inverter battery or the inverter components fail, a static bypass switch directly connects the AC loads to the AC grid power supply.

Bypass Mode Gives Flexibility

It does mean that the AC loads may be subjected to the variations and instabilities of the grid power, but this is preferable to the complete loss of power. The inverter can be repaired or replaced in static bypass mode, or the UPS battery can be recharged.

A bypass mode is a form of redundancy in that the grid AC can be switched on in the event that the UPS supplied AC is interrupted. This maintains the functionality of the connected AC appliances and devices, allowing them to be shut down in a controlled manner.

The computer systems for banks and communication centers will have several layers of redundancy to ensure that the systems that require continuous power supply can remain unaffected if one or more of the power supply sources are affected.

Inverters designed for such applications have static bypass switches incorporated into the system design to enable continuity of supply to grid-tied inverter power systems.


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Elliot has 20+ years of experience in renewable technology, from conservation to efficient living. His passion is to help others achieve independent off-grid living.

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