Voltage Variation and Usage of UV and OV Relay

For the electrical system to be safe, reliable, and efficient, it is very important to keep voltage at the right level. If the voltage is too low (undervoltage), it can cause devices to pull high currents, making them overheat. If the voltage is too high (overvoltage), it can damage the insulation. Maintaining a constant and appropriate voltage extends equipment life, minimizes damage, and ensures proper operation.

Under Voltage.

Undervoltage is a condition where the voltage in a power system drops below the nominal or rated voltage level for an extended period.

Impact on Contactors:

When a contactor coil is subjected to undervoltage, the electromagnetic coil may not generate sufficient magnetic flux to fully close the magnet gap. This leads to an open gap or chattering condition, where the magnets attempt to pull in but doesn’t make full contact. In this state:

• The coil draws excessive current (Ampere-turns).
• This excessive current generates substantial I²R losses, leading to overheating of the coil.
• Prolonged overheating degrades the coil’s insulation, potentially causing inter-turn shorts.
• Ultimately, this can result in coil burning.

An overvoltage is a voltage that exceeds the maximum value of operating voltage in an electric circuit.

When a contactor coil is subjected to overvoltage:

• The increased voltage imposes higher electrical stress across the coil's insulation.
• This excessive voltage can cause dielectric breakdown of the insulation between adjacent turns of the coil winding.
• Insulation breakdown leads to inter-turn short circuits.
• These shorted turns bypass a portion of the coil's total impedance, causing an increase in current within the shorted turns.
• The resulting I²R heating in these shorted sections can rapidly escalate, leading to coil burning.

Undervoltage (UV) and Overvoltage (OV) relays are critical protective devices designed to safeguard electrical equipment from the detrimental effects of voltage variations beyond acceptable limits. Their primary function is to continuously monitor the supply voltage and initiate protective actions when a pre-set threshold is crossed.

Continuous Monitoring: These relays employ internal voltage sensing circuits that constantly compare the instantaneous voltage against upper (OV) and lower (UV) setpoints.

Trip Logic:

• UV Relays: Are configured to trip (change contact state) when the monitored voltage drops below the specified undervoltage threshold. This protects equipment from condition like undervoltage which could lead to higher current and subsequent coil damage in contactors.
• OV Relays: Are configured to trip when the monitored voltage rises above the specified overvoltage threshold. This protects equipment from events of overvoltage, which can lead to insulation breakdown.

Protective Action: Upon tripping, the relay's contacts can be used to:

• Cut off the control supply: By controlling contactor’s coil, thus isolating the coil from the abnormal voltage. This prevents continued operation under stress.

The single diagram is shown below:-

The increase in load will trigger the alarm once the pre-set limit is hit. We can configure relay modules and employ relays to automatically trip the non-critical loads (such as parking lights or street lights) once the pre set limit is reached. Once the HVAC demand is reduced, the relays will then turn on the non-critical loads again.

So, in our example suppose if we have an incomer ACB with 4 MCCBs downstream of it as follows:

We can configure a relay module that MCCB-1, MCCB-2 and MCCB-3 are critical loads while MCCB-4 is non-critical load.

So, when the load on ACB will exceed 90%, the percentage loading feature will detect it and the AL+ AL- terminals will subsequently go high. The release communicates this to the relay module using CAN Bus Communication, which will then cut off the MCCB-4 load since it is programmed as non-critical.

Now since this is remote and automated tripping, the ACB should have communication capabilities (either through UNCO* or Modbus Module). Also, the downstream connected MCCBs should also have electrical operation (motorized MCCBs).

When the demand from MCCB-1 (HVAC Loads) decrease after a few hours, the Relay module will turn on the MCCB-4 using remote electrical operation.

L&T’s Omega ACB offers an add-on relay module that can be used for load shedding on the consumer level. When the percentage loading on the ACB exceeds a specific value i.e., the load current exceeds a certain percentage of its rated current, the relay module will operate and cut off a few loads to avoid total shutdown.

This relay module can be programmed to trigger 4 configurable outputs (2NO and 2 Changeover) and is compatible with the MTX Release 3.5 and above.