Under-Voltage (UV) Coils in Generator Circuit Breakers: Purpose and Common Failures

What is a UV Coil? An Under-Voltage (UV) coil is an essential safety component used in circuit breakers, particularly in generator and power distribution systems. Its primary purpose is to automatically trip (open) the circuit breaker when voltage drops below a certain threshold, protecting the system from operating under unsafe or unstable conditions.

Purpose of a UV Coil

1. Automatic Disconnection at Low Voltage. If the system voltage drops significantly (typically below 70–80% of nominal), the UV coil de-energizes, which causes the breaker to trip. This prevents motors and sensitive equipment from operating with insufficient voltage, avoiding damage.
2. Fail-Safe Mechanism. UV coils are often used as a fail-safe, ensuring that a breaker cannot remain closed (or be closed) unless there is adequate control voltage present — typically coming from the generator itself.
3. Interlocking and Synchronization Systems. In marine and industrial generator systems, UV coils are used in automatic transfer switches (ATS) and synchronization schemes to ensure proper breaker operation based on the system’s logic.

Blackout Test. Protections of an emergency diesel generator

Greetings! In the previous article on diesel engines, we discussed the protection of auxiliary diesel generators, which are most often tested on a ship during inspections in ports. In this article, we will consider the protections of an emergency diesel generator, as well as the procedure for testing a blackout of a ship's power plant.

At first I wanted to write that the protection of the emergency diesel generator (EDG) in ports is checked very rarely, but recently the Belgian PSC suddenly decided to check the protection of our emergency diesel. Therefore, in order to always be prepared for any checks, it is worth being aware of all the protections that this engine has.

Checking and Troubleshooting a Reactive Compensation Circuit for the Automatic Voltage Regulator in an Isolated AC Bus

One of the common issues encountered in generator systems operating on an isolated AC bus is excessive circulating current between generators. This problem can lead to system inefficiencies, potential damage to components, and unexpected shutdowns. To address this, it is essential to ensure proper setup and functionality of the Automatic Voltage Regulator (AVR) reactive droop compensation circuit.

This guide provides a step-by-step approach to diagnosing and resolving issues related to reactive power compensation in a generator system.

The Current Transformer Unit - Parallel Switch. Paralleling Different Size Generators

A switch is installed on the secondary side of the current transformer to short-circuit both the CT and the burden resistor, preventing any signal from reaching the generator. When set to the “Unit” position, the switch allows the generator to operate independently of the parallel generating system, eliminating the effects of the droop circuit.

When a generator operates independently in a parallel droop system without the short-circuit switch set to the "Unit" position, an unwanted droop occurs in the generator's output voltage. The same effect happens in a crosscurrent system, though to a lesser extent, because other burden resistors in series function as a voltage divider, proportionally reducing the voltage.

Function of Voltage Regulator and Parallel Generator Operation

The voltage regulator's primary function is to maintain a stable and precise generator voltage under no-load conditions and as loads fluctuate. When generators operate in parallel, a parallel compensation circuit is required to help voltage regulators manage reactive power distribution among the generators.

Reactive power imbalances between generators can occur when the voltage regulator adjusts the generator’s excitation due to load variations, prime mover speed changes, thermal drift, and other factors. 

Automatic Voltage Regulator. Real power, Reactive power, Apparent power. KW, KVAR, KVA

In a DC analogy with batteries in parallel, only voltage needs to be controlled for proper load sharing. Similarly, in a mechanical AC analogy, only torque requires control. However, when paralleling AC generators, both variables — torque and excitation — must be properly managed.

• Torque control regulates the division of real power (kilowatts, kW).
• Excitation control manages the division of reactive power (kilovolt-amperes reactive, kVAR).

Both must be precisely controlled to ensure stable and efficient generator operation.

Real power represents the actual work performed by the electrical energy generated. It is supplied by the prime mover in the form of torque, which the generator converts into electrical energy. This energy is then delivered to a load, where it is transformed into useful forms such as heat, light, or mechanical motion (e.g., in a motor).

Reactive power, on the other hand, is the power required by inductive or capacitive loads to store energy during each half-cycle. In the example shown in Drawing 10, the load is purely resistive, such as a heating element. Since no reactive power is needed at any point on the sine wave, the current remains directly proportional to the voltage at all times. Real power can be calculated using Ohm’s Law in this scenario.

Automatic Voltage Regulator and Parallel Operation of generators. Voltage droop

Generator sets are commonly operated in parallel to enhance fuel efficiency and increase the reliability of the power supply. When generators are connected in parallel, fuel economy is optimized by activating only the necessary generators to meet the load demand at any given moment. Operating each generator close to its full capacity ensures efficient fuel consumption.

The reliability of the power system is also improved since backup generators are available when others are in use. Additionally, protective systems can be implemented to detect faults and isolate the affected part while maintaining power to the rest of the system. If one generator fails or develops a problem, it can be shut down, with the remaining generators taking over the load.

Removing contamination from generator and electric motor windings using Electrosolv-E

Instructions for removing contamination from generator and electric motor windings using Electrosolve-E.

1. De-energize the generator. Switch off the heater. Remove the filters.

2. Check the insulation resistance of the stator. Check the insulation resistance of the rotor, observing safety precautions.

3. Provide maximum convenient access to the stator and rotor windings.

4. Apply clean undiluted Electrosolve-E to all accessible surfaces with a brush or spray for pre-soaking. The product must be able to saturate all the dirt accumulated on the windings. It is advisable to turn the rotor for good access. Do not be afraid to apply the product to the terminals and bare wires.

5. Electrosolve-E will flow down into the bottom of the generator. A drain for the accumulated product must be provided.

Automatic Voltage Regulators. What is a generator AVR or Automatic Voltage Regulator?

All modern ship alternators are equipped with voltage regulators. The regulators permit to obtain constant voltage of generators under various amount of load and power factor. Here we shall speak about AVR for brushless alternators.

Block diagram (Fig. 1) shows the regulator principle in an autonomous alternator system. The alternator voltage is fed to the AVR unit via fuses and exciter switch. The reference level of the alternator voltage is set by the voltage adjusting potentiometer. The difference between the actual voltage and reference value is amplified and converted to triggering pulses to the thyristor bridge. A-c voltage is fed from the alternator via supply transformer to the thyristor bridge.