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.

Diesel Generator Protections. A Quick Check for an Inspector

Greetings! In this article, I recommend that you familiarize yourself with the main protections of diesel generators on a ship, which inspectors like to check. There are many protections on ship diesel generators and one article is not enough to consider them all, so only the main protections will be described, which are most often demonstrated when passing a particular check.

Using the example of the Daihatsu 6DE-18 diesel engine and the Taiyo FE 647A-8 brushless synchronous generator, we will consider the protection of marine diesel generators and how to test them. Other most common diesel engines (Himsen, Yanmar, MAN B&W) found in the merchant marine fleet will also be partially covered.

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.

ACB Trouble. The generator does not connect to the main busbars. Troubleshooting

Greetings! In this article will discuss one of the problems with diesel generators, which is possible and rare, but happens on ships. In general, any problem with generators for an ETO is a sore and most discussed topic.

Also here we will briefly examine the conditions for synchronizing ship synchronous generators (alternators).

Instruction for recovery of power generating plant after blackout

A blackout on a vessel refers to a complete loss of electrical power on the ship. This situation can arise from various causes, including mechanical failures, electrical faults, or human error. 

When a blackout occurs, it impacts all systems that rely on electrical power, which can include:

1. Navigation Systems: Loss of radar, GPS, and other essential navigation tools.
2. Communication Systems: Inability to use radios, satellite phones, and other communication devices.
3. Propulsion Systems: Potential loss of control over the main engines and steering mechanisms.
4. Lighting: Both interior and exterior lights may go out, causing visibility issues.
5. Safety Systems: Systems such as fire alarms, emergency lighting, and bilge pumps may become non-functional.

The anemometer gives false readings. Troubleshooting

Greetings! Today we have a problem with the anemometer, which shows the wind speed and direction on the bridge. The readings were stuck in one direction (opposite to the wind). At this speed the wind is displayed correctly.

What is an anemometer?

An anemometer (or anemoscope) is a weather vane that consists of a generator, a synchronous motor (Synchro TX), bearings, gear guides, rings and a brush device. All this can be said in one name - transmitter.

One LinkedIn user suggested it's better to simply call a synchronous motor - "synchro," as it's not a motor, even though the manual specifically calls it a "synchronous motor." I agree, but I use the name "synchronous motor" in the article, so I'll leave it as is. Keep this comment in mind:

Wrong description for the function. Correct name is Synchro or synchro TX or transmitter. A motor moves or developes mechanical work. You can even name it a generator, but not a motor.

As example, in electric propulsion systems you may have (or not) synchronous motors that requires a rotor winded and excitation system, exactly the same than a alternator. Actually many times those motors acts with as regenerative function like alternators (or have a choke to dissipate energy instead).

So, technically you can list the device with the name of synchro motor but actually is not a motor but like a generator (Tx). Shortly: call it synchro and that's all.

Procedure of restoring of power after black out on the vessel (example)

Vessel has 3 Main Diesel Generators; Power is supplied by one DG under normal operation at sea and in port. The other two DG is in place on 1st Standby and 2nd Standby accordingly. 

Power is restored after a blackout in the following process: 

AFTER A POWER BLACK OUT, EMERGENCY GENERATOR IN AUTO STAND BY STARTS UP AUTOMATICALLY AND POWER CAN BE BACK UP BY EMERGENCY GENERATOR IN LESS THAN 15 SECONDS. 

(E/G is started by battery power. If it not starts automatically, we have to manually start by switching to "MANUAL" mode position and Push Engine "START BUTTON" switch.) 

Variable Voltage Control. Ward-Leonard control

Where fine control of both hoisting and lowering speed is required either booster control or a modified form of Ward-Leonard control is suitable and footbrakes are not essential. 

A magnetic brake provides against power failure or when returning the controller to off. Generally speaking for straightforward Ward-Leonard schemes the motor for the generator set can be either a.c. or d.c. and as the set runs continuously and in one direction only it is started in the conventional manner. If the supply is a.c. the exciter would be replaced by a static rectifier.

Under these Regulations vessels of 200 gross tons or less must have two lines of hawsers, one at the bow and the other at the stern quarter, each leading through a closed chock. Larger vessels must have at least four lines so arranged that they can be used on either side of the vessel. Two must lead from the bow and two from the stern quarters and not from the extreme bow or stern. For vessels between 200 and 300 gross tons the windlass forward and the capstan aft may be used for the two lines ahead but those leading aft must run from the main drum of power-driven winches and not from capstans. For all larger vessels all four lines must be power-operated and run from the main drum of power-driven winches and not from capstans.

What is a Shaft Generator? Automatic Control of the Shaft Generators

When the order is issued for the main engine to operate at a constant speed corresponding to the correct frequency, and if the load on the distribution network is not in excess of the maximum permissible load on the shaft generator, and if no alarm is operative on the shaft generator, this combination of conditions is indicated on the control panel by the "shaft generator ready" lamp lighting up.

The generator then rotates at the correct speed although without excitation, i. e. it generates no voltage.

If the change-over switch on the control panel is set to the "shaft generator on" position the generator will be excited and will generate a voltage. When about 85% of the normal voltage is attained, control of excitation shaft generator will be taken over by the voltage regulator in the switchgear.

Automatic Control System (Power Management System) of Electrical Power Plants

Automatic Control System for Ships Electrical Power Plants. The automatic control system is designed for controlling turbo- and diesel-driven electric generating plant, predominently onboard ships.

The purpose of the automatic control equipment is to ensure that the power demand of a ship is generated in an economical manner and also to ensure that any faults occurring will not cause damage to the machinery.

During fully automatic operation the equipment carries out starting, synchronizing and load sharing on one or several diesel alternators as the power demand increases, as well as disconnecting and stopping the diesel alternator in the event of excessive power being available.

Power Plant Control System Operation of Central Unit

Adjustable level detectors determine at which busbar load a generator shall start or stop. According to the starting sequence, selected with push-buttons at the front, start and stop orders are distributed to the basic units.

These orders are delayed one minute in order to prevent actions by temporary load variations. The delay will be bypassed when a fault occurs on a running set, overload is detected or a controlled load is required.

The unit also determines whether or not the number of connected sets is sufficient to cover the required load under normal conditions. In this case a signal to the units will permit a set with a non-critical fault to be disconnected and stopped. This signal will also permit a requested controlled load to be connected.

Lloyd's Register of Shipping Certificate for AC Generator or Motor

This is to Certify that the auxiliary propulsion AC generator/stator, particulars of which are given below, has been inspected by me at the Makers' works. The construction, workmanship and materials are good, and the machine complies with the relevant of the Society's Rules.

On completion the generator/motor was tested with satisfactory results.

This certificate is issued upon the terms of the Rules and Regulations of the Society, which provide that:

The Committees of the Society use their best endeavours to ensure that the functions of the Society are properly executed, but it is to be understood that neither the Society nor any Member of any of its Committees nor any of its Officers, Servants or Surveyors is under any circumstances whatever to be held responsible or liable for any inaccuracy in any report or certificate issued by the Society or its Surveyors, or in any entry in the Register Book or other publication of the Society, or for any act or omission, default or negligence of any of its Committees or any Member thereof, or of the Surveyors, or other Officers, Servants or Agents of the Society".

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.