Ship monitoring relays

Ship monitoring relays are not intended to substitute fuses or motor protection switches.

Instead, they signal irregularities earlier before the corresponding protective devices are tripped.
Trips from a ship’s thermal relays are linked to:

• Ship power system current imbalance
• Abnormal temperature rise of the PTC (Positive Temperature Coefficient) thermistor probes
• cosfi measurement dropping too low
• Locked rotor and over-torque detection
• Underload
• Earth fault currents
• Phase rotation reversals
• Prolonged starting

Only a few of the trips listed in the control circuits are used onboard ships and are restricted to essential electrical equipment:

• Current monitoring relays
• Voltage monitoring relays
• Phase imbalance and phase sequence relays
• PTC relays

Some other terms commonly used for this subject are:
Starting override – Measured values outside the tolerated range are ignored during startup. This is when the starting override is active. The purpose is to tolerate brief fluctuations, such as motor inrush currents.
Response delay -Threshold value violations are accepted within the delay time. A signal is only generated if the threshold value violation continues beyond the delay time.
Threshold value – If the measured value passes the set minimum or maximum, the relay will trip.
Reset value -The value where the relay picks up again.
Hysteresis – The range between the threshold value and the reset value.

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Even with a mechanical force against the magnetic system, the contacts of both contactors cannot close simultaneously.

An electrical interlock consists of two interlinked auxiliary NC contacts. While one contractor (eg 1KM1) is energizing, its interlocking contact (1KM1/21- 22) opens, preventing current flow. During this time current flows through the coil of the second contactor (1KM2).

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The markings make it possible to determine the terminals that are associated with the ship component, ie functions they are assigned to and standardize the markings for different manufacturers on ship electrical system.

Terminal markings adjacent to main contacts have a single digit reference.Three-phase contactor main contact pairs are designated with: 1-2; 3-4; 5-6.
Terminal markings adjacent to auxiliary contacts have a double digit reference numbers so that:
• The first digit is the contact’s position (the position digit) which indicates where the contact is physically situated on the block, ie 1,2,3,…, 8
• the second digit defines a contact’s function and is called the function digit.
Function digits have been given the following DIN standardized designations

Standardized designations for contactors

Break contacts (NC) have function digit 1 and 2
Late break contacts (LB) have function digits 5 and 6 also known as ‘Normally close Late’ (NCL)

Make contacts (NO) have function digits 3 and 4

Early make contacts (EM) have function digits 7 and 8 also known as’Normally open Early’ (NOE)

Change over contacts (CO) have function digits 1-2-4

Each auxiliary contact that is utilised by relays and contactors is clearly designated, the reference numbers are stamped on both sides of the contact terminals.

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To ensure cables are connected correctly they are given unique numbers.

This is vital for maintenance and troubleshooting in the event of equipment failure.

Conductors on all electrical drawings are represented by straight lines.

we can see a line break for the conductor 45, assigned with the tag identifier, this is a line break reference (=GA02/5.6) indicating where the conductor’s continuation can be found.

The reference points to a different diagram group, therefore it is preceded with a plant identification sign (=). The continuation of the conductor 45 can be located in diagram group =GA02, sheet 5, sector 6, where it is assigned with a reference =GA01/10.8.

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Equipment markings on diagrams are composed as designation blocks.

The identification blocks 1 (Plant) and 2 (Location) are always entered In the diagram’s block, for example, plant is identified as = GA02, location is identified as +27in.

The alphanumeric representation of a code generally reflects the method used for filing and locating the diagrams.

When locating an item using ship diagram sheets, where a relay coil and contacts are shown through different sheets, reference codes this diagram are used.

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Very high values of ship voltage, current, power, temperature, force, pressure etc. create the possibility of danger in an ship engineering system.

To minimise the safety risk to personnel and equipment a system must be designed and manufactured to the latest high standards and be correctly installed.

During its working life the equipment must be continuously monitored and correctly maintained by professionally qualified personnel who understand its operation and safety requirements.

Before attempting any electrical work, there are some basic safety precautions you must bear in mind.

The possible dangers arising from the misuse of electrical equipment are well known.

Electric shock and fire can cause loss of life and damage to equipment.

Regulations exist to control the construction, installation, operation and maintenance of electrical equipment so that danger is eliminated as far as possible.

Minimum acceptable standards of safety are issued by various bodies including national governments, international governmental conventions (e.9. SOLAS), national and international standards associations (e.g. BS and IEC), learned societies (e.9. IEE), classification societies (e.g. Lloyds), etc.

Where danger arises it is usually due to accident, neglect or some other contravention of the regulations.

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Now looking at lithium neat metal cobalt aluminum or lithium iron phosphate. Lithium has better stability in terms of chemical configuration and thermal stability.

Metal cobalt aluminum or lithium iron phosphate can get us to the levels of voltage we really need.
In terms of application size, the large marine applications will need 2,000-6,000V battery systems, so this scale is up on automotive, which currently uses around 300-700V.

When it comes to temperature levels, operate in 80°C+, but current ship battery technologies are rated between 30 and 50°C, so the range in this respect needs to be improved.
There’s also work that needs to be done to the ship system and electronic management of the battery pack.

Each cell is currently 10-30Ah 3.5V. To get it to where we need it in terms of packaging, it needs to be 60-100A and to get to 3,000V, essentially you need a lot of battery cells to connect together.
The mechanical design for ship battery packaging is also a challenge – it needs to be a robust design with safety first. When you get to these higher voltages and temperatures, you have to make sure you’re designing a safe system.

Source: Electric & Hybrid Marine Technology International

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The instrument will almost certainly be damaged if it is used to measure the current to motors and other power circuits.

The basic current range can be extended by using external shunts (DC) and current transformers (AC).

These accessories are generally purchased separately from the instrument manufacturers.
The procedure to be used to measure current in a small current circuit is:
PROVE the correct instrument operation.
SWITCH the instrument to the highest current range (either acA or dcA as appropriate).
TURN OFF the power to the circuit to be tested and discharge all capacitors.
OPEN the circuit in which current is to be measured – removing a fuse-link often gives a convenient point for current measurement.
ADJUST the test leads as designated for current measurement.

Securely connect the probes in series with the load in which current is to be measured.
Turn ON the power to the circuit being tested.

Note the current size on the meter display.
Turn OFF the power to the circuit being tested and discharge all capacitors.
Disconnect the test probes and switch the instrument to OFF.

Reconnect the circuit that was being tested.
Often, the most convenient way to measure current is to use a clampmeter, which is simply clamped around an insulated conductor.

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Ship constant frequency generator

The constant frequency generator is known as the isosynchronous generator. Most governors have a set point adjustment that allows for varying the no-load speed of the prime mover. By adjusting this set point, one can adjust the frequency or the load shared by one generator with the other operating in parallel.

In the isosynchronous generator, this adjustment is done by an automatic
control system with precise compensation feedback. We note two points here when operating two generators in parallel:

1. Only one generator can be isosynchronous. Two isosynchronous generators cannot work in parallel since they would have no intersection that makes the common point of stable operation. In parallel, they would conflict with each other, could overload, and self-destroy in a continuous search of an intersection.

2. Beyond the initial load sharing, all additional load is taken by the isosynchronous generator, whereas the drooping generator load remains constant. This is analogous to two mechanical springs sharing a load, with one soft spring drooping with load and the other spring infinitely stiff like a solid metal block (we may call it isoshape spring). Regardless of the initial load sharing, any additional load will be taken by the solid metal block (isoshape spring) without additional drooping of either spring.

Isosynchronous (constant frequency) generator on ship

For this reason, the load shared by the isosynchronous generator is much greater than that by the drooping generator. Therefore, the prime mover governor for an isosynchronous generator is specifically designed for isochronous operation at any load from zero to 100% load.

Such governor assures that the prime mover shares the load proportional to the generator kW rating by using direct measurement of kW.

It provides rapid response to load changes, stable system operation, ability for paralleling dissimilar-sized engine-generator sets, and fine speed regulation under 0.25%.
Droop is inherent in all prime mover speed controls, but in an isochronous generator, it is recovered in a short time.

It is a temporary droop of transient nature more commonly called the compensation.

This gives bus frequency 60 Hz from no load to full load.

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Big load change on ship generator

For example, if the generator load were suddenly changed from P1 to P2 in one step, the rotor power angle would increase from δ1 at old load P1 to δ2 at new load P2. This takes some time due to the mechanical inertia of the rotor. No matter how short or long it takes, the rotor inertia and the electromagnetic restraining torque will set the rotor in mass-spring-damper
type oscillations, swinging the rotor power angle beyond its new steady state value of δ2

If δ exceeds 90° any time during this swing, the machine stability and the power generation capability may be lost.

Load limits on ship generators

For this reason, the machine can be loaded only to the extent that, even under the worst-case load step, planned or accidental or during all possible faults, the power angle swing remains below 90° with sufficient margin. This limit on loading the machine is called the transient or dynamic stability limit, which is generally the power output for which δ is about 25° to 35° under normal steady-state operation.

For damping the transient oscillations of the rotor following a step load change, each pole face is provided with copper bars running along the length of the machine. All bars on each pole face are shorted at both ends, forming a partial squirrel cage of copper conductors on each pole surface.

Power oscillations on ship generator system

When the rotor oscillates around the synchronous speed, these bars see a relative slip with respect to the stator flux that runs exactly at the synchronous speed. This slip induces currents in the bars, as in the squirrel cage induction motor. The resulting I²R power loss in the bars
depletes the oscillation energy, cycle by cycle, until the oscillations are completely damped out and the induced currents subside.

Adjusting synchronous generator

Thus, there is a small induction motor superimposed on the synchronous generator.
It contributes damping only when the rotor oscillates around the constant synchronous speed. It is also used to start the machine as an induction motor to bring the generator near full speed before applying the dc excitation to the rotor and making it a synchronous machine then onward.

Ship ac motors and impact on 3-phase generator

Since many ac motors with direct online start are used on ships, such as
winches, the transient behavior of the generator needs close consideration. It may be necessary to select the shipboard generator with low reactance to improve the around the synchronous speed to improve transient stability limit and to minimize the voltage dips, but that may increase shortcircuit
current levels.

Step load design for protection of ship generator

Many shipboard loading events may constitute step load on the generator, such as turning on the bow and stern thruster, cargo and ballast pumps, cranes, main circulating pump on steam ships, high-power weapons on combat ships, and tripping a large load circuit breaker accidentally or for fault protection purposes.

To maintain dynamic stability under such transients, sudden loading on the generator should be limited to no more than 25% to 30% of the rated power in one step, but the exact limit can be determined by the equal area criteria.

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