Transformers, unlike a motor, have no mechanical output (expressed in kW), instead, they are characterized by the apparent nominal power kVA that they can supply continuously. However, they are also designed to supply significantly higher power. A three-phase transformer is a combination of three interconnected single-phase transformers with three primary and three secondary windings mounted on a core with three legs.
The primary and secondary windings of three single-phase transformers can be connected in the following ways:
Primary in delta – secondary in delta (D/D)
primary in delta – secondary in star (D/Y)
Three-phase delta-star 3 transformer
primary in star – secondary in star (Y/Y)
primary in star – secondary in delta (Y/D)
Low Voltage Power Network
Low Voltage (LV) power plants supply 3 x 380 V/50 Hz, 3 x 440 V/60 Hz, etc, to the power-distribution network. Smaller electrical motors require 3 x 220 V to function. Voltages are reduced from 3x 380V to 3x 220V. This distributes a reduced voltage to the consumer network.
As there is a lesser voltage in the load side (3 x 220 V) then on the power supply side (3 x 380 V), the current on the load side is greater than the current provided from the source side.
A regular LV power plant has four step-down transformers, shown in Figure 14.3 asT1 toT4.
Figure 14.3 – Ship’s LV power plant outline
T1 and T2 are located near the main switchboard .T3 and T4 are located in the vicinity of the emergency switchboard. One transformer from each side of the busbar (MSB and ESB) is kept in continuous operation while another is available as a standby unit. It is normal that the primary winding of the transformer is permanently connected to the power supply. The switching of the load is only carried out in the secondary circuit. Therefore, the standby transformer can be kept separate from the power network until required.
Transformer Parallel Operation
All transformers are hardwired to the power network during the construction process. The power plant manufacturer will have followed the requirements for parallel operation, ie:
The supply and load voltages are equal for both transformers
short circuit voltages for both transformers are equal
the load carried by both transformers is equal
the type of connection to both primary and secondary windings is equal.
Figure 14.4 illustrates features of 3 x 380 V/3 x 220 V distribution, where T1 and T2 are step-down transformers, Q7 and Q8 are automatic circuit-breakers for primary windings (supply) and Q9 and Q10 are automatic circuit-breakers for secondary windings (load).
If T1 is in operation and T2 is intended to be paralleled with T1, and T1 is scheduled to be put on standby, the following sequence for Q7 – Q10 circuit-breakers should be followed:
For bringing -T2 in parallel:
Primary winding circuit-breaker Q8 must be closed first
secondary winding circuit-breaker Q10 must be closed last, connecting -T2 under the load.
For switching off-T1:
Secondary winding circuit-breaker Q9 must disconnect the load from T1 first
primary winding circuit-breaker Q7 must then disconnect T1 from the supply voltage.
A similar sequence should be exercised for emergency switchboard transformers.
There are two types of speed sensors: magnetic pickups and proximity switches. Although they look similar there are significant differences between the operating principles and troubleshooting approach. This section is intended to explain the basic differences.
Overview at MAN B&W engine proximity switch probes
Electromagnetic Speed Sensors
Electromagnetic speed sensors are also known as Magnetic Pickups (MPUs). The operating principle is based on the fact that as the ferromagnetic pole wheel passes, the speed sensor head will alter the magnetic field in a magnetically biased coil (Figure 18.27). Based on the law of induction, an AC output voltage is generated, with a frequency and amplitude proportional to the speed of the pole wheel.
The output voltage of the magnetic pickup can be affected by the following factors:
Voltage increases as the surface speed of the monitored magnetic material increases
voltage decreases as the air gap between the magnetic head and the surface of the gear tooth is increased
voltage waveform is determined by the size and shape of the gear tooth in relation to the size and shape of the pole piece.
Figure 18.27 – Magnetic Pickup
Large engines usually have a larger air gap between the speed pickup probe and the monitored gear.
Ferromagnetic Speed Sensors with Line Amplifier
Proximity speed switches operate well on large engines as they can operate with a large air gap and low surface speeds.
The principle of operation is based on the fact that a ferromagnetic pole wheel passing the sensor head influences the voltage in an integrated HAL element magnetic sensor (Figure 18.28). The voltage is amplified to a square wave signal with the frequency dependent on the pole wheel speed. When a flywheel tooth is within the sensing range of a proximity switch probe, the output of the switch is nearly equal to the supply voltage. On passing the tooth, the sensor output switches to zero volts until another tooth comes into proximity of the sensor head. The amplitude approximates to the supply voltage and is independent of speed.
Figure 18.28 – Proximity Switch
Proximity switches require a constant external DC voltage power supply. Depending on the proximity switch probe manufacturer, it can be within a range between 10-30 V and is always applied even when an engine is stationary. This may be from the same supply source as the power for the engine control and safety system. The maximum current rating for proximity switches is 80 mA, but the actual current is much less and depends on the external load.
Speed Pickups’ Diagram Definition and Troubleshooting
There are various symbols to define speed pickups in diagrams (Figure 18.29), they are often represented as RPM-to-Frequency (n/f) converters (Figure 18.30).
The difference between the two types of speed sensors, magnetic pickup, and proximity, are as follows:
Magnetic Pickup (MPU)
Small gears and high surface speeds
high frequencies 1kHz and more
no power supply needed
sensor output signal +/-1.5 V AC (RMS).
Proximity sensor (PU)
Large gears and slow surface speeds
max switching frequency is 500 Hz
needs a power supply 10-30 V DC.
As both types of speed sensors are called pickups, it is important to determine which type it is. If it cannot be determined for example by the manufacturer’s manual, catalogs, etc, the easiest way is to count the number of conductors connected to the sensor on the diagram (Figure 18.30):
3-wires – Proximity speed sensor (173.SE1)
2-wires – Magnetic pickup (167.SE1).
As an MPU has a permanent magnetic field, it can be also checked by bringing magnetic material in proximity to the magnetic head.
When determining the type of speed probe for installations, the manufacturer’s data tables are helpful. An example of a table is given in Table 18.9 for Wartsila L38 engines.
On observing a deficiency in speed monitoring and to establish if an employed proximity speed sensor is failed and has to be replaced, the power supply across 23.1 (+) and 23.2(-), and the frequency across 23.3 (S) and 23.2 (-), should be measured (Figure 18.30).
If no voltage is detected or the voltage level has dropped below the limits defined by the manufacturer, eg an open conductor or a power supply earth failure, the frequency would be nil or the signal would become erratic.
The procedure for checking the proximity speed switch is illustrated in Figures 18.31 and 18.32.
Figure 18.31 – Measuring voltage for proximity speed switchTable 18.9 – Speed probes’ data sheet (extraction from Wartsila L38 LCS PLC data sheet)Figure 18.32 – Measuring frequency output from proximity speed switch
To confirm that an MPU is in order, Figure 18.BO, both the output voltage and frequency should be measured from the same terminals 24.1 and 24.2.
For both types of speed probes, if the frequency-carrying conductor becomes short-circuited to ground, some PLC-based applications have been known to interpret this condition as being related to over speed and can shut the engine down. Restarting became impossible until the fault was rectified.
Speed Monitoring Relays
The purpose of a speed monitoring relay is to measure the frequency taken from the speed pickup, compare it with the adjusted set point (0-100% of the relay’s frequency range) then produce an output by using the relay’s internal contacts in the event of reaching a set point.
The example given in Figure 18.33 illustrates the speed control for a Piller auxiliary engine. The frequency from one of its speed proximity sensors is shared in parallel by two-speed monitoring relays (K1 and K14). One relay acts as the ignition speed relay to cut off the starting air supply as soon as the cranking speed is reached. The safety system is engaged after the engine has reached a predetermined RPM. The second relay is used for an overspeed-related safety function. As a consequence, the relay set points are different.
Frequency to Current (F/l) Converters
The example in Figure 18.34 represents an extract from a MAN B&W engine speed monitoring diagram. U1 and U2 are frequency-to-current converters (Figure 18.35) that convert the frequency signals received from proximity speed switches into a 4-20 mA signal (required as an analog input for ABB PLC).
Figure 18.33Figure 18.34 – MAN B&W speed monitoring sectionFigure 18.35 – MAN B&W engine control system speed converters
The Miniature relay is an electro-mechanical switch, consisting of a coil, an armature, and contacts.
A current passes through the coil creating a magnetic field that attracts the armature, causing the contacts to move, either making or breaking a connection. The following contacts are known by how they can be thrown:
Normally open (NO)
When the relay is activated, NO contacts connect the circuit and will disconnect the circuit when it is inactive.
Normally closed (NC)
NC contacts disconnect the circuit when the relay is activated and connect when inactive.
Change over (CO)
The contacts have a common terminal and control two circuits. One is a NO contact and the other is an NC contact. It is also known as a ‘double-throw’ (DT).
The following contact designations are also commonly encountered
Miniature relay contacts designations
Single Pole Single-Throw (SPST)
These have two terminals that can be connected or disconnected (11-12 or 13-14) and two terminals for the coil. The terminology’SPNO’and ‘SPNC’ is sometimes used to indicate the contacts’ de-energized state (Figure 3.4a and b).
Double Pole Single-Throw (DPST)
The relay has four terminals that can be connected or disconnected. This is equivalent to two SPST switches actuated by a single coil and has two terminals for the coil (Figure 3.4c).
Single Pole Double-Throw (SPDT)
With a total of five terminals, two for the coil, a further terminal connects to two others as a CO contact (Figure 3.4d).
Double Pole Double-Throw (DPDT)
This has eight terminals with two rows of CO contacts. It is equivalent to two SPDT relays actuated by a single coil (Figure 3.4e).
Miniature relays are also subdivided by their operation:
Non-Latching (stable type)
A relay is activated when the coil is energized and turns off when de-energized. Non-latching relays are used in control applications when the switch must return to a neutral state if power is lost.
Latching type
Latching relays are used when power consumption and dissipation are limited. For example, after the initial actuation of the relay, no further power is needed to maintain the state.
One Coil Latching Type This relay uses a pulse input to a single coil and a latching mechanism to maintain the contact as either on or off. By applying signals of opposite polarities the relay is set and reset.
Two Coil Latching Type This relay uses a latching construction and two coils, one to set and another to reset. Setting and resetting is achieved by applying pulse signals of the same polarity.
Miniature relays are designed so that the casing does not become detached under normal use, as performance could be reduced. Consequently, the DIN rail mounting system allows simple replacement of malfunctioned relays. A PCB (Printed Circuit Board) miniature relay is illustrated in Figure 3.5a, socket mounted ‘ice cube miniature relays and their terminal designation are illustrated in Figures 3.5b and c.
figure 3.5a
Miniature relays are assembled into control cabinets with their sockets, which can in general be subdivided into two basic types:
Screw terminals sockets
screwless terminals sockets (for fast wiring). All sockets are either panel- or 35 mm rail-mounted (EN 60715). Socket’s terminals, as well as relays’ contact pins, are assigned with reference numbers, which should be used when troubleshooting to verify the contact’s condition in an energized and de-energized state.
Process calibrators are considered a handy tool for the calibration and troubleshooting of process control equipment.
As explained mA, mV, and frequency signals are used in a variety of electronic appliances including PLCs. Input signals are often required for calibrating and simulating processes during troubleshooting, so a Multifunction Process Calibrator is a useful instrument. A Multifunction Process Calibrator with a dual-function display (calibrator and/or multimeter) is capable of the following functions:
Constant voltage output: 0-1.5 V DC with a resolution of 0.1 mV, 0-15 V DC with a resolution of 1mV
constant current output: 0-25 mA with a resolution of 1 mA
square wave output: 0.5 Hz-5kHz with an amplitude of 5 V, +/-5 V, 12 V, and +/-12 V. The resolution is the ability of the instrument to differentiate between two signals of close frequency. It is determined by the gate time.
The following processes can be managed using a Multifunction Process Calibrator
Two-wire transducer simulation on a current loop
two-wire transmitter operational checks
R/l and U/l converters – transmitter simulation and operational checks
thermocouple output simulation and operational checks
frequency transmitter simulation and operational checks
F/l converters – operational checks and simulation
proximity transducers – operational checks and simulation.
At the end of this chapter, Below Table gives a brief overview of the different process calibrators available from various manufacturers.
When a load surge is experienced, eg when a ship’s cargo lifting equipment picks up heavy cargo, the generator voltage dips as a result. Similarly, load shedding will result in a generator overvoltage. In other words, on board, there is no such thing as a constant load. Therefore, a continuously fluctuating generator voltage must be regulated. The Automatic Voltage Regulator (AVR) regulates the generator’s output voltage by regulating the voltage of the main field (rotor’s winding).
This particular device ensures that the voltage is maintained within the required range and rapidly corrects any voltage deviations caused by load dips and shedding. In addition to regulating the generator voltage, the AVR circuitry includes protective features to ensure safe and reliable control of the generator.
Modern AVRs consist of high-quality electronic components. The components are usually cast with epoxies, which protect against high temperatures, moisture, and vibration.
A replacement AVR, if required, must be installed in accordance with the manufacturer’s installation requirements.
Troubleshooting Tips
If the generator produces either no voltage or fluctuating voltages, try to locate the problem. Check all connectors and cables in the terminal box and the exciter components for secure connections.
While tracing a problem with an AVR and/or excitation system, the following checks can be carried out.
Fault description
Action suggested
No voltage build-up when starting set
Follow separate excitation or residual voltage tests
Loss of voltage when set running
First stop and re-start set If there is no voltage or the voltage collapses after a short time, follow the separate excitation test procedure
Generator voltage high followed by collapse
Check sensing leads to AVR. Check isolating transformer secondary output – refer to the separate excitation test procedure
Voltage unstable, either on no-load or with load
Check speed. If correct, check the voltage stability setting. Refer to the load testing procedure from the manufacturer
Voltage low on load
Check speed. If correct, check the voltage stability setting. Refer to the load testing procedure from the manufacturer
Phase voltages unbalanced
Check stator winding and cables to the main circuit-breaker
Excessive voltage/speed dip on load switching
Check governor response and P-droop adjustment for the governor
Sluggish recovery on load switching
Check governor response and P-droop adjustment for the governor
