ETO COC WRITTEN EXAMINATION QUESTIONS & ANSWERS PART-5

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Q – Describe the different type of losses occurring in the power transformers?(8mark).

Answer- 1] copper losses

2] Iron losses

3] Stray losses

4] Dielectric losses

Copper losses- These losses occur in winding of the transformer, when heat is dissipated by the winding so these are also called as ohmic losses or [I.IR] . Where I is the current passing through the winding and R is internal resistance of the winding of transformer. These losses are present in both primary and secondary winding in the transformer.

Iron losses- These losses occur in core of the transformer and are generated door to variation in the flux. These losses depend upon magnetic properties of the materials which are present in the core .there are two types of iron losses in the transformer

a] Eddy current losses

b] Hysteresis losses

Stray losses- These losses are produce due to the leakage of magnetic flux in the transformer.

Dielectric losses- Dielectric losses occur in the insulating material of the transformer, that is in the oil of the transformer or in solid insulation. When oil get contaminated in solid insulation its quality decreased & because of this the efficiency of transformer is affected.

Q – With the aid of suitable line diagrams wherever required sketch and describe a suitable (MGPS) Marine growth protection system installed on vessel to prevent falling of internal sea water system mention the important precautions if any while handling the system.( 16 mark).

Ans- The Marine Growth Prevention System (MGPS) has been developed for the ships with the sole purpose of tackling marine organism growth, inhibiting it from depositing on the ship’s interior piping systems, which are continuously supplied with the sea water.

The anode in the MGPS system produces ions that spread in the seawater system, producing an antifouling & anti-corrosive layer over the internal sides of sea pipes, heat exchanger (i.e. coolers & condensers), valves in seawater system, refrigeration systems, AC units etc.

  • Sea chest in use… 2.0 V ( Cu ..2.0 v , Al 1.8 V)
  • Stand By Mode … 0.8 V Cu, Al 0.6 V

The three types of alloys used for the anodes are:

Copper Alloyed Anodes: This is the most widely used type to prevent marine fouling in piping, strainers, heat exchangers, pumps etc.

Aluminium Alloyed Anodes: This type is used in conjunction with copper alloy anodes to prevent corrosion throughout the ferrous piping system.

Ferrous Alloyed Anodes: They are used in conjunction with copper alloy anodes to prevent corrosion throughout Cu/Ni pipe work.

Anodes Mounted in strainers in the seawater pipeline: They have an advantage of replacing the anodes without affecting the seawater supply to the ship’s system.

Treatment tank setup with a spray nozzle in sea chest: In this system, a separate electrolysis tank with anodes is fitted which sprays the ion through the nozzle in the sea chest. This system is fitted on ships where sea chest or strainer mounting is not possible.

With the MGPS functioning properly, the systems using seawater will have following benefits:

  • Marine organization growth on the surface of the pipe or valve will decrease the flow. MGPS helps in keeping a good seawater circulation
  • Eliminating corrosion in the pipelines
  • Increasing the cleaning duration of seawater system which will be very frequent in the absence of the MGPS
  • Maintain the efficiency of the system & machinery supplied with the seawater
  • MGPS adds to the energy savings of the ship.

Following points to be considered when working on the MGPS system in dry-dock:

  • While opening the anode, never lift or support the anode from its cable or apply any mechanical load to the anode cable.
  • The new spare anode which is of held-in type or flange sleeves type is supplied unpainted with the rust prevention coating. Assure to remove this coating before the installation. Never use a wire brush or the mechanical instrument to remove this layer. White spirits or the turpentine can be used for the removal of this coating.
  • If anodes are fitted in a space where blast cleaning or painting is not finished, they must be suitably protected. Before flooding the spaces, check to assure that the protection has been removed. Also, remove any paint, tape adhesive residue or grease from the anode surfaces.
  • Assure the renewed anodes are mounted in the vertical position unless otherwise stated by the makers or manufacturer.
  • During dry docking for the retrofit systems, before the vessel is launched or refloated, check the isolation of each of the anode from the hull, as well as continuity of the positive feed circuit to the anode.
  • The MGPS fitted on a ship must be effective against a large variety of organisms due to the multiple numbers & complex nature of fouling communities present in waters of different parts of the world the ship ply to.
  • Any antifouling agent used within the MGPS must follow the local and international regulations & requirements, particularly about biocidal discharges.

Q – Basic principle of operation of transformer (5marks) MARCH 2019

Ans- A transformer is a static device which helps in the transformation of the electric power in one circuit to electric power of the same frequency in another circuit. The voltage can be raised or lowered in the circuit, but with a proportional increase or decrease in the current ratings.

The main principle of the operation of a transformer is mutual inductance between two circuits which is linked by the common magnetic flux. A basic transformer consists of two coils that are electrically separate & inductive, but are magnetically linked through a path of reluctance.

Electrical transformer has primary & secondary windings. The core laminations are joined in the form of the strips in between the strips you can see that there are some narrow gaps right through the cross-section of the core. These staggered joints are said to be ‘imbricated’. Both the coils have a high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to the source of alternating voltage. Most of the alternating flux produced by this coil is linked with the other coil & thus produces the mutual induced electro-motive force. The so produced electro-motive force can be explained with the help of the Faraday’s laws of Electromagnetic Induction as

                e=M*dI/dt

If the second coil circuit is closed, a current flows in it & thus electrical energy is transferred magnetically from the first to the second coil.

The alternating current supply is given to the first coil & hence it can be called as the primary winding. The energy is drawn out from the second coil & thus can be called as the secondary winding.

In short, a transformer carries the operations shown below:

1. Transfer of the electric power from one circuit to another.

2. Transfer of electric power without any change in the frequency.

3. Transfer with the principle of the electromagnetic induction.

4. The two electrical circuits are linked by the mutual induction

Q – Enumerate the different type of transformers used onboard also state their function for which they are used (5 mark) MARCH 2019

1. Step up and Step down Transformer

This type of transformer is divided on the basis of a number of turns in the primary & secondary windings & the induced emf.

Step up transformer transforms a low voltage, high current AC into the high voltage, low current AC system In this type of transformer the number of turns in the secondary winding is greater than the number of turns in the primary winding. If (V2 > V1) the voltage is raised on the output side & is known as Step up transformer.

Step down transformer converts a high primary voltage linked with the low current into a low voltage, high current. With this type of transformer, the number of turns in the primary winding is more than the number of turns in the secondary winding. If (V2 < V1) the voltage level is lowered on the output side & is known as Step down transformer.

2. Instrument Transformer

The instrument transformer is further divided into two types

  • Current Transformer (CT)
  • Potential Transformer (PT)

Current Transformer

  • The current transformer is used for measuring & also for the protection. When the current in the circuit is high to apply directly to the measuring instrument, the current transformer is utilized to transform the high current into the desired value of the current required in the circuit.
  • The primary winding of the current transformer is connected in the series to the main supply & the various measuring instruments like ammeter, voltmeter, wattmeter or protective relay coil.They have accurate, current ratio & phase relation to enable the meter accurately on the secondary side.The term ratio has a great significance in CT.

Potential Transformer

The potential transformer is also known as the voltage transformer. The primary winding is connected across the High voltage line whose voltage is to be measured, & all the measuring instruments & meters are connected to the secondary side of the transformer. The main function of the Potential transformer is to step down the voltage level to a safe limit or value. The primary winding of the potential transformer is earthed as a safety point.

Q – Write short note on negative phase sequence relay (6 mark)

Definition: A relay which protects the electrical system from negative sequence component is known as negative sequence relay or unbalance phase relay. The negative sequence relay protects the generator & motor from the unbalanced load which mainly occurs because of the phase-to-phase faults.

The negative sequence relay has a filter circuit which operates only for the negative sequence components. The relay always has a low current setting because the small magnitude over current can cause dangerous situations. The negative sequence relay has earthing which save them from phase to earth fault but not from phase to phase fault. The phase to phase fault mainly occurs because of the negative sequence components.

The construction of the negative sequence relay is shown in the figure below. The Z1, Z2, Z3, & Z4 are the four impedance of the circuit which is connected in the form of the bridge. The impedance is energized by the current transformers. The relay operating coil is connected to the midpoint of the circuit as shown in the figure below.

The Z1 and Z3 are purely resistive and the Z2 and Z4 are both resistive and inductive in nature. The impedance Z2 and Z4 are adjusted in such a manner that the current flowing through them is always lagging by an angle of 60º than those current which is flowing through Z1 and Z3. The current flowing through the junction A is split into two parts i.e. I1 and I4. The I4 lagging by an angle of 60º regards I1.

Similarly, current from phase B split at junction C into two equal components I3 and I2, I2 lagging behind I3 by 60º.

The current I4 lags by an angle of 30º to the I1. Similarly, I2 lags by an angle of 30º concerning IB and I3 leads IB by 30º. The current passing through the junction B is equal to the sum of I1, I2, and IY.

The flow of Positive Sequence Current – The phasor diagram of positive sequence components is shown in the figure below. When the load is in balanced conditions, then there is no negative sequence current. The current flows through the relay is given by the equation.

So the relay remains operative for a balanced system.

The flow of Negative Sequence Current The figure above shows that the current I1 and I2 are equal. Thus, they cancel each other. The current IY flows through the operating coils of the relay. The current setting value of the relay is kept less than the normal full load rating current because the small overload current can cause the serious conditions.

The flow of Zero Sequence Current – The current I1 and I2 are displaced from each other by an angle of 60º. The resultant of the current is in phase with the current IY. The total current twice of the zero sequence current flows through the operating coil of the relay. The relay can be inoperative by connecting the CTs in the delta. In delta connection no zero sequence current flows through the relay.

Induction type Negative Sequence Relay

The construction of the induction type negative phase sequence relay is similar as that of an induction type over current relay. This relay consists of a metallic disc normally made up of an aluminum coil, & this is rotating between two electromagnets the upper & the lower electromagnets.

The upper electromagnet has two winding- the primary winding of the upper electromagnet is connected to the secondary of the CT connected in the line to be protected. The secondary winding of the upper electromagnet is connected in series with the windings on the lower electromagnet

The primary winding of the relay has 3 terminals because of the centre tapping. The phase R energized the upper half of the relay by the help of CTs & auxiliary transformer while the lower half is energized by the phase Y. The auxiliary transformer is so adjusted that their output is lagged by an angle of 120º instead of 180º.

The operation for Positive Sequence Currents – The current IR and IY flow through the primary windings of the relay. The current flows in the opposite direction. The current IR & IY are equal in magnitude. The balanced current kept the relay inoperative.

The operation for Negative Sequence currents – The negative sequence current I flow in the primary winding of the relay because of the fault current.

The relay starts operating when the magnitude of the fault current is more than that of the relay setting.