Electrical Test part 2
ELECTRICAL TEST and MEASURING EQUIPMENT 2
(PRACTICAL MARINE ELCTRICAL KNOWLEDGE; Hall, Witherby)
QUESTION 11
QUESTION 11
A 3-phase short-circuit occurs on the main bus-bars and the short-circuit trip of the running generator breaker fails to operate. Explain how the 'undervolt' release provides a 'back-up' trip.
ANSWER 11
The short-circuit reduces the bus-bar voltage to zero which causes the U/V release to trip the breaker.
The short-circuit reduces the bus-bar voltage to zero which causes the U/V release to trip the breaker.
QUESTION 12
What is the purpose of the SHEATH on a cable?
What is the purpose of the SHEATH on a cable?
ANSWER 12
The sheath of the cable protects the insulation from damage and injury- it is not classed as an insulant. Sheath materials are required to be heat, oil and chemical resistant and flame retardant (HOFR). The sheath must also be tough and flexible.
The sheath of the cable protects the insulation from damage and injury- it is not classed as an insulant. Sheath materials are required to be heat, oil and chemical resistant and flame retardant (HOFR). The sheath must also be tough and flexible.
QUESTION 13
Will cable materials burn?
Will cable materials burn?
ANSWER 13
Yes, all organic materials will eventually burn in a fire. Cable sheath materials commonly in use are organic plastic compounds that are classed as flame retardant, i.e. will not sustain a fire. Most cable materials now achieve this property by developing chlorine gas and acid flumes to smother the flame. PVC is notorious for its release of deadly acid fumes, but PCP and CSP do the same. EPR and XLPE do not. New materials being introduced or developed also do not issue acid fumes when burning - an important feature for personnel when fire fighting. However, burning cable materials still tend to produce dense smoke.
Yes, all organic materials will eventually burn in a fire. Cable sheath materials commonly in use are organic plastic compounds that are classed as flame retardant, i.e. will not sustain a fire. Most cable materials now achieve this property by developing chlorine gas and acid flumes to smother the flame. PVC is notorious for its release of deadly acid fumes, but PCP and CSP do the same. EPR and XLPE do not. New materials being introduced or developed also do not issue acid fumes when burning - an important feature for personnel when fire fighting. However, burning cable materials still tend to produce dense smoke.
QUESTION 14
What is the purpose of a cable gland?
What is the purpose of a cable gland?
ANSWER 14
Cables are insulated, mechanically protected and watertight. They may be armoured and suitable for installation in a hazardous explosive area. A cable gland maintains these properties where the cable is terminated at an appliance, e.g. at a motor terminal box.
Cables are insulated, mechanically protected and watertight. They may be armoured and suitable for installation in a hazardous explosive area. A cable gland maintains these properties where the cable is terminated at an appliance, e.g. at a motor terminal box.
QUESTION 15
Full load line current.
A 3 phase ac generator rated at 500kW, 440V at 0,83 lag will deliver a full load line current of:
IL = P/(✔️3. VL . cosΦ) = 500000/(✔️3 . 440 . 0.83 = 790.5 Amp.
This means that each phase winding line conductor and generator circuit breaker must be capable of carrying this current continuously without exceeding their temperature limits.
Full load line current.
A 3 phase ac generator rated at 500kW, 440V at 0,83 lag will deliver a full load line current of:
IL = P/(✔️3. VL . cosΦ) = 500000/(✔️3 . 440 . 0.83 = 790.5 Amp.
This means that each phase winding line conductor and generator circuit breaker must be capable of carrying this current continuously without exceeding their temperature limits.
If the above 500kW generator circuit-breaker has an over-current trip setting of 125%, what will be the actual minimum tripping current level?
ANSWER 15
At 500kW, 0.83 lag the full load line current is 790.5A so the generator breaker over-current
coil will trip at 125/100 x 790.5 = 988A
At 500kW, 0.83 lag the full load line current is 790.5A so the generator breaker over-current
coil will trip at 125/100 x 790.5 = 988A
QUESTION 16
Figure
The power factor meter shown above has its scale divided into 4 segments - each calibrated 0-1. What is the significance of each segment?
ANSWER 16
An indication in the top half of the scale shows that the machine is generating. The bottom half of the scale indicates that the generator is 'motoring'. Both top and bottom halves are further split into lagging and leading p.f. sections.
An indication in the top half of the scale shows that the machine is generating. The bottom half of the scale indicates that the generator is 'motoring'. Both top and bottom halves are further split into lagging and leading p.f. sections.
QUESTION 17
The water cooling system on a large generator is out of service due to a faulty inlet valve. How will this affect the generator operation?
The water cooling system on a large generator is out of service due to a faulty inlet valve. How will this affect the generator operation?
ANSWER 17
The generator can only be used to supply a much reduced electrical output in order to keep the machine temperatures below their maximum permitted levels. External emergency doors in the generator's air cooling ducts may be opened in such cases. The penalty is that the normally closed air circuit of the generator is now open to the engine room atmosphere.
The generator can only be used to supply a much reduced electrical output in order to keep the machine temperatures below their maximum permitted levels. External emergency doors in the generator's air cooling ducts may be opened in such cases. The penalty is that the normally closed air circuit of the generator is now open to the engine room atmosphere.
QUESTION 18
What is likely to happen if one of the rotating diodes fails and becomes (a) an open circuit? (b) a short-circuit?
What is likely to happen if one of the rotating diodes fails and becomes (a) an open circuit? (b) a short-circuit?
ANSWER 18
In condition (a) the remaining healthy diodes would continue to supply the main field. In 'hand' control the total field current, and hence generator voltage, will be slightly reduced. Under AVR control, the exciter field current would be automatically boosted to maintain the correct generator voltage while the diode failure would probably be undetected causing the exciter to gradually overheat. A short-circuited diode is more serious as it leads to a short-circuited exciter. Rapid exciter overheating will occur.
In condition (a) the remaining healthy diodes would continue to supply the main field. In 'hand' control the total field current, and hence generator voltage, will be slightly reduced. Under AVR control, the exciter field current would be automatically boosted to maintain the correct generator voltage while the diode failure would probably be undetected causing the exciter to gradually overheat. A short-circuited diode is more serious as it leads to a short-circuited exciter. Rapid exciter overheating will occur.
QUESTION 19
What factors govern the overall voltage response of a generator to sudden (transient) load changes?
What factors govern the overall voltage response of a generator to sudden (transient) load changes?
ANSWER 19
The main opposition to changes in the field current required to correct the generator output voltage are:
(a) Inductance of main rotor field winding.
(b) Inductance of exciter field winding.
(c) Regulator (hand or automatic) response.
The main opposition to changes in the field current required to correct the generator output voltage are:
(a) Inductance of main rotor field winding.
(b) Inductance of exciter field winding.
(c) Regulator (hand or automatic) response.
QUESTION 20
What precaution must be taken when testing the insulation of generator cables and wiring connected to an AVR unit?
What precaution must be taken when testing the insulation of generator cables and wiring connected to an AVR unit?
ANSWER 20
Electronic components such as transistors, integrated circuit chips, thyristors, etc. are likely to be damaged during a high voltage (500V) megger test. To test the generator and its cables to earth and protect the electronic parts, either:
Electronic components such as transistors, integrated circuit chips, thyristors, etc. are likely to be damaged during a high voltage (500V) megger test. To test the generator and its cables to earth and protect the electronic parts, either:
(i) short-circuit all outgoing cable terminals with fuse wire during the IR test.
(ii) remove electronic card(s).
(iii) disconnect all cables at both ends and test separately.
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