Protections & Interlocks in power plants

 Interlocks: Are the programmed or hardwired control systems to protect systems and improve the operation reliability.

Protections: Are the programmed or hardwired control systems to protect the equipments, man power and systems from failure/harm.

The interlock and protection system is used to ensure safety of equipment and personnel as well as smooth & trouble free operation of the plant

This system initiates automatic corrective actions to stabilize the unit quickly. The protection scheme is developed to trip the equipment automatically with or Class A trip involves a serious electrical fault like differential, stator earth fault etc. and is considered to be the most dangerous in terms of the shock on the unit. Since it involves serious electrical faults, connections from both generator and the HV bus is immediately switched off to limit the damage at the fault point and also to isolate the healthy system. Hence the unit (turbine, generator and boiler) has to be tripped without time delay. Alarm & buzzers are generally used to alert the operator.

POWER PLANT PROTECTIONS & INTERLOCKS AND THEIR SIGNIFICANCE

Sl No.	Interlock description	Significance
A	Boiler
1	FD & SA fan trip/stop on tripping of ID fans	To avoid furnace pressurizing
2	Fuel feeding system trip/stop on trip/stop of SA fans	To avoid jamming of fuel feeding system due to no spreading air
3	FD fans trip/stop on high furnace pressure (>25 MMWC)	1-To avoid furnace leakage
		2-To avoid furnace explosion
		3-To avoid buck stay damage
4	ID fans trip/stop on low furnace pressure (-25 MMWC)	1-To avoid carryover of fuel at secondary combustion zone
		2-To avoid back end flue gas ducts explosion due to accumulation of unburnt (Unburnt results into formation of CO gas)
5	FD fan trips on low drum level (On tripping ID fans, boiler all systems like FD,SA & fuel feeding system trip)	To avoid boiler pressure parts over heating & failure
B	Steam Turbine
1	Turbine trips on high main steam pressure	To protect turbine internals & casing from high pressure damage
2	Turbine trips on low main steam pressure	To protect turbine internals from  saturated  steam (water particles in steam)
3	Turbine trips on high main steam temperature	To protect Turbine internals from creep failure (Turbine internals fail on prolonged exposure to temperature more than recommended)
4	Turbine trips on low temperature	1-To protect Turbine from uneven expansion
		2-To protect Turbine internals from water particles in steam (Low pressure & temperature steam will have water particles in it)
5	Turbine trips on high bearing temperature (>110 deg C)	To protect turbine bearing failure & other secondary system/operation interruption for long time
6	Turbine trips on high vibration (>5 mm/sec or >110 microns)	To protect turbine bearing failure & other secondary system/operation interruption for long time
7	Turbine trips on high axial displacement	To protect turbine internals from rubbing & damages
8	Turbine trips on high differential expansion	To protect turbine internals uniform thermal expansion & from rubbing & damages
9	Turbine trips on low control oil pressure	To ensure reliable operation of HP & LP actuators
10	Turbine trips on low lube oil pressure	To avoid damages to the bearings
11	Turbine trips on low trip oil pressure
12	Turbine trips on low vacuum or high exhaust pressure	To avoid damages to the rotor blades
		Note: High back pressure on rotor creates reaction force to rotation of turbine rotor
13	Turbine trips on high back pressure
14	Vacuum breaker valve opens on activation of trip interlocks like	To reduce the speed of rotor within minimum time to avoid damages to the bearings & internal parts.
	1.High bearing temperature	Note: High back pressure on rotor creates reaction force to rotation of turbine rotor
	2-High bearing vibration
	3-High axial displacement
	4-High differential expansion
	5-Low lube oil pressure
15	High hot well level	To avoid entry of water into Turbine
C	Fuel handling
1	Belt conveyor trips on operation of Zero speed switch (ZSS)	1-To avoid the further damage to the belt conveyor
		2-To avoid system disturbance & major damages to the conveyor structure
		Note: ZSS operates when belt gets cut or slips on pulley
2	Belt conveyor trips on operation of belt sway switch (BSS)	1-To avoid swaying of belt
		2-To avoid belt side edges damage
		3-To avoid fuel spillage
3	Belt Pull cord Switch (PCS)	To stop the belt conveyor during emergency situations to avoid damages to the man & system
D	Boiler feed pumps
1	Pump trips on high bearing temperature	To avoid bearing damage & secondary system damage/disturbance
2	Pump trips on high bearing vibrations	To avoid bearing damage & secondary system damage/disturbance
3	Pump trips on low suction pressure	To avoid pump cavitation
4	Pump trips on high differential pressure	To avoid pump cavitation
5	Pump trips on high balance leak off pressure	To avoid further damages to the balance & counter balance discs
6	Pump trips on lower cooling water temperature	To avoid failure of pump's bearings & seal
7	Pumps trips on over load	To avoid damages to the pump internals
8	BFP trips on Deaerator level low
E	Boiler fans
1	Fan trips on high bearing temperature	To avoid bearing damage & secondary system damage/disturbance
2	Fan trips on high bearing vibrations	To avoid bearing damage & secondary system damage/disturbance
F	Motor
1	Motor trips on higher bearing temperature	To avoid bearing damage & secondary system damage/disturbance
2	Motor trips on higher winding temperature	To protect winding
3	Motor trips on over load	To protect winding
G	Generator
1	Over current protection	Protects the generator from over load, short circuit & earth faults
2	Earth Fault Protection	To protect the generator from earth faults & short circuits
3	Generator Differential Protection	To protect the generator from winding faults or unbalance currents in winding
4	Reverse Power Protection	To avoid motoring of generator during reverse flow of power to generator from other source
5	Low Forward Power Protection	To protect the generator running under load
6	High bearing temperature	To avoid bearing damage & secondary system damage/disturbance
7	High bearing vibrations	To avoid bearing damage & secondary system damage/disturbance
8	Higher winding temperature	To protect winding
9	Higher core temperature	To protect core
10	High air temperature	To limit winding temperature
	Other protections
11	High & Low voltage protections
12	High & low frequency protection
13	Rotor earth fault protection
14	Loss of excitation

Read Power plant standard operating procedures

 Classes of STG Trips:

Class A trip

This involves serious electrical faults and is considered to be the most dangerous in terms of the shock on the unit. Since it involves serious electrical faults, connections from both generator and the EHV bus is immediately switched off to limit the damage at the fault point and also to isolate the healthy system. Hence the whole unit need to be tripped.

Class B trip

Class B primarily relates to mechanical problems. This results in tripping of turbine followed by generator.

Class C

Read Generator and Turbine inter tripping

Class C involves basically external system related problems like frequency, overvoltage etc. This does not involve instant tripping of the unit. CPP unit operates on house load

Classes of Generator protections

SL NO.	CLASS A	CLASS B	CLASS C
1	Generator Differential Protection	Loss of Excitation	Under Frequency
2	100% Stator Earth Fault Protection	Rotor Earth Fault	Over Frequency
3	Generator Over Voltage Protection	Over excitation	Pole Slipping Protection
4	95% Stator Earth Fault Protection		Tripping of unit transformer
5	Starting Over Current Protection
6	Over fluxing Protection of Generator
7	Differential Protection of GT
8	Buchholz Relay of GT
9	Trip from oil & winding temperature of generator transformer
	These protection when operated initiate tripping of Generator Circuit Breaker, Field Circuit Breaker, Generator Transformer Circuit Breakers & Unit Transformer LV Circuit Breakers and turbine.	This results in tripping of turbine followed by generator.	Class C involves basically external system related problems like frequency, over voltage etc. This does not involve instant tripping of the unit.

 Why do the Boilers explode

What do you mean by Turbine supervisory system???

Questions & Answers on Power Transformers

   

1-What do you mean by power transformer?

It is a static Electro-magnetic machine which transforms alternating current from one AC voltage to another AC voltage at same frequency & at the same apparent power (KVA).

2-What is the principle of Power transformer operation?

Power transformers work on the principle of electromagnetic induction. Which states that, EMF induced in a closed conducting circuit when the magnetic flux linking with that circuit changes in time.

3-What is the main function of a power transformer?

Generally it is used for stepping up or stepping down of Voltage to desired level

4-What are the various parts of power transformer?

General arrangement of Power Transformer

• Casing
• Core
• Primary & secondary windings
• LV & HV bushings
• Radiators & cooling system
• Conservator
• Breather
• Protection devices like Buchholz relay, relief valves, temperature sensors

5-Why does the oil conservator is placed at higher elevation?

Oil conservator is placed at a slightly higher level than that of the tank. It accommodates the contraction & expansion of oil level during lower & higher loads respectively. At higher load, oil temperature rises and hence level in the conservator rises & at lower load, oil temperature decreases & level in conservator drops down.

The above cushioning in oil level is by cushioning bag present in conservator, the air cushion in the conservator permits expansion & contraction of the oil tank without contact with moist air.

6-What is the function of breather in Transformers?

Breather is installed in a pipe from conservator. One end is connected to air cushion bag in the conservator, other end is connected to external air.

Breather is filled with dry silica jel, generally pink in colour. When oil in the conservator rises, air is let out through the breather. During low load when oil level in the conservator decreases, air is sucked into the cushion bag through breather. Silica jet absorbs the moisture & lets only dry air. Wet silica jells are blue in color.

7-What is the function of Buchholzs relay?

It is fitted in the pipe between conservator tank & main oil tank. It operates by gas during arcing or short circuits

8-What are the various cooling methods employed in Power transformers?

• ONAN: Oil Natural & Air Natural
• ONAF: Oil natural & Air forced
• OFAF: Oil forced & Air forced
• OFWF: Oil forced & water forced
• AN: Air natural

9-What is the function of Pressure relief valve in Transformers?

It is fitted on tank to vent out the gases formed in oil & hence protects the transformer from explosion

10-What do you mean by small, medium & large transformers?

• Small transformers: < 5 KVA
• Medium transformers: 5 to 500 KVA
• Large transformers :> 1 MVA

11-What do you mean by core type transformer?

In this type of transformers, winding surround the limbs of core

12-What do you mean by the Shell type transformer?

In this type of transformers, core surrounds the major portion of the windings

13-What is the relation between voltage, current and number of turns on coils in a transformer?

We have following relation:

Vp/Vs = Np/Ns = Is/Ip

Where, Vp and Vs are Voltage on primary and secondary side.

Np and Ns are No. of turns on primary and secondary coils.

IP and Is are Primary side current and secondary side current of a transformer.

14-A 10 KVA single phase 2200/220 Volts Transformer has 60 turns on secondary side, then calculate Primary current, no. of turns on coil and secondary side current.

Given that,

Vp = 2200 V

Vs = 200 V

Ns = 60

Transformer rating = 10 KVA

We know that Vp/Vs = Np/Ns

Np = 60 X 2200/220 = 600 turns

Further, KVA rating of transformer is (Vp X Ip)/1000 and (Vs X Is)/1000

10 = 2200 X Ip/1000

Therefore, current on primary side Ip = 10 X 1000/2200 = 4.54 Amps

Similarly current on secondary side Is = 10 X 1000/220 = 45.45 Amps

15-A Power transformer’s input voltage is 11 KV & output voltage is 110 KV,then calculate the number of turns on secondary side, if Primary side winding has 25 turns

We have

V1 / V2 = N1 / N2

11 / 110 = 25 / N2

N2 = 250 Nos

16-A power transformer input voltage is 11 KV & output voltage is 220 KV, then calculate the secondary side current if it has 2300 Amps of current on primary side?

We have

V1 / V2 = I2 / I1

11 / 220 = I2 / 2300

I2 = 115 Amps

17-What are the materials of composition of lamination cores?

Laminations are thin 0.2 to 0.3mm thick silicon sheets. These are further coated by varnish or insulation oxide.

18-Why do the laminations are made up of silicon steel sheets?

Higher the silicon content in steel sheets increases the resistivity & reduces the eddy current losses. But silicon percentage is restricted up to 3.25% max.to avoid brittleness of sheet.

19-What are the various tests carried out on Transformers?

Meggering

• IR value measurement
• Magnetic balance test
• Magnetizing current test
• Capacitor measurement
• Ratio test
• Vector group test
• Induced voltage test
• Temperature rise test
• BDV test
• No load current test
• No load loss test

Turbine oil and transformer oil standard testing parameters

20-What is the significance of magnetizing current test?

• This test is performed to locate the defects in the magnetic core structure,
• To detect the shifting of winding, failure
• To detect the problems in tap changers

21-What are factors which affect IR value of transformer?

• Surface condition of the terminal bushings
• Quality of oil
• Quality of winding insulation
• Temperature of oil
• Duration of application & value of test voltage

22-What are the various protections given for power transformers?

• Differential protection for earth faults
• High oil temperature & high winding temperature protection
• Over current protection
• Over fluxing protection
• Protection against fire
• Protection against lightening
• Buchholz relay for gas & arcing protection
• Pressure relief valve

23-On what factors transformer loading depends on?

• Transformer current
• Winding temperature
• Oil temperature

24-What are the effects of Transformer over loading?

Overloading results into

• Cellulose insulation becomes mechanically weak & resulting in winding failure
• Oil gets rapidly oxidized

25-What is the function of tap changer in Power transformers?

Tap changer is for adjusting the secondary voltage

26-What do you mean by No-load current?

The current flowing through the terminal of a winding when rated voltage is applied at rated frequency the other winding being left open circuited

27-What do you mean by no-load losses in Power transformers?

The active power absorbed when rated voltage at rated frequency is applied to the terminals of one winding, with other winding being left open circuited

28-What parameters of Transformer oil are tested yearly?

Transformer oil qualities are recommended as per IS-335-1963

• Density
• Kinematic Viscosity
• Flash Point
• Pour point
• Neutralization Number (Acidity)
• Sludge
• Moisture content
• Dissolved Gas Analysis (DGA)
• Dielectric Dissipation test
• Interfacial tension
• Break down Voltage

29-What is the importance of Transformer BDV test?

BDV test is done to detect moisture, dirt & conductive particles in the oil. The BDV value should be more than 50 KV

30-Briefly explain the BDV test of transformer oil?

This test applies an AC voltage of frequency 40 to 60 Hz through two polished electrodes having diameter 12.5mm to 13 mm with oil gap around 2.5 to 4.0 mm.Rise in voltage between the electrodes is at uniform rate of 2 KV/sec.Thus voltage is increased from zero to till breakdown.

The final value will be arithmetic mean of 6 consecutive tests

 

 

It's all about HP heaters (Feed water heaters) in Power plants

 1-Why do you use HP heaters in power plants?

HP heaters are used for heating the feed water, which will contribute in increasing cycle efficiency as well as reduction in fuel consumption

2-What is the design code of HP heaters?

HEI, ASME SEC VIII Div-I IBR

3-What are the advantages of using feed water heaters in power plants?

• Fuel consumption reduces
•  Reduce heat losses in the condenser
•  Lower emissions as fuel use is reduced due to improved heat rate
• Decreases the plant heat rate & hence increases the plant efficiency

4-What are the different types of feed water heaters?

Open type feed water heaters & closed type feed water heaters

5-What do you mean by open type of feed water heaters?

In open type feed water heaters steam directly mixes with feed water. Steam pressure used is lower < 5 kg/cm2

6-What do you mean by closed type of feed water heaters?

In closed type feed water heaters steam directly mixes with feed water. Steam pressure used is high > 5 kg/cm2, these are shell & tube type heat exchangers

7-What are the pipe lines connected to HP heaters?

• Bleed steam inlet line
• Feed water inlet line
• Condensate outlet line
• Feed water outlet line
• Feed water box drain & vent lines
• Shell zone drain & vent

Procedure for Boiler gauge glass line up

8-What are the MOCs of Tubes & shells used in closed type of HP heaters?

Tubes: SA 213 TP 304 (Feed water pressure up to 170 kg/cm2)

Shell: SA 516 Gr.70

9-What are the different zones of HP heaters?

• Desuper heating zone
• Condensing zone
• Sub cooling zone

10-Where does the maximum heat transfer occur out of all zones?

Maximum heat transfer occurs at condensing zones

11-Why the name Sub-cooling has come?

Here condensed steam from condensing zone is cooled by feed water entering by convective heat transfer method.

12-What is the function of drain coolers in HP heaters?

Drain Coolers are employed because of heat consumption improvement in case of drain introduction into the lower heater through the control valve.

13-Why the feed water inlet line connection is at the bottom & outlet line is at the top?

          

Feed water inlet line & outlet line are connected in such a way that to separate desuperheating zone, condensing zones & sub cooling (drain cooler) zones

14-Where do the fixed & sliding supports of HP heaters are located?

Fixed support: Towards feed water inlet & outlet line

Sliding support: Opposite side of feed water line connections

15-What would be the velocity of feed water in HP heaters tubes?

It’s around 0.6 to 0.8 m/sec

16-What is the effect of high/low condensate level in heaters shell?

Higher the condensate level lower is the performance of heater and vice versa. Heater level is always maintained in between 30–50%.

17-Briefly explain the condensate level control in HP heaters?

One of the most common causes of tube failures in a feed water heater (FWH) is the improper control of the internal liquid level, which also can cause operational and maintenance costs that might lead to premature replacement. These problems are not new, they have been experienced by many utility plants throughout the industry during the past 50 years. However in many cases, the resulting damaging phenomenon has seldom been totally understood and the loss of corporate knowledge and failure of some utilities to identify and rectify level control problems continues to bring this issue to the forefront of root causes of FWH operational failures.

In general, the performance of the Drain Cooler (DC) Zone is tied to the operational parameter of Drain Cooler Approach (DCA). DCA is a good indication of whether the DC Zone is operating properly or not, it is not the only parameter that should be considered. DCA is a measurement of temperatures

The pressure of the drains also must be known in order to determine the degree of sub cooling and whether there is a potential for flashing, either within the DC itself or the downstream piping before the level control valve. Flashing and two-phase flow in either of these areas can cause significant damage to the heater.

It is important to remember that the drain cooler is designed to be a water-to-water exchanger. It must remain that way to function properly. Any admission of vapour into the zone typically results in problems. This might be a result of a low liquid level in which steam is admitted directly from the condensing zone into the DC zone, the result of flashing within the DC zone itself, or can be the result of leakage into the zone via the endplate or shroud cracks.

18-What do you mean by drain cooler approach (DCA) in HP heaters?

DCA is the temperature difference between the drains (steam condensate) leaving the heater and the temperature of feed water entering the heater. For more cycle efficiency DCA value should be small.

19-A HP heater is used to heat the feed water from 160 °C to 180 °C by using turbine bleed steam at 15 kg/cm2 and 320 °C. The condensate returning from heater is at 170 °C, calculate the DCA of heater.

We have,

DCA = Temperature of condensate leaving the heater – Temperature of feed water entering the heater

DCA = 170 - 160 = 10 °C

Note: For best performance, heaters are designed to get DCA 3 to 5 °C at full operation capacity.

20-What do you understand by Terminal Temperature Difference (TTD)?

It is the difference between the saturation temperature at the operating pressure of the inlet steam to the heater and the temperature of the feed water leaving the heater. For more cycle efficiency TTD value should be small.

21-A HP heater is used to heat the feed water from 110 °C to 160 °C by using MP steam at pressure 13 kg/cm2 at temperature 280 °C, calculate the TTD.

We have,

TTD = Saturation temperature of inlet steam - Feed water outlet temperature

Saturation temperature of inlet steam at 13 kg/cm2g pressure = 195.6 °C

TTD = 195.6 - 160 = 35.6 °C

Note: For best performance, heaters are designed to get TTD 3 to 5 °C at full operation capacity.

22-How the DCA does affects condensate level of HP heaters?

An increase in DCA,HP heater level decreases & vice versa

23-What is flashing in heaters? How does it occur?

Flashing, by definition, is the change in state of liquid to vapour. While in most cases this change of state results from the addition of heat (as in the boiler) in a FWH the most common cause of flashing is a result of a reduction in pressure (or pressure drop). Pressure drop might be a result of the geometry of the Drain Cooler Entrance window, the fact that the drains must travel around the tubes and change direction many times due to the baffling arrangement and also due to changes in elevation and elbows in the downstream piping. If the liquid drains are not sub cooled enough, any one of these pressure drops could result in flashing and two-phase flow. Two-phase flow is known to cause problems to piping, tubing, the cage and the shell, especially in the case of carbon steel components.

24-What do you mean by fouling in heat exchangers?

Deposition of any undesired material on heat transfer surfaces is called fouling. Fouling may significantly impact the thermal and mechanical performance of heat exchangers. Fouling is a dynamic phenomenon which changes with time. Fouling increases the overall thermal resistance and lowers the overall heat transfer coefficient of heat exchangers. Fouling also impedes fluid flow, accelerates corrosion and increases pressure drop across heat exchangers.

Different types of fouling mechanisms have been identified. They can occur individually but often occur simultaneously.

Scaling:

• Particulate/Sedimentation Fouling
• Corrosion Fouling
• Chemical Fouling
• Freezing Fouling

25-What are the problems associated with HP heaters?

•  Initial two phase mixture & hammering
• Tubes failure due to wrong operation
• Level fluctuation & leakages
• Overfeeding of steam & feed water
• Operating the heaters above the operating & design parameters

Read Power plant standard operating procedure

26-Write down the initial charging process of feed water heater (HP heater)

Steps:

• Ensure all the maintenance activities on HP heaters are completed
• Ensure all inlet & outlet valves of heaters are healthy
• Ensure all field instruments are healthy
• Keep open all water box & shell side vents & drains are  open
• Ensure steam condensate outlet valve is open

Water side

• Crack Open the feed water outlet valve
• Then crack open the feed water inlet valve
• Allow to vent out the air
• Then gradually open the outlet & inlet feed water valves
• Then close the water box vents & drain valves

Steam side

• Ensure steam line drains are in opened condition
• Ensure steam parameters are as per desired values
• Ensure no water in steam line drains
• Crack open the steam inlet valve to HP heater
• Ensure there are no water particles in drain & vent line of shell. If found clear, then close the valves
• Then gradually open steam inlet valves & allow for stabilization

27-A HP heater is used to heat the 200 TPH feed water from 160 °C to 180 °C by using turbine bleed steam at 15 kg/cm2 and 320 °C. The condensate returning from heater is at 170 °C, calculate the quantity of steam used.

 Given that,

Qf = 200 TPH

Tf1 = 160 °C

Tf2 = 180 °C

Hg  at pressure 15 kg/cm2 & temperature 320 °C =735.3 kcal/kg

Enthalpy of condensate water Hf = 171 kcal/kg

Heat lost by the steam = Heat gained by feed water

Ms X (Hg-Hf) = Mw X (Tf2-Tf1)

Ms X (735.3-171) = 200 X (180-160)

Ms = 7.08 TPH

28-What are the precautions to be taken for safe operation of HP heaters?

Precautions:

• Operate the HP heaters as per SOP
• Take utmost care during initial charging
• Do not operate the heaters beyond the operating pressure & temperature
• Bypass the HP heaters during Boilers Hydraulic tests
• Conduct routine preventive maintenance

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