Study of generator and switchgear Vizag steel plant report

BY:Sushil Roy

Gvpcoe(a)

ABSTRACT

This project titled “Study of ELECTRICAL GENERATOR, SWITCHGEAR AND PROTECTION SYSTEM” covers the complete electric protections of turbo generator in a thermal power plant of Visakhapatnam steel plant. The main objective of the project is to study the various protections provided for the alternator and the necessity of each.

The study also covers verification of the existing settings used for different protective relays by actually calculating the fault currents under various conditions.

Normally the faults in generator can occur either inside like stator, rotor or external to it in the bus-ducts of feeders. While internal faults in a generator should be cleared as fast as possible to minimise the damage of the core whereas the external faults can be sustained for a considerable period to enable the respective down-stream protections to act and isolate the same from the generator. In case of downstream protections fail to clear a fault, a backup is provided in generator scheme to isolate the same. Proper coordination between these protections should also be verified.

TABLE OF CONTENTS

S.NO TABLE OF CONTENTS

01. Title of the Project

02. Certificate

03. Acknowledgement

04. Introduction

05. Description

06. Conclusion

CHAPTER - 1:

INTRODUCTION TO VSP

Visakhapatnam steel plant is the most sophisticated and modern integrated steel plant in india and the decision was announced by then prime minister smt. Indira gandhi in parliament on 17 april 1970. The government of india and ussr signed on agreement on june 12, 1979 for setting up the 3.4 million tonnes integrated steel plant.

The soviet design organisation, gipromex designed the coke oven and coal chemical plant (excluding the 7 metre battery portion) sinter plant and blast furnace. Mecon of ranchi is engineering the 7 metre tall coke oven batteries with dry quenching. The remaining facilities have been designed by dastur co., the principal consultants for vsp.

The steel plant acquires its supply of iron ore-lumps and fines from bailadialla deposits in Madhya Pradesh. Blast furnace grade lime stone from jaggayyapeta in Andhra pradesh. The steel plant produces angles, channels, bars, wire-rods and billets for re-rolling and also produces pig iron, besides normal by-products from the coke-oven and coal chemical plant.

Visakhapatnam steel plant has two blast furnaces each of 3200 m3 useful volume, which is the largest blast furnace in india each furnace capable of producing 1.7 mt of hot metal per year. The plant has 4 coke oven batteries. Each battery has 67 ovens and 68 heating chambers. The coke oven has the facility of dry cooling of hot coke instead of water cooling or air cooling as in other plants. The capacity of each coke oven is 320 pushing per day for recovery of by-products from coke oven gas. The main by-product from coke oven gas recovered in coal chemical plant are xenol, phenol compounds, tar, benzol ammonium suphate (urea), pitch and anthracine.

The main burden material to blast furnace is sinter. Sinter is regarded as the most desirable feed for blast furnace all over the world. Now a days most of the world’s efficiency run blast furnace use 90% to 100% sinter. The sinter plant at Visakhapatnam steel plant is designed to meet 80% load of iron bearing material required to the blast furnace. As the consumption of fluxed sinter for every ton of hot metal production is 1390 kg. Two sintering machines in sinter plant produce 5256 thousand tonnes every year for charging into blast furnace. The sinter machine is dwigtn lloyd type, having 313 m2 area each. It is designed to operate at the rate of 1.27/hr/m3 for 330 days in a year.3

The estimated power requirements for vsp is 280 mw at 3.0 mt stage, the peak load being 292 mw and essential load being 219 mw. The installed in-plant generation capacity is 286.5 mw, comprising of 247.5 mw from captive thermal power plant, 24 mw from gas expansion turbines utilising blast furnace high top pressure and 15 mw from back pressure turbines utilising waste heat of coke dry cooling plants. The generation from bf and coke oven will be fluctuating in nature the balance is supplied by apseb. The purchased power requirements at 3 mt stage is around 250 mva. Apart from meeting part of the power requirements, the power plant and blower house supply cold blast to blast furnace and steam at 13 ksca and 21 ksca to the plant.

The main plant equipment consists of three turbo-generator units of 60 mw each and one of 67.5 mw, three turbo-blowers each having capacity of 6067 nm3/min and five boilers each of 330 t/hr. Capacity at 101 ksca and 540o c.

1.1 MAJOR PLANT FACILITIESVsp has the following major production facilities:

• 3 coke oven batteries of 67 oven each having 41.6 m3 volume

• 2 sinter machines of 312 m2 area.

• 2 blast furnace of 3200 m3 useful volume

• Steel melt shop with three ld converters (2 operating and one stand by) of 150 t capacity each and 6 nos. Of 4 strand continuous bloom caster.

• Light and medium merchant mill of 710,000 tonnes per year capacity.

• Wire rod mill of 850,000 tonnes per year capacity.

• Medium merchant & structural mill of 850,000 tonnes per year capacity.

Extensive facilities have been provided for repair and maintenance as well as manufacture of spare parts. A power plant, oxygen plant, acetylene plant, compressed air plant etc; also from part of the plant facilities.

1.2 MODERN TECHNOLOGYIn vsp modern technology has been adopted in many areas of

production, some of them for the first time in the country. Among these are:

Selective crushing of coal

7 metre tall coke ovens

Dry quenching of coke

On ground blending of sinter basemix

Cast house slag granulation for blast furnace

100% continuous casting of liquid steel

Gas expansion turbine for power generation utilising blast furnace top gas pressure.

Hot metal desulphurisation

Extensive treatment facilities effluents for ensuring proper environmental protection

Computerisation for process control

Sophisticated, high speed and high production rolling mills

General arrangement of power distribution in vsp

Power requirement

Integrated steel plants are major consumers of electricity, with specific consumption of power at around 600-650 kwh/ton of liquid steel. The estimated annual power requirement of visakhapatnam steel plant, at full level of production in each shop (corresponding to 3.0 mt of liquid steel), is 1932 million kwh. This corresponds to an average demand of 221 mw. The estimated energy consumption and average demand of major shops is given below:

Shop Annual energy (mwh)Average demand (mw)

Rmhp 35 4.0

Co & ccp 171 19.5

Sinter plant 254 29.0

Blast furnace 210 24.0

Sms & ccm 126 14.5

Lmmm 100 11.5

Wrm 118 13.5

Mmsm 100 11.5

Crmp 35 4.0

Tpp 310 35.0

Asp 258 29.5

Com. Station & cwp 131 15.0

Auxiliary shops 20 2.5

Water supply 15 2.0

Traffic & others 7 1.0

Township 28 3.0

Losses 14 1.5

Total 1932 221.0

1.3 Sources of powerPower requirement of vsp is met through captive generation as well as supply from apseb grid. The captive capacity of 270 mw is sufficient to meet all the plant needs in normal operation time. In case of partial outage of captive generation capacity due to breakdown, shutdown or other reasons, the short fall of power is availed from abseb grid. Turbo generators of vsp normally operate in parallel with state grid. Excess generation over and above plant load is exported to apseb.

the agreement with apseb provides for a contract demand of 150 mva and permit export of power. Tariff for import, export, demand charges, penalties etc. Are stipulated. For purpose of billing, import and export energy is separately metered at main receiving station.

Apseb supply net work

The power grid corporation’s sub-station adjacent to ukkunagaram is connected to vijayawada by a 400 kv line. It is also being connected to jaipur, orissa (eastern grid) through dc back to back arrangement of 500 mw capacity and by 400 kv ac double circuit line. Power is stepped down through a 315 mva, 400/220 kv auto transformer at power grid corporation sub-station and is fed to the adjacent apseb switching station. This switching station is also connected to bommuru and gajuwaka sub-stations by 220 kv double circuit lines. Bommuru sub-station is connected to generating stations at Vijayawada, lower sileru, vijjeswaram, Kakinada and jegurupadu. Gajuwaka sub-station is connected to upper sileru. Two 1000 mw thermal power stations are expected to come up in the next few years at Visakhapatnam and close to steel plant.

Power is supplied to vsp from apseb switching station over two 220 kv lines on double circuit towers. Power is received at the main receiving station (mrs) located near main gate and further distributed to various units within the plant.

1.3.1 Extra high voltage distribution (220 kV)

Power from apseb is received at main receiving station (mrs). The entire plant is configured as five electrical load blocks and step down sub-stations are provided in each block (designated as lbss 1 to 5) with 220 kv transformers to step down power to 33/11/6.6 kv and for further distribution as indicated below:

Station designation Areas covered

Lbss1 (220/11/6.6 kv) Rmhp, co & ccp, sinter plant, bf

Lbss2 (220/11/6.6 kv) (220/33 kv)

Bf, sms, asp, crmp, comp. House-1, ladle furnace in sms

Lbss3 (220/11/11 kv) Mmsm

Lbss4 (220/11/11 kv) Lmmm, wrm, aux. Shops, adm. Building and kanithi reservoir pump house.

Lbss5 (220/11&220/11/11 kv)

Tpp, plant essential category loads, kbr & township pump houses & hospital.

Mrs (220/33 kv) Plant, township and construction network.

Power is distributed within vsp, between above major blocks and mrs over 220 kv lines on double circuit towers. Mrs and lbss5 at tpp are inter connected by three tie lines for bi-directional power flow. Lbss1 is connected to lbss5 by two radial lines. Lbss2, lbss3 and lbss4 are connected to mrs by two radial lines each.

to ensure continuity of supply and also facilitate maintenance, the stations are connected by double circuit lines. Mrs and lbss5 are designed with double bus (main bus-1, main bus-2) and transfer bus arrangement. At lbss1, 2, 3 and 4 provisions are made so that with only one 220 kv line and two transformers in service, all the loads can be catered to. The equipment installed is suitable for 15000 mva fault level. The various equipment installed in these stations include 220 kv lighting arrestors, current transformers, potential transformers, isolators, sf6/mocb circuit breakers, transformers, aluminium pipes, acsr conductors, insulators structures, relay and control panels, batteries etc.

carrier communication apparatus is provided at mrs to contact any of the apseb stations.

1.3.2 High voltage distribution (33/11/6.6 kv)

Two 220/33 kV transformers installed at MRS feed power to township step down station (called as CPRS) through 33 kV cables. Here the voltage is further stepped down to 11 kV by two nos. Of 33/11 kV transformers. Outgoing feeders from this station supply power at 11 kV through cables to township net work. 11 kv overhead construction power lines are connected to this sub-station.

The 33 kv supply form the transformer at lbss2 feeds sms ladle furnace transformer and capacitor banks through cables. This is a highly fluctuating load and the voltage dips on 220 kv systems can be felt when the furnace is in operation.

Electric power at 11/6.6 kV stepped down at above LBSS stations is distributed to smaller load block distribution station (LBDS) located in each unit. Load centre sub-station (LCSS) transformers which convert power form 11 kV to 415 v; convertor, arc furnace, 11/6.6 kV high voltage load centre (HVLC) transformers and 11 kV motors (above 200 kw rating generally) are connected to lbss/lbds, 11 kv switch boards through circuit breakers. Power supply to essential category loads of various zones is extended form tpp directly from generator switch board (gsb) at 11 kv. Cross-linked poly ethylene (elpe) cables are used for 11 kv distribution.

6.6 kv supply is mostly used for motors (of rating >200 and <200 kw generally). In the zones fed by lbss3, lbss4 stations and also where 11 kv supply from tpp is connected for motor drives, where ever required, step down 11/6.6 kv high voltage load centres (hvlc) are formed with suitable capacity transformers (20/10/6.3 mva capacity). High voltage motor control centres (hvmcc) are fed from 6.6 kv lbss/lbds/hvlc stations. 6.6 kv motors are connected through breaker to lbss/lbds/hvlc/hvmcc switch boards. Some times vacuum contactors are also used for motor switching. Cross-linked poly ethylene cables are used for 6.6 kv distribution.

The 11 kv switch gear is of 1000/750/500 MVA rating. The 6.6 kv switch gear is of 450/350 MVA rating. All the equipment selected is suitable for use with ungrounded system specifications. Zigzag connected transformers are connected to delta connected secondary windings of 220/11/6.6 kv transformers to create neutral point. The 11 and 6.6 kv system neutrals are grounded through resistance to limit the earth fault current to 1000/600 amps respectively. Appropriate protective relays provided to quickly isolate faulty feeders/apparatus with suitable discrimination. All varieties of breakers such as bulk oil/mocb/vcb/sf6/air blast are used.

1.3.3 Medium voltage distribution (below 650 v)

In each shop, to cater to medium and low voltage load, 11 kv/415 v lcss are formed. Power is fed from lcss to motors, mcc, power distribution boards (pdb), lighting distribution boards (ldb), esp transformers etc. And for further distribution. The 3 phase 415 v distribution is solidly earthed neutral system. The transformers are of standard rating 1600/1000/750/630/250 kva in plant and township areas. Pvc cables are used for medium voltage distribution generally. The secondary windings of converter transformers feed dc converters and other special equipment for variable speed ac/dc drives. The secondary voltage of these transformers suits to the particular application for which they are provided.

1.3.4 Low voltage distribution (250 v & below)

low voltage distribution consists of power supply on single phase to lighting equipment, ceiling fans, portable hand tools and domestic appliances etc. For safety reasons, 24 v distribution is also provided to cater to hand lamps etc. In some areas. Pvc cables are used.

1.4 SUPERVISORY CONTROL AND DATA ACQUISITION (SCADA)

the 220 kv 11 kv and 6.6 kv distribution system is monitored by a centralised supervisory control and data acquisition system. The plant generation, import/export and consumption in each unit are monitored through SCADA.

1.5 Essential category loadsSome of the technological process/equipment requires all time

availability of electricity. Such loads are approx. 70 mw and spread over various plant units. These include exhausters in co & ccp, cooling water pump houses in bf, sms, rolling mills, intake pump house, kanithi balancing reservoir pump house, tpp auxiliaries, township pump house, hospital etc. Disruption of supply to these loads may cause wide spread dislocation to the process, involve dangerous situation to equipment etc. These are classified as special/essential category-i loads. Power supply to them is envisaged from two sources i.e. From thermal power plant generator 11 kv switch board through cables and also from 220 kv sub-station in that area. Depending upon level of captive generation, the 220 kv system is so configured that in the event of isolation of captive generators form apseb grid, the load throw off at tpp and disturbance to plant units is minimized.

1.6 Power distribution in power plant220 kv & 11 kv load block substation-5 (lbss-5)

lbss-5 is located outdoor and it consists of 220 kv, 1250 a, 3 phase main bus-1, main bus-2 and transfer bus. Lbss-5 is having 12 bays i.e. Three nos. For the three tie lines, two nos. Of lbss-1 lines, 3 nos. For the three 50/63 mva transformers, one no. For 30/40/50 mva transformer, one for 90 mva transformer, one for bus coupler & one for bypass. Each of the above lines, transformers, bus coupler or bypass breaker can operate on bus-1 or bus-2 through 220 kv isolators 29a or 29b. In case of any difficulty in taking into service any of the 220 kv circuit breakers of transformer or lines as the case may be, bypass breaker can be taken into service in lieu of the defective breaker by charging the transfer bus. Both 220 kv main bus-1 & main bus-2 can be paralleled & transformer feeders (t1, t2, t3, t4 & t5) can be either connected to bus-1 or bus-2 or distributed between bus-1 & bus-2 depending on operational/maintenance requirement. All 220 kv circuit breakers are sf6 breakers. Synchronising facility exists only for tie lines ml1, ml2, ml3, bypass and bus coupler breakers at control & relay panel of lbss-5 located in ecr. Whenever bypass breaker is used in lieu of any of the breakers of lines & transformer feeders, the protections also should be transferred to the bypass breaker through trip transfer switch. The loads (lines or transformers) can be transferred form bus-1 to bus-2 and vice versa live through ‘on load bus transfer scheme (olbt). A typical single line diagram is enclosed.

CHAPTER – 2:

STUDY OF GENERATOR

5 TGs : ( 3 × 60MW + 2 × 67.5MW ).

2.1 Special features:

Electro Hydraulic Turbine Governing System.

Controlled extraction at 13 ata and 4 ata for process steam needs. (Only in TG 1,2&3)

Central admission of steam to reduce axial thrust.

Air cooled Generators.

2.2 Operational limits:

For analysing the operational problems and taking necessary steps, operational

limits of the generator should be known to be operator. If the operates within the limits, the

system will work without any disturbance. These are the possible occurrences of disturbance

due to some fault seen in generator.

Problems are studied to occur at the following conditions:

a. Generator field failure trip.

b. Generator negative phase sequence trip.

c. Overvoltage and overcurrent trips.

d. Fault in static extension equipment and pole slipping trips.

e. Fault in Automatic Voltage Regulator.

f. Stator or rotor temperature high.

2.3 Variation of terminal voltage:

Generator can develop rated power factor when terminal voltage changes

within +/-5% of the rated value i.e. 10.45 to 11.55 KV. The stator current should accordingly

be changed within corresponding values of the MVA outputs and stator currents are also to be

carefully observed. During operation of generator at 110% of the rated value of continuous

operation, stator current should not exceed 4130A corresponding to 105% of the rated value.

2.4 Variation in frequency:

The generator can be operated continuously at output with a frequency

variation of +3 to -5% over the rated value i.e. 47.5 to 51.5 Hz. However the performance of

the generator with frequency variation is limited by the turbine capacity. The variation in

frequency depends on the load and generation.

Generation>demand: high frequency

Demand>generation: low frequency

2.5 Overloading:

Under abnormal condition, generator can be overloaded for a short duration.

Permissible value of short time-overloads in terms of stator and rotor currents and

corresponding duration at rated power factor and rated voltage and rated parameters of cold

air and stator and rotor temperatures can be applied.

2.6 Operation under unbalanced load:

The turbo generator is capable of operating continuously. When unbalanced

system loading is provided, a continuous negative sequence current during this period shall

not exceed 5% of the rated stator current. If unbalance exceeds permissible levels, measures

are to be taken immediately to eliminate or reduce the extent of unbalance within 3 to 5

minutes. If not, the machine trips.

2.7 Synchronization:

A generator requires to be synchronized if it to be run in parallel with others.

Before it is connected electrically to energize bulbar, the following conditions must be

satisfied.

a. Equality of voltage

b. Equality of frequency

c. Synchronization of phases

With these requirements fulfilled, there will be no voltage difference between any

corresponding pairs of terminals of machines and bus bars, so that points can be electrically

connected without disturbance.

2.8 Synchronization procedures:

1. Ensure that the machine has attained the rated speed of 3000 rpm.

2. Obtain clearance from MCR to synchronize the machine.

3. Put the common “SYN SELECTOR” switch to “Manual with check” position.

4. Put the generator “Synch” switch to “Synch’ position.

5. See that bus voltage and frequency appears in “SYNCH TROLLEY”.

6. Give a closing command for the field breaker to close. Observe the “Red lamp” glows on the

control desk indicating the closing of field breaker.

7. Voltage will start building up due to field flashing and it builds up to 6 to 7KV is the

regulation is in manual mode. Then check the voltage of all 3 phases of voltages are not equal

then check the PT fuses and replace the blown one if any. If every aspect is normal, then start

increasing the voltage by giving ‘raise’ command through ‘Regulation manual’ switch in TG

control desk. Raise the voltage till it matches with the bus voltage.

8. If the regulation is in auto, voltage will automatically go to 11KV while raising the voltage in

auto, please observe carefully so that it should not go high abruptly. I if that happens then

immediately change the regulation to manual and adjust the voltage manually.

9. Once the voltage is adjusted, see that frequency of TG is approximately equal to bus

frequency. If difference is there then give impulse to the speed controller by pressing the speed

raise and lower button desk accordingly to bring the frequency approximately equal to bus

frequency.

10. Switch on the “SYNCHROSCOPE” of the frequency of TG is higher than the system

frequency, synchroscope pointer will move in clockwise direction and if the frequency is

lower, it will move in the anti-clockwise direction speed of rotation of pointer will depend upon

the difference in frequencies.

11. When the voltage and frequency match, the synchroscope will move very slowly in the

clockwise direction. This positions shown that:

Phase sequences of generated voltage and system voltage are same.

Effective values of both the voltage are same.

Frequencies of both are same.

12. Give closing impulse to generator breaker the instance when the synchroscope pointer is in

between 11 &12 0’clock position and the red lamp on “Synchronal Trolley” glows

indicating synchronized condition between TG and system.

13. Once TG breaker is closed load the machine from ECR by pressing “Speed raise” button

up to 10-15 MW.

14. Inform MCR and MRS regarding the synchronization of the set.

15. Put synchroscope switch to ‘OFF’ position. Remove the trolley and put back in proper

place. Put TG synchronizing switch to ‘OFF’ position.

16. Observe the voltage of generator and see that the generator delivers lagging MVAR. If the

generator is delivering leading MVAR then make the TG deliver lagging MVAR by

adjusting the excitation.

17. If the AVR is normal mode then adjust the ‘auto’ position till the ‘Null voltmeter’ for A/M

changeover reads ‘Zero’. Then change the regulation to auto

2.9 Asynchroscope operation:

Asynchroscope operation of the generation on field failure is allowed

depending upon the permissible degree of the voltage dip and acceptability of the system from

the stability point of view. During field failure there are important points to be noted.

Field failure with under-voltage

Field failure without under-voltage.

Field failure with under voltage will be sensed and the machine will get tripped without any

delay.

During field failure without under voltage, active load on the generator shall be decreased to

40% of rated load immediately. The generator can operate at 40% of the rated load

asynchronously for a total period of 15 minutes from the instant of failure of excitation.

Within this period, steps should be taken to establish the reasons of field failure to restore

normally. If it cannot be restored then the set has to be switched off. Then the set should

switched over to the reserve excitation.

2.10 Shutdown of generator:

Slowly bring down mechanical input to a minimum level. Then the machine is tripped using

breakers from the grid. Load is also reduced to avoid abnormality i.e.to prevent it from

affecting other systems.

CHAPTER – 3:

SWITCHGEAR AND PROTECTION SYSTEM

3.1 Generator switch board – gsb-1Generator switchboard, gsb-1 is a 11 kv, 4500 amp, 3 section board located at 0 meters in aa’ bay. Each 60 mw generator is connected to one section of the board. The bus is provided with a bus coupler and a bus coupler with 4500 amp reactor between section 1 & 2 as well as between sections 2 & 3. Interlocking is provided to close either bus coupler or bus-coupler with reactor at a time between section 1 & 2 and section 2 & 3. With all 3 generators in service, the bus can be operated only if bus couplers with reactors are in service. Power is evacuated through lbss-5 transformer 1, 2 & 3 which are connected to section 1 and 3 of the board.

All category-i loads of the steel plant i.e. Water supply, bf, sms and coke ovens are connected to the gsb through 11 kv cable. In addition, all tpp auxiliaries are also connected to gsb-1. All outgoing feeders are connected to gsb through link, reactor and mob in addition to earthing switches.

Other than 3 generator operation along with both bus coupler reactors in service, the following combinations are possible:

Bypass bus coupler between any two sections can be closed only if one of the two generators connected to the sections is not in service.

Both bypass bus couplers can be closed only when one 60 mw set is in service.

The typical single line diagram is enclosed.

3.2 Synchronisation

Synchronisation facility exists for any of the incoming generators, 50/63 mva transformer 1, 2 & 3, bus couplers and bus couplers with reactors at 11 kv. The synchronising operation is to be carried out using synchronising trolley in ecr.

3.3 Switchgear

all the generators, incomers, transformer feeders, bus couplers and bus coupler with reactors are provided with 4500 a sf6 gas circuit breakers while all outgoing feeders are provided with 2000 a minimum oil circuit breakers. Interlocking arrangement exists between breakers, links and earth switches. All the disconnecting switches are motor operated (3 phases, 415 v supply) while 220 v dc supply is used for closing/tripping/indication and spring charging motors. Ventilation fans are provided for cooling of reactors in the gsb in addition to general ventilation.

3.4.1 5lbds7 switch board

5 lbds7 switchboard is 11 kv distribution station located in a’a” bay of annexe building. From this board, 11 kv feeders are distributed to various transformer sub-stations 57lc1 to 57lc13 & 57hvlc1, 57hvlc1 ext. These sub-stations cater to power supply requirements of various auxiliary units of power plant. List of load centers feeding from 5lbds7 are

Sno Load center

Transformer Connected loads

Qty, rating (kva)

01 57lc1 3 x 1600 Boiler, tg and tb lt drives and auxiliaries.

02 57lc2 3 x 1600 Common utilities like chp, ventilation & a/c systems, cranes.

03 57lc3 2 x 1600 Ash water pump house

04 57lc4 3 x 1600 Cooling towers

05 57lc5 3 x 1600 Chemical water treatment plant & deaeration plant

06 57lc6 2 x 630 Pump house & lt auxiliaries

07 57lc7 2 x 1000 Chilled water plant – ii

08 57lc8 2 x 1600 Ash slurry pump house

09 57lc9 2 x 1600 Esp-1 & esp-2

10 57lc10 2 x 1600 Esp-3 & esp-4

11 57lc11 2 x 1600 Esp-5

12 57lc12 2 x 1000 Lighting of main machine building and boilers.

13 57lc13 2 x 1000 Pump house-4 extension-cooling towers and auxiliaries.

14 57lc14 2 x 1600 Turbo generator 4 drives and auxiliaries.

there are three bus sections in this board as shown in single line diagram. There are three incomers designated as incomer-i coming from gsb-1 section-i, incomer-ii coming from gsb-1 section-ii, incomer-iii coming from gsb-1 section-iii.

Considering the loads connected to this sub-station, one can understand that this sub-station is lifeline of power plant. One must take utmost precaution while issuing shutdown/normalisation on the incomers as well as outgoing feeders on this sub-station.

3.4.2 05hvlc1 load centre substation

This has a 6.6 kv switch board located in a’a” bay of annexe building. This is a high voltage transformer sub-station provided with 3 nos. Of 11 kv/ 6.6 kv, 16 mva / 20 mva transformers. These transformers designated as transformer-1, transformer-2 and transformer-3 are getting their incoming 11 kv supply from gsb-1 section-1, section-2 and section-3 respectively.

This sub-station is a triple ended sub-station with four bus-sections. Section-1 is fed from secondary of transformer-1, incomer-2 and incomer-3 are fed from secondary of transformer-2 and incomer-4 is fed form secondary of transformer-3. There is a bus-coupler between section-1 & section-2 and another bus-coupler between section-3 and section-4 as shown in single line diagram. This sub-station caters to ht auxiliaries of all five boilers and boiler feed pumps distributed over ht mccs of the boilers. This sub-station is again very critical for stability of power plant.

3.5 Boiler ht mcc power supply distribution

Boiler ht mcc power supply distribution for various boilers are shown in the single line diagrams. From the single line diagram, one can understand that each boiler mcc is having two incomers coming from different sections of 05hvlc1, or two different transformers of the sub-station. Whenever any power supply failure occurs in any section of gsb-1 one section of running boilers get affected depending on the power supply distribution. This causes dislocation in many mcc boards and which causes difficulty in quick normalisation.

Since four boilers are kept in operation, the distribution is so arranged that three boilers will get power supply form one transformer and the fourth boiler is fed from two transformers of 05hvlc1 sub-station. Accordingly feed pump distributions were also so changed and a practice is established to run four boiler feed pumps so that at least one fed pump is loaded on to each of the transformers in 05hvlc1. Boiler lt mcc distribution was also designed to keep an identical distribution with respect to ht mcc. At present, we are keeping the bus-couplers, closed in boiler ht and lt mccs and incomer configurations are changed from time to time depending on the combination of boilers being run. This has made normalisation process easier during power outages.

3.6 Gsb-2 & gsb-3 11 kv switch board

gsb-2 and gsb-3 11 kv switchboard is located in a’a” at bay +0 m in annex building. In gsb-2 switchboard, back pressure turbine station generator no.1 and gas expansion turbine station generator no.1 are connected. In gsb-3 switchboard, back pressure turbine station generator no.2 and gas expansion turbine station generator no.2 are connected. No outgoing feeders are connected in both these board. These generators are connected to system through a three winding 220 kv/11 kv/11kv 31.5/40/50 mva transformer with one 11 kv winding connected to gsb-2 and other 11 kv winding connected to gsb-3. Bpts generators are getting synchronised at 11 kv in bpt station & get generators are getting synchronised at 11 kv at get station. Typical single line diagram is enclosed for the explanation of the system.

3.7.1 57lc1 & 57lc2 substation

out of all the fourteen load centre sub-stations located in tpp area, the most critical and vital sub-stations for stable operation of power plant are 57lc1 and 57lc2. From 57lc1 sub-station the entire auxiliary drives of boiler, turbo-generator and turbo-blower are connected. All the feeders in this board are so arranged that the boiler ht mcc distribution and lt mcc distribution are identical with respect to gsb-1 distribution. This distribution has helped in keeping boiler mcc incomers, dedicated each to one section of gsb-1. Tg and tb mcc incomers are also distributed in this sub-station so that each section is connected to one transformer in this sub-station.

57lc2 sub-station is very critical because incomers of coal handling plant, acdb-1 and acdb-2 are connected to this sub-station. Acdb-1 and acdb-2 boards cater to the common facilities such as acw pumps, phosphate and hydrazine pumps, battery chargers, common valves, elevators etc. This sub-station distribution is identical to 57lc1 sub-station and there is only one 11 kv feeder for corresponding transformer of 57lc1 and 57lc2. All ventilation and air conditioning mccs are also connected in this sub-station.

3.7.2 57hvlc1 sub-station cum mcc

57hvlc1 sub-station cum mcc is an indoor 11kv/6.6 kv triple ended sub-station cum motor control centre located in pump house-4. The above sub-station consists of three nos. Of 16/20 mva, 11 kv/6.6 kv transformers designated as t-1, t-2 and t3. The 11 kv incoming supply for transformer-1 is from section-iii of 5lbds7, transformer-2 from section-i and transformer-3 from section-ii of 5lbds7. The 11 kv supply from 5lbds7 is received at 11 kv isolator in pump house-4 (isolator is an 11 kv mocb with only manual operation) and from isolator supply is looped out through 11 kv cables to primary of respective transformers. Secondary of transformer-1 feeds to 6.6 kv bus section-ii of 57hvlc1. The sub-station has three 6.6 kv incomers and two 6.6 kv bus coupler and three 6.6 kv bus sections (single line diagram enclosed). The out going feeders from this sub-station are:

Re-circulating water pumps of system-1 & 2.

Air compressors

Chilled water plant-ii.

The incomers and bus coupler breakers are minimum oil circuit breakers (mocb) with six poles (two poles per phase) and all the outgoing feeders have 3 pole mocbs. But for the third section incomer and bus-coupler are sf6 breakers. The 11 kv isolators are mocb with manual operation only. The above sub-station is also very critical from operation point of view. Any disruption of supply here results in loss of cooling water to all the equipment, which totally throws power plant operation out of gear.

3.8 Auto mode operation

normally at 57hvlc1 all the 6.6 kv incomers are in on condition and bus couplers are off. In auto mode, in the event of loss of power supply to a particular section due to a fault external to the sub-station, the corresponding incomer trips and bus coupler closes automatically and the drive which would have been decelerating by then restarts. The above sub-station normally operates on auto mode with both incomers on and bus-coupler off. Auto/manual selector switch is located on bus-coupler panel.

3.9 57 hvmcc1

57hvmcc1 is a 6.6 kv motor control centre located in chilled water plant-ii. It is having two 6.6 kv incomers with two 6.6 kv sections and a bus-coupler. Section-i of 57hvmcc1 gets 6.6 kv supply form section-2 of 57hvlc1 and section-2 from section-1 of 57hvlc1. The above mcc caters to 3 nos. Of chilled water supply pumps to steel plant. This mcc is normally kept in manual mode with both incomers ‘on’ and bus-coupler off. All incomers, bus-coupler and outgoing feeder breakers are mocbs. 220 v dc control supply for above mcc is fed from a 100 ah battery and chargers located in cwp-ii.

3.10 Lt distribution of power in mccs

Following supply systems are involved in various auxiliary controls/drives in power plant.

415v supply for all auxiliary drives, all motor operated valves through lt mccs.

220v dc supply for emergency oil pump, emergency lighting, unit critical power supplies, control of ‘x’ valves, interlock and protection of drives not covered under ehtr/hima.

230v ac ups supply for all instrumentation of boiler, turbine and blower including recorders, uc-99/spec 200 and vibration monitoring systems.

24v dc supply for boiler plc, turbine ehtr and blower hima plc systems.

415v motor control centre (mcc) is divided into two sections – each section deriving power form different section of 57 lc1. The distribution of power supply to lt mccs from 57lc1 is shown in the drawing at annexure. It can be noted that all important drives are divided between the two sections and any disruption of control supply or power supply to one section does not cause tripping of equipment provided the standby equipments take over in auto.

Valve mccs are fed by two independent feeders form main mcc from each section with a provision to select either one source. Normally valve mcc is connected to section-i feeder and feeder from section-ii as standby.

2. 220 v dc supply to is used for following functions.

Emergency oil pumps of turbines.

Control supplies to ht switchgear.

Control of ‘x’ – valve functions like:

Auto operation of valve

Tripping of x-valve at limits to avoid over travel.

Interlock & protections not covered under ehtr/hima like (i & p, bhopal).

I. Cep controls - starting interlocks.

- tripping interlocks

Auto starting.

Ii. Valve controls - boiler, tg and tb steam and

drain valves.

- auto operating of drain valves

Hph valves controls

Extraction valves controls etc.

Deaerator controls

Bfp interlocks.

I & p (bhopal) panel is situated at 10.5 mtr. Level in mer. 220 v dc supply is taken form 1600 ah battery/charger system which is known as “pp & bh station auxiliary battery system”. The supply distribution for 220 v dc network is shown in annexure. Two independent feeders from each charger are taken to each of the four boiler1,2&3, boiler 4&5, turbo-generator and turbo-blower dc distribution boards (dcdb). Dcdb is made up of 3 sections interconnected by bus couplers and the extreme section (sec-i & sec-iii) are fed by feeders form the charger. Each section of the dcdb caters to supply requirements of corresponding boiler/tg/tb i.e. Sec-i to boiler-1/boiler-4/tg-1/tb-1, sec-ii to boiler-2/boiler-5/tg-2/tb-2,sec-iii to boiler3/tg-3/tb-3. Any one section can be independently isolated without affecting the other two section.

In addition to the above there are other battery systems installed in power plant as follows:

350ah battery system - for lbss5, gsb1, gsb2, gsb3, 5lbds7 and 05hvlc1 switchgear control supplies.

100ah battery systems - 3 nos. One each in pump house-4, chilled water plant ii and ph4 extention for switchgear control supplies.

170ah battery system - for turbo-generator 4 requirements of eop, emergency lighting, control and protection supplies etc.

3. Schematic of plant ups supply system is as shown in annexure. Two sets of 1700 ah battery and battery charger are provided for supply of 220 v ac uninterrupted power supply (ups) for proper functioning of controls and instruments. The system consists of battery, battery charger, inverter and bypass. Inverter is used for converting the dc output of battery/charger to ac supply and bypass is a servo controlled voltage stabliser, which will take over the load when there is a fault in inverter. The change-over inverter to bypass or vice versa is carried out with in 10 milli-seconds so that no disruption is noticed in the output. Six nos. Of inverters are provided each of the five catering to acdbs located at 6 mtr. Elevation near col.35 of bc bay and the sixth inverter is kept as a stand-by for use during breakdowns / periodic maintenance. The change over form main inverter to stand by a vice versa is carried out without any break in output supplies.

The loads connected to ups are:

‘x’ valve control desk for indication lamps and instrumentation.

Boiler control desks for instrumentation.

Turbine control desks of tg/tb for instrumentation.

Uc99/spec 200 and vibration monitoring systems of all blowers and common control desk cp 1000.

Vibration monitoring systems of tg and boiler fans.

Annunciation panels of boiler/turbine.

Acs control panels of boiler/tg/tb.

3.11 Necessity of Generator protections

In order to generate power and transmit it to customers millions of rupees must be spend on power system equipment. This equipment is designed to work under specified normal conditions however a short circuit may occur due to failure of insulation caused by:

1. over voltage due to switching

2. over voltage due to the direct and indirect lightning strokes

3. Bridging of conductors by birds

4. Breakdown of insulation due to decrease of its dielectric strength

5. Mechanical damage to the equipment

In modern power system it is eliminate faults to a large degree by careful system design. Careful insulation coordination, correct operation and maintenance, it is not obviously possible to ensure cent percent reliability and therefore the possibility of faults must be accepted.

Protective relays are the device that detect abnormal conditions in electrical circuit by constantly measuring the electrical quantities which are different under normal and fault conditions. The basic electrical quantities which may change during fault condition are voltage, current, phase angle (direction) and frequency. Having detected the fault the relay operates to complete the trip circuit which results in the opening of the circuit breaker which results in the opening of the circuit breaker and therefore in the isolation of the fault circuit. A typical relay circuit is illustrated in figure no.1.

The connections are divided into 3 main circuits. First one is the primary winding of current transformer which is connected in series with the main circuit to be protected. The second circuit consists of secondary winding. Third circuit is the tripping circuit, which may be either ac or dc.

Basic requirements of protective relaying

A well designed and efficient protective relaying should have:

1. Speed

2. Selectivity

3. Sensitivity

4. Reliability

5. Simplicity

6. Economy

Speed:-

Protective relaying should disconnect a faulty element as quickly as possible.

This is desirable for many reasons.

a. Improves power system stability

b. Decreases the amount of damage incurred.

C. Lessors annoyance to electric power consumers and decreases total outage time for power consumers.

D. Decreases the likelihood of development of one type of fault into other more severe type.

E. Permits use of rapid reclosure of circuit breakers to restore service to customers.

To decrease the time taken to disconnect the faulty element of the system, high speed protection should be operated in conjunction with high speed circuit breakers. The time interval within which a faulty system element-i disconnected from the system or “clearing time” in the sum of operating time of the protective relaying and breaker interrupting time.

Modern high-speed protective relaying has operating time 0.02 to 0.04 sec. And cb’s have interrupting time 0.05 to 0.06 sec. Hence clearing time may be about 0.07 to 0.10 sec.

Selectivity:-

It is ability of the protective systems to determine the point at which the fault occurs and select the nearest of circuit breaker tripping of which will lead to clearing of fault with minimum or no damage to the system.

Assume a fault to occur at point f1. In this case breakers 9 and 13 should be opened. In fact, opening of any other breaker to clear the fault will lead to greater part of the system being isolated. In case of fault f4, breakers 6, 7, 8 and 9 should be opened and so on.

Therefore every time a fault occur, only those breakers which are nearest to the fault should be opened. This gives us an idea about dividing the power system into protective zones which can be adequately protected with minimum part of the system isolated. Any failure occurring within a given zone will cause the opening of all breakers within that zone. The system can be divided into the following protective zones.

a. Generator or generator transformer units.

b. Transformers

c. Bus bars

d. Transmission lines

e. Distribution circuits

Sensitivity:-

It is the capability of the relaying to operate reliably under the actual conditions that produce the least operating tendency. It is desirable to have the protection as sensitive as possible in order that it shall operate for low values of actuating quantity.

Reliability :-

The protection relaying must be ready to function reliable and correct in operation at all times under any kind of fault and abnormal conditions of the power system for which it has been designed.

Simplicity:-

Simplicity of construction and good quality of the relay, correctness of design and installation qualified maintenance and supervision etc. Are the main factors which influence protective reliability as a rule, the simple the protective scheme and the lesser the no. Of relays, circuits and contacts it contains, the greater will be its reliability.

Economy: -

As with all good engineering design economics play a major role. Too much protection is as bad as too little and the relay engineer must strike a sensible with due regard to practical situation considered.

Types of protections: -

Two types of protection:

1. Primary protection

2. Backup protection

Primary protection:-

Primary protection is the first line of defence and primary relays clear faults in the protected section as fast as possible. 100% reliability is not guaranteed for protective scheme and also for associated ct’s, pt’s and cb’s. Therefore some sort of backup protection must be provided.

Backup protection:-

Backup relays operate if the primary relays fail and cover not only the local station but the next one also and have a time delay long enough for the primary relays to operate if they can.

An example of the remote backup protection is provided by simple time graded relays as shown in figure-3. A fault at f would normally seen by relay r1 and isolated by the cb at c. In the event of failure of the relay or associated equipment at c the fault would be isolated by the operation of the relay r2 cb at b. Hence r2 is the back up relay of r1 and its characteristic is thus shifted up wards. Similarly r3 is the back up of r2.

3.12 Classification of relays:-

Protective relays must be classified depending upon their construction and principle of operation such as:

I. Ordinary electromagnetic relays: - moving plunger, moving iron, attracted armature hinged and balanced beam types. Eg. Such relays are actuated by dc or ac quantities.

Ii. Electromagnetic induction or simply induction relays: - they use the principle of induction motor in their operation. These relays are actuated by ac quantities only.

Iii. Electro thermal relays: - thermal over load protection wing bimetallic strip.

Iv. Physic electric relays: - e.g. Buchholz relay

V. Static-relays: - these will employ thermionic valves, transistors or magnetic amplifiers to obtain the operating characteristics.

Vi. Electro-dynamic relays: - they operate on the same principle as moving coil instrument.

Relays may also be classified depending upon their application:

I. Under-voltage, under-current, and under-power relays in which operation takes place when the voltage, current or power falls below a specified value (mostly instantaneous or induction relays).

Ii. Over voltage over current and over power relays in which operation takes place when the voltage, current or power rises above a specified value (mostly instantaneous or induction relays).

Iii. Directional or reverse current relays in which operation occurs when the applied current assumes a specific phase displacement with respect to the applied voltage and the relay is operated for fall in voltage (induction current relays).

Iv. Directional or reverse power relays: - operation occurs when the applied current and voltage assume a specific phase displacement and no compensation is allowed for fall in voltage (power relays).

V. Differential relays: - operation takes place at some specific phase or magnitude difference between two or more electrical quantities.

Vi. Distance relays: - operation depends upon voltage to current ratio

Relays classified according to time of operation: -

A. Instantaneous relays: - operation takes place after a negligibly small interval of time from the instant of current or other quantity which causes operation.

B. Definite time lag relay: - time of operation is quite independent of the magnitude of current or other quantity which causes operation.

C. Inverse time lag relays: - time of operation is approximately inversely proportional to magnitude of current or other quantity which causes operation.

D. Inverse definite minimum time lag (Idmt relay) :- time of operation is approximately inversely proportional to smaller value of current or other quantities causing operation and tends to definite minimum time if the value increases without limit.

3.13 Protective practices based on the probability of failure:-

protective practices are based on the probability of failure to the extent that present day practices are the results of years of experience in which a frequency of failure undoubtedly has played a part however the probability of failure seldom ever enters directly into the choice of a particular type of relaying equipment except when for one reason or another one finds it most difficult to apply. The type that otherwise would be used. If any event the probability of failure should be considered only together with the consequence of failure should it occur. It has been said that the justification for a given practice equals the likely hood of trouble times the cost of trouble. Regardless of the probability of failure, no portion of a system should be entirely without protection even if it is only backup relaying.

Study of protections provided in generator

Protection of the stator winding:

Phase to phase faults.

Phase to earth faults.

Short circuit between turns.

Open circuit in winding.

Overheating due to inadequate cooling

Phase to phase faults: - circulating current differential system of protection is used to clear faults between phases.

3.14 Types of relays:-

A. Attracted armature type relay.

B. Biased relay.

Differential protection involves the basic principle of merz price differential scheme.

ct’s with suitable values are interposed at both ends of the protected unit and their secondary are connected as shown through pilot wire at the electrical mid pint a relays is connected this relay carries the difference current between ct secondary for normal condition i1 = i2 and (i1 - i2 = 0). For an internal fault i1 and i2 are different (i1 – i2) 0. So relay operates on the magnitude of this difference. It is shown in fig. 4.

3.14.1 Biased relay:-

in this scheme biased circulating current protection is employed such a protection provides a biasing feature which automatically increases relay setting in proportion to the load or through fault current i.e. The relay set operates not at a definite current but at a certain percentage of the through current by suitably proportioning the ratio of restraining coil turns to the operating turns any amount of biasing can be obtained. It is shown in fig 5.

Phase to earth faults:-

Earth faults may cause the melting of laminations due to temperature of high arc currents.

Methods used:-

I. Generator neutral is earthed through a suitable resistance limiting the earth fault current.

Ii. Modern practice is to earth the generator neutral through a distribution transformer with a resistance loaded secondary. In this voltage relay is connected across secondary of loading resistor which is frequency compensated and time delayed to avoid unwanted operation due to 3rd harmonic voltage.

Transverse differential protection: -

This is used to detect inter-turn faults. On external faults no current flows in pilot wire, the ct’s being connected in operation, but on internal faults current flows through both relays. It is shown in fig 6.

Stator turn fault protection - split phase relaying.

Main causes:-

I. Ventilation failure

Ii. Over loading

Iii. Short circuited laminations and failure of core bolt insulation.

Stator is equipped with thermo couples to give temperature alarm.

B. Protection of the rotor

Rotor earth faults.

Inter turn faults.

These faults will cause the vibrational troubles. Sustained unbalance of the stator phase currents characterised by the presence of a negative phase sequence components in the stator current gives rise to local overheating of the rotor mass.

Loss on reduction of excitation:-

Rotor of the effected machine will heat rapidly at high loads. Generator looses synchronism with insufficient excitation for this field failure protection is necessary.

A. By the use of the reactive power measuring relay (mvar relay).

B. Offset mho relay.

These will ensure tripping of the machine for a sustained pole slipping or synchronous running condition.

Sudden loss of load and high speed:-

a turbo generator must be equipped with over speed protection to prevent the rotor attaining dangerously high speed. Generator will attain over speed when:

I. It is disconnected from system

Ii. It is provided of major part of the load.

Backup protection: -

Backup relays are those which will trip a generator in the event of the main protection which would normally clear a fault, failing to operate.

Over current relays:-

Clear fault current in feed to the generator. The group has gone to Visakhapatnam steel plant to study the protections provided 60 mw turbo generators in their captive power plant. Thermal power plant has a total of 4 units’ three 60 mw units and one of capacity 67.5 mw. The protections provided and their settings are as shown in the figure 7.

Generator technical data: -

Type : tn n218226/l

Apparent output : 7500 kw

Active output : 0.8 pf lagging

Rated voltage : 11000 v

Rated current : 492 a

Rated speed : 3000 rpm

Rated frequency : 50 1% hz

Generator phase connection : star

Critical speed of generation : 1900 rpm

Generator differential protection : -

causes:

I. Stator conductor insulation failure due to fault between conductors or between a conductor and the iron core.

Ii. Insulation breakdown due to over voltage and over heating.

Iii. Overheating can be caused by unbalanced currents, ventilation troubles etc.

Iv. Inter turn shorts.

V. Ageing of insulation.

Effects:-

A. Phase shorter circuits are usually accompanied by heavy currents flowing through the point of the fault and leading to the formation of an arc which burns the insulation and current carrying part of the windings. Sometimes melting and fusing the stator laminations.

B. Stator winding faults to frame is a fault to earth, because the stator frame is earthed when flowing to earth, the fault current also passes through the magnetic circuit of the stator, causing severe burns to the steel laminations. Damage to steel lamination requires completed and time consuming repairs.

Principle of operation: -

it works on the principle of ‘merz price differential scheme’. Ct’s with suitable values are inter posed at both ends of the protected unit and their secondary’s are connected as shown through pilot wire at the electrical mid-point a relay is connected the relay carries the difference current between ct secondaries for normal condition i1 = i2 and ( i1 – i2 = 0 ) for an internal fault i1 and i2 are different i1 – i2 0. So relay operates on the magnitude of the difference as shown in figure no. 7.

Generator earth fault protection :-

Causes:-

This relay operates on core balance ct operating on capacitance balance principle. During normal operating condition, line to neutral capacitance charging currents balance and so no current flow to earth. Due to earth fault on any phase, capacitance to earth of that particular phase disappears and earth leakage currents flows in sensitive earth fault relay in ungrounded neutral system, under a single line to ground fault the voltage to earth of the two heating phases rise from their normal phase to neutral voltage to fall line value. Capacitive currents in the two phases increases to 3 times the normal value. The capacitive current in the faulty phase in 3 times its normal value.

Second rotor earth fault: -

The field circuit of our generator comprises the rotor winding. The armature of the exciter, field circuit breaker and connecting cables. This total system is an isolated one and if an earth fault occurs, fault current will not follow with double earth fault, rotor winding turns will get short circuited which decreases the field circuit resistance and increases the current flowing through it. This current over heats the rotor winding and the exciter supplying it, causes further destruction at the points of faults and may lead to burning of the rotor insulation.

On occurrence, of the first rotor earth fault, selector switch sw1 is kept in position 2 connecting coarse control potentiometer across field winding. After making sure that 2nd rotor earth fault relay caem 33 is not operating in the test position, selector switch sw1, selector switch sw1 is turned to position 4, thereby taking caem 33 relay into service. This relay is having a fixed pickup setting of 1ma as shown in figure no.10.

Field failure relay (loss of excitation):-

Failure of field system results in generator losing synchronous speed. It will then operate as an induction generator. The main flux being produced by watt-less stator current drawn from the system. Excitation under these condition require a large reactive component which may be comparable with or even exceeding the normal rating of the generator. When excitation is lost from generator, the flux dies away slowly during which period the ratio of generated to system voltage is decreasing and the rotor angle of the machine is increasing. Results in a decrease of impedance and also a change is power factor.

Mho type impedance relay ycgf observes the change in impedance from the normal load value. This relay may also operate due to rapid changes in system input to the turbine in avr manual mode. If operator fails to adjust excitation. It is shown in figure no. 11.

Effects:-

Abnormal heating of the rotor and overloading of the stator winding. Voltage surges may appear in field winding.

Pole slipping relay (zto): -

a generator may lose synchronising with power system without failure of excitation system, because of severe fault disturbance or operation at a high load with leading power factor and hence a relatively weak field. A system power shock may make a generator rotor oscillate. In case the angular displacement of rotor exceeds the stable limit, the rotor will slip a pole pitch. During this time machine should be isolated form the system.

The relay consists of impedance characteristic enclosing on operating area and observes the change in impedance during power swings. The sequential operation is provided by auxiliary relays tripping being initiated only when the power swing locus enters third zone.

Effects: -

1. Oscillation will create severe vibrations in machine and subsequent damage of machine bearings.

2. If generator breaker fails to open on loss of excitation, machine starts working as an induction generator.

Over voltage – 2nd stage : -

Due to sudden load throw off, reactive magnetic flux in the stator disappears and machine gains speed which will in turn raise generator terminal voltage, such a rise in generator terminal voltage will be taken care by:

Load shedding relay

Automatic speed governor

And automatic voltage regulator

During load thrown off governing system acts and momentarily closes control valves, and cuts admission of steam into turbine.

Effects: -

1. Over voltage may cause stator winding insulation failures.

2. Induced ac voltage in the field winding will cause rotor insulation failures.

Low forward power: -

When a generator set is shutdown in an emergency there is a risk of over speed if the generator circuit breaker opens before the steam valves are completely closed. Even if these actions are simultaneous, steams trapped in the causing of a large turbine may be sufficient to cause over speed. A sensitive low forward power relay retains the generator on load until the set of ‘motoring’ and then operates to open the generator circuit breaker. To prevent operation due to power swings when the machine is being synchronised, definite time delay unit is incorporated. External to low forward power relay. The low forward power relay is sensitive to operate at 0.5% of rated power at unity power factor and less than 1% of rated power at 75o phase angle.

Reverse power relay: -

When a generator operating in parallel with others loses its driving force or failure of prime mover, it remains in synchronism with the system and continues to run as a synchronous motor, drawing sufficient power to drive the prime mover. This condition may not be dangerous to generator but it will cause damage to turbine.

The avr component that the generator was handling before prime move failure changes very little on motoring but the motorising current phase angle changes to as high as 85o which may be either leading or lagging. The protective relay must therefore be a true watt metric device, as the angle compensation used with other directional relay is not permissible here. A single element relay is sufficient as the electrical condition are balanced, unlike those arising from electrical faults. However in bpts, a sensitive polyphase relay cn11 is used which has an operating setting less than 0.5% at unity power factor. A sensitive relay will operate for any power saving severe enough to cause the power component to pass through zero. To avoid unnecessary trippings due to power swings afixed time delay has been introduced to reverse power relay.

Negative phase sequence current relay: -

Negative phase sequence currents results due to:

1. Unsymmetrical short circuits outside generator differential zone.

2. Opening of one or two phases of the circuit connecting the load to the generator.

3. Unbalanced loads.

A three phase unbalanced load produces a reaction field which, to a first approximation, is constant and rotates synchronously with the rotor field system. Any unbalanced condition can be solved into positive, negative and zero sequence components. The positive sequence components is similar to the normal balanced load. The zero sequence component produced no main armature reaction. The negative sequence component is similar to the positive sequence component, except that the resulting reaction field rotates counter to the dc field system and hence produces a flux which cuts the rotor at twice the rotational velocity. Thereby currents in the fields inducing double frequency currents in the field system in the rotor body. The resulting eddy currents are very large and cause severe heating of the rotor. This is shown in fig no.12.

Over current with voltage restrain protection:-

Causes:-

1. External short circuits: - generator supplies fault current higher than rated current. Normally these short circuits are cleared by the protective system of faulty zone. If their protection system fails to respond, generator will carry sustained short circuit current and trips.

2. Under voltage/low voltage frequency operation will result in generator stator currents raise.

3. Loss of excitation will result in the rise of stator current.

4. Operation of generator with leading power factor due to reduced field current will cause stator over current.

Effects: -

1. Sustained short circuits or overloads may increase stator windings core and frame temperatures with increase in temperature, stator conductor’s insulation may fail.

2. Over currents sometimes may cause damage to circuit breakers breaker oil may lose its properties. Figure shown in fig no.13.

Stator frame at 100o c: -

For the protection of the turbo generator against any possible fire accident twelve fire detector relays are provided on either side of the stator winding. These relays have a set of normally open contacts. The set of contacts will close when the temperature surrounding the fire relay exceeds 80o c. The other relay set of contacts close when the temperature exceeds 100o c. These contacts are wired to co2 fire extinguishing system.

Co2 gets discharged due to any of the following causes maintained below:

Contact of the 100o c fire detector closed.

Generator restricted earth fault relay operates.

Generator differential relay operates.

Co2 discharge system activated from turbo generator control test or manual to the box or push button located near the generator.

They will actuate co2 release mechanism atop the co2 cylinder resulting in co2 gas flooding into the generator air ducts.

Under frequency relay: -

Normally for generator, the operation of the machine at 5% of rated frequency is permitted and during this range the kva output of the machine is kept the same. Further reduction of frequency below 5% to as low as 10% require the reduction in kva output of the generator. Normally reduction of frequency is in proportion to the operating speed thus reducing kva at the rated proportion to the square of the speed. The continuous synchronous run of the turbo generator is not permissible during that regime.

Under frequency relay operation results due to reduced generation without corresponding reduction in the load.

Under voltage relay: -

Turbo generator should operate with the variation of voltage within the limit of 5% of rated voltage at rated load, but further reduction of voltage, it is necessary to control the generator stator current so as not to cause the excessive heating while delivering the rated output. Incase the operation of the generator below 95% of rated voltage is required; the load on the machine should be reduced proportionally.

Under voltage relay operates during severe system voltage drop and will trip the machine if the system fault fails to clear within the time delay of 3 seconds.

3.15 Upgradation in generator protection

Multi-function generator protection relay with voltage control (im3g-v):-

3 phase thermal over load voltage restrained over current and unbalance relay suitable for protection of generators. The relay im3g-v measures the true rms value of 3 phase currents and of 3 phase to neutral voltages. By means of proper algorithms the relay computes the positive and negative sequence components of current system. The measuring dynamics of input currents is from 0.01 through 12 times in the functions here below listed are performed:

F1 50v/51v (i>) voltage restrained over current element instantaneous and programmable definite time or inverse time.

f2 50 v / 51 v (i>>) voltage restrained over current instantaneous element and delayed element programmable definite time.

f49 (t>) thermal image with programmable alarm and trip levels.

f146 (1is) current unbalance, i2t = k programmable curves.

f246 (2is) current unbalance instantaneous element and delayed element programmable definite time.

f1 27/59 under/over voltage programmable definite time operation.

f2 27/59 under/over voltage programmable definite time operation.

F 18i under/over frequency programmable definite time operation.

F 28i under/over frequency programmable definite time operation

F 40 alternator loss of field or out-of-step (directional under impedance).

F 32 ( lr >) reverse power

F 37 (p<) under power programmable definite time operation.

Programmable input quantities

Fn = system frequency = (50 – 60) hz

In = rated primary current of phase ct’s = (0-9999) a, step 1 a

Uns = pts rated secondary voltage = (100-125) v, step 1 v

ib = generator rated current = (0,5 – 1,1) ln step 0,1 in

F 49 = thermal image

Rated thermal current: ib

Warming up time constant tc = (1-400) min. Step 1 min.

Temperature pre alarm ta/n = (50-110) % tn step 1% tn

F1 50v/51v (i>) low set over current

If set to on, the actual operation level i> is changed from the set value [ i>] according to the curve.

if set to off, voltage restrain does not operate.

Trip level i> = (1-2,5) ib, step 0,01 ib

Independent time delay: f (i>) = p : t1 >= (0,05 – 30) s, step 0,01 s

Dependent time delay: f (1>) = s1

T = ( 5a-1) t1 > / (i/i>) 9-1 ; t1 >= time delay at 5 x i >

a = 0.02 normal inverse time curve

f 40 loss of field.

Circle offset: k2 = (5-50) % zb, step 1%

Circle dia : k1 = (50-300) % zb, step 1%

Independent time delay: t2f = (0,1 – 60)s, step 0,1 s

Integration time ti = (0-10) s, step 0,1 s

U/v inhibition: v < 0,3 vn

U/c inhibition: i < 0,2 in

f 32 : reverse active power

Trip level: ir >= (0,02 – 0,2) ib, step 0,01 ib

Independent time delay: tlr >= (0,1-60) s, step 0,01 s

F 46 : current unbalance (negative sequence)

Continuous negative sequence current level

1is = (0, 05-0,5) ib, step 0,01 ib

Time/current curves

Time multiplier: KS = (5-80)s, step 1s

Cooling time tcs = (10-1800) s

Negative sequence alarm current level:

2is = (0, 03-0, 5) ib, step 0, 01 ib

Independent delay: t2is = (1-100) sec. Step 1s

Suggestions & conclusion

Multi-function generator protection relay

3 phase multifunction generator protection relay. The relay im30-g measures the true rms value of 3 phase currents and of the neutral current, plus the true rms value of one phase to phase voltage. By means of a proper algorithms the relay computes the positive and the negative sequence components of the current system.

The relay is suitable for current inputs 1a and 5a, voltage inputs adjustable from 100-125 v. The neutral current input circuit of the relay includes a 3rd harmonic active filter.

The functions listed below are performed.

F1 50/51 v (i>) instantaneous element and delayed element programmable definite or inverse time.

F2 50/51 (i>>) instantaneous element and delayed element programmable definite or inverse time.

F1 46 (1is) current unbalance, i2t = k programmable curves

F2 46 (2is) current unbalance instantaneous element and delayed element programmable definite time.

F 64s (0>) machine stator ground fault current

F 32 (ir>) reverse power

F 40 alternator loss of field or out of step (directional under impedance)

Programmable input quantities:

Fn = system frequencies (50-60) hz

In = rated primary current of phase cts : (0-999) a, step 1a

On = rated primary current of earth fault detection ct: (0-9999) a, step 1a

Vns = pts rated secondary voltage : (100-125 v), step 1v

Ib = generated rated current : (0,5-1,1) in, step 0,1 in

CONCLUSIONThe microprocessor based relays are now a days commercially acceptable. It can process and displays signals very efficiently, accurately and in a fastest possible manner. Due to their programmable characteristics microprocessor based relays can be applied extensively in protection of electrical systems. Moreover, one microprocessor unit may be able to perform relaying function of several systems. However the microprocessors should be properly shielded from external influences and the system earthing must be very good from which they receive their control voltage.

REFERENCES1) Switchgear and protection by Badri Ram, second edition, Tata

McGraw Hill Education.2) Theory and Performance of Electrical Machines , J.B.GUPTA, S. K.

Kataria & sons publications.

Study of generator and switchgear Vizag steel plant report

Education

Transcript of Study of generator and switchgear Vizag steel plant report