design, manufacturing, testing, installation … · february 22nd - 23rd, 2017, scope complex,...

February 22nd - 23rd , 2017, Scope Complex, Lodhi Road, New Delhi, India

6th International Conference on Large Power Transformers – Modern Trends in

Application, Installation, Operation & Maintenance

by

DESIGN, MANUFACTURING, TESTING,

INSTALLATION AND COMMISSIONING OF

LARGE RECTIFIER TRANSFORMERS

FEEDING 3.4 MW RECTIFIER UNITS FOR

NUCLEAR RESEARCH APPLICATIONS

DR.R.D.KULKARNI

1

BHABHA ATOMIC RESEARCH CENTRE

[email protected]




2

Outline of Presentation• Working of Typical Nuclear Power Plant

• Simulation of Safety Related R&D Studies in Experimental Facilities

• Specifications of High Power Rectifier Units

• Single Line Diagram of Electrical Power System feeding Rectifier Units

• Schematic Block Diagram of 3.4 MW Rectifier Unit

• Approach for Design of High Power Rectifier Transformer

• Simulation of 12 Pulse, 3.4 MW Rectifier Unit

• Factors considered for the Design of High Power Rectifier Transformer

• Parametric values of rectifier transformer and mineral oil

• Testing methodology of Rectifier Transformer

• Testing, site installation and commissioning of rectifier transformer

• Conclusions




3

Working of Typical Nuclear Power Plant




4

Simulation of Safety Related Studies in Experimental Facilities

3-D view of ITL Phase-II




5

Simulation of Safety Related Studies in Experimental Facilities

Old FRCS

New FRCS




6

Specifications of High Power Rectifier Units• Input: 33±10% kV, 3 Phase, 50±3% Hz, AC

• Output: Maximum 10,000 kA at 340 Volts DC

• Output Power: 3,400 kW

• Current/power stability: ± 0.25% of set value

• Response time: < 100 milli second from 10% to 100% of set value

• Type of control: Secondary control incorporating phase angle control

method using SCR as a control element in a closed loop manner

• Mode of control: CC/CP adjustable from 10% to 100% of rated value

• Type of load: Resistive (Direct heating of Inconel tubes of Fuel Rod Cluster

Simulator (FRCS) of NPP)

• Cooling: Step down Power Rectifier Transformer is ONAN, Inter Phase

Transformer is forced air cooled & SCR assemblies are DM water cooled

• Circuit configuration: Twelve pulse, fully controlled bridge rectifier system

• DC current sensor: Clamp-on type DCCT to sense, monitor, control current

• Power monitor: Incorporation of isolated, true RMS power transducers




7

Single Line Diagram of Electrical Power System feeding Rectifier Units




8

Schematic Block Diagram of 3.4 MW Rectifier Unit




9

Approach for Design of High Power Rectifier Transformer• Rectifier transformer supply DC load currents.

• Secondary windings of rectifier transformers interfaced with rectifier circuits

based on semiconductor devices and thus, generate harmonic currents.

Secondary current is highly discontinuous and flows part of the cycle.

• Transformer primary current is continuous, however, THD exceed the limit

mentioned in IEEE: 519 guidelines and hence need attention.

• Harmonic currents causes overheating, induce voltage stresses, core

saturation and offer serious problems if they are not evaluated properly.

• Design of magnetic circuit parameters- operating flux density.

• Calculation of true RMS magnitude of voltage & current

• Sizing of copper conductor of windings

• Modeling/software simulation of the transformer and rectifier together to

extract the harmonic spectrum of the current.

• Designer use this approach for optimal design of rectifier transformer.

• IEEE standard No. C57.18..10.1998 verifies usefulness of this methodology.




10

Simulation of 12 pulse, 3.4 MW Rectifier Unit




11

Simulation Result of DC Output Current Waveform




12

Simulated Result of Secondary Current Waveform




13

FFT Analysis of Secondary Current Harmonic Content in Secondary Current

Harmonic Order Frequency(Hz)

Magnitude (% of Fundamental)

Fundamental 50 100

3rd 150 0.39

5th 250 19.85

7th 350 13.39

11th 550 8.27

13th 650 6.34

17th 850 4.54

19th 950 3.50

23rd 1150 2.58

25th 1250 1.92

29th 1450 1.35

31st 1550 0.94

35th 1750 0.56

37th 1850 0.32

47th 2050 0.25

49th 2150 0.25




14

Simulated Results of Primary Current Waveform




15

FFT Analysis of Primary Current Harmonic Content of Primary Current

Harmonic Order Frequency(Hz)

Magnitude (% of Fundamental)

Fundamental 50 1003rd 150 0.355th 250 0.337th 350 0.349th 450 0.31

11th 550 8.0613th 650 6.4715th 750 0.2817th 850 0.2719th 950 0.2921st 1050 0.2123rd 1150 2.4525th 1250 0.227th 1350 0.1529th 1450 0.1431st 1550 0.135th 1750 0.5237th 1850 0.3447th 2350 0.2249th 2450 0.27




16

Problems caused due to harmonics

Generally, transformers are designed to operate at rated frequency.

The effects of harmonics on transformers are commonly unnoticed

and disregarded until over heating & pre-mature failure happens.

Transformers that have operated adequately for long periods have

failed in a comparatively short time.

Main causes are:

Overheating of windings

Over - stressing of the insulation of the windings

Core deformation and extra vibration

Skin effect

Nuisance tripping of circuit breaker

Triple-n harmonics currents circulate in delta windings




17

Effect of harmonics on losses




19

Derating of transformer due to presence of harmonics

• To estimate by how much a standard rectifier transformer should bede-rated so that the total loss on harmonic load does not exceed thefundamental design loss. The De-rating Factor can be given by

where:e – the eddy current loss at the fundamental frequency divided by the loss due to a DC current equal

to the RMS value of the sinusoidal current, both at reference temperature.n – the harmonic orderI –the RMS value of the sinusoidal current including all harmonics given by RMS value of the sinusoidal

current formulaIn – the magnitude of the nth harmonicI1 – the magnitude of the fundamental currentq – exponential constant that is dependent on the type of winding and frequency typically 1.5 to 1.7.




20

Life of rectifier transformer handling harmonic currents• To estimate life reduction of transformer, the deterioration rate of its insulation materials has to be

considered, too. About fifty per cent of life reduction of transformer is caused by thermal stresses created by non-linear load

• The most important factor in life reduction of transformer is hotspot. The temperature of the winding is calculated from the following equation-

• Where θHS = Hot spot temperature• θA = Ambient temperature• θTO = Top oil rise Over ambient temperature under non-linear load current• θTO-R = Top oil rise under rated condition• θg = Hot spot rise over top oil temperature under non linear load current• θg-R = Hot spot temperature rise over top oil rise under rated condition• PLL = Load loss under Non linear condition• PLL-R = Load loss under rated condition• PNL = no load loss




21

Life of rectifier transformer handling harmonic currents

• The maximum allowed current for decreasing rated power and capacityof transformers with harmonic load can be determined that this actionbe called de-rating for estimate reduced transformer life, degradationrate of insulation material must considered. 50% reduction oftransformer life is due by thermal stress is caused by harmonic currents.Most important factor in reducing the life of transformers is θHS pointtemperature. Reduction of life and real life of a transformer can becalculated from the following relations.

• Real life = Life (pu) × normal insulation life




22

Factors considered for design of rectifier transformer • Current density

• Operating flux density

• Sizing of core cross section

• De-rating capacity of the transformer

• Transformer utilization factor

• Provision of electrostatic shield between primary & secondary

windings

• Limit on temperature rise of winding

• Adequate fixing/bolting of core to take care of vibrations, etc.

• Individual ratings of primary & secondary windings

• Calculation of losses and efficiency

• Use of appropriate insulation materials

• Consideration of skin effect at harmonic frequency

• Estimation of life of the transformer




Parametric values of rectifier transformerParameter Description Value

Number of phases 03 nos.

Rated voltage of high voltage winding (Primary winding) 33 kV ± 10%

Rated voltage of low voltage winding (Secondary winding) LV1 310.2 V

Rated voltage of low voltage winding (Secondary winding) LV2 313.83 V

Rated frequency of both primary and secondary side 50 Hz ± 5%

Rated capacity of high voltage winding (Primary winding): Delta connected 4235 kVA

Rated capacity of low voltage winding (Secondary winding) LV2: Star connected 2192 kVA

Rated current of high voltage winding (Primary winding) 74.09 A

Rated current of low voltage windings (Secondary windings) LV1 and LV2 4080 A

Insulation Class ‘A’

Short Circuit Impedance (Z) 7.49%

Vector Group Dd0y1

Ambient Temperature 450 C

Relative Humidity 95% Non-condensing max.

Guaranteed Maximum Temperature Rise of Oil and Windings 350 C & 450 C

Cooling method ONAN

Max. No Load Losses 5.2 kW

Max. Copper Losses 32.5 kW

Total Load Losses in Rectifier Operation 45 kW

Weight of Core & Winding (Un-tanking Weight) 8450 Kgs.

Transformer (Mineral) Oil (As per IS: 335-2005) with Quantity 4900 Liters (4350 Kgs.)

Total Weight of Transformer 19100 Kgs.

List of applicable codes, standards and specifications IEC 60076, IS 2026

Dimensions 3750 (W)×3570 (D)×3825(H)

List of Accessories As per GA drawing




24

Parametric Values of Mineral (Transformer) Oil of Rectifier TranformerSr. No. Parameter

(Test Description)

Test Value as per IS: 1866-2000 Test Method

Required Actual

1 Appearance -- ---- Clear Visual

2 Density at Room Temperature g/cc 0.89 Max. 0.8150 IS 1448 (P16) - 2007

3 Kinematic Viscosity at 270 C cSt 27 Max. 11.28 IS 1448 (P25) - 2007

4 Flash Point (PMCC) 0 C 140 Min. 147.5 IS 1448 (P21) - 2007

5 Pour Point 0 C -6 Max. -27 IS 1448 (P10) - 2007

6 Total Acid Number mg KOH/g 0.03 0.03 IS 1448 (P1 Sec 1) - 2007

7 Inorganic Acidity mg KOH/g ---- Nil IS 1448 (P2) - 2007

8 Water Content ppm 20 Max. 13.5 ASTM D 6304 - 04

9 Corrosive Sulphur % by Wt ---- Non corrosive ASTM D 1275

10 Total Sediments % by Wt 0.1 Max. Nil Centrifuge Method

11 Break Down Voltage kV 40 Min. 60.7 ASTM D 877

12 Interfacial Tension at 270 C mN/m 35 Min. 45 IS 6104 - 1971

13 Dielectric Dissipation Factor (Tan δ) at 900 C -- 0.015 Max. 0.0081 IS 6262 - 1971

14 Specific Resistance at 270 C Ω-cm ---- 1678.05 × 1012 IS 6103 - 1971

15 Specific Resistance at 900 C Ω-cm ---- 62.15 × 1012 IS 6103 - 1971

16 Dissolved Gas Analysis (DGA) Obtained Value (ppm)

IS 10593 - 2006

&

IS 9434 - 1992

a) Hydrogen (H2) Nil

b) Oxygen (O2) 5934

c) Nitrogen (N2) 16748

d) Methane (CH4) Traces (<1)

e) Ethylene (C2H4) Nil

f) Ethane (C2H6) Nil

g) Acetylene (C2H2) Nil

h) Propylene + Propane (C3H6 + (C3H8) Nil

i) Carbon Dioxide (CO2) 107

j) Carbon Monoxide (CO) 2




25

Testing Methodology of Rectifier Transformer• Type tests of transformer:

Transformer winding resistance measurement

Transformer ratio test

Transformer vector group test

Measurement of impedance voltage/short circuit impedance and load loss

(Short circuit test)

Measurement of no load loss and current (Open circuit test)

Measurement of insulation resistance

Dielectric tests of transformer

Temperature rise test of transformer

Tests on on-load tap-changer

Vacuum tests on tank and radiators




26

Testing Methodology of Rectifier Transformer

• Routine tests of transformer:

Transformer winding resistance measurement

Transformer ratio test

Transformer vector group test

Measurement of impedance voltage/short circuit impedance and load loss

(Short circuit test)

Measurement of no load loss and current (Open circuit test)

Measurement of insulation resistance

Dielectric tests of transformer

Tests on on-load tap-changer

Oil pressure test on transformer to check against leakages and gaskets




27


• Special tests of transformer:

Dielectric tests

Measurement of zero-sequence impedance of three-phase transformers

Short-circuit test

Measurement of acoustic noise level

Measurement of the harmonics of the no-load current

Measurement of the power taken by the fans and oil pumps

Tests on bought out components/accessories such as buchhloz relay,

temperature indicators, pressure relief devices, oil preservation system, etc.




28


• Tests to be done at user site:

Pre-commissioning tests: The transformer also goes through some other

tests, performed on it, before actual commissioning of the transformer at

site. The transformer testing performed before commissioning the

transformer at site is called pre-commissioning test of transformer. These

tests are done to assess the condition of transformer after installation and

compare the test results of all the low voltage tests with the factory test

reports.

Periodic/condition monitoring tests: These tests include the periodical

checking the health of the transformer and monitoring the signatures of

various parameters of the transformer.

Emergency tests: There are especially carried out during the breakdown

of the transformer as well as during the observance of some of the

abnormality in the parametric value of the transformer.




29

Tests performed on Rectifier Transformer at factory• Routine Tests: Visual Inspection for quality of workmanship, finish, dimensional checks, etc.

Transformer oil testing as per IS: 1866 and IS: 335

Winding Temperature Indicator / Oil Temperature Indicator Electrical and Calibration Tests

Gas Operated Relay and Pressure Relief Valve Electrical Tests

Insulation Resistance Tests and Measurement of Polarization Index

Measurement of Winding Resistance (As per IS: 2026 - Part 1, Clause 16.2)

Turns Ratio Test (As per IS: 2026 - Part 1, Clause 16.3)

Magnetic Balance Test

Capacitance and Tan Delta Measurement

Induced Over Voltage Withstand Test (As per IS: 2026 - Part 3, Clause 11.1 and 11.2)

Vector Group Test for Dd0y1 Transformer

Dielectric Test (As per IS: 2026 (Part 1- Clause 16.7 and 16.6)

Separate Source Voltage Withstand Test (As per IS:2026(Part3-Clause 10.1,10.2,10.3,10.4&10.5)

Measurement of No Load Losses and Current (As per IS: 2026 - Part 1, Clause 16.5)

Measurement of Voltage & Current Harmonics at No Load (as per IS:2026-Part1,Clause 16.13)

Measurement of Acoustic Noise Level (As per IS: 2026 - Part 1, Clause 16.12)

Measurement of Impedance Voltage/Short Circuit Impedance & Load Losses (As per IEEE

C57.18.10-1998 and IS: 2026- Part 1, Clause 16.3)

96 hours No Load Test

Oil Leakage Test as per CBIP Standards




30

Testing performed on Rectifier Transformer at factory

• Type Tests:

Heat Run (Temperature Rise) Test (As per IS: 2026- Part 2): The heat

run test has been carried out on the rectifier transformer by

simulation of copper loss for minimum period of 2 hours after the

stabilization of temperature (variation within ± 10 C in one hour) at all

measuring locations including hot spots. The locations for the

temperature measurement have been mutually decided between the

supplier and the user. The temperature at any location has not

exceeded the design value.

Impulse Voltage Withstand Test




31

Assembled view of rectifier transformer without oil tank




32

Assembled view of rectifier transformer with oil tank




33

Installation, on site testing and commissioning of Rectifier Transformer

• Following low voltage routine tests have been performed on site as

part of commissioning processes. Polarity, phase angle displacement and phase sequence

Voltage ratio

Winding resistance

Windings and core insulation resistance measurements (Megger test)

No-load losses and excitation current at 90%, 100% and 110% of rated voltage (AC

separate source)

Load losses with reduced current (separate AC source)

Short-circuit impedances

• The following HV tests have been performed on site: AC applied voltage

Longtime induced voltage (one hour) with electrical and acoustic monitoring and

measurement of Partial Discharges and voltage level up to 150% of rated voltage

feed by a separate controllable source

No-load energization with rated voltage during 24 hours with electrical and acoustic

monitoring and measurement of Partial Discharges and voltage level up to 115% of

rated voltage feed by a separate controllable source.




34

Conclusions

Rectifier transformer has been designed carefully determining various design

parameters by performing combined simulation.

Due to presence of harmonics, the parameters like current density, magnetic

flux density, selection of transformer core, sizing, etc. have been worked out.

Overcoming satisfactorily the issues of overheating, over fluxing, core

saturation, voltage stresses, losses & efficiency, etc.

Manufacturing of rectifier transformer referring design data sheet derived from

the specifications.

Factory tested to evaluate the performance which was found satisfactory.

Rectifier transformer has been installed at the R&D site considering standard

guidelines and safety practices.

On-site testing carried out to confirm the technical specifications.

First charging of rectifier transformer at no load was carried out successfully

at site following the check list.

Full load commissioning is performed gradually step by step manner up to the

rated capacity and the performance has been found satisfactory.




35

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