Marine and Offshore Report Final

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By Students:

Hasala Dharmawardena

Praveen Shrestha

Derenik Gemalmazyan

https://www.facebook.com/prvnsrta14

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Table of Contents

1. Introduction .............................................................................................................................. 3

1.1Task Description ................................................................................................................... 3

1.2 Objective............................................................................................................................. 3

2. System description and component data ................................................................................... 3

2.1 System description............................................................................................................... 3

2.2 Component data .................................................................................................................. 4

3. Power flow calculations ............................................................................................................. 5

3.1 2 GTs running. ..................................................................................................................... 5

3.2 3 GTs running. ..................................................................................................................... 5

4. Characteristics for induction machines ....................................................................................... 6

5. Short circuit calculations ............................................................................................................ 7

5.2 Tasks ................................................................................................................................... 7

5.3 Influence of Automatic Voltage Regulator (AVR) ...................................................................17

5.4 Questions ...........................................................................................................................18

5.5 Calculation of DC offset of the short circuit current:..............................................................19

5.6 Calculation of k factor of the short circuit current .................................................................21

6. Short circuit calculations by hand ..............................................................................................21

6.1 Calculation of short circuit current for generator ..................................................................21

6.2 Calculation of k-factor .........................................................................................................23

7. Motor starting analysis .............................................................................................................24

8. Transient Stability.....................................................................................................................24

8.1 Contingency Analyses..........................................................................................................24

8.2 Critical clearing time ...........................................................................................................25

9. Optional task............................................................................................................................26

9.2 Power supply from shore - voltage control via converter .......................................................26

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1. Introduction

1.1 Task Description This is the report for Mini-project in the course TET 4200 Marine and Offshore Power Systems. The

tasks include the common part and the optional part.

The common part is an electrical power system for an offshore platform. In this system four tasks

should be done, which including:

Power flow simulation

Short-circuit analysis by both hand calculation and simulation

Motor starting analysis

Contingency analysis

In the optional task, task B Power Supply from Shore-Voltage Control via Shore Converter has be

done.

The basic tool used in the mini-project is Power Factory-DigSILENT.

1.2 Objective The objective of the mini-project is to increase the physical understanding of the phenomena and

challenges that are addressed in the lecture in TET4200.

Furthermore is to get familiar with the electrical system analysis tool - Power Factory-DigSILENT.

Learn to analysis the physical phenomena in an electric power system may meet.

2. System description and component data

2.1 System description The electrical power system to be analyzed in part 1 of the Mini -project (electrical system of a

thought offshore platform) is depicted in Figure 1. The system consists of thre e turbine/generator

sets, eight induction machines, three passive loads, three transformers, two cables and breakers.

There are three different voltage levels in this system: 13.8, 6.0 and 0.44 kV, respectively. The rated

frequency of the system is 60 Hz.

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Figure 1 Single line diagram of electrical system on offshore platform

System characteristics:

Three-phase system

Frequency: 60 Hz

Voltage levels: 13.8, 6.0 and 0.44 kV

About grounding of system and components’ neutral: The generators are earthed via

resistance. In different tasks there will be different requirement and will be shown in the

specific tasks.

In normal operation of the system two of the three turbine/generator sets are running. In some of

the analyses, G/T set No 3 is also to be put into operation.

2.2 Component data See the Appendixes 2.

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3. Power flow calculations

3.1 2 GTs running. The complete SLD for the PF calculation, in normal production (2GT) is available in

Appendix A.

Figure 2 Overloaded GTs in Normal Production

As per the above figure, which shows a part of the SLD for PF, the GTs are getting overloaded if

operated under normal production in normal loading condition. Thus in this setting it is not possible

to operate the plant continuously. It is recommended to upgrade the GT capacity by at least by 10%

to facilitate a design margin and increase the flexibility of operation.

3.2 3 GTs running. The complete SLD for the PF calculation, in normal production (2GT) is available in

Appendix B.

Figure 3 3 GTs in Normal Production

As per above figure taken from the SLD for 3 GTs running case the generators are running within

operation margins. The only recommended operation mode is 3 GTs running in parallel.

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Figure 4 Undervoltage on bus 5

The above figure, taken from Appendix B, shows that Bus 5 is at a voltage less than 95%. It is

recommended to use a -5% tap in T4-5 tap changer to increase the bus voltage.

Figure 5. Bus 5,After changing T4-5 Tap to -5%

The simulated results show that it is possible to achieve a recommended voltage of 0.98 pu at the

bus.

4. Characteristics for induction machines

The torque speed and current speed characteristics generated via simulations is as follow:

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Figure 6 Torque speed characteristics & Current speed characteristics:

5. Short circuit calculations

5.2 Tasks

a) Three phase faults (symmetrical faults) shall be studied for the following cases:

i) Condition:

1 G/T set running without load. A three phase fault occurs on bus 1.

Figure 7 RMS Simulation for short-circuit 1 GT

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Figure 8 EMT Simulation for short-circuit 2 GT

The calculation above shows that the maximum RMS and instantaneous value of the fault

current is 4.251kA and 11.392 kA respectively at 0.995 second.

ii) Condition:

2 G/T sets running without load. A three phase fault occurs on bus 1.

Figure 9 RMS Simulation for short-circuit 2 GT

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Figure 10 EMT Simulation for short circuit 2 GT

The plot above shows that the maximum RMS and instantaneous values of the fault current

is 8.501 kA and 23.212 kA at 0.995 second.

The comparison of fault current for the same fault condition but with different numbers of

identical generator sets in action shows that, the fault current increases with the increase in

number of generator sets in action. In ii) the fault current is nearly double that in i).

iii)

Condition:

2 G/T sets running with normal load. A three phase fault occurs on bus 1.

Figure 11 RMS Simulation short-circuit 2 GT normal load (bus 1)

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Figure 12 EMT Simulation short-circuit 2 GT normal load (bus 1)

The plot above shows that the maximum RMS and instantaneous value of the fault current

for generator G1 is 4.083 kA and 11.669 kA.

With the load connected the fault current increases then with no load condition.

Condition:


Figure 13 RMS Simulation short-circuit 2 GT normal load (bus 4)

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Figure 14 EMT Simulation short-circuit 2 GT normal load (bus 4)

The plot above shows that the maximum RMS and instantaneous value of the fault currents

are 6.150kA and 15.713 kA at 0.995 second.

Condition:


Figure 15 RMS Simulation 3 Phase fault on bus 5

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Figure 16 EMT Simulation 3 Phase fault on bus 5


are 26.267kA and 64.336 kA at 0.995 second.

b) Two-phase faults:

Condition:

2 G/T sets running without load. A two phase fault (phase to phase fault without ground

connection) occurs on bus 1.

Figure 17 RMS Simulation 2 Phase fault

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Figure 18 EMT Simulation 2 Phase fault


are 6.708 kA and 21.581 kA at 0.995 second.

This is smaller than the three phase fault on bus 1 for the same condition.

c) Single-phase-to-ground faults shall be studied for different cases:

From the NORSOK Standard E-001, maximum current must be limited to 20A RMS per generator.

The zero sequence current in this context means I0, that is the zero sequence current flowing in one

phase of the synchronous machine. If it is assumed that the synchronous generator is the only

system component that is involved in the zero sequence system, then the current in the generator

neutral is 3*I0.

Condition : 1-phase fault on bus 1

Figure 19 Earth fault RMS solidly grounded

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Figure 20 Earth fault RMS grounded via resistance

Plot above shows that the 1-phase fault on bus 1 causes 3.346 kA current to flow through the solidly

grounded neutral. When the grounding resistance of 398.3 ohms is kept on both generators G1 and

G2 the RMS fault current is limited to 20 A.

Condition: 1-phase fault on bus 3

Figure 21 Neutral current in G1 for earth fault in bus 3

In the case of 1-phase fault at bus 3 the generator neutral current is further decreased to 0.011 kA.

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d) Three phase faults (symmetrical faults) - instantaneous value calculation:

Condition:


Figure 22 2 GT with normal load chort-circuit on bus 1

The plot above shows that the maximum peak value of the fault current is 64.209 kA at 0.995

second.

e) Contribution from the induction machines to the resulting fault current:

Condition:


Figure 23 Total Fault current

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Figure 24 Fault current provided by one generator.

The contribution from the induction machines to the resulting fault current can be calculated by

subtracting the total fault current by the fault current provided by two generators. i.e. 11.889 kA –

2*4.083 =3.723 kA.

Figure 25 Fault Contribution from Induction Motor M1.

Condition:


The contribution from the induction machines to the resulting fault current is 6.15 kA – 2 * 1.195 kA

= 3.76 kA.

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Figure 26 Fault Contribution from Induction Motor M5

5.3 Influence of Automatic Voltage Regulator (AVR) 2 G/T sets running with normal load. A three phase fault occurs on bus 1:

With AVR for the synchronous generators, the result we got for the condition:

Figure 27 EMT Simulation short-circuit at bus 1 with AVR

Without AVR for the synchronous generators, the result we got for the condition:

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Figure 28 EMT Simulation short-circuit at bus 1 without AVR

Due to three phase short circuit fault, the terminal voltage of the generator decreases. Without AVR

no control is provided to boost the terminal voltage. When AVR is connected, it tries to boost up the

terminal voltage by increasing the internal voltage. Due to this the steady state fault current

increases when AVR is put in service than without AVR.

5.4 Questions

1. It can be ensured that the stationary short-circuit current is minimum 3*IN for generators by

increasing the upper limit of the saturation zone of AVR.

2. In figure 1, breakers are shown on all incoming and outgoing feeders ref. the different bus bars.

Assuming that a short circuit occurs on the load side (downstream) of the breaker of the outgoing

feeders:

a) For a fault on the downstream of bus 1, at the load P1,Q1 , it will be fed by all the induction

machines while for other faults, they will be fed by one induction machine less. Hence, from

bus 1 downstream, the CB of the load P1,Q1 will carry the highest short circuit current.

b) For a fault on the downstream of bus 4, at the load P2,Q2 , it will be fed by all the induction



c) For a fault on the downstream of bus 5, at the load P3,Q3 , it will be fed by all the induction



d) When the fault appears on the bus-bar side of the circuit breaker only one generator feeds

the fault through the breaker. When the fault occurs on the generator side of the breaker

other remaining generator feeds through the breaker. So the breaker rating should be

selected when the fault occurs on the generator side when all 3 generators are in operation.

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5.5 Calculation of DC offset of the short circuit current:

Case a) iii)-2 G/T sets running, normal load, three-phase symmetrical fault on bus 1 (RMS-Simulation)

Figure 29 RMS short-circuit plot for G1

Figure 30 EMS short-circuit plot for G1

This plot above shows the total current short circuit current, including both the DC and AC current

components.

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Tabulating the envelope

Time (sec) Lower Value

(Amp) Upper Value

(Amp) Average (DC offset) Amp

1,00 0,00 34,66 17,33

1,10 2,61 28,94 15,78

1,20 1,38 15,47 8,43

1,30 -2,68 12,00 4,66

1,40 -3,86 9,50 2,82

1,50 -4,47 7,96 1,75

1,60 -5,06 6,67 0,80

1,70 -5,37 6,30 0,47

1,80 -5,53 6,07 0,27

1,90 -5,58 5,92 0,17

2,00 -5,64 5,82 0,09

2,10 -5,66 5,76 0,05

2,20 -5,67 5,72 0,03

2,30 -5,66 5,69 0,01

The dc component current is drawn below.

Figure 31 DC component of G1 fault current

0

2

4

6

8

10

12

14

16

18

20

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3

Time

(A)

(S)

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5.6 Calculation of k factor of the short circuit current

The maximum value of the total s/c current is 34.532 kA at time 1.007 seconds. At this time the

maximum RMS value of fault current is 11.889 kA.

Kappa factor is given by the relation:

√

√ = 2.021

6. Short circuit calculations by hand

6.1 Calculation of short circuit current for generator

In the hand calculation, for the simplifying, armature resistance is neglected.

i ) 2G/T sets running, no load condition, three symmetrical fault on Bus No 1.

Since the fault located at Bus 1, also the terminals of the Gen1 and Gen2, so the fault current can be

limited to the generator current only.

The current base value is

√

√

According to

When there is no load connected the current will be 0. Therefore the pre -fault sub-transient

voltage , the initial AC current (in RMS) will be

The real value of the fault current is

The hand calculation above shows that the maximum RMS value of the fault current is 4.445 kA.

In the above simulation for 5.2 a ii) the RMS value of short circuit current for one generator is

. They are close to each other. And the reason the hand calculation

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answer is a little bit higher than the simulation is that during hand calculation is considered zero

and there is no decay but in the simulation is a non-zero value.

( ) √

Where

(

)

Therefore maximum current observed at 8.33 ms and is of magnitude,

( √ )

ii ) 2G/T sets running, normal load condition, three symmetrical fault on Bus No 1.

For the normal load condition, each generator supply power

Choosing the generator base value 17 MVA

The pu value of is

The normal load condition current is

( )

( )

( )

In the dq plane the angle of the current refer to the q-axis is

Then according the figure shown (

)

(

)

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√

The real value of

√

This is the short circuit current in one generator, for two generators the short circuit current (also the

short current on the Bus1) is

√ 9.38 kA

The hand calculation above shows that the maximum RMS value of the fault current supplied by

generator is 3.38kA. The maximum RMS current calculated from the simulation is 4.083 kA which is

slightly higher than the hand calculated value. Also the maximum fault current during normal load

condition is lower than during no load condition.

6.2 Calculation of k-factor No load values are used for evaluation of the theoretical k-factor for this system. It will be interesting

to see how the literature values compare with the hand calculated and simulated k-factor values for

this case for we can get a feel of how correct they will be when used in a rough calculation and how

much extra design margin will be gained by using them.

√

√

According to the simulation result the kappa factor is 2.021, which nearly matches the hand

calculated one.

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7. Motor starting analysis

The results for the motor starting analysis conducted for the Motor M1 is attached in the Appendix D

(both full load and no load cases).

The key characteristics of interest derived from the detailed results are as per below.

No load Full load Lowest Voltage in pu 0.78 0.78

Start up time in s 4.8 8.5 Maximum Frequency deviation in Hz

1.2 2

As per the simulation results (Appendix D, E) no significant change of speed in the other machines

are observed for M1 motor starting period.

In order to compensate for the voltage dip, a capacitor bank of 30 MVar is connected to bus 1 during

the startup time period. Both pre and post capacitor bank starting results are attached in Appendix

F, G (two graph sheets). The average reactive power drawn by M1 in the pre-capacitor bank analysis

is used as the basis for dimensioning the starting capacitor. The voltage dip at start up becomes

negligible with capacitor start up, however a high transient is observed just after M1 startup which

points to the importance of using a fine tuned controller starter controller for this system.

8. Transient Stability

8.1 Contingency Analyses For a 2 G/T sets running (normal operation), normal load, the contingency analysis is done by

disconnecting motor M1.

There is almost no voltage variation.

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Figure 32 Generator bus voltage for disconnection of M1

The frequency variation is: 1.576 Hz (increase)

Figure 33 Bus frequency for disconnection of M1

8.2 Critical clearing time Critical clearing time is dependent on the large machines of the system not losing speed after fault is

cleared.

The simulations results for both pre and post critical clearing fault clearance cases are attached in

Appendix H, I.

The estimated critical clearing time is 35 ms.

The critical clearing time for operation with 3 GTs is 50 ms and the simulation results for both pre

and post critical time is attached in Appendix J, K.

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9. Optional task

9.2 Power supply from shore - voltage control via converter A transmission voltage of 24 kV is selected for shore to platform connection. The resulting power

flow for 100% loading is attached in Appendix L.

Some key parameters of interest are below.

On Offshore platform On shore

Voltage in pu 1 1.3

Reactive power consumption in MVar

21 32

However using this method motor M1 will not start as observed from simulations attached in

Appendix M.

The Onshore voltage variation at motor M1 start up is attached in Appendix N. The stalling motor

will draw locked rotor current resulting in a voltage collapse as observed in the current time

characteristic for the cable.

As a solution to this problem it is suggested to increase the Offshore main bus voltage to 120% for a

time period of 40 seconds taken for motor start up. The simulation results, shown in Appendix O

show a slow but successful motor starting.

The other solution is to connect a starting capacitor which will provide a successful startup as

observed in previous task.

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Ap

pen

dix A

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Ap

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dix B

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Ap

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dix C

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Ap

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dix D

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Ap

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dix E

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Ap

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dix F

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Ap

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dix G

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Ap

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dix H

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Ap

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dix I

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Ap

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dix J

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Ap

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dix K

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Ap

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dix L

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Ap

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dix M

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Ap

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dix N

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Ap

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dix O

Marine and Offshore Report Final

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