DESIGN OF 31-BUS INTERCONNECTED INTERCONNECTED POWER SYSTEMS.
Felani Stefanzie Choe1, Azizan Mohd Maharudin2, Mohamad Na’im Mohd Nasir 3, Muhamad Faizal Abd Razak 4 Master student Of Electrical Engineering Faculty, University Technology of Malaysia, Skudai, Johor . Abstract - This paper presents, a way of designing a 31-bus interconnected power systems. The main objective for the design is to meet the certain specification determined by the assignment for Power System Control. This paper will cover the design process, calculation process, simulation using power world software and the results. In the design process, it will cover the steps taken to choose the tower, conductor and transformer types. Calculations have been done to determine the value of resistance, inductance and capacitance for the lines. In the simulation part, the result is shown and discussed. 1.0
INTRODUCTION
The rapidly increasing transmission voltage in recent decades is a result of growing demand in electrical energy. The purposed of transmission network is to transfer electric energy from generation to distribution systems which ultimately supply power to the load. The excellent performance of transmission line might have high efficiency which it have no or low power losses, high reliability which it can carry high power and safe and economic system. Several aspects must be considered in design transmission lines which are load capacity, power and voltage transfer capacity that will carry, size of cable, types of tower, the distance of transmission line and so forth. This report described the design of a 31 bus interconnected interconnected power system. This design contains 31 busses with different voltages, 11 transformer, 4 generators and different loads at certain busses.
2.0
DESIGN PROCES
The selection of tower, transmission line conductor and transformers is based on several assumption and the specifications given.
2.1
Types of tower
The selection of tower is an important aspect in design the power system. The line inductance and capacitance value is depending on the selected tower of the system. There are several consideration must be taken into account in selection of tower for the system which are voltage rating, distance, environmental and so forth. Five types of tower that were used which are 69 kV H-frame and 69 kV double circuit steel tower for 69 kV transmission line voltage rating, 115 kV single circuit steel tower and 115 kV double circuit steel tower for 115 kV transmission line voltage rating and 230 kV double circuit steel tower for 230 kV transmission line voltage rating. Figure 1 shows the types of tower that were used in design the power system. Single circuit steel tower is used for transmission line distance less than 50km because it will reduce the cost of transmission line system. While, double circuit steel tower is used for transmission line distance more than 50km because it is good for the expansion plans in the future and it can carry more power.
2.2
Conductor type.
The selection of transmission conductor is other aspect aspect the must be taken into account. account. The value of line inductance and resistance is also depending on the selected cable. Table 1 shows the conductors that are available for the design. The biggest size of conductors was selected for all transmission line. This conductor was selected because it has lowest resistance, resistance, high gradient mean radius (GMR or DS) and it can transfer more power. Besides that, it also a long term investment. In the future, the system can be expanded more easy when the demand of the electricity i
While, gradient mean radius (GMR) is depending on the types of cable.
3.1.1
H- Frame and single circuit Tower
For H-frame and single circuit tower, the GMD of the system is calculated by using equation (1) and the distance was determined as shown in Figure 2(a) and Figure 2(b) for H-frame and single circuit tower respectively. GMD = (D12D23D13)1/3 ft
(1)
GMR
(a)
Figure 1: Types of transmission tower.
(b) Table 1: Electrical Characteristics of Bare Aluminum Conductors, Steel-Reinforced (ACSR).
Figure 2: (a) Distance determination for H-frame tower, (b) Distance determination for single circuit steel tower
The GMR for H-frame and single circuit steel tower was obtained from Table 1 which is 0.04983 ft and it is fixed.
3.1.2 3.0
LINE PARAMETER CALCULATIONS
The design is started by determined the value of capacitance and inductance for all types of line transmission system. There is several steps necessarily to be followed. 3.1
mean
distance
(GMD)
For double circuit steel tower, the GMD of the system is calculated by using equation (2) and the distance was determined as shown in Figure 3.
GMD = (Da1b1Da1b2Da1c1Da1c2D b1c1D b1c2)1/6 m
GMD and GMR Calculations
Gradient
Double Circuit Steel Tower
is
(2)
3.2
Line Inductance and Capacitance
The line inductance is calculated by using the formula below: L
Figure 3: Distance determination for double circuit steel tower
3
The line capacitance is calculated by using the formula below: C
0.0556
ln
r A r B r C
r Da1a 2
r B
r Db1b 2
r C
r Dc1c 2
GMD
uF / km
r
The calculated line inductance and capacitance table for each transmission line is shown in Appendix (Table A).
b
r A
mH / km
r
Ds= 0.0403 ft (from Table 1)
The GMR for double circuit steel tower is calculated using equation as follows: GMRc
GMD
0.2 ln
b
b
3.3
r b= r (radius of cable) = 0.04983 ft 3.1.3
GMD and GMR value for each types of tower
The calculated GMD and GMR value that was used is shown as in Table 2 and Table 3 respectively. Table 2: Calculated GMD for towers used in power system design
Per-unit line impedance
Given that the MVA base was selected at 100MVA. In order to simulate the 31 bus system, all parameters must be in per-unit system. Below are the equations to change all parameters into per-unit system. The per-unit value of any quantity is defined as:
Quantity in per unit
actual quantity
base value of quantity
Vb
Types of tower
GMD
69 kV H-Frame
13.86
69 kV double circuit steel tower
16.09
Zb
15.74
115 kV double circuit steel tower
18.27
230 kV double circuit steel tower
Ib
2
,
Zb
K .Vb
Y
MVAb Y
Admitance
B
Susceptanc e
Ib
jB
32.54
B PU Table 3: Calculated GMR for 115 kV and 230 kV double circuit steel tower
Types of tower 115 kV double circuit steel tower 230 kV double circuit steel tower
Vb
2
Zb
115 kV single circuit steel tower
3
GMR 1.190251 1.580439
X L
B
Y base
, B
2 fL, B PU
2 fc
B
Zb
Impedance base for 69 kV single circuit steel tower is 47.61 ohm, 115 kV Double circuit Steel Tower is 132.25 ohm and 230 kV Double
circuit Steel Tower is 529 ohm. Admittance base for 69 kV single circuit steel tower is 0.021, 115 kV Double circuit Steel Tower is 7.56 x10 -3 and 230 kV Double circuit Steel Tower is 1.89 x10 -3. For MVA limit is calculated by multiply the maximum current of the cable with the tower rating between the busses:
S 3 phase 4.0
3 V L
I
SELECTION OF APPROPRIATE TRANSFORMER VOLTAGE AND MVA RATING AND PER-UNIT
The selection of transformer voltage and MVA rating is based on the location, function and load the transformer carries. Transformer also function as a zoning the power consumption of the location. Usually the transformer are located near to the generator or located at interconnection area. Table 4: The per-unit line impedance Start Bus 1 1 3 4 5 6 7 8 8 8 9 11 13 15 15 16 19 21 24 25 26 26 29
End Bus 2 5 4 8 6 7 24 11 19 29 10 12 14 17 26 17 21 22 25 26 27 31 30
XL(p.u) 0.018594 0.007438 0.099228 0.070877 0.016735 0.014875 0.013016 0.080877 0.085053 0.074137 0.61661 0.087617 0.224191 0.016735 0.024172 0.026032 0.067397 0.074137 0.009297 0.022313 0.014875 0.020454 0.087617
R(p.u) 0.006625 0.00265 0.018549 0.01325 0.005962 0.0053 0.004637 0.031799 0.015899 0.029149 0.117774 0.034449 0.103052 0.005962 0.008612 0.009275 0.026499 0.029149 0.003312 0.00795 0.0053 0.007287 0.034449
B(p.u) 0.152737 0.061095 0.014049 0.010035 0.137463 0.12219 0.106916 0.05075 0.012042 0.046521 0.011821 0.05498 0.025084 0.137463 0.198558 0.213832 0.042292 0.046521 0.076369 0.183284 0.12219 0.168011 0.05498
As a result the MVA rating must be higher enough to carry the big amount of power. For selection transformer at the downstream location, the MVA rating selected based on the load they have to carry which is connected to the bus. The selection of voltage rating for the transformer is depending on the both side of line or substations that have to be connected. The transformers have capability of tap-changing the voltage during the losses on the line. When the voltage through the transformer is not match the desired value of voltage. The transformers will automatically tap-changing the voltage to meet the desired voltage input or output in order to get the stable voltage in the system. For this simulation the transformer tap range is between 0.90 to 1.10 steps of 2.5 percent. The transformer per unit calculation must be determined before simulation can be made. From Table 5, the impedance will be chosen is middle between minimum and maximum value of impedance limit. The reason is for to have the acceptable value, not too high or too low in order to have the best value and able to compute in the simulation. The equation for calculate the new impedance of transformer are applied below. new pu
Z
old pu
Z
S Bnew S Bold
Table 5: Transformer Impedance Data High Voltage Winding (kV)
Low Voltage Winding (kV)
115 230 230
69 69 115
Impedance Limit in Percent
Min 9 12.5 14
Max 14 18 20
Table 6: Transformer Voltage and MVA Rating FROM BUS 2
TO BUS 3
FROM (KV) 230
TO (KV) 115
MVA RATING 50
X pu (old) 0.17
X pu (new) 0.34
8 7 19 16 11
9 9 20 18 13
115 230 115 230 115
69 69 69 69 69
10 50 10 15 50
0.115 0.1525 0.115 0.1525 0.115
1.15 0.305 1.15 1.017 0.23
27 23 30 14
28 24 31 15
230 69 115 69
69 230 230 230
10 10 50 50
0.1525 0.1525 0.17 0.1525
1.525 1.525 0.34 0.305
14
16
69
230
50
0.1525
0.305
bus line that capacitance effect can be neglected. However, when dealing with interconnected system with many buses then it cannot be neglected. Table 6 shows the MVA rating that being selected for each transformer in every line buses. Generally the selection is based on calculating on a given bus using apparent power demand. By determining the rated current and load demand at the particular bus, each transformer can be rated based on that calculation. It is actually not an absolute value but it is just for initial implementation. 6.1
5.0
GENERATION SCHEDULE
The total generators of all generators must be the same as the total loads of the line together with the losses at the line. The most importance is the total active power generated must be equal to active load plus with the active losses at the line. Table 7 shows the generation schedule for each generator and their voltage profile in per-unit. Details for the generation schedule can be shown in appendix (Table B). Table 7: Generation schedule for each generator Bus no. 3 12 17 30
6.0
Gen MW 55 250 350 133.91
Gen Mvar 126.22 28.23 130.2 120.6
Min MW 0 0 0 0
Max MW 300 250 350 400
Min Mvar -145.3 -82.17 -169.5 -193.73
Max Mvar 145.3 82.17 169.5 193.73
SIMULATION AND DISCUSSION
In order to simulate the 31 bus system, all parameters must be in per-unit system. All line parameters calculated in per-unit system are tabulated in Table 4. The susceptance values are derived from capacitance values that being calculated previously. On earlier assumption, the system is short transmission line type which theoretically the capacitance effect can be neglected. However, when performing the simulation result in the entire system was blackout. This has come into our understanding that capacitance value cannot be neglected because the simulation involves 31 bus interconnected systems. It is true when calculating just one to one
Before Compensation
After simulation being made, the area which demands more power would be compensated. The impedance of transformer is calculated on 100MVA basis. From parameters and requirement condition given in assignment 3, power world simulation of 31-bus system is simulated as shown in Appendix (Figure A). This is before any compensation being made or any additional transformer being added. It can be observes that there are three red spot transformer on bus 3 to 2, bus 30 to 31 and bus 11 to 13. Table 8: Transformer Rating From No. 2 7 9 11 14 14 16 19 23 30
To No. 3 9 8 13 16 15 18 20 24 31
% of MVA 160 31 43 102 65.5 42 64.8 20 25 209
It indicates that the particular transformers have exceeded its capacity limitation. The transformers MVA value is chosen based on current rating that flows through the given line buses. However, this system is interconnected with other buses which makes the current rating calculation on a given line buses is not entirely dependent on load at that particular bus. Table 8 shows each transformer’s MVA rating that being utilized by the system.
Table 9: Buses Voltage Magnitude Bus no. 1 2 3 4 5 6 7 8 9
Norm KV 230 230 115 115 230 230 230 115 69
Actual KV 206.365 208.303 115 107.49 206.118 206.003 103.285 65.402 59.603
PU Volt 0.89724 0.90567 1 0.9347 0.89616 0.89508 0.89566 0.89813 0.94785
From Table 9, it shows that most of the buses are out of desired ±0.05p.u of voltage magnitude error. The data table is partially shown where the rest of the data is shown on appendix. The desired voltage magnitude is ranging from 0.95p.u to 1.05p.u. However buses 1 to 9 except bus 3 are out of desired voltage magnitude range. It is shown with red color marked. Therefore, compensation has to be introduced in the system to mitigate the problems. There are several methods to compensate the voltage magnitude as well as to increase the transformer capacity. In this project the solution would be by adding capacitor at certain buses area. As for transformer, there would be another transformer with the same rating connected in parallel with the existed transformer. In this ways, voltage magnitude can be kept within range and avoid red spot on each and every transformer. 6.1
After Compensation
Based on previous simulation, there are things that exceed the power system specification limit such as voltage range limit and transformer MVA limit. There are several busses that ranging not between the specified limit. In order to installed capacitor bank to which busses, the location is selected considering the value of bus voltage that outside the limit. In reality, it is not practical to install every capacitor bank to all busses which its bus voltage outside the range in order to fix the voltage value due to the expensive cost of capacitor bank. Figure B in Appendix shows the simulation after the correction after capacitor bank and reactor being installed. In this design the method to select the particular busses as the location of capacitor bank installment is by simply select the particular bus that locate in
between two busses that not within the specified system voltage. For an example, by referring to Figure B in Appendix, the chosen bus for installing the capacitor bank is bus 5. It can be seen that bus 1 and bus 6 which is connected to bus 5 is outside the voltage range. So the proper location to install the capacitor bank is bus 5 so that it can supply voltage to bus 5 and both bus 1 and 6 simultaneously and efficiently. The value of the capacitor bank and reactor determined by simply adjusting the nominal Mvar for capacitor until the bus voltage reach the specified range. This method is applied to other busses in the system. Table C in appendix shows the range of voltage after correction is 0.95
Bus 5 8 10 13 18 19 20 22 25
Nom kV 230 115 69 69 69 115 115 230 230
Compensation type Shunt Capacitor Shunt Capacitor Shunt Capacitor Shunt Capacitor Shunt Capacitor Shunt Capacitor Reactor Shunt Capacitor Shunt Capacitor
Mar value 23.8 25.03 5.05 33.22 11.39 18.56 -2.15 37.32 18.46
Table 11: Detail of correction regarding transformer
Line (2-3) (11-13) (30-31)
Transformer 230/115 kV 115/69 kV 115/230 kV
Existing (MVA) 1x50 1x50 1x50
Correction (MVA) 2x50 2x50 2x50
Table 12: Transformer MVA limit after correction From Bus No. 2 2 11 11 30 30
7.0
To Bus No. 3 3 13 13 31 31
% of MVA 94 94 53 53 97 97
CONCLUSION
The data given in the design such as line parameters and reactive power generated by the generator give poor condition in this designed power system. Thus, if this paper not considered capacitance effect on the line, the system must be black out because there is not enough reactive power to supply the load. This paper has considered capacitance effect for the sake of running the simulation regardless the unnecessary consideration for capacitance effect because all the line is short which is below than 80km. Capacitor bank is used to increased the voltage of the bus and reactor is used to decreased the voltage in the bus. The used of tap changing transformer in terms of automatic voltage regulator (AVR) is to varies the tap until meet the specification of certain requirement to regulate bus voltage.
APPENDIX Table A: The line configuration detail of each busses and line
Start Bus
End Bus
Distance (kM)
GMD (ft)
1
2
50
230kV Double cct
1
5
20
230kV Double cct
3
4
35
4
8
25
5
6
45
6
7
40
7
24
8
11
8 8
R ( Ω/km)
R ( Ω)
L (mH/km)
L (mH)
21.54066
0.626199
31.30996
0.07009
3.5045
0.009161
0.919049
9.836315
288.7279
230
529
0.018594
0.006625
0.152737
402.355
21.54066
0. 626199
12.52399
0.07009
1.4018
0.009161
0.36762
3.934526
115.4912
230
529
0.007438
0.00265
0.061095
402.355
115KV single cct
18.82153
1.193474
41.77159
0.07009
2.45315
0.00937
0.338135
13.12293
106.2281
115
132.3
0.099228
0.018549
0.014049
201.178
115KV single cct
18.82153
1.193474
29.83685
0.07009
1.75225
0.00937
0.241525
9.373523
75.87723
115
132.3
0.070877
0.01325
0.010035
201.178
230kV Double cct
21.54066
0.626199
28.17897
0.07009
3.15405
0.009161
0.827144
8.852684
259.8551
230
529
0.016735
0.005962
0.137463
402.355
230kV Double cct
21.54066
0.626199
25.04797
0.07009
2.8036
0.009161
0. 73524
7.869052
230.9823
230
529
0.014875
0.0053
0.12219
402.355
35
230kV Double cct
21.54066
0.626199
21.91698
0.07009
2.45315
0.009161
0.643335
6.885421
202.1095
230
529
0.013016
0.004637
0.106916
402.355
60
115kV Double cct
11.4
0.567439
34.04633
0.07009
4.2054
0.010234
1.221503
10.69597
383.7464
115
132.3
0.080877
0.031799
0.05075
201.178
19
30
115KV single cct
18.82153
1.193474
35.80422
0.07009
2.1027
0.00937
0.28983
11.24823
91.05268
115
132.3
0.085053
0.015899
0.012042
201.178
29
55
115kV Double cct
11.4
0.567439
31.20913
0.07009
3.85495
0.010234
1.119711
9.804639
351.7675
115
132.3
0.074137
0.029149
0.046521
201.178
9
10
80
69 kV H-Frame
11
1.16807
93.44557
0.07009
5.6072
0.010302
0.790322
29.35679
248.2871
69
47.61
0.61661
0.117774
0.011821
120.707
11
12
65
115kV Double cct
11.4
0.567439
36.88352
0.07009
4.55585
0.010234
1.323295
11.5873
415.7253
115
132.3
0.087617
0.034449
0.05498
201.178
13
14
70
69kV Double cct
10.19804
0.485365
33.97556
0.07009
4.9063
0.010449
1.677084
10.67374
526.8716
69
47.61
0.224191
0.103052
0.025084
120.707
15
17
45
230kV Double cct
21.54066
0.626199
28.17897
0.07009
3.15405
0.009161
0.827144
8.852684
259.8551
230
529
0.016735
0.005962
0.137463
402.355
15
26
65
230kV Double cct
21.54066
0.626199
40.70295
0.07009
4.55585
0.009161
1.194764
12.78721
375.3462
230
529
0.024172
0.008612
0.198558
402.355
16
17
70
230kV Double cct
21.54066
0.626199
43.83395
0.07009
4.9063
0.009161
1.286669
13.77084
404.219
230
529
0.026032
0.009275
0.213832
402.355
19
21
50
115kV Double cct
11.4
0.567439
28.37194
0.07009
3.5045
0.010234
1.017919
8.913308
319.7887
115
132.3
0.067397
0.026499
0.042292
201.178
21
22
55
115kV Double cct
11.4
0.567439
31.20913
0.07009
3.85495
0.010234
1.119711
9.804639
351.7675
115
132.3
0.074137
0.029149
0.046521
201.178
24
25
25
230kV Double cct
21.54066
0.626199
15.65498
0.07009
1.75225
0.009161
0.459525
4.918158
144.3639
230
529
0.009297
0.003312
0.076369
402.355
25
26
60
230kV Double cct
21.54066
0.626199
37.57196
0.07009
4.2054
0.009161
1.102859
11.80358
346.4735
230
529
0.022313
0.00795
0.183284
402.355
26
27
40
230kV Double cct
21.54066
0. 626199
25.04797
0.07009
2.8036
0.009161
0.73524
7.869052
230.9823
230
529
0.014875
0.0053
0.12219
402.355
26
31
55
230kV Double cct
21.54066
0.626199
34.44096
0.07009
3.85495
0.009161
1.010954
10.81995
317.6007
230
529
0.020454
0.007287
0.168011
402.355
29
30
65
115kV Double cct
11.4
0.567439
36.88352
0.07009
4.55585
0.010234
1.323295
11.5873
415.7253
115
132.3
0.087617
0.034449
0.05498
201.178
Type of tower
C (µF/km)
8
C (µF)
XL
B (µS)
Voltage (kV)
Zbase
XL(p.u)
R(p.u)
B(p.u)
MVA limit
Table B: Generation schedule
9
Table C: System bus voltage after correction
Bus no.
Nom kV
PU Volt
Volt (kV)
Angle (Deg)
1
230
0.97781
224.897
-11.57
2
230
0.98826
227.301
-11.07
3
115
1
115
-1.86
4
115
0.97269
111.86
-1.47
5
230
0.97562
224.393
-11.61
6
230
0.96758
222.542
-11.4
7
230
0.96158
221.164
-10.92
8
115
0.96286
110.729
-0.26
9
69
1.04475
72.088
-11.83
10
69
1.00488
69.337
-16.76
11
115
0.95704
110.059
8.19
12
115
1
115
21.81
13
69
0.95941
66.199
3.66
14
69
0.98156
67.728
-1.28
15
230
0.9819
225.837
-3.29
16
230
0.9899
227.677
-0.88
17
230
1
230
-0.33
18
69
0.97414
67.216
-6.07
19
115
0.96339
110.789
-5.12
20
69
1.03646
71.516
-5.59
21
115
0.95172
109.448
-8.11
22
115
0.96592
111.08
-9.85
23
69
1.04781
72.299
-12.7
24
230
0.95915
220.605
-10.31
25
230
0.96071
220.962
-9.62
26
230
0.96322
221.541
-7.42
27
230
0.95423
219.472
-8.1
28
69
1.04439
72.063
-9.91
29
115
0.97732
112.391
-0.5
30
115
1
115
0
31
230
0.96723
222.464
-7.45
10
Figure A: Power world simulation of 31-Bus power system before compensation
11
Figure B: Power world simulation of 31-Bus power system after compensation
12
