Design of Radial Network

7/22/2019 Design of Radial Network

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Student Name/ID: Muhammad Ali Qaiser/26561999 Module: T&D ELEC6116

Coursework #1: OHL Design Date: 04-Mar-2014

Table of ContentsA. INTRODUCTION........................................................................................................................................... 1

A.1. Changing Transmission Paradigm ....................................................................................................... 1

A.2. Network Element Assumptions........................................................................................................... 1

B. Part 1: SIMPLE RADIAL NETWORK .............................................................................................................. 2

B.1. Q1: Network Plot................................................................................................................................. 2

B.2. Q3 (a): Summary Base Network Evaluation........................................................................................ 3

B.3. Q3 (b): Equipment Upgrade Choices................................................................................................... 3

C. Part 2: ALTERNATIVE TRANSMISSION NETWORK ....................................................................................... 7

C.1. Q4: Network Design Considerations ................................................................................................... 7

C.2. Q5: Techno-Commercial Comparison of Alternatives....................................................................... 10C.3. Q6: Load Extension Margins..............................................................................................................12

APPENDIX 1 Q2: Simulator Output Files....................................................................................................... i

APPENDIX 2 Q3 (a): Detailed Base Network Evaluation..............................................................................iii

APPENDIX 3 References...............................................................................................................................vi

Report Statistics:

Word Count [6,471] No. of Tables [8]

Main Pages [15] No. of Figures [10] Appendix Pages [6] Line spacing [1]

References [7] Font size [12]





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A. INTRODUCTIONThis section establishes practical utility of coursework and basic modelling assumptions.

A.1. Changing Transmission Paradigm

The grid was established in the UK in 1926, with the aim of interconnecting centralized generation

stations [1]. The basic design philosophy of electric grid has remained unchanged, namely that the

flow of power has traditionally been unidirectional, from high to low voltage levels [2]. However,

recent years have seen two fundamental changes in electrical energy generation:

1. Renewable energy sources are often non-dispatchable (as in case of wind and solar PV), and

also tend to be distributed, not centralized, in small power output clusters all over the grid.

2. Micro-stations (especially CHP1) have given rise to concept of embedded generation, which

fundamentally disturbs the concept of unidirectional power flow. Netherlands is a prime

example, where 52% of electricity is achieved from cogeneration [3].

The above factors will eventually lead to a paradigm shift in design, planning, protection and

operation of traditional grid to transform it into a self-diagnosing smart grid. Also as gridinfrastructure starts to outlive its initial design life, the issues of transmission losses will gain

increasing focus and may prompt refinements more complex than simple renovation [4]. It is

therefore very important to understand the efficiencies and economic trade-offs involved in the

design of various transmission topologies, as well as the modelling of faults and voltage correction.

A.2. Network Element Assumptions

For individual elements, the specific rating parameters provided by instructor were used; for

remaining parameters, default value-sets supplied by PowerWorld were used in the basic network

setup. A summary of important element characteristics for basic radial configuration is given below.

1. Buses: HV levels 11 kV and LV levels 0.415 kV, with no regulation devices attached; intake busset as system slack

2. Lines: lengths set to assigned values (km); R = 0.543 Ω/km, X = 0.395 Ω/km; current limit of

185 A results in MVA limit of 3.525 at 11 kV (Rabbit type); no line capacitance defined

Line ID L1 L2 L3 L4 L5 L6 L7

Assigned Length (km) 1 1.5 2.5 1 1.5 2.5 1

3. Transformers: winding configuration kept as Delta-Grounded Wye; line length and resistance

values assumed 0, rated at 3 MVA with X = 0.15 pu ( JP_B type); no automatic tap control,

shunt or line capacitance defined

4. Generator: attached to intake bus; total given load value is 6.9 MW + 2.9 MVAr so generation

capacity limit was set at 10 MW + 5 MVAr5. Loads: attached to all LV buses as constant power type, with provided P + Q values; set as

available for auto-generation control (so fixed, excessive generation is avoided) and lumped

at end of respective line

6. NGR: neutral-earthing resistor values for generator and transformers set at 0;

7. PU Base: network base rating was set to 10 MVA; SI units were followed (lengths in km)

1 Combined Heat and Power





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B. Part 1: SIMPLE RADIAL NETWORKAs a first step, a two-trunk radial network was set up using paths L1 to L7 only (see Figure 1).

Associated state output files (requested in Question 2 of coursework) are provided in Appendix 1.

B.1. Q1: Network Plot

Figure 1: Basic Radial ConfigurationLEGEND OF DISPLAYED VALUES

Buses: V value, V pu, δ load swing

Lines: i flow, S flow, P flow, P losses

Transformers: S flow

Loads: P value, Q value

Pie Charts: display share of actual to rated S capacity





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B.2. Q3 (a): Summary Base Network Evaluation

A loadflow was conducted on the basic radial network, which is asymmetric due to right trunk having

longer distribution path and higher load attachment (L4 to L7) . It was noted that line overloading

errors are reported by the simulator at normal operational conditions (see Figure 1).

A more exhaustive technical analysis of the network loadflow is carried out in Appendix 2. Here onlyconclusions are presented; the reader is encouraged to refer to Appendix 2 for detailed derivation.

1. Voltage Drops: It can be readily observed that as we traverse down the radial trunks, busbar

voltage drops increase in magnitude. It is a matter of simple postulation that as total distance

increases between source and load take-off points, the amount of V = iR drop will increase

(as R ∝ l ), adding on to previous drop. In terms of harmonized European regulations, none

of the buses show a voltage value below 90% [5].

2. Line Loading: Both trunks of the network experience severe line overloading at the

originating feeders, as they carry the total load for their respective circuits. The right side

feeder (L4 at 123%) again shows a higher overload than left side (L1 at 101%), due to the

greater total load being served by it (3.8 MW + 1.5 MVAr vs. 3.1 MW + 1.4 MVAr). The

magnitude of line loading decreases as we travel down the radial trunks, since current

branches off to intermediate load consumption points.

3. Power Losses: Absolute values of power loss (MW and MVAr) decrease down the trunk; this

is explained by decrease of carried current as loads branch off along the way. Higher total

loss values are observed on right trunk (L4 to L7); this is congruent with the higher loading

explained above. From a system-wide perspective, real losses compare as 3.9% of real power

flow whereas reactive losses form 11.4%2 of reactive power flow (see Table 2). This means

that any improvement actions on the transmission infrastructure should consider reactive

compensation to prevent this value from rising as network grows.

B.3. Q3 (b): Equipment Upgrade Choices

In light of Section B.2 above, it can be concluded that base case equipment is not sufficient for normal

operation of the radial network. It is further proposed that line loading and reactive power losses be

decreased3 by replacing lines L1, L4 and L5 with thicker conductors. Lines L1 and L4 are clearly

overloaded, whereas L5 shows 81% carriage; the latter is not an immediate issue but can become

problematic as resistivity and heat removal properties change in summer months. In order to offset

cost hike caused that will be caused by re-conductoring, transformer T7 (at lower loading) could be

replaced by cheaper one ( JP_A type) having higher inductance.

Various equipment changes were tried; the final choices are listed and justified below.

1. Re-Conductoring of L1: Increase of conductor by one size (from Rabbit to Horse) brought line

loading down from 101% to 70%. Another possibility could be to use in parallel, 2 Ferret or 2

Rabbit conductors; however, that would be economically more expensive than a single Horse.

2 Computed by dividing total losses to total “clean load” P and Q values respectively3 Decreased real losses improve busbar Vpu, which in turn assists in decreasing reactive power losses





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2. Re-Conductoring of L5: The same reasoning applies, as above for L1. Although in base case,

the conductor was not overloaded, it was nevertheless above 80%. By upgrading cable size,

loading is brought to a safe value of 56%, in harmony with other lines.

3. Re-Conductoring of L4: By attempting to upgrade to Horse, it was seen that loading was

decreased from 122% to 82%; this is still not very encouraging. Other options included anupgrade to Dog, or using 2 parallel conductors. Proposing 2 Ferret conductors in parallel gives

a much better value for money. While their cost4 is 8 against 10 of Dog, they cut loading to

70% against 79% of Dog5. This also carries the advantage of feeder L4 becoming identical in

loading to its mirror image feeder L1. Extra cost is further avoided by using common circuit

breakers at both ends of the two parallel conductors.

4. Degradation of T7: In order to offset cost of re-conductoring above, it was decided to de-rate

transformer T7 to type JP_A, as its load flow is 0.71 MVA, which in turn is well below the rated

capacity of 1 MVA for type JP_A. Other transformers cannot benefit from this option, as their

loads are very close to or greater than 1 MVA.

Figure 3: Upgrade Effect on Voltage Profile

5. Economic Viewpoint: The impact of equipment upgrade is summarized by comparison

against original base case, and presented in Table 1. It can be seen that the upgrade choices

discussed above align so that a very minor change in total cost is experienced. This rise in cost

from 300 to 303 computes to only 1.0%.

6. Technical Viewpoint: The performance impact of upgrade is summarized by comparison

against original base case, and presented in Table 2. It can be clearly seen that total losses,

both real and reactive, have decreased. This occurs primarily because reduced lineresistances (from re-conductoring) decrease both voltage drop (∝ iR) and copper losses (∝

i 2R). When VD reduces, busbar V pu rises and consequently prevents current inrush for power

compensation. With reduced current component, reactive circulation in the system is also

controlled. In addition, all lines are loaded to a value less than 80%; in fact the maximum

4 Unit cost 4 x 2 cables x 1 km = 85 Load/Rating = 222/278 = 79%

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

Source

(B1)

Centre

(B10)

Long

(B13)

Far

End

(B15)

Radial Bus Voltage Profile (V pu): TrunkL4 to L7

Base Equipment Upgraded Equipment

0

20

40

60

80

100

120

140

Near End (L4) Centre (L5) Long (L6) Far End (L7)

Line Current Loading (% of Rated):Trunk L4 to L7

Base Equipment Upgraded Equipment

Figure 2: Upgrade Effect on Carriage Profile





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loading that is seen is at L4 (source line), which is only about 70%. In doing a cost-benefit

analysis of the upgrade, it could be favourably argued that a mere 1.0% cost increase has led

to a loading improvement of 52% (from 122% to 70%). These effects can also be graphically

appreciated by comparing voltage loading profile of right-side trunk (L4 to L7) before and

after equipment upgrade (see Figure 2 and Figure 3). Both the VD and current loadingsituations show visible improvement.

Finally it may be proposed that at this stage (after equipment upgrade), capacitive compensation is

not required. The MVAr losses, as seen earlier (ref. Table 2), have fallen compared to base scenario.

The power factor seen by generator is 0.91, calculated as cos[tan-1 (QG/PG)] using values from Table

2. Not only is this within acceptable industrial limits, it is also slightly better than base case scenario.

Upgraded network plot is shown in Figure 4 for reader’s reference.

ID Device UoM Base Case Equipment Upgraded Equipment

TypeUnit

Cost Qty

Total

Cost Type

Unit

Cost Qty

Total

Cost

L1 OHL km Rabbit (50 mm2) 5 1 5 Horse (70 mm2) 7 1 7

L2, L3 OHL km Rabbit (50 mm2) 5 4 20 Rabbit (50 mm2) 5 4 20

L4 OHL km Rabbit (50 mm2) 5 1 5 Ferret (40 mm2) 4 2 8

L5 OHL km Rabbit (50 mm2) 5 1.5 7.5 Horse (70 mm2) 7 1.5 10.5

L6, L7 OHL km Rabbit (50 mm2) 5 3.5 17.5 Rabbit (50 mm2) 5 3.5 17.5

T1 - T6 Xfmr ea JP_B (x = 0.15 pu) 25 6 150 JP_B (x = 0.15 pu) 25 6 150

T7 Xfmr ea JP_B (x = 0.15 pu) 25 1 25 JP_A (x = 0.20 pu) 20 1 20

CB CB ea Circuit Breaker 10 7 70 Circuit Breaker 10 7 70

(CB for T&D only) Grand Total 300 (CB for T&D only) Grand Total 303

Table 1: Cost Impact of Equipment Upgrade( NOTE: For CB, only those involved in T&D are accounted for, 1 per line with switch on other end;

transformers CB are assumed included in latter’s cost; load-side and generator CB are ignored)

Component Quantity Symbol UnitsValue

(base network)

Value

(upgraded)

Generator Total Generation: Real PG MW 7.17 7.11

Generator Total Generation: Reactive QG MVAr 3.23 3.2

Generator Observed Power Factor cos[tan-1QG/PG] - 0.91 0.91

Loads Total Load: Real PL MW 6.9 6.9

Loads Total Load: Reactive QL MVAr 2.9 2.9

Loads Applied Power Factor cos[tan-1QL/PL] - 0.92 0.92

Transmission Total Losses: Real PLO = PG - PL MW 0.27 0.21

Transmission Total Losses: Reactive QLO = QG - QL MVAr 0.33 0.3

Transmission Waste Fraction: Real PLO/PL % 3.9% 3.0%

Transmission Waste Fraction: Reactive QLO/QL % 11.4% 10.3%

Line Max OHL Loading: L4 Sactual/Srated % 122.6% 69.6%

Table 2: Summary of Network Losses





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Figure 4: Upgraded Radial Equipment





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C. Part 2: ALTERNATIVE TRANSMISSION NETWORKThis section aims to increase the reliability of the basic radial design of previous section. Further

changes to equipment will be made in addition to the upgrades already performed in Section B.3.

C.1. Q4: Network Design Considerations

In order to increase reliability, a system design methodology had to be followed to efficiently zoom

in on a cost-effective solution from amongst the many possible corridor combinations. Firstly, a

conceptual comparison was carried out between the 4 major topologies [6] to decide the general

direction that alternative design plan would take.

1. Simple Radial: This is the coursework’s provided base case. It was seen in Section B.2 that its

equipment was not sufficient for load carriage, and had to be upgraded in Section B.3. Not

only is it asymmetric with respect to the two trunks, it does not cater to segmental

disruptions.

2. Cross-Linked Radial: This is an improved version that increases reliability by cross-linking

trunks; it has the advantage of providing alternate outage paths, or decreasing load of normal

paths if the additional feeders are switched on during normal operation as well.

3. Ring: This topology provides a continuous loop back to sources with loads distributed along

the path. In the current case, extra OHL would only be required at L8 to complete the loop,

hence introducing a very small cost increment. The benefit of offering 100% increase in

redundancy (as both trunks now cover each other with alternate routing in case of segmental

interruption) conclusively makes this a strong candidate.

4. Grid: This is a meshwork providing a high degree of inter-connection between various

distribution buses; however the cost of OHL and allied circuit-breakers increases

exponentially (as does reliability) so cost-benefit analysis would be negative unless verysensitive consumers with zero-tolerance were involved.

From the foregoing discussion, it seems obvious that options 1 and 4 are inappropriate extremes of

the choice spectrum, being unreliable or overdesigned (hence uneconomical) respectively. It would

be more likely to investigate further on options 2 and 3, and perhaps model a hybrid network

structure from these.

As a departure case for network modelling, a symmetric ring topology was completed by adding path

L8 with same line specifications as L7 (Rabbit). Further equalization was done by converting L2 to

Horse like L5, and by branching L1 into 2 Ferret feeders like L4 from earlier upgrade in Section B.3;

running load flow confirms balanced condition on both sides of the ring. The symmetric6 ring wasthen divided into zones for testing reference (see Figure 5). Turn-wise disruptions were caused by

opening breakers on left trunk from Zones 4 to 1, and their overloading effects on right-side OHL

noted (Table 3). Since the ring is symmetrical, the same results would be obtained by reversing the

faults along left side, so such testing is not required.

6 With respect to left and right sides of network hexagon





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Figure 5: Indicative Additional Laying and Zonal Division

This testing was repeated for additional cross-linking feeders as standby routes, and the final solution

was reached iteratively in the manner described below (refer to Table 3 throughout this discussion).

1. Option A (ring derived from simple radial) demonstrates that overloading is not a problem

for transmission if ring is broken by disruption in farther zones (Zone 3, 4); this explained by

the fact that the loads drawn by farther end are small compared to bulk of consumption in

upper zones. However, above 100% loading is seen for upper zone faults, which is not

acceptable even for short periods. Therefore, it appears that improved cross-linkage could be

attempted in these zones (Zone 1, 2).

2. Pursuant to above, Option B is constructed by adding crosslinks along paths L10+L11 and

L13+L14; for demonstrative purpose, Ferret equipment is used for OHL. The severity of

overloading in case of faults drastically decreases due to these additional routes; however

undesirable loading above 90% is seen in some instances, which may make this configuration

unsuitable for long fault periods.

3. A more diverse cross-linkage is obtained by Option C, which installs feeders along bent

corridors. This “diagonal” cross-linkage is performed in upper zones for two reasons: firstly itis the upper feeders that carry greater load and secondly, path L12 (upper) is shorter than

L15 (lower) so that equipment and laying cost would be saved. Severity of overloading falls

visibly, and it may be postulated that further strengthening of feeders nearer to the source

(such as L1, L4) may get rid of any remaining load capacity issues. Therefore, an optimum

combination of cross-linked radial and ring topologies is reached.





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Further tweaking is done to prevent overloading in case of severe disruption even in Zone 1, as

well as to allow double faults in lower zones (Zones 3, 4). This is achieved by using Horse instead

of Ferret cables for lines L1 and L4; double feeders are used for each line for reasons earlier

discussed in Section 0B.3, and are operated by common breakers in order to cut back unnecessary

cost incurred by separate breakers. The final solution is presented in Figure 6, with its alliedequipment list in Table 4. It may be noted that whilst path L12 is common to both cross links, a

common middle trunk is not made for two technical reasons (i.e. both cross feeders are kept

separate along L12): firstly, having a common trunk would introduce a single point of failure for

both cross-links as well as requiring thicker conductor to carry joint load; secondly, extra jointing

breakers would increase the cost more than using continuous separate double lines.

Option Description Loading on Right Trunk upon Disruptions in Left Trunk

Zone Disconnect Point L4 L5 L6 L7

Option

A

Upgraded radial as earlier, but

with symmetrical feeders added

as follows to give ring topology:

L8=L7, L2=L5, L1=L4

Zone 4 CB-B6-L8 70% 56% 49% 21%

Zone 3 CB-B4-L3 87% 76% 78% 49%Zone 2 CB-B2-L2 111% 106% 121% 91%

Zone 1 CB-B1-L1 152% 155% 191% 158%

Option

B

Additional corridors upon Option

A (Ferret) to link left-right trunks:

L10+L11

L13+L14

Zone 4 CB-B6-L8 65% 51% 49% 21%

Zone 3 CB-B4-L3 70% 60% 78% 49%

Zone 2 CB-B2-L2 91% 99% 57% 29%

Zone 1 CB-B1-L1 131% 78% 50% 21%

Option

C

Additional corridors upon Option

A (Ferret) to link left-right trunks:

L10+L12+L14

L11+L12+L13

Zone 4 CB-B6-L8 66% 42% 49% 21%

Zone 3 CB-B4-L3 72% 56% 78% 49%

Zone 2 CB-B2-L2 87% 42% 58% 30%

Zone 1 CB-B1-L1 130% 84% 57% 28%Table 3: Design Iterations for Network Reliability (>80% yellow, >100% red)

ID Device UoM Alternative Design Equipment List

Type Unit Cost Qty Total Cost

L1, L4 OHL km Horse (70 mm2) 7 4 28

L2, L5 OHL km Horse (70 mm2) 7 3 21

L3, L6 OHL km Rabbit (50 mm2) 5 5 25

L7, L8 OHL km Rabbit (50 mm2) 5 2 10

L10 + L12 + L14 OHL km Ferret (40 mm2) 4 3.5 14

L11 + L12 + L13 OHL km Ferret (40 mm2) 4 3.5 14

T 1 - T 6 Xfmr ea JP_B (x = 0.15 pu) 25 6 150

T7 Xfmr ea JP_B (x = 0.15 pu) 25 1 25

CB CB ea Circuit Breaker 10 10 100

Grand Total 387

Table 4: Bill of Materials for Alternative Design





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Figure 6: Alternative Reliable Network Design with Worst-Case Disruption

( NOTE: Whereas red squares in diagram indicate circuit breakers, in actual proposed design, each

network OHL has 1 circuit breaker at load-side and 1 normally-closed throw switch near source-side)

C.2. Q5: Techno-Commercial Comparison of Alternatives

Comparison of improved radial network (Section B.3) and alternative design (Section C.1) shall now

be done to establish which option is better from a technical and commercial point of view.

Economic Comparison: The final costs of improved radial and alternative designs are 303 and 387

units respectively. This represents a 27.7% increase, which is roughly speaking, a third of original. An





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informed cost-benefit analysis would now consider if the technical benefits accrued represent an

improvement by more or less than a third.

Technical Comparison: A survey by CIGRE7 concludes that equipment at higher voltage is more prone

to failure, as is that handling larger current flows (circuit breakers, cables etc.) [7] It is seen from load

flows presented earlier in this work, that busbar voltage falls as we traverse the transmission network

away from the source. In addition, current carriage in feeders nearer the source is greater compared

to far end lines, because load branching occurs along the way.

This means the probability of failure by electrical fatigue is greater in upper zones (see

Figure 5). Even is probability of failure in any given line is considered equal to all others, the upgraded

radial network (Question 3) does not provide alternate routing in case of disrupting faults anywhere

in the network. It with therefore necessitate downstream outage until the fault is repaired (e.g.

breaker failure, OHL damage or grounding).

The higher upstream a fault occurs, the more number of consumers are affected downstream. In

addition, although the coursework scenario does not mention so, there may be some highly sensitive

consumers in the network who can tolerate either no outage or a very brief one. Examples could be

dairy processing or polyester extrusion industrial units.

The alternative design (Question 4) offers 4 major advantages:

1. Routing redundancy is provided in case of disruption faults against outage; this situation can

be automated so that the standby path breakers are interlocked to operate immediately in

case of rise in busbar voltages or fall in feeder current so that only a momentary loss is

experience by consumers. The ring topology increases path provision by 100%, whereasstandby cross-linkages strengthen the network against overload.

2. Network overloading is prevented in upper zone disruptions; even in worst case, where loss

of entire first feeder is experienced on one side (see Figure 6) of the ring, the maximum

loading occurs around 84%; since it is a low-probability scenario, it is acceptable for brief

periods of time (as that required for repair) and does not necessitate expensive upgrades.

3. Load symmetry is maintained on both sides during normal operation; this allows equal wear

and tear of equipment.

4. Standby cross-links can also be operated during normal running, even when there is no fault.

This will serve to reduce loading in all upper zone lines, and can be useful as a temporary

relief measure if hot weather increases copper losses or if unexpected load surge occurs.

The above benefits qualitatively make a strong business case for proceeding with the alternative

design proposed in Section C.1; in addition, since 100% redundancy is provided, it may be

quantitatively argued that the benefits far outweigh the cost increment of around 28%.

7 International Conference for Large Electrical Systems





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C.3. Q6: Load Extension Margins

This section firstly examines the available load increase cushion for PQ1, PQ 5 and PQ 7. It then takes

a look at overall future growth.

A review of network layout shows that PQ7 resides furthest from source; any increase in it will

therefore affect all the lines. The iterations followed to check load limits are listed below.

1. PQ7 is increased first to allow loading higher zones; this is then supplemented by increasing

PQ5 and PQ1.

2. Loads are scaled up by % proportion, not by absolute value.

3. When increasing load, both P and Q components (MW and MVAr respectively) are raised by

same ratio; this maintains the aspect ratio of the power triangle, and hence does not change

the power factor. Therefore, the central assumption in testing load growth is that we

maintain the original power factor of each load point.

4. The smaller transformer T7 is upgraded back to original JP_B type to allow 3 MVA power

rating, otherwise PQ7 will reach its limit fairly quickly. This change will cost an extra 5 units.

5. Iterative increases in load points is done, and loading is measured on selected lines (upstream

of expanding load points) on both ring trunks. The iterations and final values reached are

listed in Table 5. It can be readily observed that each of the load points had a margin of

increase of more than double (>200%).

The real load growth limitation in network model turned out to be due to the transformers (see

Steps 4 and 6 of Table 5), not the OHL equipment. The maximum line loading is at L5 at 96%, and

it is demonstrated that by opening the standby cross-links, relief is obtained by lowering it to 75%

(see Step 7 of Table 5).

The network is capable of handling outages even in upper zones to some extent, despite the

increased load values of PQ1, PQ 5 and PQ 7. An example of power flow with increased loads and

outage in upper zone (and standby links turned on), is shown in Figure 7 . Although lines are not

overloaded above 100%, they are highly loaded (above 80%). This is acceptable for short

durations, as that required for fault repair. Network can be additionally made resilient to more

or longer disruptions by re-conductoring the cross-link paths (from Ferret to Horse for instance).

Finally, if a forecast is made that annual load growth will be 1.5% for 40 years, then the total load

at end of that period would have grown by a factor of:

= (1 + 0.015) = 1.81

The load nearly doubles (or grows by 181%). The original base case values of 6.9 MW + 2.9 MVAr

will therefore become 12.5 MW + 5.2 MVAr, assuming power factor remains same.

It has earlier been demonstrated that the network can withstand load increases of over 200% for

PQ 1, 5 and 7. The original base case values of these 3 loads is 3 MW + 1 MVAr, which represents





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a share of 43% and 34% of total network’s real and reactive loads respectively. Therefore, when

all remaining loads will start to grow along with PQ 1, 5 and 7, the overall network cushion will

fall below 200%. Taking real power as limiting factor 8, an increase in all loads simultaneously

would make the expansion margin fall to less than 86% (i.e. 0.43 x 200% = 86%).

Since the total load growth is 181%, following network upgrade steps would have to be taken to

cope with future demand:

1. Upgrade of transformers, as they reach power limits earlier than OHL under the present

configuration. Either load distribution across a pair of 3 MVA transformers, or a larger 6 MVA

step-down device would be required for each load take-off point.

2. Uniform load distribution, as a factor that should be kept in mind when planning for growth

load connections; while it is not always controllable 9, zonal planning can be done to some

extent so that main trunks and branching laterals carry uniform load density. This has the

advantage of decreasing effective line length to half for purpose of voltage drop calculations.

3. Upgrade of conductor thickness for main lines in lower zones of the network, and for busbar

cross-connections in upper zone of the network.

4. Utilization of capacitive correction to control reactive power, so that power flow is freed up

for real load consumption. The original load Q/P ratio gives a power factor of 0.92 (see Table

2), which can be improved to perhaps 0.98 by injective capacitor banks at major busbars, and

shunt capacitance on longer transmission lines.

5. Advanced redundancy scheme at generator busbar (such as breaker-and-a-half ) to prevent

source disruption.

On the whole, comparing the figures of 181% with 86% is telling in that nearly a final capacity

double-up of all load carrying equipment would be required (transformers, lines, breakers).

The above improvements can be planned incrementally, in perhaps 10 or 15 year blocks; or they

can be implemented presently after further reviewing and confirming the forecast study. The

former approach carries the advantage of amortizing the financial burden over time, whereas

the latter approach capitalizes on lower present value of money rather than waiting for

inflationary effects of future.

8 By introducing capacitive injection to control real power, we can eliminate its limitation9 For example, an area may be declared an industrial zone and so will carry a lop-sided load consumption compared to

neighboring residential areas





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Step

No.

Load Increment Steps Standby

Links

PQ1 PQ5 PQ7 L1 L5 L3 L8 Observation

MW MVAr MW MVAr MW MVAr Loading

Original Load Values 1.3 0.5 1 0.4 0.7 0.1

Original Power Factor cos[tan-1(Q/P)] 0.93 0.93 0.99

1 PQ7 increased by 50% Off 1.3 0.5 1 0.4 1.05 0.15 40% 52% 43% 16%

2 PQ1 & PQ5 increased by 50% Off 1.95 0.75 1.5 0.6 1.05 0.15 47% 62% 45% 18%

3 PQ7 increased by 40% Off 1.95 0.75 1.5 0.6 1.47 0.21 50% 66% 52% 24%

4 PQ1 & PQ5 increased by 40% Off 2.73 1.05 2.1 0.84 1.47 0.21 59% 79% 54% 27% T1 reaches 99% so PQreaches limit at transfo

OHL still has capacity

5 PQ7 increased by 30% Off 2.73 1.05 2.1 0.84 1.91 0.27 62% 84% 61% 34%

6 PQ5 increased by 30% Off 2.73 1.05 2.73 1.09 1.91 0.27 63% 96% 65% 37% T5 reaches 100% so PQ

reaches limit at transfo

L5 also near limit

7 Final Load Values On 2.73 1.05 2.73 1.09 1.91 0.27 65% 75% 63% 35% Relief on L5 observed,

loading falls to 75%

Increase from Original 1.43 0.55 1.73 0.69 1.21 0.17

Final % Increase 210% 210% 273% 273% 273% 273%

Original Power Factor cos[tan-1(Q/P)] 0.93 0.93 0.99

Table 5: Testing to Increase PQ1, 5, 7



Page | 15

Figure 7: Increased Loads PQ1, 5, 7 and Outage on L2



Page | i

APPENDIX 1 Q2: Simulator Output FilesReference data tables are provided here. To fit A4 size, unnecessary columns have been removed

(Note: all buses are in Area 1, all lines in Circuit 1; all breakers kept closed during loadflow solution).

Name Nom kV PU Volt Volt(kV)

Angle(Deg)

LoadMW

LoadMVAr

GenMW

GenMVAr

B1 11 1 11 0 7.17 3.23

B2 11 0.98067 10.787 -0.21

B3 0.41 0.97275 0.404 -1.38 1.3 0.5

B4 11 0.96326 10.596 -0.36

B5 0.41 0.95686 0.397 -1.29 1 0.4

B6 11 0.94944 10.444 -0.41

B7 0.4 0.94139 0.377 -1.18 0.8 0.5

B8 11 0.97664 10.743 -0.31

B9 0.41 0.96869 0.402 -1.49 1.3 0.5B10 11 0.95325 10.486 -0.63

B11 0.41 0.94678 0.393 -1.59 1 0.4

B12 11 0.92968 10.226 -0.97

B13 0.4 0.92145 0.369 -1.78 0.8 0.5

B14 11 0.92593 10.185 -1.09

B15 0.4 0.92424 0.37 -1.8 0.7 0.1

Table 6: Bus State Records

From

Name

To

Name

Branch

Device Type XfrmrMW

From

MVAr

From

MVA

From

MVA

Limit

% of MVA

Limit

MW

Loss

MVAr

Loss

B1 B2 Line NO 3.2 1.5 3.5 3.5 100.5 0.06 0.0

B1 B8 Line NO 4 1.7 4.3 3.5 122.6 0.08 0.0

B2 B3 Transformer YES 1.3 0.5 1.4 3 46.8 0 0.0

B2 B4 Line NO 1.8 1 2.1 3.5 58.8 0.03 0.0

B4 B5 Transformer YES 1 0.4 1.1 3 36.1 0 0.0

B4 B6 Line NO 0.8 0.5 1 3.5 27.3 0.01 0.0

B6 B7 Transformer YES 0.8 0.5 1 3 31.7 0 0.0


B8 B10 Line NO 2.6 1.1 2.8 3.5 79.9 0.06 0.0

B10 B11 Transformer YES 1 0.4 1.1 3 36.1 0 0.0

B10 B12 Line NO 1.5 0.6 1.7 3.5 47.2 0.03 0.0

B12 B13 Transformer YES 0.8 0.5 1 3 31.7 0 0.0

B12 B14 Line NO 0.7 0.1 0.7 3.5 20.1 0


Table 7: Line State Records



Page | ii

Name Bus 1 Bus 2 Bus 3 Bus 4 Bus 5 Bus 6 Bus 7 Bus 8 Bus 9 Bus 10 Bus 11 Bus 12 Bus 13 Bus 14 Bus 2

B1 29.14 -

j21.20

-14.57 +

j10.60

-14.57 +

j10.60

B2 -14.57 +

j10.60

24.29 -

j24.33

-0.00 +

j6.67

-9.71 +

j7.07

B3 -0.00 +

j6.67

0.00 -

j6.67

B4 -9.71 +

j7.07

15.54 -

j17.97

-0.00 +

j6.67

-5.83 +

j4.24

B5 -0.00 +

j6.67

0.00 -

j6.67

B6 -5.83 +

j4.24

5.83 -

j10.91

-0.00 +

j6.67

B7 -0.00 +

j6.67

0.00 -

j6.67

B8 -14.57 +

j10.60

24.29 -

j24.33

-0.00 +

j6.67

-9.71 +

j7.07

B9 -0.00 +

j6.67

0.00 -

j6.67

B10 -9.71 +

j7.07

15.54 -

j17.97

-0.00 +

j6.67

-5.83 +

j4.24

B11 -0.00 +

j6.67

0.00 -

j6.67

B12 -5.83 +

j4.24

20.40 -

j21.51

-0.00 +

j6.67

-14.57 +

j10.60B13 -0.00 +

j6.67

0.00 -

j6.67

B14 -14.57

+

j10.60

14.57 -

j17.27

-0.00

j6.67

B15 -0.00 +

j6.67

0.00

j6.67

Table 8: Bus Admittance Matrix (Y-Bus)



Page | iii

APPENDIX 2 Q3 (a): Detailed Base Network EvaluationPursuant to the summary conclusions on base network listed in Section B.2, a more exhaustive

analysis is presented here. The reader should refer to Figure 1 during this discussion.

1. Voltage Drops: It can be readily observed that as we traverse down the radial trunks, busbar

voltage drops increase in magnitude. This is further represented graphically by profiling the

VD per-unit along two trunks (see Error! Reference source not found.). It is a matter of simple

postulation that as total distance increases between source and load take-off points, the

amount of V = iR drop will increase (as R ∝ l ), adding on to previous drop (even though

current loading decreases after each load take-off). Similarly, each PQ load along the way

contributes to reactive loss10, causing the subsequent busbars to see greater volt drops (as

transmission system’s supply handling capacity decreases with loading space taken up by

non-performing reactive current). In real-life networks, reactive power mismanagement can

cause runaway effect on VD due to compensatory current inrush, leading to transmission

collapse. This situation would not be experienced in the current network model because the

loads are set to draw constant (not variable) PQ power. Referring to Error! Reference sourcenot found.8, it can be seen that the VD effect along radial length is more pronounced in right-

hand side trunk (L4 to L7) than left-hand side one. This is again due to the factors explained

above, namely higher current flow in right trunk due to greater number of attached reactive

loads. In terms of harmonized European regulations, none of the buses show a voltage value

below 90% [5], so in that respect, the network is within limits.

Figure 9: Network Line Loading Profile

10 Q values are inductive for the loads

Figure 8: Network Busbar Voltage Profile

0

20

40

6080

100

120

140

Near End (L1, L4) Centre (L2, L5) Long (L3, L6) Far End (L7

Line Current Loading (% of Rated)

Trunk L1 to L3 Trunk L4 to L7

0.88

0.9

0.920.94

0.96

0.98

1

1.02

Source

(B1)

Centre

(B4,

B10)

Long

(B7,

B13)

Far

End

(B15)

Radial Busbar Voltage Profile (V pu)

Trunk L1 to L3 Trunk L4 to L7



Page | iv

If however, action needs to be taken to raise the busbar voltages, several options could be deployed:

a. Re-conductoring to increase thickness or utilize a better conductive material; this

would have the effect of decreasing resistance, and hence lowering value of V = iR

b. Reactive compensation by adding shunt capacitance at requisite bus points, toeffectively supply voltage (or capacitance in series with load to cancel inductive

element)

c. Increase of generation voltage to perhaps 5% above its present value

2. Line Loading: Both trunks of the network experience severe line overloading at the

originating feeders, as they carry the total load for their respective circuits. The right side

feeder (L4 at 123%) again shows a higher overload than left side (L1 at 101%), due to the

greater total load being served by it (3.8 MW + 1.5 MVAr vs. 3.1 MW + 1.4 MVAr). The

magnitude of line loading decreases as we travel down the radial trunks, since current

branches off to intermediate load consumption points (see Error! Reference source not found. ) whereas the current ratings are kept same for all lines at 185 A. It is therefore clear

the conductors for L1 and L4 would have to be refactored to a thicker size and/or better

conductive material to allow loading carriage to come down. Similarly line L5, although not

overloaded, shows 82% carriage. This could become a problem in hot weather conditions so

depending on the geographical location of the network, it may also require re-conductoring.

3. Power Losses: Both real and reactive power losses are seen from the “Branches State” table

in the simulator’s “Network Explorer” function. The pattern observed while traversing down

the radial trunks is as follows:

a. Absolute values of power loss (MW and MVAr) decrease down the trunk; this is

explained by decrease of carried current as loads branch off along the way. Therefore,

the copper and reactive losses decrease with current (i 2 Z).b. The comparative ratio of power loss to power flow also decreases (i.e. MW loss to

MW flow, and MVAr loss to MVAr flow) down the trunk; this means that the loss

values fall more rapidly than power flow. Again it is simple to see why; loss depends

on factor of i 2 (quadratic) while power depends on i (linear).

c. Higher total loss values are observed on right trunk (L4 to L7); this is congruent with

the higher loading explained above.

d. From a system-wide perspective, real losses compare as 3.9% of real power flow

whereas reactive losses form 11.4%11 of reactive power flow (see Table 2). This means

that any improvement actions on the transmission infrastructure should consider

reactive compensation to prevent this value from rising as network grows.

It should be explained that the reactive loss is caused by a combination of load Q as well as

line and transformer X values (inductance); a similar comment can be made for real loss (load

P and line R). These losses are compensated by increased generation; for example, while

11 Computed by dividing total losses to total “clean load” P and Q values respectively



Page | v

“attached” loads total up to 6.9 MW + 2.9 MVAr, the “Generators” table shows a supply of

7.17 MW + 3.23 MVAr.

4. Bus Voltage Angles: As we compare values down the radial trunks, some interesting

observations can be made about the load angle at busbars:

a. Load angles are higher for LV buses (415 V) in general compared to HV (11 kV); this is

explained by the inverse relation between i and V for supplying requisite power. A

lower voltage would require higher current, which in turn would increase IZ swing12

in a phasor diagram (see Error! Reference source not found.).

b. Load angles tend to increase as we proceed down a radial trunk; this is occurs because

busbar voltage drops, and therefore a higher current is required to serve requisite

power to consumer load.

c. Right side trunk (L4 to L7) shows more load angle swing compared to left side (L1 to

L3) for comparable points along the path; this is congruent with the fact that a higher

current loading with cause greater phasor shift.

12 Z = R + jX

Figure 10: Load Angle Development (V S

o – V L

o )



APPENDIX 3 References

[1] B. M. Weedy, B. J. Cory, N. Jenkins, J. B. Ekanayake and G. Strbac, Electric Power Systems, Sussex: John

Wiley & Sons, 2012.

[2] M. Bello and C. Carter-Brown, “Impact of Embedded Generation on Distribution Networks,” Energize,

pp. 32-34, Jul 2010.

[3] COGEN Europe, “European Cogeneration Review - The Netherlands,” www.cogeneurope.eu, Brussels,2013.

[4] J. Cooper, “Our Outdated Electrical Grid: An Intolerable Situation,” The Energy Collective, 03 Jul 2012.

[Online]. Available: http://theenergycollective.com/john-cooper/90021/intolerable-situation-outdated-

paradigm. [Accessed 22 Feb 2014].

[5] European Parliament, “Directive 2006/95/EC on Electrical Harmonisation,” Council of Dec-2006,

Strasbourg, 2006.

[6] T. Gonen, “Design of Subtransmission Lines and Distribution Substations,” in Electric Power Distribution

System Engineering, CRC Press, 2008, pp. 170-172.

[7] C. E. Solver, “Cigre Surveys on Reliability of HV Equipment,” 2004. [Online]. Available:

http://www.mtec2000.com/cigre_a3_06/Rio/past.pdf. [Accessed 27 Feb 2014].

Documents

Design of Radial Network