Converting AC Distribution Lines to DC to
Increase Transfer Capacities and DG
Penetration
Srinath.N.S
March 12, 2019
Srinath.N.S
Converting AC Distribution Lines to DC to IncreaseMarch
Transfer
12, 2019
Capacities
1 / 14
and
Outline
1
Overview
2
Existing Distribution systems
3
Conversion Methodology
4
Merits
5
Conclusion & De-Merits
Srinath.N.S
Converting AC Distribution Lines to DC to IncreaseMarch
Transfer
12, 2019
Capacities
2 / 14
and
Overview
Transfer capacities of existing AC distribution lines needs to be
increased to meet the increase in load demands.
On the other hand large scale integration of dispersed renewable
sources poses a challenge to voltage control
So improving the transfer capacities considering the accomodation of
DG sources can be taken as an issue to be addressed
Srinath.N.S
Converting AC Distribution Lines to DC to IncreaseMarch
Transfer
12, 2019
Capacities
3 / 14
and
Existing Distribution System
Distribution lines in urban areas are generally close to each other
At times of fluctuating load conditions,power tends to change
continuously
For two adjacent lines, if one line is lightly loaded and other one is
heavily loaded,power losses and voltage deviations will be high in
heavily loaded line.
Loads needs to be shed in-order to avoid over-loading
DG curtailment needs to be performed in lightly loaded lines to avoid
over-voltage problems
Flexible power shift between two adjacent lines will address this issue
effectively to increase the overall transfer capacity
Srinath.N.S
Converting AC Distribution Lines to DC to IncreaseMarch
Transfer
12, 2019
Capacities
4 / 14
and
Conversion Methodology
Base line configuration
Conversion of AC line to DC is constituted based on the fact that 3 AC
wires are used as positive DC wire,negative DC wire and a neutral wire
respectively.
Configuration types
1
Symmetrical configuration with neutral wire
2
Symmetrical configuration without neural wire
3
Asymmetrical configuration with a neutral wire
Configuration examples
Srinath.N.S
Converting AC Distribution Lines to DC to IncreaseMarch
Transfer
12, 2019
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and
Conversion Methods
DC Voltage calculation
Generally in urban distribution networks 3-core underground cables
with double circuit configuration is widely used to limit corridor
spaces.
When converting AC cable to DC operation for symmetrical
configurations,total DC voltage is
2UDC =
√
2
U AC
(1)
Since both ’+’ve and ’-’ve poles are with in the same cable,maximum
dc voltage is
√
UDC = 2/2UAC
(2)
For asymmetrical configuration, the DC voltage is
√ √
UDC = 2/ 3UAC
Srinath.N.S
(3)
Converting AC Distribution Lines to DC to IncreaseMarch
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and
DC Line Resistance
The resistance of DC lines are smaller than those of AC lines due to
skin effect(ys )andproximityeffect(yp )whichcanbegivenby
rAC = rDC (1 + ys + yp )
(4)
where ys = 0.02&yp = 0.001respectively .
Therefore DC Resistance is
rDC = 0.98rAC
Srinath.N.S
(5)
Converting AC Distribution Lines to DC to IncreaseMarch
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12, 2019
Capacities
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and
DC Current
Current dissipation capacity of both AC and DC conductor is the
same,
2
2
IDC
∗ rDC = IAC
∗ rAC
(6)
Therefore maximum allowed DC Current is
IDC = 1.01IAC
Srinath.N.S
(7)
Converting AC Distribution Lines to DC to IncreaseMarch
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and
Maximum Transfer Capacities for Single
Circuit OH and UG Cables
Srinath.N.S
Converting AC Distribution Lines to DC to IncreaseMarch
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12, 2019
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and
Maximum Transfer Capacities for Double
Circuit OH and UG Cables
Srinath.N.S
Converting AC Distribution Lines to DC to Increase
March
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12, 2019
Capacities
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and
Network Losses
Power losses of a conductor depends on the heat dissipation capability
of the conductor.
Maximum allowed losses for AC system in given by
2
P AC loss = (Pl2 + Ql2 /UAC
)rAC
(8)
Since there is no reactive power in DC system,full current rating can
be used to transfer real power.
Therefore power loss in DC lines for configurations a,b and c are
2
P DC loss = (Pl2 /2UDC
)rDC
DC
2
P loss = (Pl2 /4UDC
)rDC
2
P DC loss = (2Pl2 /3UDC
)rDC
(9)
(10)
(11)
Using the above equations and considering a power factor of 0.9, the
ratio of losses between AC and DC are 0.794,0.397,0.794 respectively
for 3 cases.
Srinath.N.S
Converting AC Distribution Lines to DC to Increase
March
Transfer
12, 2019
Capacities
11 / 14
and
Voltage Drop
AC voltage drop under Pl canbecalculatedas
∆UAC = (Pl .rAC + Ql .xAC )/UAC
(12)
For DC, voltage drop for configurations a,b& c are as follows :
∆UDC = (Pl /2UDC )rDC
(13)
∆UDC = (Pl /4UDC )rDC
(14)
∆UDC = (2Pl /3UDC )rDC
(15)
The ratio of voltage drop between DC and AC using above equations are
0.539,0.27 & 0.311 respectively for 3 cases.
Srinath.N.S
Converting AC Distribution Lines to DC to Increase
March
Transfer
12, 2019
Capacities
12 / 14
and
Merits
Through the conversion,the maximum power transfer can be
increased by 83.2% for symmetrical configuration single circuit cables
For double circuit cables,maximum transfer capacity is increased by
58.7%
Hybrid AC/DC networks have flexible power scheduling and power
balance
Transient currents can be avoided through the hybrid configuration
thus giving better performance than AC
Srinath.N.S
Converting AC Distribution Lines to DC to Increase
March
Transfer
12, 2019
Capacities
13 / 14
and
Conclusion & De-Merits
Proposed method analyses and quantifies the capacity increases of
various methods to convert AC lines to DC
Major contributions includes:
1. Large utilization of line capacity
2. Along with VAR optimization, active power optimization is
designed to accommodate DG’s and maximize load supply
Also major drawbacks includes:
1.Electro-Magnetic interference(EMI)
2.DC Stress of materials designed for AC
3.Creepage on Insulators
4.Corrosion of structures in absence of return conductor
failing to provide current path return
Srinath.N.S
Converting AC Distribution Lines to DC to Increase
March
Transfer
12, 2019
Capacities
14 / 14
and