Amount of Variable
Renewable Energy (Solar and Wind) that can be connected to the Pakistani Grid
System
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RE (Renewable energy)or VRE ( Variable RE) interconnection requires that the grid must
add reactive loads; both capacitive and inductive loads are required to provide
voltage stability in over and under voltage conditions. SVCs or FACT devices
are required to provide speedy in time insertion of these inductive loads to
prevent voltage and system collapse. The
other factor related to solar is that there is need for a large geographic
area, with possibly multiple time zones, so that day light hours are stretched.
The wind turbine in itself presents technical problems but by far the greatest
matter of concern is the supply variability of both wind and solar , this
necessitates that the grid have suitable spinning reserves, flexible base load
plants and inductive loads coupled to FACT
or SVC devices to inset reactive load in abnormal conditions .
Pakistan’s grid network has suffered frequent voltage and
system collapse during the last 15 years. These have been at times system wide
and at times have been restricted of the Southern Network. Quetta-Baluchistan
network also suffers for instability and voltage collapse.( SVC has not been
installed in Quetta , instead NTDC has proposed to add 220kV Sibi-Mastung-Quetta, Loralai
double circuit transmission lines
it is not clear if this will address the instability in Quetta system NTDC in a submission to NEPRA stated that voltage
profile as low as 180kV and 170kV instead of the nominal voltage of 220kV at
Sibbi and Quetta Grid stations , respectively. 170 kV represents 0.77 pu.
voltage which translates to 0.6 pu. capacitance , SVC would add capacitance in
time to avert system and voltage collapse . NTDC has also decided
to include in their investment program, construction of the new
220kV, Guddu-Sibbi single circuit transmission line, for improvement of power
supply system in southern areas at a total cost of Rs8.36bn, it does not seem
that the steps proposed by NTDC will rectify the deficiency in the transmission
system, although these step will mitigate the issue somewhat). A SVC at Lahore has been installed to maintain
the stability of the primary system, another one planned at Quetta has not materialized.
The causes of this instability are, many and include in the last 10 years the
lack of any spinning reserves and the fact that the system was operated in
violation of the frequency requirements set by the Gird code. Some incidents were caused due to equipment
and relaying failures. A fully functional
grid system is absolutely necessary for VRE capacity to be connected to a large
integrated grid system. VRE addition requires serious upgrades in the grid, establishing
communication protocols and deploying control and protection systems that cater
to this rise in VRE capacity,
RE presents a challenge in so far
as: properties of wind and solar include
the constraints imposed by the weather conditions, VRE plants are smaller in
size as compared to conventional generators and connects in a more dispersed
manner and VRE uses equipments that connects to the grid using power conventional
technologies which are different to conventional generators and therefore
present a challenge to grid integration. VRE present less of a challenge in
systems where there is a good match between demand and VRE supply, further
systems that are perkier and cater to varying supplies over the year are better
placed to absorb VRE. Our system with a large hydroelectric supply and large
power flows from either the North or South to the Central load centers should
have the ability to cater to integration of a reasonable amount of VRE capacity
It should also be mentioned that geographic location of a wind plant will
affect the benefit it has for the system, at different location wind plants
will have widely varying benefits to the system. Additional system costs (other
than “normal” interconnection costs) needed to integrate VRE to the grid system
adds about US$400/kW to the cost of a typical wind plant, this translates to an
additional 1.45 c/kWh to the cost of generation.
GOPA study presents an analysis
of the Pakistan Grid with reference to RE interconnection. The conclusions
reached are:
1. 2224
MW wind and solar capacity can be added, with the following grid additions: 25
MVAr capacitors at Bhan Saedabad grid station; 120 MVAr Thyristor controlled reactor
(TCR) inductive 200 MVAr MSC Capacitive SVC at Lal Suhanra.; Power system
stabilizers (PSS) at two synchronous generators at Hub and Jamshoro. ; And
operation of some renewable generators in voltage droop control mode would be
beneficial.
2. In
the next phase major additional reinforcements are required these would allow
the full 4067 (phase I) of renewable generation to be added but it will also
facilitate addition totally 9332 MW (including the 4067 MW capacity discussed
earlier) of renewable energy to be added. Improvements required are : Lal Suhanra region requires reinforcement of
220kV system where a new collection substation is to be installed and two radial 220kV lines are closed to form
a ring This requires addition of 80 km
double circuit 220kV transmission lines , 5 km single circuit 220kV
transmission line and 5 additional 220/132 kV power transformers including
transformer bays . Reinforcements of the Southern wind corridor requires a new
500kV Jhampir grid station , 90 km 220kV and 500kV transmission lines , a
200MVAr capacitor at Jhampir and 100 MVAr shunt capacitors at Gharo are
required . 600MVAr SVC (600MVAr TCR and
300MVAr TSC) at Shikarpur are also required.
3. Spinning
reserve requirements will increase to 1500 MW 500MW more than required by the
system in 2018, Thus RE will need an additional 500 MW spinning reserves.
4. Dispatch
will, require to: re-execute wind and OV prediction at 1-4 hours ahead; and shorten
the dispatch cycle from 30 min to 15 min.
5. Wind
and PV generators need to have provisions for operation in defined conditions.
(Modern VRE plants connect to the grid using electronic power converters or inverters;
these can be programmed to allow the way in which a VRE power plant behaves on
the power grid to be controlled.)
6. Addition
of specified levels of RE generation results in a higher NPV as compared to the
one without RE capacity.
There is consensus on the fact
that addition of up to 30% RE capacity is possible with needed inductive loads
and SVCs but higher than that capacity will require new approaches on operating
and extending grids. Variability of RE due to weather Introduces uncertainty in
generation output .These could affect 70% of solar capacity due to cloud cover
and 100% of wind capacity due to still days. This requires base load capacity
that can follow load to be interconnected, these are costs. To reduce reserves
and spinning reserve costs it is recommended that: there needs to be
improvement in weather and wind forecast accuracy; forecasts should be for shorter
periods than a day
Features of VRE that pose challenges to grid
managers:
1)
Variability:
This is the biggest and most vexing
Power plants that run on fuel
(along with some hydro and geothermal plants) can be ramped up and down on
command. They are, in the jargon, "dispatch able." But VRE plants
produce power only when the wind is blowing or the sun is shining. Grid operators
don't control VRE; they accommodate it, which requires some agility. The figure
above shows one week of electricity
supply and demand (details and location not particularly important). The green
at the bottom is power coming in from wind. The yellow at the top is total
demand. The orange in the middle is the gap between the two, the amount that
has to be supplied by conventional power plants.
Another way of looking at it:
from the perspective of the grid operator, who has control over a set amount of
dispatch able power, VRE energy supply is functionally equivalent to reduction
in demand — large, rapidly rising and falling fluctuations in demand for
dispatch able power.
On the chart above, "shorter
peaks" refers to times when conventional plants are supplying the
day's "peak load," which is when power is most valuable. VRE reduces
or "shaves" the peak, thus screwing with the economics of conventional
plants. "Steeper ramps" refers to times when conventional plants have
to increase or decrease their output quickly in response to fluctuations in VRE
— often more quickly than they are designed or regulated for. And "lower
turn-down" means that in times of high VRE supply, conventional plants
will have to run at the lowest output they are capable of, i.e., "minimum
load." All these effects of variability pose challenges to the rules and
economics that govern existing power infrastructure.
2) Uncertainty:
The output of VRE plants cannot be predicted with perfect accuracy in day-ahead
and day-of forecasts, so grid operators have to keep excess reserve running
just in case.
3) Location-specificity:
Sun and wind are stronger (and thus more economical) in some places than in
others and not always in places that
have the necessary transmission infrastructure to get the power to where it's
needed.
4) Non synchronous generation:
Conventional generators provide voltage support and frequency control to the
grid. VRE generators can too, potentially, but it's an additional capital
investment.
5) Low capacity factor:
VRE plants only run when sun or wind cooperates. The average capacity factor —
production relative to potential — for utility-scale solar PV was around 28
percent; for wind, 34 percent. (By way of comparison, the average capacity
factor of nuclear power was 92 percent;
those plants are almost always producing power.) Because of the low capacity
factor of VRE, conventional plants are needed to take up the slack, but because
of the high output of VRE in peak hours, conventional plants sometimes don't
get to run as often as needed to recover costs.
The
challenges to integrating high levels of VRE into the grid are technically
solvable:
Regional grid integration studies conducted to
date have indicated that there is nearly always a technological fix that can be
adopted at some cost (e.g., a change in operation or piece of hardware that can
be added to the grid). So, simply deploying extremely large amounts of
transmission and storage (or some other set of technologies), and modifying the
RE generation to maintain system operational parameters could enable 100%
penetration of wind and solar. So the VRE carrying capacity of a grid is
technically 100 percent, if cost is no issue. But cost tends to be an issue. So
NREL posits a difference kind of carrying capacity:
The limit to RE penetration is primarily
economic, driven by factors that include transmission availability and
operational flexibility, which is the ability of the power grid to balance
supply and demand. This limit can be expressed as economic carrying capacity, or the level of variable RE
generation at which that generation is no longer economically competitive or
desirable to the system or society.
This notion of "economic carrying
capacity" clarifies our original question. Technically speaking, we can
integrate as much VRE as we want, as long as we're willing to keep spending
more money on grid-integration solutions. The question is, at what point is it
cheaper, from a total cost-benefit perspective, to resort to low-carbon
alternatives to VRE? And wherever that point is, will it still be there when we
actually reach it?
Solutions for integrating solar and wind into the grid...
Improved planning and
coordination: This is the first step, making sure that VRE is matched up with
appropriately flexible dispatch able plants and transmission access so that
energy can be shared more fluidly within and between grid regions.
Flexible demand and storage: To
some extent, demand can be managed like supply. "Demand response"
programs aggregate customers willing to let their load be ramped up and down or
shifted in time. The result is equivalent, from the grid operator's
perspective, to dispatch able supply. There's a whole range of
demand-management tools available
and more coming online all the time.
Similarly, energy storage, by
absorbing excess VRE at times when it's cheap and sharing it when it's more
valuable, can help even out VRE's variable supply. It can even make VRE
dispatch able, within limits. (For example, some concentrated solar plants have
molten-salt
storage,
which makes their power available 24 hours a day.)
Flexible conventional generation:
Though older coal and nuclear plants are fairly inflexible, with extended
shut-down, cool-off, and ramp-up times, lots of newer and retrofitted
conventional plants are more nimble — and can be made more so by a combination
of technology and improved practices. Grid planners can favor more flexible
non-VRE options like natural gas and small-scale combined heat and power (CHP)
plants.
Cycling conventional plants up
and down more often does come with a cost, but the cost is typically smaller
than the fuel savings from increased VRE.
Flexible VRE:
New technology enables wind turbines to "provide the full spectrum of
balancing services (synthetic inertial control, primary frequency control, and
automatic generation control)," and both wind turbines and solar panels
can now offer voltage control.
Interconnected transmission
networks: Wind
and solar resources become less variable if aggregated across a broader region.
The bigger the geographical area linked up by power lines, the more likely it
is that the sun is shining or the wind is blowing somewhere within that area.
Power
system planning. Power system planning
needs to shift from a conventional methodology where base load, intermediate
load and peak load capacity is added to fit the demand curve. A new approach
necessitates that capacity be added firstly through VRE sources and then base load,
intermediate load and peak load capacity be added, demand shift is also
factored into the planning regime.
Conclusions and Recommendations
Pakistani grid could absorb about
10000 MWs of RE capacity( excluding roof tops, net metering and isolated grids using wind or solar) in the next 10 years, but to
do that there is need to carry out improvements in the grid system including
placing inductive loads that can be switched on by FACT or SVC devices. There
is also need to modify the specifications of the wind turbine permitted to be
added to the system. Development of the
grid system is the key to enhanced absorption of VRE into the energy mix.
Solutions for adding sizeable VRE capacity include: use smart inverters with
advanced functionality; mimic synchronous generators; and provide active power,
reactive power, voltage, and frequency control.
Wind capacity in the Gharo-Jampir wind
corridor will depend upon the transmission transfer capacity between the South
and North. Wind capacity should also be added in the three other wind zones (In
Baluchistan and KP). Solar can and should be added all over the country,
perhaps instead of aiming at economy of scale (cost reduction due to size) more dispersed , smaller plants be installed
all over the solar zone in all Provinces(
smaller plants will add to operational issues though, especially under abnormal
conditions) ..
