Amount of Variable Renewable Energy (Solar and Wind) that can be connected to the Pakistani Grid System

Amount of Variable Renewable Energy (Solar and Wind) that can be connected to the Pakistani Grid System

Ads

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) ..