Our country is wide spread over North to South and East to West. So, daytime in eastern side area is about 2 hours ahead of western side area. Power requirement varies according to local day night period. This time diversity of demand is favourable to manage local peak and off peak demands. Similarly climatic variations particularly in temperature from northern to southern area is favourable to manage local seasonal demand. Higher heating load is in winter but no cooling load in summer in northern area but contrary in southern area having no heating load in winter but higher cooling load in summer.
Maximum hydro power is available during monsoon from runoff river projects – whereas it is available at fag end of monsoon whilst dam is overflowing in case of irrigation liked projects but is available at onset of summer in northern area due to melting of ice. Thermal generation is hampered during monsoon due to lumpy coal/lignite feeding. Thermal generation drop due to higher ambient temperature in summer causes insufficient condenser cooling and low vacuum. Gas turbine generation drops due to low density air. Effect is predominant in more such plants. Holidays are different in different areas according to local race and religions. Extent duration and time period of power demand for lift irrigation varies in different areas as per crop pattern and irrigation requirements.
In addition to these periodic variations, there may be stray incidences causing variations in supply or demand of power. Generation is likely to be affected due to forced outages of generators, fault on evacuating lines, disturbance in fuel supply chain on account accident or agitation in transport or fuel source. Power demand is also affected due natural calamities like cyclone, drought, flood, earth quack etc. It is also affected due to agitations like bandh, rasta-roko, strike etc.
A situation may arise due to variations in supply and demand that some area is comfortable or surplus in power whereas other area is facing shortage or starving of the power. Providing power support to starving area is easy in wider grid system.
Cost of power from various sources is different depending on fuel price, transportation cost, operating cost, resource availability, taxes etc. National grid facilitates optimal overall economical grid operation in addition to equitable utilization of available resources.
Grid Development
Original power systems were DC standalone catering to only local loads. Parallel operation of generators was possible only at local bus of power plant. Load catering at long distance was not sensible due to high losses and large voltage drop. So, it had no scope for grid consideration.
But this constraint is solved with development of device for transformation of voltage level. Power transfer over long distance became sensible by transforming to high voltage but low current. Losses and voltage drop fairly controlled due to low current. Feasibility of power transport to long distance and parallel operation of generators at different locations made a way to grid system. But voltage transformation was possible in AC system only. Hence existing standalone DC power plants were converted to AC system and interconnected.
Power system expanded with numbers of power plants operating in synchronism and catering loads in wide spread area. Expansion continued to form state grid of single ownership. Later on central sector power plants come up as pooled source having allotted share for various power systems. State grids were extended to draw power share from these power plants. Ultimately regional grid was established by indirect and direct links with other power systems. Efforts made for inter region connections. But trial operations were not fruitful at this stage.
Thereafter regional grids were interconnected with HVDC links to start with national grid benefits. These regional asynchronous interconnections continued few years. Finally. synchronous connections amongst regions were established in steps forming synchronous national grid as at present.
Pros and Cons of Synchronous Link
- Synchronous interconnection is straightforward by transmission line. No need of specific equipment or set-up.
- Power assistance is instantly automatic in case of exigency in any part of the grid.
- Frequency excursions are narrow and slow due to increased stiffness of wider grid system having larger bias and higher inertia.
- Increase in Fault Level may require higher capacity equipment at some locations.
- System control is difficult with multiple partners.
- Probability of disturbance spreading from one to other regions.
- Power flow in transmission network is automatically governed by system parameters, available network, injections and extraction of active reactive power at various locations. Task is intricate to relieve critically loaded element to avoid untoward incidents.
- Frequency remains common for all in the grid. Abnormal frequency operation is detrimental to equipment at power plants and consumers. Regions capable to operate system at comfortable frequency level are dragged for risky operation.
HVDC Link
HVDC Links were established between regions in past when synchronous operation was not practicable at the time. Such asynchronous links were operative few years till synchronous links established.
Pros and Cons of Asynchronous Link
- Power transfer on this link is controlled and regulated as desired.
- One regions’ system frequency is independent of other regions.
- No scope of spreading disturbance in one region to other due to isolation.
- Regions can be operated at desired frequency.
- System operation is better coordinated with limited partners.
- Require specific set up, equipment and operation expertise.
- System is slow because of manual operation. Time spent for formalities and implementations.
This implies that neither of two options is just right for the purpose. Both the alternatives have favourable and unfavourable characteristics. Now option is to modify either of the links to overcome constraint. One option is auto regulated DC link.
Auto Regulator DC Link
Present Indian power system is synchronous national grid. Inter regions HVDC links established earlier are already available and operated in hybrid mode to manually regulate power flow on inter region AC links.
Operating characteristic of high voltage long transmission line was unfavourable at load other than surge impedance loading. However, it could be made flexible for operation at any loading by integrating with FACTS devices. Similar HVDC link can be made versatile for operation as auto regulator link. Following criteria stand as the basic requirements:
- Fast Scanning Frequency Transducers installed at one of the ends transmitting data to control unit.
- Fast Scanning Frequency Transducers installed at other ends transmitting data to control unit via data channel.
- Actual Power Flow signal from HVDC controller.
- Control unit developed with following basic features.
- Three Displays for Local Frequency, Remote Frequency and Actual Power Flow.
- Three Control Settings for Transfer Bias, Maximum Power and Ramping Rate.
- Data links with local and remote frequency sensor and HVDC controller.
- Usual modems, power supply etc.
Functioning
- Controller program evaluate Expected Power Transfer.
- Expected Power Transfer X = Transfer Bias × (Local Frequency – Remote Frequency)
- X is restricted to maximum power setting.
- Deviation D = Required Power Transfer X – Actual Power Flow P i.e. D = X – P
- Signal pass to HVDC controller to raise export as per ramping.
- Actual power flow P increase from high frequency region to low frequency region.
- Frequency drops in high frequency region due to export.
- Frequency rises in low frequency region due to import.
- Expected Power Transfer X reduces due to reduction of frequency difference.
- Deviation D reduces as expected power transfer X decrease and actual power flow P increase.
- Repeated after each doze, till deviation D is less than allowed tolerance.
- Power transfer at this level continues till changes in load / generation in either region.
Power transfer increase when D is positive due to following changes.
- Power injection increase in exporting region
- Load decrease in exporting region
- Power injection reduce in importing region
- Load increase in importing region
Power transfer decrease when D is negative due to following changes.
- Power injection decrease in exporting region
- Load increase in exporting region
- Power injection increase in importing region
- Load decrease in importing region
Fiscal Impact
Frequency in exporting region is always higher than importing region. Unscheduled interchange rate of exporting region is lower than that of importing region. Power sent at export point and received at import point differs due to line loss. UI charges payable to export region is less than receivable from import region. The balance of two is transfer gain for the link.
Analytical Equation
Consider HVDCAR link between regions S operating at higher frequency and region R operating at lower frequency.
- System bias is Bs MW/Hz and Br MW/Hz respectively.
- Frequency in isolation is Fsn Hz and Frn Hz respectively.
- Transfer Bias setting is Bt MW/Hz.
After close of the link power P flows in the link.
System Bias MW/Hz Bs Br
Frequency (Open Link) Hz Fsn Frn
Frequency (Close Link) Hz Fsc Frc
Transmission Bias MW/Hz Bt
Let power transfer is X MW from region S to region R
Frequency drop from Fsn to Fsc in region S due to loss of X MW transferred to Region R
X = Bs (Fsn-Fsc)
Frequency rise from Frn to Frc in region R due to gain of X MW received from Region S
X = Br (Frc-Frn)
Power transfer X is proportional difference of frequency between regions.
X = Bt (Fsc-Frc)
Solving these three simultaneous equations for X, Fsc and Frc in term of Bs, Rs, Bt, Fsn and Frn.
We get
X = (Fsn-Frn)BtBsBr/BsBr/(+BtBs/(+BtBr))
Fsc = Fsn – X/Bs
Frc = Frn + X/Br
Calculation: Consider isolated operating regions as under.
- Region S has bias 2400 MW/Hz operating at 50.00 Hz.
- Region R has bias 1800 MW/Hz operating at 49.00 Hz.
- Transfer bias of link is set at 1000 MW/Hz
- Power transfer and frequencies when HVDCAR link closed.
- Actual Power transfer P = 507 MW
- Region S Frequency Fsc = 49.789 Hz
- Region R Frequency Frc = 49.282 Hz
- Changes in power transfer and frequencies when load / generation changes in any region.
- Consider 600 MW generation drop in region S.
- Expected isolated frequency drop in region S = 600/2400=0.25Hz
- Expected isolated frequency of region S = 50.00–00.25= 49.75 Hz.
- Revised power transfer and frequencies recalculate is as under as
- Actual Power transfer P = 380.3 MW
- Region S Frequency Fsc = 49.592 Hz
- Region R Frequency Frc = 49.211Hz
In this way revised power transfer and frequencies can be calculated when changes in any region.
- Performance: Transfer bias setting establishes solidarity of connection. Low bias allows less power transfer and wide frequency difference whereas higher setting allow more power transfer and narrow frequency difference. Maximum power selection is useful to set limit for power export based on spare resources and internal network loading condition.
Conclusion
The proposed link has all the benefits of HVDC connection plus flexibility for automatic power transfer with limit to region in deficit. Regional grids are rigid enough to tolerate most probable disturbances. Acute disturbance could not be prevented even in strong synchronous grid but may have wide repercussion.
The proposed link provides optimum balance between system rigidity and complexity. Synchronous connection is like partnership company having unlimited liability whereas HVDCAR connection is like limited company having limited liability.
The comfortable region automatically assists one in need but with safeguard of own system. Crisis in other regions do not jeopardize helping region as exporter frequency is always higher than importer region – and there is limit setting for export. This works as sure islanding contrary to tentative success in planned islanding in synchronous interconnection.
Existing available HVAC links are useful with back-to-back interconnections or useful for redial assistance to region in need.
Future Scope: Design and development of hardware and software for Controller Unit.
Er. Natvar D. Makwana is a retired senior engineer and served last 26 years on various positions at state load dispatch center and post-retirement 7 years as visiting faculty of Parul Institute of Engineering and Technology.
Paresh R. Modha is perusing Ph D from CVM University, Vallabh Vidyanagar. He had joined academic filed with CHARUSAT University, Changa, during his early stage of career development. At present he is working as an Assistant Professor in the Department of Electrical Engineering, ADIT, New Vallabh Vidyanagar.
