Flexible HVDC
Flexible HVDC
National Power Grid: 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 favorable to manage local peak and off peak demands. Similarly climatic variations particularly in temperature from northern to southern area is favorable 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 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 cause insufficient condenser cooling and low vacuum. Gas turbine generation drops due to low density air. Effect is predominant where more such plants. Holidays are different in different area 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 effected 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 also effected due natural calamities like cyclone, drought, flood, earth
quack etc. Power demand also effected due to agitations like bandh, rasta-roko,
strike etc.
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 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 has 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 location 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. Letter 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 is
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. This regional asynchronous interconnection
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 equipments 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 equipments at some location.
·
System control is difficult with multiple partners.
·
Probability of disturbance spreading from one to other region.
·
Power flow in transmission network is automatic 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 incident.
·
Frequency remains common for all in the grid. Abnormal frequency
operation is detrimental to equipments 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.
·
Regions system frequency is independent of other regions.
·
No scope of spreading disturbance in one region to other due to
isolation.
·
Region can be operated at desired frequency.
·
System operation is better coordinated with limited partners.
·
Require specific set up, equipments and operation expertise.
·
System is slow because of manual operation. Time spent for formalities
and implementation.
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 unfavorable 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. Basic requirement is
followings.
·
Fast
Scanning Frequency Transducers installed at one of the end transmitting data to
control unit.
·
Fast
Scanning Frequency Transducers installed at other end 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
Settings controllers for Transfer Bias, Maximum Power and Ramping Rate
·
Data
link with local and remote frequency sensor and HVDC controller.
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.
1.
Power
injection increase in exporting region
2.
Load
decrease in exporting region
3.
Power
injection reduce in importing region
4.
Load
increase in importing region
Power transfer decrease when D is
negative due to following changes.
1.
Power
injection decrease in exporting region
2.
Load
increase in exporting region
3.
Power
injection increase in importing region
4.
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
Frequency rise from Frn to Frc in
region R due to gain of X MW received from Region S
X = Br
Power transfer X is proportional
difference of frequency between regions.
X = Bt
Solving these three simultaneous
equations for X, Fsc and Frc in term of Bs, Rs, Bt, Fsn and Frn.
We get
X =
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 where as 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: In addition to merits of HVDC link, automatic power transfer is feasible along with safety margin. Synchronous connection is like partnership company having unlimited liability whereas HVDCAR connection is like limited company having limited liability. Comfortable region automatically assist one in need but with safeguard of own system. Crisis in other region do not jeopardize helping region. Exporter frequency is always higher than importer 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 lines are useful for redial
assistance to other region in need.