GO4 - Asyn Interconnection
Grid
Operation
Part 4
System
Interconnection
Asynchronous
Synchronous grids were
developed up to regional level. There were five regional grids WR, ER, NR, SR
and NER. National grid was envisaged with following advantages.
Our country is wide spread from
Kashmir in north to Kanyakumari in south and from Kutchh in west to Arunachal in
east with variety of culture, festivals, weather and time zones.
There is diversity in power
demand due time zones from east to west and temperature zones from north to south
during seasons and time of the day.
Northern area is hilly having
more hydro resources. Maximum hydro power is available during post winter
period because of flood in rivers due to melting of ice. Availability of hydro
power in other areas may be during monsoon.
Thermal generation is also
effected due to local seasonal issues. Coal stacked in open yard gets wet and
lumpy in monsoon due to rain. Generation drops due to problems in grinding and
feeding to boilers. High ambiance effects cooling of condenser and drop in
vacuum cause reduced output. Similarly air volume expands at high temperature. So
less quantity of air is accommodated in combustion chamber of gas turbine. Gas
injection is also regulated accordingly to maintain air fuel ratio and
ultimately generation drops. In this way generations in different area is
affected due to different issues related to climate. In addition to such
foreseen reasons, generation is also effected due incidental unpredictable problems
like tripping of power outlets, disruption in fuel supply chain, strike, storm,
station bus fault, multiple simultaneous breakdowns of generators, etc.
Demand of power is also affected
due various reasons. There is wide variation in power demand for agricultural
load due to rain. Quantum of load drop depends on numbers and size of pump sets
and total rain in the area. Load drop due to national holiday is common everywhere
but is different in different areas due to local festival holidays. Heaters
load may be very high in winter in northern area. Load rise due to fans, air
coolers and air conditioners is very high in other area during summer. In
addition to such foreseen reasons, demand is also affected due natural
calamities like flood, cyclone, draught, earth quack etc and incidental unpredictable
events like strike, line tripping, public agitation, etc.
Sun rise and sun set is earlier
in eastern area and it follows in area from east to west. Local load pattern depends on day/night
timings. Peak demand of the day occurs at different times in these areas. This
phenomenon is beneficial to meet peak demand of the area when demands in other
areas are low.
Generation cost is different in
different areas. Hydro power has no input cost except operational cost. It may
be USE OR LOOSE condition in case of Run-off River or over flowing dams. Coal
based power at pithead is cheaper as no cost of transportation. Cost of power
from thermal plant using coal, lignite, gas or liquid fuel is different.
Ultimately there may be
instances when some systems are facing power shortage or starving of power
whereas other systems are comfortable or surplus in power condition because of variations
in power availability and demand due to above and similar causes. All these can
be well managed for overall equitable supply and optimal energy cost by
integration into National grid.
Therefore National grid was essential
and initiated to interconnect regions. First interconnection tried between WR
and SR. But performance of synchronous operation was not encouraging due to
following.
1. Weak
links between regions. Likely to trip and cause disturbance in both regions
2. Lack
of commercial mechanism for power exchange
3. Undisciplined
operation by constituent with acute power shortage.
4. No
effective authority for inter regional coordination.
Alternatively radial assistance
without interconnection was tried. Some load in border area of deficit region
is isolated and connected to other region. In this way deficit region get
relief by transferring load on other region. Power exported to this area is
radial power assistance by other region. Energy received in this way is payable
by receiving region to exporting region as per mutually agreed rates. But this
was also not workable solution as load in concern area gets power interruption while
transferring to other region and again back to own region. Hence some other
alternative was required.
Solution to this appears as in
our water usual supply control system.
Modification of set up is just
replacement of interconnecting valve IC by Bi-Directional Pump BDP
In this condition transfer of water between the systems A and B is not automatic by level difference. But it is possible to transfer water from system A to B or from system A to B as required by use of bi-directional pump. Both the systems operate independently and when water level in both the systems can be different. Water will not flow from high level tank to low level tank. Water will flow between the systems when pump is operated. Direction of water flow depends on mode of pump and not on levels in the tanks. It may be possible to transfer water from the tank at low level to the tank at higher level.
Such arrangement in power
system was possible by HVDC system. In this system high voltage AC at sending
end is converted to high voltage DC by converter (rectifiers). Power is
transmitted online as high voltage DC. High voltage DC at receiving end is
converted to high voltage AC by converter (inverters). Both the systems remains
interconnected for power transfer but there is no direct AC link. So this is
known as asynchronous interconnections. Practically both the systems are
operating as independent systems. Frequency of both the systems can be
different. Logically sending end is like load in its system whereas receiving
end has sources of generation in its system.
Transfer of power is not
automatic from comfortable system to deficit system as in synchronous
interconnection. It is regulated power transfer as per operation of HVDC
system. Transfer of power is not necessarily from high frequency to low
frequency system. Technically it is possible to transfer power from the system
at low frequency to the system at high frequency. Of course generally such
operation is not required unless in some specific conditions.
HVDC interconnections have
three types of setups. DC system required two conductors in line but monopole
system has only one conductor and ground as second conductor for return path.
But monopole and other homo pole setup is not popular. Most of the HVDC systems
are bipolar type with two line conductors as in the figure.
Concern stations are connected by high voltage DC line through converter/inverter and control block at both ends. AC is converted to high voltage DC at sending end and sent to receiving end on DC line. High voltage DC is converted to matching AC at receiving end. It is possible to transfer power in any direction as required. Quantum and direction of power transfer is as per operators.
Advantages of HVDC
interconnections.
·
Both the systems operate
independently. Disturbance in one system do not reflect on other system. Consider
as Galvanic Isolation.
·
No rise in fault level except
small one as per power imported.
·
Power transfer as
desired. Can be regulated
·
Reduced transmission
losses as compared to Synchronous interconnection.
·
Connected systems can operate
at different frequency.
But again one more hurdle in
the way. Operation and control of high voltage direct current line was problem
at the time. Alternative of this was to use AC line but asynchronous interconnection
through HVDC link. Both ends blocks are installed side by side at a place in any
region and having direct connections without DC line. AC line from other region
is extended up to this. Logically virtual station of other system is created
near this junction and connected via HVDC system as in the figure. This is referred
to as back to back connection.
In this arrangement, all the benefits of asynchronous connection are available like independent operation of each region with controlled power transfer.
Once direct current system was
hurdle for transfer of power at a distance due to high power loss and huge voltage
drop but now direct current system at high voltage turn out to be the solution for
interconnection of power systems and bulk power transfer.
After the development of synchronous
national grid, such HVDC links continue in operated but not as asynchronous
link. Mixed operation of HVDC and AC links is hybrid mode. Hybrid means two
stations which already have synchronous connection via other root and also interconnected
by HVDC link. In this set up the basic function of independent working of
systems is not available. Systems are operating in synchronous mode having
common frequency in both the system and total tie line flow as per ACEs of the
systems. Here HVDC link’s function is to divert the power flow in the network.
Consider two systems A and B
having hybrid interconnection as under. There is AC line between station k in
system A and station e in system B. Because of synchronous interconnection both
systems are operating at same frequency and tie line flows are as per systems
errors. There is other link of HVDC system also between station p in system A
and station t in system B as in the figure.
Broken line between station k and p in system A and between station e and t in system B indicates local network through which they are connected in respective systems. This may be single line as appear in figure or many lines and stations in between.
Assume 120 MW power flow on AC
line from system a (k) to B (e) as per operating systems condition when no
power flow on HVDC link.
Under this condition if 50 MW
power is sent on HVDC link from system A(p) to B(t), than power flow on AC link
reduces to 120-50=70MW.
Or in above condition if 30 MW
power is sent on HVDC link from system B (t) to A (p), than power flow on AC
link increases to 120+30=150MW.
Here HVDC link in hybrid mode
is diverting power flow.
For more general concept, consider
four power systems A, B, C and D having interconnections as under.
A to B, B to C and C to D have
synchronous interconnections trough AC lines as in the figure.
System A and D have no direct link. But have synchronous interconnections via other systems. So provision of HVDC link between system A and D is effectively hybrid connection that can divert flow in the network. Frequency of all the four system remains the same.
In absence of HVDC
link (OFF), the power flow on tie lines are as under.
From system A to
system B is 50 MW.
From system B to
system C is 20 MW.
From system D to
system C is 15 MW.
This is based on power
generation and power demand in all the four systems at particular instant. Net
power import or export by each system is as under.
System A net export is 50 MW.
System B net import is 30 MW.
System C net import is 35 MW.
System D net export is 15 MW.
This condition will continue
till there is no change in power generation and demand in all the four systems.
Change in generation or demand in any one or more system may alter the net
export/import of all the four systems. But transfer of power on HVDC link has
no effect on net export/import of the systems.
In this condition
transfer of 25 MW on HVDC from system A to system D will change tie line flows
as under.
From system A to
system B will be 25 MW.
From system C to
system B will be 05 MW.
From system D to
system C will be 40 MW.
But import/export of each
system remains the same as earlier.
Power flow on various links has
changed as under.
Power flow from system A to B
has reduced from 50 MW to 25 MW.
Power flow from system B to C
has reversed from 20 MW to -o5 MW.
Power flow from system D to C
has increased from
15 MW to 40 MW.
Instead of transferring
25 MW from system A to D, when 20 MW transferred from system D to system A on
HVDC link, the link line flow will be as under.
From system A to
system B is 70 MW.
From system B to
system C is 40 MW.
From system C to
system D is 05 MW.
Power flow on various links has
changed as under.
Power flow from system A to B
has increased from 50 MW to 70MW
Power flow from system B to C
has increased from 20 MW to 40MW.
Power flow from system D to C
has reversed from 15 MW to -05MW
System operator may decide based
on power flow in the network the requirement of power transfer on DC link for safe
operating condition.
Systems having hybrid interconnection behave like synchronous connections. Direction and amount of power flow between the systems is according to control errors of the systems. Power transfer on DC link diverts the route of power flow. This phenomenon is helpful for management of conjunction in tie lines or even internal network of any system.
Similar effect of diversion of
power flow can also be achieved by using Phase Shifting
Transformers (PST).
Phase shifting transformers are
the device wherein phase is shifted between primary and secondary. Normal
delta/star transformers of vector group Dy01 or Dy11 has phase shift of 30
degree lead or lag. But this phase shift is fixed. Phase shift is adjustable in
degree and direction (lead or lag) as required by phase shifting transformer. Application
is similar as discussed for HVDC link.
There are two types of phase
shifting transformers.
Quadrant Booster Type.
Secondary phase voltage of
transformer is boosted by quadrant voltage derived by magnetic coupling from
remaining two phases as in the figure. Only one phase arrangement is shown for
simplicity of diagram. Reversing switch is to toggle between leading and
lagging boost. Tapping on booster transformer is to adjust the phase shift
angle.
Other
phases have similar arrangement. Reversing switch and tapping switch of booster
transformer have gang operation for all the three phases. Phase shift occurs in
steps as per tapping in booster transformer.
Moving Secondary Type
Primary
and secondary windings of the transformer are on stator rotor type arrangement.
Rotor element is not free to spin. It is locked with gear to shift its position
with respect to stator element as in the figure. Phase shift is achieved by
changing relative position of rotor element by gear. Leading or lagging phase
shift is according to clockwise or anticlockwise shift of rotor element. In
this case phase shift is smooth without steps.
This
type of phase shifting devices for diversion of power flow is better compared
to HVDC system in cost, construction, installation and operation.
Now National
grid has HVDC links operating as hybrid interconnections. All these HVDC links
were installed earlier for asynchronous interconnections amongst regional grids
to form National grid when synchronous operation was not feasible. After
formation of synchronous National grid, these HVDC links are useful as power
flow diversion to manage conjunction in the network. But any new link is
expected to be normal AC link or AC with PST where need of power flow diversion
is expected.