GO2-Grid System
Grid
Operation
Part 2
Grid System
Standalone systems were small
and unstable as expansion was limited to local load only. But transformer changed
the scenario. Current for specified load is inversely proportional to the
operating voltage. Power loss on line is proportional to square of the current
and voltage drop on line is also proportional to line current. So power loss
and voltage drop problem can be solved by sending power at high voltage. It was
possible to change voltage as required by transformer. Also voltage at
receiving end can be adjusted as required by transformer tapping. So feeding
loads at faraway become practically feasible without any technical hitch. Sending
end voltage was raised by step up transformer and power transmitted to faraway
loads at high voltage. Receiving end voltage again reduced by step down
transformer as required and fed to the loads. In this way third element Transmission was added in power system and systems
expansion started.
Parallel operation of generators
at different location also became feasible by interconnection of such expanded
systems. It was a step toward grid formation and thereafter more and more generating
stations and load centers were integrated to form large grid systems and
ultimately forming state grid. During the process individual licenses were
discouraged and extinguished. State electricity board emerged as single license
for the state. It was transformation from private sector to public sector. State electricity board became the single largest
licensee for the state with state grid. Occasionally, this is referred as
vertical integration of power systems. Power plants were demarcated as base
load stations and peaking stations for regulating generation with the load.
Cheap power plants included in base load group and costlier power plants in peaking
group for optimum economic operation. Accordingly peaking stations were operated
for regulation with active governor mode whereas base load stations were operated
with full load.
Review of water supply system
with multiple in/out flows
Now let outflow increased to 45
liters per minute by opening valve CLA. Now total inflow is 40+50=90 liters per
minute whereas total outflow is 45+50=95 liters per minute. So there is shortage
of 95-90=5 liters per minute. Hence water level of the tank starts dropping. Consequently
head HG will increase and inflow from CGA as well CGB also increase to 40+d and 50+d
respectively. So also head HL will decrease and outflow from CLA as well CLB
also decrease to 45-d and 50-d
respectively. This will continue till total inflow and total outflow match.
Thereafter it will be stable with water at that lower level.
Water level had dropped also
but drop is smaller than earlier with single inflow and single outflow lines. But
now drop in level and change in head is effective to both inflows and both
outflows. So the corrective action is almost double. If there were multiple inflow
lines and multiple outflow lines, corrective action would be very fast and level
drop is very small. This will happen in any case of load adding/tripping or
generation adding/tripping.
Power system also has similar
effect. Grid system has better control on frequency as compare to standalone
system. Frequency may drop due to addition of load anywhere in the system. But
frequency drop activates all the generators in the system to pickup generation.
Similarly power demand by all the loads in the system drops according to their
response index. Ultimately drop in frequency is very fast and very small as
compared to standalone system as hereunder in the chart. Stiffness of the
system is better with larger grid.
Broken lines represent condition of standalone system. System may stabilize at N’ after increase in load from DO to DX and frequency drops from FO to F’N. Solid lines represent condition in grid mode. System stabilize at N after increase in load from DO to DX and frequency drops from FO to FN. Slop of generation line and demand line is high due to multiple corrective responses in grid mode. Hence frequency drop is small and fast.
Advantage of grid mode
Higher System Bias --> Frequency
Deviation is Restricted
Frequency changes when there is
mismatch in power generation and demand
Change in frequency = Unbalance
in load and generation / System bias
Higher the system bias smaller is
the change in frequency for the same unbalance.
Similarly
For linear motion, Force F =
mass × acceleration = m × dv/dt
For angular motion, Force F =
Moment of Inertia × dω/dt
Frequency is angular velocity
of electricity.
Unbalance in load and
generation is operating force
Unbalance = df/dt ×
System Inertia.
But the unbalance is reducing
due to rise in generation and drop in demand with drop in frequency.
So df/dt, the
rate of change of frequency is also reducing tending to zero as under
Higher the System Inertia.-->
Slower the Frequency Variation
Slow variation in frequency has
scope for corrective action.
Synchronous grid system has
major advantage that load anywhere in the system can be met by generation
anywhere in the system. This enables system to operate at most economical way
by merit order generation scheduling. Of course care has to be taken for system
losses and network constraint.
Grid mode frequency control criteria.
Power injection PI = Power
generated by generator – auxiliary consumption.
Gross Load GL = Power Demand by
active loads in the system including system losses.
Unbalance = Net injection –
Gross Load.
Frequency is normal and steady when
Unbalance is zero.
Frequency increase when Unbalance
is positive.
Frequency decrease when Unbalance
is negative.
