GO1-Standalone Systems
Grid
Operation
Part 1
Standalone
Systems
Elementary power systems were very simple having generator and loads connected to lines from generator. Mostly these were direct current systems. There was limitation on generator capacity as power generated in armature has to be drawn via sliding contacts of commutator brush assembly. High current through sliding contact cause heavy sparking. Letter on alternating current system were available with higher capacity generators. Till this time power systems has only two elements Generation and Distribution. Adding generator and operate in parallel was necessitated with increased load beyond capacity of single generator. These systems were standalone type catering local loads in town/city. Catering load at faraway was not feasible due constraint of high power loss and high voltage drop. Such power systems were developed, operated and mange by private party as commercial activity. Roads and streets had telephone line on one side and electric line on other side. So it was not practically feasible to have more electricity distributors in any area. But other area had other distributor. So it was monopoly business requiring license to generate and distribute electricity and such companies were identified as licensee. All licensees had demarcated area for its service where other operator cannot supply electricity.
In the chart, X axis represents frequency and Y axis active power generation and load. Generation line is droop characteristic representing variation of generation with respect to frequency. Initial load is L1 represented by horizontal load line. Intersection of the two lines at 1 is operating point at frequency F1 and having load L1 and generation also L1. Increased load L2 is represented by horizontal line shifted upward. New operating point is at 2 the intersection of new load line with generation line. Here with frequency dropping to F2 but generation picking L2. This is how the stand alone system having mostly lighting load was operating.
Developed systems have variety of loads. Motors were widely used for driving various types of loads. Frequency regulation was better in such developed system. Study of working of water supply system here under may elucidate the concept.
Layout is as per diagram in which a water tank of 5000 liters capacity is having 2500 liters of water i.e. water level is at 50% in the tank. Water to this tank is supplied from very big water reserve via incoming pipe line extended up to bottom of tank and inflow control by valve CGA. Similarly outgoing line to some industrial process has outflow control valve CLA. Initially opening of the valves CGA and CLA are adjusted such that water inflow is 40 liters per minute and outflow is also 40 liters per minute. So water in tank is maintained 50% level.
After some time, water
requirement in process increased to 45 liters per minute. Accordingly flow
increased by opening valve CLA only. Water level in the tank starts dropping
due to shortage of 45-40=5 liters per minutes. It is presumed that tank will be
empty after 2500/5 = 500 minutes.
Before any conclusion in the matter let us review phenomenon observed in routine. Most of the houses have ground level water tank in back yard for household work like cloth washing, utensil cleaning etc. Water is drawn through tap at lower part of the tank. Rate of water outflow is regulated by tap opening. It is observed that water rush very fast even with partial opening of the tap when tank is full. But water flow is very slow even with full opening of the tap when tank is about to empty. This fact is known to everyone who are not engineer like house maid, house wife or children etc. Observed slow flow signals concern to refill the tank before it is completely empty. This means flow of water is influence by opening of valve as well pressure of water head.
Same thing happens in above water supply system also. Shortage of 5 liters of water per minute causes water level in tank to drop. This drop in water level has two fold effects.
Head HGA increases. Inflow
through valve CGA increases even though no change in its opening. Let inflow be
20+d liters per minute.
Head HLA decreases. Outflow
through CLA decreases even though no change in its opening. Let outflow be 25-d liters
per minute.
This process of drop in water
level, increase in inflow and decrease in outflow will continue till inflow and
outflow match. Thereafter system will be stable with new parameters. Inflow and
outflow may be somewhere between 20 and 25 liters per minute with water level lower
than 50%.
Consider other possibility when
requirement of process water reduced from 20 liters to 15 liters per minute. Water
level in the tank will rise due surplus inflow of 20–15=5 liters per minute. Consequently
inflow through valve CGA will decrease to 20-d due to
decrease in head HGA. Outflow through valve CLA will increase to 15+d due to
increase in head HLA. Process of rising water level, decreasing inflow and
increasing outflow will continue till inflow and outflow match somewhere
between rate 20 and 15 liters per minute with water level above 50%. Thereafter
system will be stable with new parameters.
Similarly auto corrective
action occurs when there is no change in process water requirement but inflow
line valve CGA is inadvertently operated and reduced inflow to 15 liters per
minute. This causes level reducing, head HGA increasing, inflow increasing to
15+d, head HLA decreasing, outflow decreasing to 20-d till
both flow match with rate between 15 and 20 liters per minute and water level lower
than 50%.
In case inadvertent operation
of CGA has increased inflow, then also it would have been stable after changes
of inflow and out flow to match at water level higher than 50%.
Above analogy represents grid
system substituting the elements as under.
Water tank is grid system.
50% level is indicative of 50
Hz frequency.
CGA is governor setting
HGA is droop characteristic.
CLA is load in service.
HLA is frequency sensitive load
characteristic.
Water inflow is power injected
in the grid.
Water outflow is power drawn by
load in service.
Frequency Sensitive Load
Characteristic’s explanation is as under.
About load and its power
demand.
Load draws power when connected
to supply. Power drawn by load at any time is its power demand. Generally it is
considered that power demand by load depends on its capacity. But in fact other
factors also influence power drawn by load. Ten horse power motor expected to
draw about 15 Amps as per rating. But at no load and partial load current may
be less than 15 Amps. Power drawn by lamp depends on supply voltage. Reduction
of supply voltage to fan reduces fan speed but reduction of supply voltage to
pump set increase the load current. Similarly power drawn by loads depends on
supply frequency also. Response of frequency on power demand is also different
with different type of loads. Broadly loads can be classified in four
categories.
Category fn0 type.
(n0 stands for nearly 0)
Frequency response of such load
is (fa/fn)0.
fa = actual frequency,
fn= normal frequency
There is no change in power
demand by such loads due to change in supply frequency. Mostly resistive load
like lamps, heaters etc have no effect of frequency on power drawn.
Category fn1 type.
(n1 stands for nearly 1) Frequency
response of such load is (fa/fn)1.
Power demand by such load changes
in proportion to change in frequency. Motor coupled to constant torque load
have such response.
Motor Revolution per Minute (RPM)
= Hz × 60 × 2/poles.
Revolution per second = Hz × 2/p
One revolution = 2 × π radians
Angular velocity = ω = 2 × π × Hz
× 2/p radians per second
Power = angular velocity
multiply by torque. = ω × T = 2 × π × Hz × 2/p × T
= 4 × π × T × Hz/p radians per second
Power is proportional to
frequency.
Category fn2 type. (n2
stands for nearly 2) Frequency response
of such load is (fa/fn)2.
Power demand of such load changes
proportional to square of the change in frequency. Motor coupled to air
handling systems may have such response.
Category fn3 type. (n3
stands for nearly 3) Frequency response of such load is (fa/fn)3.
Power demand of such load changes
proportional to cube of the change in frequency. Motor
coupled to pumping load have such response.
Load response frequency index
n0, n1, n2 & n3 are not exactly 0, 1, 2 & 3. There may be slight
deviation depending upon setup and coupled devices. For example effective
resistance changes with change in frequency due to change in skin effect. Power
= V2/R. Therefore power drawn by such load may change with change in
frequency but effect is negligible and ignored. It means n0 is not exactly zero.
Actual load of any power system
is mix of above categories. Proportion of such load is not known and not fixed.
Proportion is also changing with time. During night hours lighting load is more
and industrial load is less. During monsoon irrigation pumping load is almost
out. Summer has more load of fan and compressors of air conditioners. Exact
value of n0, n1,n2 & n3 is also not known. So it is rather impossible to
evaluate analytically overall response on power system due to frequency
variation. Only way is to derive from observed results. However as thumb rule
system demand changes 3 to 4 percent per Hz change in frequency. This is known
as system bias. I have derived system bias from load data somewhere in 1998-99.
It was 3.5% and was adopted in western region.
Power system also behaves the
way as seen in water supply system. Power system is stable and operating at 50
Hz when power input by generator and power drawn by load are equal. System
frequency drops while load is added in the system because input power is less
than output.
Due to drop in frequency governor
increases mechanical input to prime mover to generate more power by generator.
Also drop in frequency cause
drop in system demand due to frequency dependant load as discussed above.
Dropping frequency, increasing power
input and reducing power drawal continue till power input match with power
drawn. Thereafter system will be stable at lower frequency.
Reverse will happen when there
is load drop. Frequency is increasing, generation is decreasing and demand is increasing.
This may continue till match and stable at higher frequency.
Reduction in generation has also
similar reactions. System frequency is dropping, generation on other units
picking due to governor and power demand by loads are reducing. Finally system
is stabilizing by matching of generation and demand at lower frequency.
In view of the above frequency chart now modify as above. Generation line is droop characteristic of generator as usual indicating variation in generation with frequency due to governor control. Demand line indicates variation in demand with frequency at specified load in circuit. System is stable at point O with generation G0 equal to demand Do and frequency FO. With addition of load, demand line is shifted upward as shown. Operating point is temporary shifts to point X on shifted demand line. Frequency is still FO and generation is GO but demand has increased from DO to DX. This is unstable condition as demand is higher than generation and frequency is dropping. Generation improves from GO to GN moving from O to N and demand drops from DX to DN moving from X to N while frequency drop from FO to FN till stable condition when GN = DN. Frequency would have dropped up to FM. Frequency drop is restricted due to frequency dependant load.
Stand alone system has following limitations.
Small System Bias –>
Frequency variation is large with small mismatch in
load
and generation.
Low System Inertia –>
Frequency variation is fast leaving no scope for corrective action.
System is unstable due to large
and fast frequency variations. Possibility of system collapse is more.
Solution of this is with grid system.