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System Security andAncillary Services
Daniel Kirschen
Daniel Kirschen 2005 2
Introduction
Electricity markets rely on the power system infrastructure
Participants have no choice to use a different system
Cost to consumers of outages is very high
Consumer have expectations for continuity of service
Cost of this security of supply must match its benefit
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System security
System must be able to operate continuously if situationdoes not change
System must remain stable for common contingencies
u Fault on a transmission line or other component
u Sudden failure of a generating unit
u Rapid change in load
Operator must consider consequences of contingencies
Use both:
u Preventive actions
u Corrective actions
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Preventive actions
Put the system in a state such that it will remain stable ifa contingency occurs
Operate the system at less than full capacity
Limit the commercial transactions that are allowed
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Corrective actions
Taken only if a disturbance does occur
Limit the consequences of this disturbance
Need resources that belong to market participants
Ancillary services that must be purchased from themarket participants by the system operator
When called, some ancillary services will deliver someenergy
However, capacity to deliver is the important factor
Remuneration on the basis of availability, not energy
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Outline
Describe the needs for ancillary servicesu Keeping the generation and load in balance
u Keeping the security of the transmission network
Obtaining ancillary servicesu How much ancillary services should be bought?
u How should these services be obtained?
u Who should pay for these services?
Selling ancillary servicesu Maximize profit from the sale of energy and ancillary services
Needs for ancillary services
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Balancing production and consumption
Assume that all generators, loads and tie-lines areconnected to the same bus
Only system variables are total generation, total load andnet interchange with other systems
Generation Load Interchanges
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Balancing production and consumption
If production = consumption, frequency remains constant
In practice:
u Constant fluctuations in the load
u Inaccurate control of the generation
u Sudden outages of generators and interconnectors
Excess load causes a drop in frequency
Excess generation causes an increase in frequency
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Balancing production and consumption
Generators can only operate within a narrow range offrequencies
u Protection system disconnects generators when frequency is toohigh or too low
u Causes further imbalance between load and generation
System operator must maintain the frequency withinlimits
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Balancing production and consumption
Rate of change in frequency inversely proportional tototal inertia of generators and rotating loads
Frequency changes much less in large interconnectedsystems than in small isolated systems
Local imbalance in an interconnected system causes achange in tie-line flows
Inadvertent flow
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Balancing production and consumption
Inadvertent flows can overload the tie-lines
Protection system may disconnect these lines
Could lead to further imbalance between load andgeneration
Each system must remain in balance
Inadvertent flow
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Balancing production and consumption
Minor frequency deviations and inadvertent flows are notan immediate threat
However, they weaken the system
Must be corrected quickly so the system can withstandfurther problems
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Example: load over 5 periods
0
50
100
150
200
250
300
1 2 3 4 5 Period
Load [MW]
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Example: energy traded
0
50
100
150
200
250
300
1 2 3 4 5 Period
Load [MW]
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Example: energy produced
0
50
100
150
200
250
300
1 2 3 4 5 Period
Load [MW]
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-150
-100
-50
0
50
100
1 2 3 4 5 Period
Imbalance [MW]
Example: imbalance
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-150
-100
-50
0
50
100
1 2 3 4 5 Period
Imbalance [MW]
Example: imbalance with trend
Random loadfluctuations
Slower loadfluctuations Outages
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Example (continued)
Differences between load and energy traded:
u Does not track rapid load fluctuations
Market assumes load constant over trading period
u Error in forecast
Differences between energy traded and energy produced
u Minor errors in control
u Finite ramp rate at the ends of the periods
u Unit outage creates a large imbalance
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Balancing services
Different phenomena contribute to imbalances
Each phenomena has a different time signature
Different services are required to handle thesephenomena
Exact definition differ from market to market
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Regulation service
Designed to handle:
u Rapid fluctuations in load
u Small, unintended variations in generation
Designed to maintain:
u Frequency close to nominal
u Interchanges at desired values
Provided by generating units that:
u Can adjust output quickly
u Are connected to the grid
u Are equipped with a governor and usually are on AGC
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Load following service
Designed to handle intra-period load fluctuations
Designed to maintain:
u Frequency close to nominal
u Interchanges at desired values
Provided by generating units that can respond at asufficient rate
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Reserve services
Designed to handle large and unpredictable deficitscaused by outages of generators and tie-lines
Two main types:
u Spinning reserve
Start immediately
Full amount available quickly
u Supplemental reserve
Can start more slowly
Designed to replace the spinning reserve
Definition and parameters depend on the market
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Classification of balancing services
Regulation and load following services:
u Almost continuous action
u Relatively small
u Quite predictable
u Preventive security actions
Reserve services:
u Use is unpredictable
u Corrective security actions
u Provision of reserve is a form of preventive security action
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Example: Outage of large generating unit
49.20
49.30
49.40
49.50
49.60
49.70
49.80
49.90
50.00
50.10
12:2
4:00
12:2
4:30
12:2
5:00
12:2
5:30
12:2
6:00
12:2
6:30
12:2
7:00
12:2
7:30
12:2
8:00
12:2
8:30
12:2
9:00
12:2
9:30
Primary response
Secondary response
Gas turbines
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Network issues: contingency analysis
Operator continuously performs contingency analysis
No credible contingency should destabilize the system
Modes of destabilization:
u Thermal overload
u Transient instability
u Voltage instability
If a contingency could destabilize the system, theoperator must take preventive action
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Types of preventive actions
Low cost preventive actions:
u Examples
Adjust taps of transformers
Adjust reference voltage of generators
Adjust phase shifters
u Effective but limited
High cost preventive actions:
u Restrict flows on some branches
u Requires limiting the output of some generating units
u Affect the ability of some producers to trade on the market
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Example: thermal capacity
Each line between A and B is rated at 200 MW
Generator at A can sell only 200 MW to load at B
Remaining 200 MW must be kept in reserve in case ofoutage of one of the lines
A B
Load
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Example: emergency thermal capacity
Each line between A and B is rated at 200 MW
Each line has a 10% emergency rating for 20 minutes
If generator at B can increase its output by 20 MW in 20minutes, the generator at A can sell 220 MW to load at B
A B
Load
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Example: transient stability
Assumptions:
u B is an infinite bus
u Transient reactance of A = 0.9 p.u., inertia constant H = 2 s
u Each line has a reactance of 0.3 p.u.
u Voltages are at nominal value
u Fault cleared in 100 ms by tripping affected line
Maximum power transfer: 108 MW
A B
Load
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Example: voltage stability
No reactive support at Bu 198 MW can be transferred from A to B before the voltage at B
drops below 0.95 p.u.
u However, the voltage collapses if a line is tripped when powertransfer is larger than 166 MW
The maximum power transfer is thus 166 MW
A B
Load
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Example: voltage stability
25 MVAr of reactive support at Bu 190 MW can be transferred from A to B before the outage of a
line causes a voltage collapse
A B
Load
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Voltage control and reactive support services
Use reactive power resources to maximize active powerthat can be transferred through the transmission network
Some of these resources are under the control of thesystem operator:
u Mechanically-switched capacitors and reactors
u Static VAr compensators
u Transformer taps
Best reactive power resources are the generators
Need to define voltage control services to specify theconditions under which the system operator can usethese resources
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Voltage control and reactive support services
Must consider both normal and abnormal conditions
Normal conditions:u 0.95 p.u. V 1.05 p.u.
Abnormal conditions:u Provide enough reactive power to prevent a voltage collapse
following an outage
Requirements for abnormal conditions are much moresevere than for normal conditions
Reactive support is more important than voltage control
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Example: voltage control under normal conditions
Load at B has unity power factor
Voltage at A maintained at nominal value
Control voltage at B?
A B
Load
X=0.6 p.u.R=0.06 p.u.
B=0.2 p.u. B=0.2 p.u.
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-40
-20
0
20
40
60
80
0 20 40 60 80 100 120 140 160 180 200 220
Active Power Transfer [MW]
Reac
tive
Pow
er In
ject
ion
[MV
Ar]
0.9
0.95
1
1.05
1.1
Vol
tage
[p.
u.]
Example: voltage control under normal conditions
Reactive injection at BVoltage at B
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Example: voltage control under normal conditions
Controlling the voltage at B using generator at A?
Local voltage control is much more effective
Severe market power issues in reactive support
A B
Load
Power Transfer [MW] VB [p.u.] VA [p.u.] QA [MVAr]49.0 1.05 0.95 -68.3
172.5 0.95 1.05 21.7
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Example: reactive support following line outage
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140
Power Transfer [MW]
Post
-con
tinge
ncy
reac
tive
pow
er in
ject
ion
at b
us B
[M
VA
r]
A B
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Example: pre- and post-contingency balance
A B
130 MW0 MVAr
68 MW
13 MVAr 0.6 MVAr136 MW
26 MVAr
68 MW
13 MVAr
65 MW
0.6 MVAr
65 MW
1.2 MVAr
0 MW
1.0 p.u.1.0 p.u.
A B
130 MW0 MVAr
145 MW
40 MVAr
145 MW
40 MVAr 67 MVAr
130 MW
67 MVAr
0 MW
1.0 p.u.1.0 p.u.
Pre-contingency:
Post-contingency:
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Other ancillary services
Stability services
u Intertrip schemes
Disconnection of generators following faults
u Power system stabilizers
Blackstart restoration capability service
Obtaining ancillary services
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Obtaining ancillary services
How much ancillary services should be bought?
How should these services be obtained?
Who should pay for these services?
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How much ancillary services should be bought?
System Operator purchases the servicesu Works on behalf of the users of the system
Services are used mostly for contingenciesu Availability is more important than actual usage
Not enough servicesu Cant ensure the security of the system
u Cant maintain the quality of the supply
Too much servicesu Life of the operator is easy
u Cost passed on to system users
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How much ancillary services should be bought?
System Operator must perform a cost/benefit analysisu Balance value of services against their cost
Value of services: improvement in security and servicequality
Complicated probabilistic optimization problem
Should give a financial incentive to the operator toacquire the right amount of services at minimum cost
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How should services be obtained?
Two approaches:
u Compulsory provision
u Market for ancillary services
Both have advantages and disadvantages
Choice influenced by:
u Type of service
u Nature of the power system
u History of the power system
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Compulsory provision
To be allowed to connect to the system, generators maybe obliged to meet some conditions
Examples:
u Generator must be equipped with governor with 4% droop
All generators contribute to frequency control
u Generator must be able to operate from 0.85 lead to 0.9 lag
All generators contribute to voltage control and reactive support
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Advantages of compulsory provision
Minimum deviation from traditional practice
Simplicity
Usually ensures system security and quality of supply
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Disadvantages of compulsory provision
Not necessarily good economic policy
u May provide more resources than needed and causeunnecessary investments
Not all generating units need to help control frequency
Not all generating units need to be equipped with a stabilizer
Discourages technological innovation
u Definition based on what generators usually provide
Generators have to provide a costly service for free
u Example: providing reactive power increases losses and reducesactive power generation capacity
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Disadvantages of compulsory provision
Equityu How to deal with generators that cannot provide some services?
Example: nuclear units cant participate in frequency response
Economic efficiencyu Not a good idea to force highly efficient units to operate part-
loaded to provide reserve
u More efficient to determine centrally how much reserve is neededand commit additional units to meet this reserve requirement
Compulsory provision is thus not applicable to allservices
How to deal with exceptions that distort competition?
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Market for ancillary services
Different markets for different services
Long term contracts
u For services where quantity needed does not change and availabilitydepends on equipment characteristics
u Example: blackstart capability, intertrip schemes, power systemstabilizer, frequency regulation
Spot market
u Needs change over the course of a day
u Price changes because of interactions with energy market
u Example: reserve
System operator may reduce its risk by using a combination of spotmarket and long term contracts
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Advantages of market for ancillary services
More economically efficient than compulsory provision
System operator buys only the amount of service needed
Only participants that find it profitable provide services
Helps determine the true cost of services
Opens up opportunities for innovative solutions
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Disadvantages of market for ancillary services
More complex
Probably not applicable to all types of services
Potential for abuse of market power
u Example: reactive support in remote parts of the network
u Market for reactive power would need to be carefully regulated
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Demand-side provision of ancillary services
Creating a market for ancillary services opens up anopportunity for the demand-side to provide ancillaryservices
Unfortunately, definition of ancillary services often stillbased on traditional practice
In a truly competitive environment, the system operatorshould not favour any participant, either from the supply-or demand-side
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Advantages of demand-side provision
Larger number of participants increases competition andlowers cost
Better utilization of resources
u Example:
Providing reserve with interruptible loads rather than partly loadedthermal generating units
Particularly important if proportion of generation from renewablesources increases
Demand-side may be a more reliable provider
u Large number of small demand-side providers
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Opportunities for demand-side provision
Different types of reserve
u Interruptible loads
Frequency regulation
u Variable speed pumping loads
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Who should pay for ancillary services?
Not all users value security and quality of supply equally
u Examples:
Producers vs. consumers
Semi-conductor manufacturing vs. irrigation load
Ideally, users who value security more should get moresecurity and pay for it
With the current technology, this is not possible
u System operator provides an average level of security to all users
u The cost of ancillary services is shared by all users on the basisof their consumption
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Who should pay for ancillary services?
Sharing the cost of ancillary services on the basis ofenergy is not economically efficient
Some participants increase the need for services morethan others
These participants should pay a larger share of the costto encourage them to change their behaviour
Example: allocating the cost of reserve
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Who should pay for reserve?
Reserve prevents collapse of the system when there is alarge imbalance between load and generation
Large imbalances usually occur because of failure ofgenerating units
Owners of large generating units that fail frequentlyshould pay a larger proportion of the cost of reserve
Encourage them to improve the reliability of their units
In the long term:
u Reduce need for reserve
u Reduce overall cost of reserve
Selling ancillary services
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Selling ancillary services
Ancillary services are another business opportunity forgenerators
Limitations:
u Technical characteristics of the generating units
Maximum ramp rate
Reactive capability curve
u Opportunity cost
Cant sell as much energy when selling reserve
Need to optimize jointly the sale of energy and reserve
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Example: selling both energy and reserve
Generator tries to maximize the profit it makes from thesale of energy and reserve
Assumptions:
u Consider only one type of reserve service
u Perfectly competitive energy and reserve markets
Generator is a price-taker in both markets
Generator can sell any quantity it decides on either market
u Consider one generating unit over one hour
Dont need to consider start-up cost, min up time, min down time
u No special payments for exercising reserve
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pi1
pi 2
x2
x1
Pmin
Pmax
Rmax
C1(x1)
C2 (x2 )
Notations
: Market price for electrical energy (/MWh)
: Market price for reserve (/MW/h)
: Quantity of energy bid and sold
: Quantity of reserve bid and sold
: Minimum power output
: Maximum power output
: Upper limit on the reserve (ramp rate x delivery time)
: Cost of producing energy
: Cost of providing reserve (not opportunity cost)
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Formulation
f (x1, x2 ) = pi1x1 + pi 2x2 C1(x1)C2 (x2 )
x1 + x2 Pmax
x1 Pmin
Objective function:
Constraints:
x2 Rmax Rmax < Pmax Pmin(We assume that )
l(x1, x2 , 1, 2 , 3 ) = pi1x1 + pi 2x2 C1(x1)C2 (x2 )+ 1(P
max x1 x2 ) + 2 (x1 Pmin ) + 3(R
max x2 )
Lagrangian function:
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Optimality conditions
lx1
pi1 dC1dx1
1 + 2 = 0
lx2
pi2 dC2dx2
1 3 = 0
l1
Pmax x1 x2 0
l2
x1 Pmin 0
l3
Rmax x2 0
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Complementary slackness conditions
1 (Pmax x1 x2 ) = 0
2 (x1 Pmin ) = 0
3 (Rmax x2 ) = 0
1 0; 2 0; 3 0
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Case 1:
No binding constraints
Provide energy and reserve up to the point wheremarginal cost is equal to price
No interactions between energy and reserve
1 = 0; 2 = 0; 3 = 0
lx1
pi1 dC1dx1
1 + 2 = 0 dC1dx1
= pi1
lx2
pi2 dC2dx2
1 3 = 0!!!!dC2dx2
= pi2
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Case 2:
Generation capacity fully utilized by energy and reserve:
Marginal profit on energy equal to marginal profit on reserve
1 > 0; 2 = 0; 3 = 0
lx1
pi1 dC1dx1
1 + 2 = 0
lx2
pi2 dC2dx2
1 3 = 0
x1 + x2 = Pmax
pi1 dC1dx1
= pi2 dC2dx2
= 1 0
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Case 3:
Unit operates at minimum stable generation
Marginal profit on reserve
Marginal loss on energy minimized by operating at minimum
KKT conditions guarantee only marginal profitability, not actual profit
1 = 0; 2 > 0; 3 = 0
lx1
pi1 dC1dx1
1 + 2 = 0
lx2
pi2 dC2dx2
1 3 = 0
x1 = Pmin
dC1dx1
pi1 = 2
dC2dx2
= pi2
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Cases 4 & 5: 1 > 0; 2 > 0; 3 = 0 1 > 0; 2 > 0; 3 > 0
Rmax < Pmax PminSince we assume that these cases are not interesting because the upper and lower limits cannot be binding at the same time
x1 + x2 Pmax1 :
x1 Pmin2 :
x2 Rmax3 :
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Case 6:
Reserve limited by ramp rate
Maximum profit on energy
Profit on reserve could be increased if ramp rate constraint could berelaxed
1 = 0; 2 = 0; 3 > 0
lx1
pi1 dC1dx1
1 + 2 = 0
lx2
pi2 dC2dx2
1 3 = 0
dC1dx1
= pi1
pi2 dC2dx2
= 3
x2 Rmax
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Case 7:
Maximum capacity and ramp rate constraints are binding
Sale of energy and sale of reserve are both profitable
Sale of reserve is more profitable but limited by the ramp rateconstraint
1 > 0; 2 = 0; 3 > 0
lx1
pi1 dC1dx1
1 + 2 = 0
lx2
pi2 dC2dx2
1 3 = 0
pi1 dC1dx1
= 1
pi2 dC2dx2
= 1 + 3
x2 = Rmax
x1 + x2 = Pmax
x1 = Pmax Rmax
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Case 8:
Generator at minimum output and reserve limited by ramp rate
Sale of reserve is profitable but limited by ramp rate constraint
Sale of energy is unprofitable
Overall profitability needs to be checked
1 = 0; 2 > 0; 3 > 0
lx1
pi1 dC1dx1
1 + 2 = 0
lx2
pi2 dC2dx2
1 3 = 0
pi1 dC1dx1
= 2
pi2 dC2dx2
= 3
x2 = Rmax
x1 = Pmin