Can a slack bus have load on it? While simulating in MATLAB I placed load on a slack bus and nothing was wrong in simulation. But I am curious about the theory of slack bus here and I assume that its theoretically wrong to place a load on a slack bus. But I dont have a valid explanation for it.
In the General Load Flow Problem the Bus wit Largest Generating Capacity is taken as the Slack Bus or Swing Bus.The Slack Bus Voltage is taken to be 1.0 + j 0 P.U. and should be capable of supplying total Losses in the System.As such we may think that if it supplies Local Load,the Slack Bus Voltage Restriction may be Violated.Not so,because this Local Load can also be treated as Loss to be supplied by the Generation at the Slack Bus.
The statement of Chittesh V.C that But usually the generator bus are only having station auxiliary which may be only up to 3% of total generation is only a Thumb Rule. If the Generation at Slack Bus is More it can take More Load connected to the Slack Bus.
Hence We can treat that as stated by Junjion Qi, A slack bus is usually a generator bus with a large real and reactive power output. It is assumed that its real and reactive power outputs are big enough that they can be adjusted as required in order to balance the power in the whole system so that the power flow can be solved. A slack bus can have load on it because in real systems it is actually the bus of a power plant, which can have its own load. It also takes care of the Line losses
But we have a Different Situation when we go for Economic Load Dispatch in Systems having Renewable Energy Sources also.This is an interesting idea.
Distributed generation has been growing rapidly in power systems. Previously, one dominant source, the substation, existed in distribution systems. Now, the consistent supply of energy from DGs, the number of sources and the percen
tage of real power injections from DGs have significantly increased.
Refer to the Dissertation:
Distributed Slack Bus Model for Qualitative Economic Load Dispatch
Soumya Ranjan Panda
Link: http://ethesis.nitrkl.ac.in/4877/1/109EE0292.pdf
Economic Load Despatch(ELD) scheme searches for a load flow solution
out of many which optimally satisfies all the technical and most of the economic constraints of the power system. Since total cost of generation is a function of indi-
vidual production of the generating units, the system constraints will decide
the total cost of the system
BURDEN ON SLACK BUS The slack bus accounts for two functions in a load flow. It can serve as a virtual reference for other buses in the system with its phase being assigned as zero. The second purpose is of acting as a source (as well as sink) for the unaccounted acti ve and reactive power, which is the system loss.In a
system where the incremental fuel cost has to be kept equal on all buses, by asking one bus to carry all the extra burden , this idea is certainly violated.. Thus the existing optimum solution is not optimum at all for the slack bus. In the previous ELD schemes, nothing can be done about it, because the insertion of slack bus in the jacobian matrix will make it singular and the solution will fail.
Thus the idea is of distributing the burden on the slack bus to other generator buses in the system in order to increase the quality of Economic Load Dispatch.
The Concept of DISTRIBUTED SLACK BUS is used in such case.
To distribute the burden on the slack bus among other voltage controlled buses a constraint named as Participation Factor is introduced into the existing ELD method. A participation factor is a simple algebraic ratio. It is thefactor attached to each generator bus such that, the total unaccounted or loss power shall be distributed to that bus multiplied by that factor.
P.S.
A slack bus is usually a generator bus with a large real and reactive power output. It is assumed that its real and reactive power outputs are big enough that they can be adjusted as required in order to balance the power in the whole system so that the power flow can be solved. A slack bus can have load on it because in real systems it is actually the bus of a power plant, which can have its own load. To check this you can look at the data files in Matpower, such as case 39, case57, case2383wp, case2736sp, case2746wop, and case2746wp.
@junjian Qi, thank you for the answer. ya it is possible in simulation. But usually the generator bus are only having station auxiliary which may be only upto 3% of total generation. Other than this the generating station is connected to a substation before feeding some load. So i would like to know if there is any theoretical or practical restriction for having a load on slack bus.
As pointed out by Mr. Junjian, the slack bus automatically adjusts the real and reactive powers to obtain the power balance during loadflow. It also takes care of the Line losses. If you see the Power flow in ETAP, you have to specify the type of generation on the source model itself (Alternator, Wind etc.) and not on any bus. In case you have Plant auxiliaries, then you will have to consider the power output of the plant as Total generator output - Plant auxiliary power.
In the General Load Flow Problem the Bus wit Largest Generating Capacity is taken as the Slack Bus or Swing Bus.The Slack Bus Voltage is taken to be 1.0 + j 0 P.U. and should be capable of supplying total Losses in the System.As such we may think that if it supplies Local Load,the Slack Bus Voltage Restriction may be Violated.Not so,because this Local Load can also be treated as Loss to be supplied by the Generation at the Slack Bus.
The statement of Chittesh V.C that But usually the generator bus are only having station auxiliary which may be only up to 3% of total generation is only a Thumb Rule. If the Generation at Slack Bus is More it can take More Load connected to the Slack Bus.
Hence We can treat that as stated by Junjion Qi, A slack bus is usually a generator bus with a large real and reactive power output. It is assumed that its real and reactive power outputs are big enough that they can be adjusted as required in order to balance the power in the whole system so that the power flow can be solved. A slack bus can have load on it because in real systems it is actually the bus of a power plant, which can have its own load. It also takes care of the Line losses
But we have a Different Situation when we go for Economic Load Dispatch in Systems having Renewable Energy Sources also.This is an interesting idea.
Distributed generation has been growing rapidly in power systems. Previously, one dominant source, the substation, existed in distribution systems. Now, the consistent supply of energy from DGs, the number of sources and the percen
tage of real power injections from DGs have significantly increased.
Refer to the Dissertation:
Distributed Slack Bus Model for Qualitative Economic Load Dispatch
Soumya Ranjan Panda
Link: http://ethesis.nitrkl.ac.in/4877/1/109EE0292.pdf
Economic Load Despatch(ELD) scheme searches for a load flow solution
out of many which optimally satisfies all the technical and most of the economic constraints of the power system. Since total cost of generation is a function of indi-
vidual production of the generating units, the system constraints will decide
the total cost of the system
BURDEN ON SLACK BUS The slack bus accounts for two functions in a load flow. It can serve as a virtual reference for other buses in the system with its phase being assigned as zero. The second purpose is of acting as a source (as well as sink) for the unaccounted acti ve and reactive power, which is the system loss.In a
system where the incremental fuel cost has to be kept equal on all buses, by asking one bus to carry all the extra burden , this idea is certainly violated.. Thus the existing optimum solution is not optimum at all for the slack bus. In the previous ELD schemes, nothing can be done about it, because the insertion of slack bus in the jacobian matrix will make it singular and the solution will fail.
Thus the idea is of distributing the burden on the slack bus to other generator buses in the system in order to increase the quality of Economic Load Dispatch.
The Concept of DISTRIBUTED SLACK BUS is used in such case.
To distribute the burden on the slack bus among other voltage controlled buses a constraint named as Participation Factor is introduced into the existing ELD method. A participation factor is a simple algebraic ratio. It is thefactor attached to each generator bus such that, the total unaccounted or loss power shall be distributed to that bus multiplied by that factor.
P.S.
@Subramanyam Sir.. Interesting explanation. Though slack bus is well known for being an 'infinite' source, I never wondered if it can act as a sink also..
One more thing, is it may be possible that including slack bus in Jacobian renders it singular for ELD studies only? I mean does it ever happen in other studies like stability as well?
There is a Popular Question in viva,"Is it possible to get Bus Impedance Matrix by inverting the Bus Admittance Matrix,,always?". The Answer is "no ,because in forming Y Bus Matrix we take Ground as reference and in Z Bus we take some other Bus as Reference, in such case for example if the Y Bus Matrix is of the order 4x4 then z Bus Matrix will be of the order 3x3. it will be 4x4 ,only if Ground is taken as Reference to form Z Bus also.
In the case of using Newton Raphson method for load flow, for a four bus system if bus1is taken as reference and bus 4 as voltage control bus then the Jacobian matrix will be of the order 5x5 and not 6x6 because
Voltage controlled buses: In the polar form of the power flow equations voltage controlled buses are easily taken into account. For ex: if bus 4 of the four bus system is voltage controlled, then |v4| has a specific constant value and voltage correction must always be zero consequently the sixth column of the Jacobian is always multiplied by zero, so it may be removed also since Q4 is not specified the mismatch in Q4 can not be defined and hence the sixth row is also omitted. Of course Q4 can be calculated after the power flow solution is available................. Ref: 9.4 Newton Raphson power flow solution and ex:9.5, pg: 353 in the book POWER SYSTEM ANALYSIS by J. Grainger and William D. Stevenson, JR (TMH).
P.S.
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