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use of If-Then-Constraint

  • 20 October 2021
  • 7 replies
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 Good morning all,

Please, can someone tell me why I keep getting this error :point_down_tone4:? Both SOC(agg,t) and P_Charging(agg,t) are variables.

Thanks so much.

 

 

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Best answer by mohansx 20 October 2021, 23:08

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7 replies

Userlevel 5
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@olusegun.adeniji 

 

The problem is with your “else” block. This is a constraint and not a variable definition, so 

else

P_Charging(agg, t)

endif

does not make any sense. You can simply have

 

IF SOC(agg, t-1) > 0.949 THEN

P_Charging(agg, t) = 0

ENDIF

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Thanks @mohansx for your response.

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Thanks @mohansx ,

I tried that suggestion and  it still gives error of missing “ELSE” part. Sincerely this looks so simple to tackle personally considering what I have done in AIMMS but I really dont know why I cant resolve it. Any further help would be greatly appreciated.

 

Thanks

 

Userlevel 5
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Hello @olusegun.adeniji 

 

Apologies, I missed that AIMMS requires IF blocks in constraints to have ELSE conditions too. 

 

You have two options -

 

  1. Create another variable with same index domain as P_Charging(agg, t), call is temp_Charging(agg, t). Give it the below definition

    IF SOC(agg, t-1) > 0.949 THEN

    0

    ELSE

    P_Charging(agg, t)

    ENDIF


    Use temp_Charging(agg, t) in the objective function or other constraints/definitions instead of P_Charging.
  2. Use the modelling trick described in section 7.4 of the AIMMS Optimization Modelling guide.

    Your requirement is SOC(agg, t-1) > 0.949, then P_Charging(agg, t) must be 0 → which is equivalent to only one of SOC(agg, t-1) <= 0.949 and P_Charging(agg, t) <= 0 can be true. 

    use a binary variable Y(agg, t) and upper limits for SOC - U1 and upper limit for P_Charging - U2. Introduce two constraints with index domain (agg, t)

    constraint1



    SOC(agg, t-1) <= 0.949 + U1 * Y(agg, t)



    constraint2



    P_Charging(agg, t) <= 0 + U2 * (1 - Y(agg, t))

If SOC(agg, t-1) needs to be greater than 0.949, Y(agg, t) must be 1 which will force the second constraint to become P_Charging(agg, t) <= 0 + 0 = 0 (assuming the range of P_Charging is nonnegative). 

I hope this helps.

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Hello @mohansx ,

Thanks for your response. My system went for some maintenance and just returned with it. I will let you know if your suggestion solves my problem.

Thanks again

Olusegun

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Hi @mohansx ,

Sorry to bother you again on the above subject. 

Please I want to ask what happens if range of P_Charging is nonpositive, that is it must be negative or zero?

Please, what will now be U2?

Thanks in advance for your prompt response.

Olusegun

Userlevel 5
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@olusegun.adeniji In that case, instead of upper limit U2, you use the lower limit of P_Charging with range [L2, 0].

P_Charging(agg, t) <= 0 + U2 * (1 - Y(agg, t))

will become something like

P_Charging(agg, t) >= L2 * (1 - Y(agg, t))

 

When Y is 0, this will be P_Charging >= L2 and when Y = 1 - P_Charging >= 0 which will force it to be P_charging = 0 because of the range L2 <= P_Charging <= 0

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