ETAP Knowledge Base
Modeling Modeling of Swing Sources in L oad Flow Description: Different methods of modeling swing sources in ETAP Category:
Load Flow, Swing Sources
Type:
Modeling
Keywords:
Load Flow, Swing, Parallel Generators, Load Sharing
ETAP Load Flow models the sources as ideal (infinite) voltage sources when they are configured as “Swing” machines. This is the traditional way of handling sources in load flow calculations. Under this model you would expect that all the generators connected to the same bus would share the load equally since their output voltage is fixed (see the image below).
Image1: Configuration showing 4 swing sources operating in parallel.
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Modeling of Swing Sources in L oad Flow In ETAP you have the following modes of operation which assume the operation of governor and exciters as described below:
Image2: Information about ETAP modes of operation for different sources.
This document is confidential and proprietary to Operation Technology, Inc. and may not be reproduced, published or disclosed to others without the written authorization of Operation Technology, Inc., 17 Goodyear, Suite 100, Irvine, CA 92618, USA. © 2010 by Operation Technology, Inc. All Rights Reserved.
© Operation Technology, Inc.
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Modeling of Swing Sources in L oad Flow
Image3: More on the modes of operation of sources in ETAP. You can use the modes of operation described in Image 1 & 2 above to assign the power output to each machine. In Voltage control mode, the MW can be set fixed. In Mvar control mode both the MW and Mvar can be set to a fixed value, but you have to have at least one more source that is operating in Swing mode. With these modes you cannot limit the amount of power flow from a generator or power grid operating in Swing mode (according to the configuration shown in Image1). Most likely you will have to use a combination of the different modes of operation to get the desired power output from each machine. Using the modes of operation described above would be the recommended way to distribute the power flow to each machine according to their individual ratings, but there is no way that you can achieve a power flow proportional to their MW rating if all of them are set to Swing mode. One way to achieve the MW sharing effect in Swing mode is to place very small impedance in front of the generators. The impedance should be proportional to the amount of output power that you desire for the particular machine. This would have to be done since all five machines are configured as “swing” sources. The smaller impedance would be in front of the larger generator and the larger impedance in front of the smaller machine. Of course all of the impedances are relatively small to cause any considerable power losses. This document is confidential and proprietary to Operation Technology, Inc. and may not be reproduced, published or disclosed to others without the written authorization of Operation Technology, Inc., 17 Goodyear, Suite 100, Irvine, CA 92618, USA. © 2010 by Operation Technology, Inc. All Rights Reserved.
© Operation Technology, Inc.
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Modeling of Swing Sources in L oad Flow The load flow results that you obtain when added the additional small impedances are shown below:
Image 4: MW sharing for all swing parallel operating generators after adding proportional impedances between the generator and the common bus. You will notice from Image 4 that the power flow is shared according to the amount of impedance specified for Z1 to Z5. It should be noted that the impedance values can be very small in magnitude so that their effect in the system is negligible (as far as power losses are concerned), but will help in distributing the power according to the generator ratings (since the impedance is proportional to the generator size. One last general comment is that the impedance values that can be added to the system are for the purpose of illustrating the modeling of swing sources in ETAP. You may want to use this approach for modeling Load Flow studies only if you find that the additional impedances do not have a significant effect in your system. Of course in a real life system, generators will have load-sharing controls that operate through their governors or some other form of CT control. ETAP models this type of load sharing mechanisms in more advanced studies like Transient Stability.
This document is confidential and proprietary to Operation Technology, Inc. and may not be reproduced, published or disclosed to others without the written authorization of Operation Technology, Inc., 17 Goodyear, Suite 100, Irvine, CA 92618, USA. © 2010 by Operation Technology, Inc. All Rights Reserved.
© Operation Technology, Inc.
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