The "fixed load" is modeled in PSCAD as a function of voltage and frequency. It is a ZIP load model in which the user is given freedom to choose the portion of each type (Z,I,and P) in the load model. When the user wants the load to be a constant power type by choosing dP/dV = 0, the load will remain constant power type (non-linear) for the voltage between 0.8-1.2 pu. Whereas, for the voltages outside this range, the load behaves as a constant impedance type. This is true and well-observed for a balanced 3-phase voltage. However, for an unbalanced voltage (obtained by creating an unbalanced fault), the behavior of load is unpredictable. Also, I couldn't find any documentation on this. My understanding and observations are as follows:
- When the voltage at phase-A is less than 0.8 pu, and other phase voltages are normal, the load is supposed to draw a current that a constant impedance load would draw at phase-A, and the load is supposed to draw a constant power type current in other phases (B and C). But, based on my observation, load is not doing so.
- OR, maybe, whenever one of the phases experiences a voltage drop, the load should draw current in all phases that a constant impedance load would draw. My observation is that, when I take measurements of currents and voltages at each phase of the load, the ratio V/I is the same for all three phases, which implies a constant impedance load. However, the ratio V/I is not equal to the impedance of the load based on nominal power and voltage ratings. Furthermore, this ratio V/I changes with terminal Voltage.
It would be a great help if you could provide me with the explanation about how this model of load behaves on unbalanced disturbance. Thank you.
This component models the load characteristics as a function of voltage magnitude and frequency, where the load real and reactive power our considered separately using the well known expressions:
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And,
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Where,
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Equivalent load real power
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Rated real power per phase
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Load voltage
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Rated load voltage (RMS, L-G)
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dP/dV Voltage index for real power
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dP/dF Frequency index for real power
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Equivalent load reactive power
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Rated reactive power (+inductive) per phase
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dQ/dV Voltage index for reactive power
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dQ/dF Frequency index for reactive power
NOTE: dQ, dP, dV and dF are in all per-unit quantities
If this component is used in single-phase mode, every half cycle (at the appropriate voltage zero), the shunt resistor is updated to reflect the non-linear load effect as a function of RMS voltage and frequency. Similarly, the shunt inductor or capacitor, in parallel to the load resistor, is updated every half cycle (at its voltage zero) to reflect its non-linear characteristic.
If used in a 3-phase, single-line mode, the RLC values of all phases are changed when the voltage of any one of the phases goes through a zero crossing. The load remains in constant impedance form until 10 cycles of the fundamental frequency is passed. The non-linear characteristic is effective within ±20% of the rated RMS voltage and ±DF% of the rated frequency (DF is defined as minimum of 10, 90/KPF and 90/KQF). Outside this range, the load reverts to constant impedance.
PLEASE NOTE: The initial values of R, L and C are computed based on the rated conditions supplied by the user. These values are kept constant until 10 cycles of the fundamental frequency has passed.
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Fixed Load (1-Phase, L-L)
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