1. EE653 Power distribution system modeling, optimization and simulation
ECpE Department
EE653 Power distribution system modeling, optimization and simulation
Dr. Zhaoyu Wang
1113 Coover Hall, Ames, IA

2. Modeling Shunt Components –Loads and Caps
Acknowledgement: The slides are developed based in part on Distribution System Modeling and Analysis, 4th edition, William H. Kersting, CRC Press, 2017

3. Load Models
The loads on a distribution system are typically specified by the complex power consumed. This demand can be specified as kVA and power factor, kW and power factor, or kW and kvar. The voltage specified will always be the voltage at the low-voltage bus of the distribution substation. This creates a problem since the current requirement of the loads can not be determined without knowing the voltage. For this reason, modified ladder iterative technique must be employed.
Loads on a distribution feeder can be modeled as wye connected or delta connected. The loads can be three phase, two phase, or single phase with any degree of unbalanced. The ZIP models are
Constant impedance (Z)
Constant current (I)
Constant real and reactive power (constant P)
Any combination of the above
ECpE Department

4. Load Models
The load models developed are to be used in the iterative process of a power-flow program where the load voltages are initially assumed.
One of the results of the power-flow analysis is to replace the assumed voltages with the actual operating load voltages.
All models are initially defined by a complex power per phase and an assumed line-to-neutral voltage (wye load) or an assumed line-to-line voltage (delta load). The units of the complex power can be in volt-amperes and volts or per-unit volt-amperes and per-unit voltages.
For all loads, the line currents entering the load are required in order to perform the power-flow analysis.
Actually, for all shunt components, including static loads, caps and induction machines, what we need is the line currents entering the shunt components. These line currents will be used in the forward-backward sweep method. The approaches to calculate these line currents depend on the components and on wye/delta connections.
ECpE Department

5. Fig.1 Wye-connected load
Wye-Connected Loads
Fig.1 shows the model of a wye-connected load.
Fig.1 Wye-connected load
ECpE Department

6. Fig.1 Wye-connected load
Wye-Connected Loads
The notation for the specified complex powers and voltages are as follows:
Phase a:
(1)
𝑆 𝑎 ∠ 𝜃 𝑎 = 𝑃 𝑎 +j 𝑄 𝑎 and 𝑉 𝑎𝑛 ∠ 𝛿 𝑎
(2)
𝑆 𝑏 ∠ 𝜃 𝑏 = 𝑃 𝑏 +j 𝑄 𝑏 and 𝑉 𝑏𝑛 ∠ 𝛿 𝑏
Phase b:
𝑆 𝑐 ∠ 𝜃 𝑐 = 𝑃 𝑐 +j 𝑄 𝑐 and 𝑉 𝑐𝑛 ∠ 𝛿 𝑐
(3)
Phase c:
Fig.1 Wye-connected load
ECpE Department

7. Constant Real and Reactive Power Loads
The line currents for constant real and reactive power loads (PQ loads) are given by
𝐼𝐿 𝑎 = 𝑆 𝑎 𝑉 𝑎𝑛 ∗ = 𝑆 𝑎 𝑉 𝑎𝑛 ∠ 𝛿 𝑎 − 𝜃 𝑎 = 𝐼𝐿 𝑎 ∠ 𝛿 𝑎
𝐼𝐿 𝑏 = 𝑆 𝑏 𝑉 𝑏𝑛 ∗ = 𝑆 𝑏 𝑉 𝑏𝑛 ∠ 𝛿 𝑏 − 𝜃 𝑏 = 𝐼𝐿 𝑏 ∠ 𝛿 𝑏
(4)
𝐼𝐿 𝑐 = 𝑆 𝑐 𝑉 𝑐𝑛 ∗ = 𝑆 𝑐 𝑉 𝑐𝑛 ∠ 𝛿 𝑐 − 𝜃 𝑐 = 𝐼𝐿 𝑐 ∠ 𝛿 𝑐
In this model, the line-to-neutral voltages will change during each iteration until convergence is achieved.
ECpE Department

8. Constant Impedance Loads
The “constant load impedance” is first determined from the specified complex power and assumed (e.g., nominal voltage) line-to-neutral voltages:
𝑍 𝑎 = 𝑉 𝑎𝑛 𝑆 𝑎 ∗ = 𝑉 𝑎𝑛 𝑆 𝑎 ∠ 𝜃 𝑎 = 𝑍 𝑎 ∠ 𝜃 𝑎
𝑍 𝑏 = 𝑉 𝑏𝑛 𝑆 𝑏 ∗ = 𝑉 𝑏𝑛 𝑆 𝑏 ∠ 𝜃 𝑏 = 𝑍 𝑏 ∠ 𝜃 𝑏
(5)
𝑍 𝑐 = 𝑉 𝑐𝑛 𝑆 𝑐 ∗ = 𝑉 𝑐𝑛 𝑆 𝑐 ∠ 𝜃 𝑐 = 𝑍 𝑐 ∠ 𝜃 𝑐
The load currents as a function of the “constant load impedances” are given by
𝐼𝐿 𝑎 = 𝑉 𝑎𝑛 𝑍 𝑎 = 𝑉 𝑎𝑛 𝑍 𝑎 ∠ 𝛿 𝑎 − 𝜃 𝑎 = 𝐼𝐿 𝑎 ∠ 𝛼 𝑎
𝐼𝐿 𝑏 = 𝑉 𝑏𝑛 𝑍 𝑏 = 𝑉 𝑏𝑛 𝑍 𝑏 ∠ 𝛿 𝑏 − 𝜃 𝑏 = 𝐼𝐿 𝑏 ∠ 𝛼 𝑏
(6)
𝐼𝐿 𝑐 = 𝑉 𝑐𝑛 𝑍 𝑐 = 𝑉 𝑐𝑛 𝑍 𝑐 ∠ 𝛿 𝑐 − 𝜃 𝑐 = 𝐼𝐿 𝑐 ∠ 𝛼 𝑐
In this model, the line-to-neutral voltages will change during each iteration, but the impedance computed in Equation (5) will remain constant.
ECpE Department

9. Constant Current Loads
𝐼𝐿 𝑎 = 𝑆 𝑎 𝑉 𝑎𝑛 ∗ = 𝑆 𝑎 𝑉 𝑎𝑛 ∠ 𝛿 𝑎 − 𝜃 𝑎 = 𝐼𝐿 𝑎 ∠ 𝛿 𝑎
𝐼𝐿 𝑏 = 𝑆 𝑏 𝑉 𝑏𝑛 ∗ = 𝑆 𝑏 𝑉 𝑏𝑛 ∠ 𝛿 𝑏 − 𝜃 𝑏 = 𝐼𝐿 𝑏 ∠ 𝛿 𝑏
(4)
𝐼𝐿 𝑐 = 𝑆 𝑐 𝑉 𝑐𝑛 ∗ = 𝑆 𝑐 𝑉 𝑐𝑛 ∠ 𝛿 𝑐 − 𝜃 𝑐 = 𝐼𝐿 𝑐 ∠ 𝛿 𝑐
In this model, the magnitudes of the currents are computed according to Equations (4) and then held constant while the angle of the voltage (δ) changes resulting in a changing angle on the current so that the power factor of the load remains constant:
𝐼𝐿 𝑎 = 𝐼𝐿 𝑎 ∠ 𝛿 𝑎 − 𝜃 𝑎
𝐼𝐿 𝑏 = 𝐼𝐿 𝑏 ∠ 𝛿 𝑏 − 𝜃 𝑏
(7)
𝐼𝐿 𝑐 = 𝐼𝐿 𝑐 ∠ 𝛿 𝑐 − 𝜃 𝑐
where
δabc represents the line-to-neutral voltage angles
θabc represents the power factor angles
ECpE Department

10. Combination Loads
Combination loads can be modeled by assigning a percentage of the total load to each of the three aforementioned load models. The total line current entering the load is the sum of the three components.
ECpE Department

11. Example 1
The complex powers of a wye-connected load are
𝑆 𝑎𝑏𝑐 = ∠ ∠ ∠ 𝑘𝑉𝐴
The load is specified to be 50% constant complex power, 20% constant impedance, and 30% constant current. The nominal line-to-line voltage of the feeder is kV.
A. Assume the nominal voltage and compute the component of load current attributed to each component of the load and the total load current. The assumed line-to-neutral voltages at the start of the iterative routine are
𝑉𝐿𝑁 𝑎𝑏𝑐 = 7200∠0 7200∠− ∠120 𝑉
The component of currents due to the constant complex power is
𝐼𝑝𝑞 𝑖 = 𝑆 𝑖 ∙1000 𝑉𝐿𝑁 𝑖 ∗ ∙0.5= ∠− ∠− ∠ 𝐴
ECpE Department

12. Example 1
The constant impedances for that part of the load are computed as
𝑍 𝑖 = 𝑉𝐿𝑁 𝑖 2 𝑆 𝑖 ∗ ∙1000 ∙0.5= 𝑗 𝑗 𝑗 Ω
For the first iteration, the currents due to the constant impedance portion of the load are
𝐼𝑧 𝑖 = 𝑉𝐿𝑁 𝑖 𝑍 𝑖 ∙0.2= 62.1∠− ∠− ∠ 𝐴
The magnitudes of the constant current portion of the load are
𝐼𝑀 𝑖 = 𝑆 𝑖 ∙1000 𝑉𝐿𝑁 𝑖 ∗ ∙0.3= 𝐴
ECpE Department

13. Example 1
The contribution of the load currents due to the constant current portion of the load is
𝐼𝐼 𝑖 = 𝐼𝑀 𝑖 ∠ 𝛿 𝑖 − 𝜃 𝑖 = 93.2∠− ∠− ∠ 𝐴
The total load current is the sum of the three components:
𝐼 𝑎𝑏𝑐 = 𝐼 𝑝𝑞 + 𝐼 𝑧 + 𝐼 𝐼 = ∠− ∠− ∠ 𝐴
B. Determine the currents at the start of the second iteration. The voltages at the load after the first iteration are
𝑉𝐿𝑁 = ∠− ∠− ∠ 𝑉
The steps are repeated with the exceptions that the impedances of the constant impedance portion of the load will not be changed and the magnitude of the currents for the constant current portion of the load change will not change.
ECpE Department

14. Example 1
The constant complex power portion of the load currents is
𝐼𝑝𝑞 𝑖 = 𝑆 𝑖 ∙1000 𝑉𝐿𝑁 𝑖 ∗ ∙0.5= ∠− ∠− ∠ 𝐴
The currents due to the constant impedance portion of the load are
𝐼𝑧 𝑖 = 𝑉𝐿𝑁 𝑖 𝑍 𝑖 ∙0.2= 59.1∠− ∠− ∠ 𝐴
The currents due to the constant current portion of the load are
𝐼𝐼 𝑖 = 𝐼𝑀 𝑖 ∠ 𝛿 𝑖 − 𝜃 𝑖 = 93.2∠− ∠− ∠ 𝐴
The total load currents at the start of the second iteration will be
𝐼 𝑎𝑏𝑐 = 𝐼 𝑝𝑞 + 𝐼 𝑧 + 𝐼 𝐼 = ∠− ∠− ∠ 𝐴
ECpE Department

15. Example 1
Observe how these currents have changed from the original currents. The currents for the constant complex power loads have increased because the voltages are reduced from the original assumption. The currents for the constant impedance portion of the load have decreased because the impedance stayed constant but the voltages are reduced. Finally, the constant current portion of the load has indeed remained constant. Again, all three components of the load have the same phase angles since the power factor of the load has not changed.
ECpE Department

16. Fig.2 Delta-connected load
Delta-Connected Loads
The model for a delta-connected load is shown in Fig.2.
The notations for the specified complex powers and voltages in Fig.2 are as follows:
Phase ab:
𝑆 𝑎𝑏 ∠ 𝜃 𝑎𝑏 = 𝑃 𝑎𝑏 +j 𝑄 𝑎𝑏 and 𝑉 𝑎𝑏 ∠ 𝛿 𝑎𝑏
(8)
Phase bc:
𝑆 𝑏𝑐 ∠ 𝜃 𝑏𝑐 = 𝑃 𝑏𝑐 +j 𝑄 𝑏𝑐 and 𝑉 𝑏𝑐 ∠ 𝛿 𝑏𝑐
(9)
Phase ca:
𝑆 𝑐𝑎 ∠ 𝜃 𝑐𝑎 = 𝑃 𝑐𝑎 +j 𝑄 𝑐𝑎 and 𝑉 𝑐𝑎 ∠ 𝛿 𝑐𝑎
(10)
Fig.2 Delta-connected load
ECpE Department

17. Constant Real and Reactive Power Loads
The currents in the delta-connected loads are
𝐼𝐿 𝑎𝑏 = 𝑆 𝑎𝑏 𝑉 𝑎𝑏 ∗ = 𝑆 𝑎𝑏 𝑉 𝑎𝑏 ∠ 𝛿 𝑎𝑏 − 𝜃 𝑎𝑏 = 𝐼𝐿 𝑎𝑏 ∠ 𝛼 𝑎𝑏
𝐼𝐿 𝑏𝑐 = 𝑆 𝑏𝑐 𝑉 𝑏𝑐 ∗ = 𝑆 𝑏𝑐 𝑉 𝑏𝑐 ∠ 𝛿 𝑏𝑐 − 𝜃 𝑏𝑐 = 𝐼𝐿 𝑏𝑐 ∠ 𝛼 𝑏𝑐
(11)
𝐼𝐿 𝑐𝑎 = 𝑆 𝑐𝑎 𝑉 𝑐𝑎 ∗ = 𝑆 𝑐𝑎 𝑉 𝑐𝑎 ∠ 𝛿 𝑐𝑎 − 𝜃 𝑐𝑎 = 𝐼𝐿 𝑐𝑎 ∠ 𝛼 𝑐𝑎
In this model, the line-to-line voltages will change during each iteration resulting in new current magnitudes and angles at the start of each iteration.
ECpE Department

18. Constant Impedance Loads
The “constant load impedance” is first determined from the specified complex power and line-to-line voltages:
𝑍 𝑎𝑏 = 𝑉 𝑎𝑏 𝑆 𝑎𝑏 ∗ = 𝑉 𝑎𝑏 𝑆 𝑎𝑏 ∠ 𝜃 𝑎𝑏 = 𝑍 𝑎𝑏 ∠ 𝜃 𝑎𝑏
𝑍 𝑏𝑐 = 𝑉 𝑏𝑐 𝑆 𝑏𝑐 ∗ = 𝑉 𝑏𝑐 𝑆 𝑏𝑐 ∠ 𝜃 𝑏𝑐 = 𝑍 𝑏𝑐 ∠ 𝜃 𝑏𝑐
(12)
𝑍 𝑐𝑎 = 𝑉 𝑐𝑎 𝑆 𝑐𝑎 ∗ = 𝑉 𝑐𝑎 𝑆 𝑐𝑎 ∠ 𝜃 𝑐𝑎 = 𝑍 𝑐𝑎 ∠ 𝜃 𝑐𝑎
The delta load currents as a function of the “constant load impedances” are
𝐼𝐿 𝑎𝑏 = 𝑉 𝑎𝑏 𝑍 𝑎𝑏 = 𝑉 𝑎𝑏 𝑍 𝑎𝑏 ∠ 𝛿 𝑎𝑏 − 𝜃 𝑎𝑏 = 𝐼𝐿 𝑎𝑏 ∠ 𝛼 𝑎𝑏
𝐼𝐿 𝑏𝑐 = 𝑉 𝑏𝑐 𝑍 𝑏𝑐 = 𝑉 𝑏𝑐 𝑍 𝑏𝑐 ∠ 𝛿 𝑏𝑐 − 𝜃 𝑏𝑐 = 𝐼𝐿 𝑏𝑐 ∠ 𝛼 𝑏𝑐
(13)
𝐼𝐿 𝑐𝑎 = 𝑉 𝑐𝑎 𝑍 𝑐𝑎 = 𝑉 𝑐𝑎 𝑍 𝑐𝑎 ∠ 𝛿 𝑐𝑎 − 𝜃 𝑐𝑎 = 𝐼𝐿 𝑐𝑎 ∠ 𝛼 𝑐𝑎
In this model, the line-to-line voltages will change during each iteration until convergence is achieved.
ECpE Department

19. Constant Current Loads
𝐼𝐿 𝑎𝑏 = 𝑆 𝑎𝑏 𝑉 𝑎𝑏 ∗ = 𝑆 𝑎𝑏 𝑉 𝑎𝑏 ∠ 𝛿 𝑎𝑏 − 𝜃 𝑎𝑏 = 𝐼𝐿 𝑎𝑏 ∠ 𝛼 𝑎𝑏
𝐼𝐿 𝑏𝑐 = 𝑆 𝑏𝑐 𝑉 𝑏𝑐 ∗ = 𝑆 𝑏𝑐 𝑉 𝑏𝑐 ∠ 𝛿 𝑏𝑐 − 𝜃 𝑏𝑐 = 𝐼𝐿 𝑏𝑐 ∠ 𝛼 𝑏𝑐
(11)
𝐼𝐿 𝑐𝑎 = 𝑆 𝑐𝑎 𝑉 𝑐𝑎 ∗ = 𝑆 𝑐𝑎 𝑉 𝑐𝑎 ∠ 𝛿 𝑐𝑎 − 𝜃 𝑐𝑎 = 𝐼𝐿 𝑐𝑎 ∠ 𝛼 𝑐𝑎
In this model, the magnitudes of the currents are computed according to Equations (11) and then held constant while the angle of the voltage (δ) changes during each iteration. This keeps the power factor of the load constant:
𝐼𝐿 𝑎𝑏 = 𝐼𝐿 𝑎𝑏 ∠ 𝛿 𝑎𝑏 − 𝜃 𝑎𝑏
𝐼𝐿 𝑏𝑐 = 𝐼𝐿 𝑏𝑐 ∠ 𝛿 𝑏𝑐 − 𝜃 𝑏𝑐
(14)
𝐼𝐿 𝑐𝑎 = 𝐼𝐿 𝑐𝑎 ∠ 𝛿 𝑐𝑎 − 𝜃 𝑐𝑎
ECpE Department

20. Line Currents Serving a Delta-Connected Loads
Combination Loads
Combination loads can be modeled by assigning a percentage of the total load to each of the three aforementioned load models. The total delta current for each load is the sum of the three components.
Line Currents Serving a Delta-Connected Loads
The line currents entering the delta-connected load are determined by applying Kirchhoff's current law (KCL) at each of the nodes of the delta. In matrix form, the equations are
𝐼𝐿 𝑎 𝐼𝐿 𝑏 𝐼𝐿 𝑐 = 1 0 −1 − −1 1 ∙ 𝐼𝐿 𝑎𝑏 𝐼𝐿 𝑏𝑐 𝐼𝐿 𝑐𝑎
(15)
Two-Phase and Single-Phase Loads
In both the wye- and delta-connected loads, single-phase and two-phase loads are modeled by setting the currents of the missing phases to zero. The currents in the phases present are computed using the same appropriate equations for constant complex power, constant impedance, and constant current.
ECpE Department

21. Shunt Capacitors
Shunt capacitor banks are commonly used in distribution systems to help in voltage regulation and to provide reactive power support. The capacitor banks are modeled as constant susceptances connected in either wye or delta. Similar to the load model, all capacitor banks are modeled as three-phase banks with the currents of the missing phases set to zero for single-phase and two-phase banks.
ECpE Department

22. Fig.3 Wye-connected capacitor bank
The model of a three-phase wye-connected shunt capacitor bank is shown in Fig.3.
The individual phase capacitor units are specified in kvar and kV. The constant susceptance for each unit can be computed in Siemens. The susceptance of a capacitor unit is computed by
𝐵 𝑐 = 𝑘𝑣𝑎𝑟 𝑘𝑉 𝐿𝑁 2 ∙1000 𝑆
(16)
Fig.3 Wye-connected capacitor bank
ECpE Department

23. Fig.3 Wye-connected capacitor bank
With the susceptance computed, the line currents serving the capacitor bank are given by
𝐼𝐶 𝑎 =𝑗 𝐵 𝑎 ∙ 𝑉 𝑎𝑛
𝐼𝐶 𝑏 =𝑗 𝐵 𝑏 ∙ 𝑉 𝑏𝑛
(17)
𝐼𝐶 𝑐 =𝑗 𝐵 𝑐 ∙ 𝑉 𝑐𝑛
Fig.3 Wye-connected capacitor bank
ECpE Department

24. Fig.4 Delta-connected capacitor bank
The model for a delta-connected shunt capacitor bank is shown in Fig.4.
The individual phase capacitor units are specified in kvar and kV. For the delta-connected capacitors, the kV must be the line-to-line voltage. The constant susceptance for each unit can be computed in Siemens. The susceptance of a capacitor unit is computed by
𝐵 𝑐 = 𝑘𝑣𝑎𝑟 𝑘𝑉 𝐿𝐿 2 ∙1000 𝑆
(18)
Fig.4 Delta-connected capacitor bank
ECpE Department

25. Delta-Connected Capacitor Bank
With the susceptance computed, the delta currents serving the capacitor bank are given by
𝐼𝐶 𝑎𝑏 =𝑗 𝐵 𝑎𝑏 ∙ 𝑉 𝑎𝑏
𝐼𝐶 𝑏𝑐 =𝑗 𝐵 𝑏𝑐 ∙ 𝑉 𝑏𝑐
(19)
𝐼𝐶 𝑐𝑎 =𝑗 𝐵 𝑐𝑎 ∙ 𝑉 𝑐𝑎
The line currents flowing into the delta-connected capacitors are computed by applying KCL at each node. In matrix form, the equations are
𝐼𝐶 𝑎 𝐼𝐶 𝑏 𝐼𝐶 𝑐 = 1 0 −1 − −1 1 ∙ 𝐼𝐶 𝑎𝑏 𝐼𝐶 𝑏𝑐 𝐼𝐿 𝑐𝑎
(20)
ECpE Department

26. Three-Phase Induction Machine
The analysis of an induction machine (motor or generator) when operating under unbalanced voltage conditions has traditionally been performed using the method of symmetrical components. Using this approach, the positive and negative sequence equivalent circuits of the machine are developed and then, given the sequence line-to-neutral voltages, the sequence currents are computed. The zero sequence network is not required since the machines are typically connected delta or ungrounded wye, which means that there will not be any zero sequence currents or voltages. The phase currents are determined by performing the transformation back to the phase line currents. The internal operating conditions are determined by the complete analysis of the sequence networks.
A method whereby all of the analysis can be performed in the phase frame will be developed. The analysis will be broken into two parts. The first part will be to determine the terminal voltages and currents of the motor and the second part will be to use these values to compute the stator and rotor losses and the converted shaft power.
ECpE Department

27. Fig.5 Sequence equivalent circuit
Induction Machine Model
The sequence line-to-neutral equivalent circuit of a three-phase induction machine is shown in Fig.5.
The circuit in Fig.5 applies to both the positive and negative sequence networks. The only difference between the two is the value of the “load resistance” RL as defined in the following:
𝑅𝐿 𝑖 = 1− 𝑠 𝑖 𝑠 𝑖 ∙𝑅𝑟 𝑖
(21)
where
Positive sequence slip:
𝑠 𝑖 = 𝑛 𝑠 − 𝑛 𝑟 𝑛 𝑠
(22)
where
ns is the synchronous speed
nr is the rotor speed
Fig.5 Sequence equivalent circuit
ECpE Department

28. Induction Machine Model
Negative sequence slip:
𝑠 2 =2− 𝑠 1
(23)
Note that the negative sequence load resistance RL2 will be a negative value that will lead to a negative shaft power in the negative sequence.
If the value of positive sequence slip (s1) is known, the input sequence impedances for the positive and negative sequence networks can be determined as
𝑍𝑀 𝑖 = 𝑅𝑠 𝑖 +𝑗 𝑋𝑠 𝑖 + (𝑗 𝑋𝑚 𝑖 )( 𝑅𝑟 𝑖 + 𝑅𝐿 𝑖 +𝑗 𝑋𝑟 𝑖 ) 𝑅𝑟 𝑖 + 𝑅𝐿 𝑖 +𝑗( 𝑋𝑚 𝑖 + 𝑋𝑟 𝑖 )
(24)
where
i = 1 for positive sequence
i = 2 for negative sequence
ECpE Department

29. Induction Machine Model
Once the input sequence impedances have been determined, the analysis of an induction machine operating with unbalanced voltages requires the following steps:
Step 1: Transform the known line-to-line voltages to sequence line-to-line voltages:
𝑉𝑎𝑏 0 𝑉𝑎𝑏 1 𝑉𝑎𝑏 2 = 1 3 ∙ 𝑎 𝑎 𝑎 2 𝑎 ∙ 𝑉 𝑎𝑏 𝑉 𝑏𝑐 𝑉 𝑐𝑎
(25)
In Equation (25), Vab0 = 0 because of Kirchhoff's voltage law (KVL).
Equation (25) can be written as
(26)
𝑉𝐿𝐿 = 𝐴 −1 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐
Step 2: Compute the sequence line-to-neutral voltages from the line-to-line voltages:
𝑉𝑎𝑛 0 = 𝑉𝑎𝑏 0 =0
(27)
Equation (27) will not be true for a general case. However, for the case of the machine being connected either in delta or ungrounded wye, the zero sequence line-to-neutral voltage can be assumed to be zero:
𝑉𝑎𝑛 1 = 𝑡 ∗ ∙𝑉𝑎𝑏 1
(28)
𝑉𝑎𝑛 2 = 𝑡 ∗ ∙ 𝑉𝑎𝑏 2
(29)
𝑡= ∙∠30
(30)
where
ECpE Department

30. Induction Machine Model
(27)
𝑉𝑎𝑛 0 = 𝑉𝑎𝑏 0 =0
(28)
𝑉𝑎𝑛 1 = 𝑡 ∗ 𝑉𝑎𝑏 1
(29)
𝑉𝑎𝑛 2 = 𝑡 ∗ 𝑉𝑎𝑏 2
Equations (27) through (29) can be put into matrix form:
𝑉𝑎𝑛 0 𝑉𝑎𝑛 1 𝑉𝑎𝑛 2 = 𝑡 ∗ 𝑡 ∙ 𝑉𝑎𝑏 0 𝑉𝑎𝑏 1 𝑉𝑎𝑏 2
(31)
Equations (31) can be written as
(32)
𝑉𝐿𝑁 = 𝑇 ∙ 𝑉𝐿𝐿 012
Step 3: Compute the sequence line currents flowing into the machine:
(33)
𝐼𝑎 0 =0
𝐼𝑎 1 = 𝑉𝑎𝑛 1 𝑍𝑀 1
(34)
𝐼𝑎 2 = 𝑉𝑎𝑛 2 𝑍𝑀 2
(35)
ECpE Department

31. Induction Machine Model
Step 4: Transform the sequence currents to phase currents:
𝐼 𝑎 𝐼 𝑏 𝐼 𝑐 = 𝑎 2 𝑎 1 𝑎 𝑎 2 ∙ 𝐼 0 𝐼 1 𝐼 2
(36)
Equation (36) can be written as
(37)
𝐼 𝑎𝑏𝑐 = 𝐴 ∙ 𝐼 012
The four steps outlined earlier can be performed without actually computing the sequence voltages and currents. The procedure basically reverses the steps.
Define
(38)
𝑌𝑀 𝑖 = 1 𝑍𝑀 𝑖
The sequence currents are
(39)
𝐼 0 =0
(40)
𝐼 1 = 𝑌𝑀 1 ∙𝑉𝑎𝑛 1 = 𝑌𝑀 1 ∙ 𝑡 ∗ ∙𝑉𝑎𝑏 1
(41)
𝐼 2 = 𝑌𝑀 2 ∙𝑉𝑎𝑛 2 = 𝑌𝑀 2 ∙ 𝑡 ∗ ∙𝑉𝑎𝑏 2
Since I0 and Vab0 are both zero, the following relationship is true:
(42)
𝐼 0 = 𝑉𝑎𝑏 0
ECpE Department

32. Induction Machine Model
(39)
𝐼 0 =0
(40)
𝐼 1 = 𝑌𝑀 1 ∙𝑉𝑎𝑛 1 = 𝑌𝑀 1 ∙ 𝑡 ∗ ∙𝑉𝑎𝑏 1
(42)
𝐼 0 = 𝑉𝑎𝑏 0
Equations (39), (40), and (42) can be put into matrix form:
𝐼 0 𝐼 1 𝐼 2 = 𝑡 ∗ ∙𝑌𝑀 𝑡∙ 𝑌𝑀 1 ∙ 𝑉𝑎𝑏 0 𝑉𝑎𝑏 1 𝑉𝑎𝑏 2
(43)
Equation (43) can be written in shortened form as
𝐼 = 𝑌𝑀 012 ∙ 𝑉𝐿𝐿 012
(44)
From symmetrical component theory,
𝑉𝐿𝐿 = 𝐴 −1 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐
(45)
𝐼 𝑎𝑏𝑐 = 𝐴 ∙ 𝐼 012
(46)
Substitute Equation (45) into Equation (44) and substitute the resultant equation into Equation (46) to get
𝐼 𝑎𝑏𝑐 = 𝐴 ∙ 𝑌𝑀 ∙ 𝐴 −1 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐
(47)
ECpE Department

33. Induction Machine Model
Define
𝑌𝑀 𝑎𝑏𝑐 = 𝐴 ∙ 𝑌𝑀 ∙ 𝐴 −1
(48)
Therefore
𝐼 𝑎𝑏𝑐 = 𝑌𝑀 𝑎𝑏𝑐 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐
(49)
The induction machine “phase frame admittance matrix” [YMabc] is defined in Equation (48). Equation (49) is used to compute the input phase currents of the machine from a knowledge of the phase line-to-line terminal voltages. This is the desired result. Recall that [YMabc] is a function of the slip of the machine so that a new matrix must be computed every time the slip changes.
Equation (49) can be used to solve for the line-to-line voltages as a function of the line currents by
𝑉𝐿𝐿 𝑎𝑏𝑐 = 𝑍𝑀 𝑎𝑏𝑐 ∙ 𝐼 𝑎𝑏𝑐
(50)
where
𝑍𝑀 𝑎𝑏𝑐 = 𝑌𝑀 𝑎𝑏𝑐 −1
(51)
As was done in Chapter 8, it is possible to replace the line-to-line voltages in Equation (50) with the “equivalent” line-to-neutral voltages:
𝑉𝐿𝑁 𝑎𝑏𝑐 = 𝑊 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐
(52)
ECpE Department

34. Induction Machine Model
Define
𝑊 = 𝐴 ∙ 𝑇 ∙ 𝐴 −1
(53)
The matrix [W] is a very useful matrix that allows the determination of the “equivalent” line-to-neutral voltages from a knowledge of the line-to-line voltages. Equation (50) can be substituted into Equation (52) to define the “line-to-neutral” equation:
𝑉𝐿𝑁 𝑎𝑏𝑐 = 𝑊 ∙ 𝑍𝑀 𝑎𝑏𝑐 ∙ 𝐼 𝑎𝑏𝑐
(54)
𝑉𝐿𝑁 𝑎𝑏𝑐 = 𝑍𝐿𝑁 𝑎𝑏𝑐 ∙ 𝐼 𝑎𝑏𝑐
where
𝑍𝐿𝑁 𝑎𝑏𝑐 = 𝑊 ∙ 𝑍𝑀 𝑎𝑏𝑐
(55)
The inverse of Equation (54) can be taken to determine the line currents as a function of the line-to-neutral voltages:
𝐼 𝑎𝑏𝑐 = 𝑌𝐿𝑁 𝑎𝑏𝑐 ∙ 𝑉𝐿𝑁 𝑎𝑏𝑐
(56)
where
(57)
𝑌𝐿𝑁 𝑎𝑏𝑐 = 𝑍𝐿𝑁 𝑎𝑏𝑐 −1
ECpE Department

35. Induction Machine Model
𝑉𝐿𝑁 𝑎𝑏𝑐 = 𝑊 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐
(52)
𝐼 𝑎𝑏𝑐 = 𝑌𝐿𝑁 𝑎𝑏𝑐 ∙ 𝑉𝐿𝑁 𝑎𝑏𝑐
(56)
Care must be taken in applying Equation (56) to insure that the voltages used are the line-to-neutral and not the line-to-ground voltages. If only the line-to-ground voltages are known, they must first be converted to the line-to-line values and then use Equation (52) to compute the line-to-neutral voltages.
Once the machine terminal currents and line-to-neutral voltages are known, the input phase complex powers and total three-phase input complex power can be computed:
(58)
𝑆 𝑎 = 𝑉 𝑎𝑛 ∙ 𝐼 𝑎 ∗
(59)
𝑆 𝑏 = 𝑉 𝑏𝑛 ∙ 𝐼 𝑏 ∗
(60)
𝑆 𝑐 = 𝑉 𝑐𝑛 ∙ 𝐼 𝑐 ∗
(61)
𝑆 𝑇𝑜𝑡𝑎𝑙 = 𝑆 𝑎 + 𝑆 𝑏 + 𝑆 𝑐
Many times the only voltages known will be the magnitudes of the three line-to-line voltages at the machine terminals. When this is the case, the Law of Cosines must be used to compute the angles associated with the measured magnitudes.
ECpE Department

36. Fig.6 Equivalent T circuit
Once the terminal line-to-neutral voltages and currents are known, it is desired to analyze what is happening inside the machine. In particular, the stator and rotor losses are needed in addition to the “converted” shaft power. A method of performing the internal analysis can be developed in the phase frame by starting with the sequence networks. Fig.5 can be modified by removing RL, which represents the “load resistance” in the positive and negative sequence networks. The resulting networks will be modeled using A, B, C, and D parameters. The equivalent T circuit (RL removed) is shown in Fig.6. This circuit can represent both the positive and negative sequence networks. The only difference (if any) will be between the numerical values of the sequence stator and rotor impedances.
Fig.6 Equivalent T circuit
ECpE Department

37. Fig.6 Equivalent T circuit
In Fig.6,
𝑌𝑚= 1 𝑗𝑋𝑚
(62)
Define the sequence stator and rotor impedances:
𝑍𝑠 𝑖 = 𝑅𝑠 𝑖 + 𝑗𝑋𝑠 𝑖
(63)
𝑍𝑟 𝑖 = 𝑅𝑟 𝑖 + 𝑗𝑋𝑟 𝑖
(64)
The positive and negative sequence A, B, C, and D parameters of the unsymmetrical T circuit of Fig.6 are given by
𝐴𝑚 𝑖 =1+ 𝑌𝑚 𝑖 ∙𝑍𝑠 𝑖
(65)
𝐵𝑚 𝑖 = 𝑍𝑠 𝑖 + 𝑍𝑟 𝑖 + 𝑌𝑚 𝑖 ∙𝑍𝑠 𝑖 ∙𝑍𝑟 𝑖
(66)
𝐶𝑚 𝑖 = 𝑌𝑚 𝑖
(67)
𝐷𝑚 𝑖 =1+ 𝑌𝑚 𝑖 ∙𝑍𝑟 𝑖
(68)
where
i = 1 for positive sequence
i = 2 for negative sequence
Fig.6 Equivalent T circuit
ECpE Department

38. Equivalent T circuit (69) (70) (71) (72) Note
𝐴𝑚 𝑖 ∙ 𝐷𝑚 𝑖 − 𝐵𝑚 𝑖 ∙ 𝐷𝑚 𝑖 =1
(69)
The terminal sequence line-to-neutral voltages and currents as functions of the rotor “load voltages” (Vr) and the rotor currents are given by
𝑉𝑠 𝑖 𝐼𝑠 𝑖 = 𝐴𝑚 𝑖 𝐵𝑚 𝑖 𝐶𝑚 𝑖 𝐷𝑚 𝑖 ∙ 𝑉𝑟 𝑖 𝐼𝑟 𝑖
(70)
Because of Equation (69), the inverse of Equation (70) is
𝑉𝑟 𝑖 𝐼𝑟 𝑖 = 𝐷𝑚 𝑖 −𝐵𝑚 𝑖 −𝐶𝑚 𝑖 𝐴𝑚 𝑖 ∙ 𝑉𝑠 𝑖 𝐼𝑠 𝑖
(71)
Equation (71) can be expanded to show the individual sequence voltages and currents:
(72)
ECpE Department

39. Equivalent T circuit (72) (73) (74) (75) (76) (77)
Equation (72) can be partitioned between the third and fourth rows and columns. In reduced form by incorporating the partitioning, Equation (72) becomes
(73)
[ 𝑉𝑟 012 ] [ 𝐼𝑟 012 ] = [ 𝐷𝑚 012 ] [ 𝐵𝑚 012 ] [ 𝐶𝑚 012 ] [ 𝐴𝑚 012 ] ∙ [ 𝑉𝑠 012 ] [ 𝐼𝑠 012 ]
Expanding Equation (73),
(74)
𝑉𝑟 012 =[ 𝐷𝑚 012 ]∙ 𝑉𝑠 [ 𝐵𝑚 012 ]∙ [ 𝐼𝑠 012 ]
(75)
𝐼𝑟 012 =[ 𝐶𝑚 012 ]∙ 𝑉𝑠 [ 𝐴𝑚 012 ]∙ [ 𝐼𝑠 012 ]
Equations (74) and (75) can be transformed into the phase domain:
(76)
𝑉𝑟 𝑎𝑏𝑐 =[𝐴]∙ 𝑉𝑟 012 =[𝐴]∙[ 𝐷𝑚 012 ] ∙ 𝐴 −1 ∙ 𝑉𝑠 𝑎𝑏𝑐 +[𝐴]∙[ 𝐵𝑚 012 ] ∙ 𝐴 −1 ∙ 𝐼𝑠 𝑎𝑏𝑐
(77)
𝐼𝑟 𝑎𝑏𝑐 =[𝐴]∙ 𝐼𝑟 012 =[𝐴]∙[ 𝐶𝑚 012 ] ∙ 𝐴 −1 ∙ 𝑉𝑠 𝑎𝑏𝑐 +[𝐴]∙[ 𝐴𝑚 012 ] ∙ 𝐴 −1 ∙ 𝐼𝑠 𝑎𝑏𝑐
ECpE Department

40. Equivalent T circuit
𝑉𝑟 𝑎𝑏𝑐 =[𝐴]∙ 𝑉𝑟 012 =[𝐴]∙[ 𝐷𝑚 012 ] ∙ 𝐴 −1 ∙ 𝑉𝑠 𝑎𝑏𝑐 +[𝐴]∙[ 𝐵𝑚 012 ] ∙ 𝐴 −1 ∙ 𝐼𝑠 𝑎𝑏𝑐
(76)
𝐼𝑟 𝑎𝑏𝑐 =[𝐴]∙ 𝐼𝑟 012 =[𝐴]∙[ 𝐶𝑚 012 ] ∙ 𝐴 −1 ∙ 𝑉𝑠 𝑎𝑏𝑐 +[𝐴]∙[ 𝐴𝑚 012 ] ∙ 𝐴 −1 ∙ 𝐼𝑠 𝑎𝑏𝑐
(77)
Therefore
𝑉𝑟 𝑎𝑏𝑐 =[ 𝐷𝑚 𝑎𝑏𝑐 ]∙ 𝑉𝑠 𝑎𝑏𝑐 +[ 𝐵𝑚 𝑎𝑏𝑐 ]∙ 𝐼𝑠 𝑎𝑏𝑐
(78)
𝐼𝑟 𝑎𝑏𝑐 =[ 𝐶𝑚 𝑎𝑏𝑐 ]∙ 𝑉𝑠 𝑎𝑏𝑐 +[ 𝐴𝑚 𝑎𝑏𝑐 ]∙ 𝐼𝑠 𝑎𝑏𝑐
(79)
where
𝐴𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐴𝑚 012 ] ∙ 𝐴 −1
𝐵𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐵𝑚 012 ] ∙ 𝐴 −1
(80)
𝐶𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐶𝑚 012 ] ∙ 𝐴 −1
𝐷𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐷𝑚 012 ] ∙ 𝐴 −1
The power converted to the shaft is given by
(81)
𝑃 𝑐𝑜𝑛𝑣 = 𝑉𝑟 𝑎 ∙ 𝐼𝑟 𝑎 ∗ + 𝑉𝑟 𝑏 ∙ 𝐼𝑟 𝑏 ∗ + 𝑉𝑟 𝑐 ∙ 𝐼𝑟 𝑐 ∗
The useful shaft power can be determined from a knowledge of the rotational (FW) losses:
𝑃 𝑠ℎ𝑎𝑓𝑡 = 𝑃 𝑐𝑜𝑛𝑣 − 𝑃 𝐹𝑊
(82)
ECpE Department

41. Equivalent T circuit 𝑃 𝑟𝑜𝑡𝑜𝑟 = 𝐼𝑟 𝑎 2 ∙𝑅𝑟+ 𝐼𝑟 𝑏 2 ∙𝑅𝑟+ 𝐼𝑟 𝑐 2 ∙𝑅𝑟 (83)
The rotor “copper” losses are
𝑃 𝑟𝑜𝑡𝑜𝑟 = 𝐼𝑟 𝑎 2 ∙𝑅𝑟+ 𝐼𝑟 𝑏 2 ∙𝑅𝑟+ 𝐼𝑟 𝑐 2 ∙𝑅𝑟
(83)
The stator “copper” losses are
𝑃 𝑠𝑡𝑎𝑡𝑜𝑟 = 𝐼𝑠 𝑎 2 ∙𝑅𝑠+ 𝐼𝑠 𝑏 2 ∙𝑅𝑠+ 𝐼𝑠 𝑐 2 ∙𝑅𝑠
(84)
The total input power is
𝑃 𝑖𝑛 =𝑅𝑒[ 𝑉𝑠 𝑎 ∙ 𝐼𝑠 𝑎 ∗ + 𝑉𝑠 𝑏 ∙ 𝐼𝑠 𝑏 ∗ + 𝑉𝑠 𝑐 ∙ 𝐼𝑠 𝑐 ∗ ]
(85)
ECpE Department

42. Example 2
To demonstrate the analysis of an induction motor in the phase frame, the following induction motor will be used:
25 hp, 240 V operating with slip = 0.035
Plossrotation = 0.75 kW
Zs = j Ω
Zm = 0 + j Ω
Zr = j Ω
The “load” resistances are
𝑅𝐿 1 = 1− ∙0.098= Ω
𝑅𝐿 2 = 1−(1.965) ∙0.0908=− Ω
The input sequence impedances are
𝑍𝑀 1 =𝑍𝑠+ 𝑍𝑚∙(𝑍𝑟+ 𝑅𝐿 1 ) 𝑍𝑚+𝑍𝑟+ 𝑅𝐿 1 = 𝑗 Ω
𝑍𝑀 2 =𝑍𝑠+ 𝑍𝑚∙(𝑍𝑟+ 𝑅𝐿 2 ) 𝑍𝑚+𝑍𝑟+ 𝑅𝐿 2 = 𝑗 Ω
ECpE Department

43. Example 2 The positive and negative sequence input admittances are
𝑌𝑀 1 = 1 𝑍𝑀 1 =0.3461−𝑗 𝑆
𝑌𝑀 2 = 1 𝑍𝑀 2 =0.8255−𝑗 𝑆
The sequence admittance matrix is
𝑌𝑀 012 = −𝑗 −𝑗 𝑆
Applying Equation (48), the phase admittance matrix is
𝑌𝑀 𝑎𝑏𝑐 = −𝑗 −0.0999−𝑗 𝑗 𝑗 −𝑗 −0.0999−𝑗 −0.0999−𝑗 𝑗 −𝑗 𝑆
The line-to-line terminal voltages are measured to be
𝑉 𝑎𝑏 =235 𝑉, 𝑉 𝑏𝑐 =240 𝑉, 𝑉 𝑐𝑎 =245 𝑉
ECpE Department

44. Example 2
Since the sum of the line-to-line voltages must equal zero, the law of cosines can be used to determine the angles on the voltages. Applying the law of cosines results in
𝑉𝐿𝐿 𝑎𝑏𝑐 = 235∠0 240∠− ∠ 𝑉
The phase motor currents can now be computed:
𝐼𝑠 𝑎𝑏𝑐 = 𝑌𝑀 𝑎𝑏𝑐 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐 = ∠− ∠− ∠ 𝐴
It is obvious that the currents are very unbalanced. The measure of unbalance for the voltages and currents can be computed as [1]
𝑉 𝑢𝑛𝑏𝑎𝑙𝑎𝑛𝑐𝑒 = max⁡_𝑑𝑒𝑣𝑖𝑎𝑡𝑖𝑜𝑛 𝑉 𝑎𝑣𝑔 ∙100= ∙100=2.08%
𝐼 𝑢𝑛𝑏𝑎𝑙𝑎𝑛𝑐𝑒 = max⁡_𝑑𝑒𝑣𝑖𝑎𝑡𝑖𝑜𝑛 𝐼 𝑎𝑣𝑔 ∙100= ∙100=14.27%
[1] American National Standard for Electric Power Systems and Equipment–Voltage Ratings (60 Hertz), ANSI C , National Electrical Manufacturers Association, Rosslyn, VA, 1996.
ECpE Department

45. Example 2
This example demonstrates that the current unbalance is approximately seven times greater than the voltage unbalance. This ratio of current unbalance to voltage unbalance is typical. The actual operating characteristics including stator and rotor losses of the motor can be determined using the method developed in Ref. [2]
The equivalent line-to-neutral voltages at the motor are computed using the [W] matrix:
𝑉𝑠 𝑎𝑏𝑐 = 𝑊 ∙ 𝑉𝐿𝐿 𝑎𝑏𝑐
𝑉 𝑎𝑛 𝑉 𝑏𝑛 𝑉 𝑐𝑛 = 1 3 ∙ ∙ ∠0 240∠− ∠ = ∠− ∠− ∠91.4
The input complex power to the motor is
𝑆 𝑖𝑛 = 𝑘=1 3 𝑉𝑠 𝑎𝑏𝑐 𝑘 ∙ 𝐼 𝑎𝑏𝑐 𝑘 =19.95+𝑗13.62
𝑆 𝑖𝑛 =24.15
𝑃𝐹=0.83 𝑙𝑎𝑔𝑔𝑖𝑛𝑔
[2] Kersting, W.H. and Phillips, W.H., Phase frame analysis of the effects of voltage unbalance on induction machines, IEEE Transactions on Industry Applications, 33, 415–420, 1997.
ECpE Department

46. Example 2
The rotor currents and voltages can be computed by first computing the equivalent A, B, C, and D matrices according to Equations (80). The first step is to compute the sequence A, B, C, and D parameters according to Equations (65) through (68):
𝐴𝑚 𝑖 =1+ 𝑌𝑚 𝑖 ∙𝑍𝑠 𝑖 =1.0381−𝑗0.0161
𝐵𝑚 𝑖 = 𝑍𝑠 𝑖 + 𝑍𝑟 𝑖 + 𝑌𝑚 𝑖 ∙𝑍𝑠 𝑖 ∙𝑍𝑟 𝑖 = 𝑗0.3742
𝐶𝑚 𝑖 = 𝑌𝑚 𝑖 =−𝑗0.2067
𝐷𝑚 𝑖 =1+ 𝑌𝑚 𝑖 ∙𝑍𝑟 𝑖 =1.0381−𝑗0.0188
The sequence matrices using Equations (72) are
𝐴𝑚 012 = −𝑗 −𝑗0.0161
𝐵𝑚 012 = −0.1746−𝑗 −0.1746−𝑗0.3742
𝐶𝑚 012 = 𝑗 𝑗0.2067
𝐵𝑚 012 = −𝑗 −𝑗0.0188
ECpE Department

47. Example 2
Equation (80) gives us the final phase domain A, B, C, and D matrices:
𝐴𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐴𝑚 012 ] ∙ 𝐴 −1 = −𝑗 −0.346+𝑗 −0.346+𝑗 −0.346+𝑗 −𝑗 −0.346+𝑗 −0.346+𝑗 −0.346+𝑗 −𝑗0.0107
𝐵𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐵𝑚 012 ] ∙ 𝐴 −1 = −0.1164−𝑗 𝑗 𝑗 𝑗 −0.1164−𝑗 𝑗 𝑗 𝑗 −0.1164−𝑗0.2494
𝐶𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐶𝑚 012 ] ∙ 𝐴 −1 = 𝑗 −𝑗 −𝑗 −𝑗 𝑗 −𝑗 −𝑗 −𝑗 𝑗0.1378
𝐷𝑚 𝑎𝑏𝑐 = [𝐴]∙[ 𝐷𝑚 012 ] ∙ 𝐴 −1 = −𝑗 −0.346+𝑗 −0.346+𝑗 −0.346+𝑗 −𝑗 −0.346+𝑗 −0.346+𝑗 −0.346+𝑗 −𝑗0.0125
ECpE Department

48. Example 2
With the matrices defined, the rotor voltages and currents can be computed:
𝑉𝑟 𝑎𝑏𝑐 =[ 𝐷𝑚 𝑎𝑏𝑐 ]∙ 𝑉𝑠 𝑎𝑏𝑐 + 𝐵𝑚 𝑎𝑏𝑐 ∙ 𝐼𝑠 𝑎𝑏𝑐 = ∠− ∠− ∠83.9
𝐼𝑟 𝑎𝑏𝑐 =[ 𝐶𝑚 𝑎𝑏𝑐 ]∙ 𝑉𝑠 𝑎𝑏𝑐 +[ 𝐴𝑚 𝑎𝑏𝑐 ]∙ 𝐼𝑠 𝑎𝑏𝑐 = 42.2∠− ∠− ∠79.1
The converted electric power to shaft power is
𝑃 𝑐𝑜𝑛𝑣𝑒𝑟𝑡 = 𝑘=1 3 𝑉𝑠 𝑎𝑏𝑐 𝑘 ∙ 𝐼 𝑎𝑏𝑐 𝑘 ∗ =18.5 𝑘𝑊
The power converted in units of horsepower is
ℎ𝑝= 𝑃 𝑐𝑜𝑛𝑣𝑒𝑟𝑡 =24.8
Note how the shaft power in horsepower is approximately equal to the input kVA of the motor. This is typically the case so that a good assumption for a motor is that the rated output in horsepower will be equal to the input kVA.
ECpE Department

49. Thank You!
ECpE Department