RTDS/RSCAD FX – Induction Motor

This article is translated from RSCAD’s introductory tutorial.pdf Chapter 5 Induction Machine

For 5-RTDS/RSCAD FX – Induction Motor.

1 Introduction

Induction motors make up a large portion of industrial loads. Modeling asynchronous motor loads can be important in RTDS simulation cases when the impact of motor starting on the power supply system is of concern. The impact of motor loads on system operation can also be of interest.

2 Induction Motor Model

The induction motor model available on RTDS is a three-phase motor that can operate as a generator or a motor. The stator windings of the induction motor are connected to the three-phase bus as shown in Figure 5.1. Control signals are used as inputs to the motor model to specify the mechanical torque or speed input. Control signals are also used to select whether the torque or speed input is active.

Figure 5.1: Induction motor simulation example

When an induction motor is in “Torque” mode, a positive value of the input torque is required for the motor to operate as a generator (i.e. energy flows from the motor into the AC system). Negative values ​​of torque result in motor operation (i.e. energy flows into the motor). In “Speed” mode

1. If the input speed is greater than the synchronous speed, generator operation occurs
2. The motor runs when the speed is less than the synchronous speed.

**Note: The speed is allowed to be negative; corresponding to the motor braking

3 Induction motor parameters

The mechanical and electrical parameters of various induction motors can be found in technical papers and textbooks on electric machines. Data on various induction motors are given in reference [1]. Table II of reference [1] lists data on 31 different induction motors, ranging in size from 4 kW to 4 MW. The inertia is the moment of inertia J in kgm2 instead of the inertia constant H in MW−sec/MVA required by the RTDS induction motor model. The conversion from J to H can be done using the following relationship –

Reference [1] does not include saturation characteristics for the motor. Saturation curves are included in the default data for the RTDS induction motor component. However, the default saturation characteristics may not accurately represent the saturation characteristics of all induction motors.

In this example, a 150 hp (≈112 kW) induction motor with a double cage configuration is used. The data are given in Section 7.2.6. References [2].

The parameters of induction motor are as follows;

Figure 5.2: Configuration menu

Figure 5.3: Initial Conditions Menu

Figure 5.4: Trends menu

Figure 5.5: Control signal input menu

Figure 5.6: Motor electrical parameters menu

Figure 5.7: Mechanical parameters menu

Figure 5.8: Enabling monitoring in the RunTime menu

Figure 5.9: Signal names in the Runtime menu

4 Torque-speed characteristics

Graphs showing motor torque versus motor speed are often used to illustrate the operating characteristics of induction motors. Using an RSCAD/Draft circuit of the form shown in Figure 5.10, a torque-speed curve can be generated using RTDS. In this case, the induction motor model is connected to a 3-phase bus driven by an infinite power supply. The power supply model is used to maintain the bus voltage at 1 p.u. throughout the test range. A simple control circuit is used to generate a speed signal that increases linearly from 0.0 Hz to 60.0 Hz over a period of 6 seconds.

Figure 5.10: Induction motor induced draft circuit

In RSCAD/Runtime, the speed signal is started to rise through the SPD switch (as shown in Figure 5.11).

Create a Plot component in RSCAD/Runtime and select the SPEED signal as the x-axis variable and the torque signal as the y-axis variable.

In the Case->Options menu, set the plot completion time to 8 seconds and the Pre-trigger time to 5%.

Figure 5.11: RSCAD/Runtime torque-speed diagram

5 Torque load

When an induction motor is used as a motor, the load can be expressed as torque. The load of fans and pumps is usually proportional to the square of the speed.

The constant k is a property of the specific load. The friction torque load is constant with speed.

It is interesting to plot the load torque curve on the same grid as the motor torque-speed characteristic (see Figure 5.12). The intersection of the two curves defines the steady-state operating point of the motor. The intersection should be such that the operating point does not exceed the full-load operating point of the motor. The full-load capacity of a motor is usually given at the rated speed, which is usually close to the synchronous speed. Refer to Table 2.

[1] gives the full load capacity of the motor per unit rated slip (SN). The synchronous speed unit slip of the reference induction motor is –

Therefore, the rated unit slip of a 60 Hz motor is 0.025, which corresponds to a rotor speed of −377 − 0.025 * 377 = 367.58 rad/sec or 58.5 Hz

Figure 5.12 shows the torque-speed curves for various rotor resistance values, along with the load torque characteristics. The torque-speed curves are generated by varying the first cage and second cage rotor resistances.

Figure 5.12: Motor torque-speed and load torque-speed curves

For each rotor resistance, right click on the curve title in RunTime and click Save to save the speed vs torque curve. The curve can be saved in “.mpb”, “csv” format. Once the curve is saved in “csv” format, it can be plotted in Microsoft excel or other plotting software. When the curve is saved in mpb format, a .out file is also generated.

The torque-speed curves in Figure 5.12 are generated by varying the rotor resistance while keeping the bus voltage constant. Note that varying the induction motor terminal voltage can also be used to generate a set of torque-speed curves. Power electronics-based circuits for varying the terminal voltage are commonly used for induction motor control.

6 Examples

Figure 5.13 shows an example simulation case that includes an induction motor.

Figure 5.13: Example of a simulation case involving an induction motor

The induction motor is connected to a 400v busbar which is connected to the grid via a 13.8kV substation. The substation is connected to the high voltage grid (230.0kV) via a 10km line. The parameters of the circuit can be entered as follows.

The induction motor is based on the data provided in reference [2]. It includes a double rotor cage structure and is rated at 150 hp. The induction motor data is the same as the data used in the previous example, except for the initial conditions. The initial power flow conditions for the induction motor used in this example are shown in the figure below.

Figure 5.14: Trend menu

Transformer 1 – y -Gnd−Δ, 10.0MVA, 230.0kV / 13.8kV, leakage reactance = 0.15pu

Transformer 2 – y -Gnd−Δ, 0.18MVA, 13.8kV / 400V, leakage reactance = 0.15pu

Line 1−100km default Bergeron Line

Line 2 – 10km, ideally replace the Bergeron distribution line, the conductor and ground wire configuration is shown in the figure below.

Figure 5.15: Line and tower parameter options

Figure 5.16: Line conductor parameters

Figure 5.17: Line grounding parameters

The traveling wave line model cannot be used to represent lines that are shorter than one simulation time-step. For a 50μsec time step, this is 15km. A 10km transmission line cannot be modeled with a traveling wave line. In the line calculation block, there is an option to force lines shorter than one time step to be modeled as pi-sections, as shown below.

Figure 5.18: Line Calculation Block “Force Use PI Section Model” Option

The results of simulating a motor start (initial speed = 0) are shown in Figure 5.19. The initial state of the circuit breaker connecting the induction motor to the 400v bus is open. Closing the 400v circuit breaker (in this case, the circuit breaker element closing resistance is 0.0001Ω) initiates the capture of the plot shown in Figure 5.19. Interestingly, starting the induction motor in this manner results in a large current being drawn by the induction motor. The initial current draw causes a significant drop in the bus voltage (upper grid of Figure 5.19). Other loads in the vicinity may be adversely affected by this voltage drop. Once the induction motor speed approaches its rated slip, the bus voltage drop subsides.

Figure 5.19: Motor starting simulation results

7 References

1) Thiringer, “Comparison of Reduced-Order Dynamic Models of Induction Machines,” IEEE Trans. on Power Systems, Vol 16 No.1, pp-119-126, Feb.2011.

2) Kundar, Power System Stability and Control, cGraw-Hill, 1994.