Location

This example model can be found in the software under Distributed Generation > VFD_PMSM.ecf

Description

Background

Variable frequency drive-based applications are gaining more popularity recently due to higher energy efficiency and increased control. This demo example demonstrates the two-quadrant operation (forward motoring and reverse motoring) of various applications like elevators, cranes etc. For instance, an elevator moving up with people inside represents the forward motoring mode wherein the load is lifted against the gravity, and coming down empty with gravity-assist as reverse motoring mode.

This example shows the implementation of Variable Frequency Drive (VFD) based Permanent Magnet Synchronous Machine (PMSM) example where the converters are implemented using: a) Switching Bridges and b) Switching Function. The converters used in this example are the 2-level Voltage Source Converters. The advantage of using Switching function based model for real time execution performance is demonstrated.

For more details about the converters model please refer to Switching Device and Switching Function

Model description

The model consists of a 380 V, 60 Hz voltage source (emulating a grid) feeding the PMSM through back to back voltage source converters as shown in Figure 1. The PMSM carries a load whose torque is proportional to the mechanical speed and is given by the equation: T_load=0.04 w_m.

The example consists of 2 separate models, one with Switching Device and the other with Switching Function. The control architecture and network parameters are the same for both these implementations. The only difference being the type of converter used and the respective controller. Switching function based converter uses the PWM average generator, which is a pulse generator averaged over the time step of the simulation. It uses an interpolation technique to calculate the transition moment within a sample time of the model. The switching bridge model uses the standard PWM generator using carrier-based pulse width modulation (PWM) converters. Both the converters are operating at the switching frequency of 4.2 kHz.

Figure 1. Schematic diagram

Each converter of the model has a specific objective, as given below:

Grid-side converter control

The grid side converter (GSC) is operated to control the DC link voltage amplitude and the reactive power . The outer voltage loop maintains the DC ink voltage at 700 V, while the reactive power control loop is set to unity power factor. Below figures show the control architecture used.

Outer loop DC link voltage regulator

Inner loop Current regulator

Where,

Id : d-axis component of source current

Iq : q- axis component of source current

Vd : d-axis component of source voltage

Vq : q-axis component of source voltage

Machine-side converter control

The machine side converter (MSC) controls the generator speed. LC filters connected shunt to the network are used to eliminate the harmonics.

Outer loop Speed regulator

Inner loop torque and current regulator

Where,

Id : d-axis component of machine stator current

Iq : q-axis component of machine stator current

Φ : Magnetic flux of the machine

p : Number of poles

Model parameters

System parameters

S. No	Name	Unit	Value

S. No	Name	Unit	Value
1	Source voltage	V	380
2	Series resistance- Rs	Ω	1
3	Series inductance - Ls	H	10
4	GSC Filter - F_GSC [R C]	[Ω uF]	[14.27 15.6]
5	GSC Choke - L_GSC	mH	20
6	DC Link Capacitor - C_dc	uF	300
7	MSC Choke - L_MSC	mH	10.46
8	MSC Filter - F_MSC [R C]	[Ω uF]	[10.95 2.42]

Machine general parameters

S No.	Name	Unit	Value

S No.	Name	Unit	Value
1	Rated voltage - V	V	380
2	Rated power - S	kVA	3
3	Nominal frequency - f	Hz	60
4	Number of poles - p	-	6

Machine electrical parameters

S No.	Name	Unit	Value

S No.	Name	Unit	Value
1	Armature resistance - R_s	pu	0.068559
2	Armature leakage reactance - XI	pu	0.032
3	Zero sequence reactance - X_o	pu	0.0652
4	d-axis synchronous reactance - X_d	pu	0.32574
5	q-axis synchronous reactance - Xq	pu	0.4469
6	D1 damper leakage reactance - XID1	pu	0.13
7	D2 damper leakage reactance - XID2	pu	0.13
8	D1 damper resistance - RD1	pu	0.054
9	D2 damper resistance - RD2	pu	0.054
10	Q1 damper leakage reactance - XIQ1	pu	0.13
11	Q2 damper leakage reactance - XIQ2	pu	0.13
12	Q1 damper resistance - RQ1	pu	0.108
13	Q2 damper resistance - RQ2	pu	0.108
14	Permanent flux magnet	Wb	0.4832

Machine mechanical parameters

S No.	Name	Unit	Value

S No.	Name	Unit	Value
1	Inertia constant - H	s	0.0265
2	Absolute damping coefficient - K_d	Nm/rad/s	0.0
3	Stiffness coefficient - Kij	Nm/rad	0.0
4	Self damping coefficient - D	Nm/rad/s	0.0
5	Mutual damping coefficient - Dij	Nm/rad/s	0.0
6	Fraction external torque - F	-	0.0

Converter parameters

S No.	Name	Unit	Value
1	Switching frequency	Hz	4200
Switching device converter parameters
2	Rclose	m Ω	1
3	Rsnubber	Ω	500
4	Csnubber	nF	100
Switching function converter parameters
5	Rclose	m Ω	1
6	Rsnubber	Ω	500
7	Rsource	MΩ	1

Controller parameters

S No.	Name	Unit	Value
DC voltage regulator - GSC
1	Kp	--	0.04
2	Ki	--	6.35
Current regulator - GSC
1	Kp	--	60
2	Ki	--	2000
Speed regulator - MSC
1	Kp	--	5.035
2	Ki	--	1258.75
Current regulator - MSC
1	Kp	--	67.52
2	Ki	--	3300

Scenarios

The following scenarios demonstrate the modes of operation of the PMSM based VFD example and the results are shown in the figure below. The sequence of the mode change is shown in the table below.To demonstrate the performance of the model few fault scenarios introduced during forward and reverse motoring modes.

Operating points	From	To	Vdc (V)	Wref (rad/s)	Scenario	Fault type
1	0 s	1.5 s	700	125.66	Forward motoring	-
2	1.5 s	3 s	700	0	Motor stop (Stand-still)	-
3	3 s	4.5 s	700	-100	Reverse motoring	-
4	4.5 s	4.51 s	700	125.66	Reverse motoring	L-G fault applied
5	4.51 s	6 s	700	125.66	Forward motoring	-
6	6 s	6.1 s	700	125.66	Forward motoring	L-G fault applied
7	6.1 s	8 s	700	125.66	Forward motoring	-

Simulation and Results

The demo example is simulated at 20us. Figures below shows the performance of both model implementations and their comparison. It can be seen that the switching function model performance is very close to switching bridge model.

The results in Red color are Switch device based PMSM model results and results in Blue color are Switch function based PMSM model

Figure 2. Results at 20 us

Figure 3.Results at 20 us (zoomed in at 4 sec)

Figure 4. Stator current of the machine

Real-time Performance

The real time execution performance of the models is given in the below table. It can be seen that the switching function model takes lesser execution time when compared to bridge model.

HYPERSIM Model	Execution Time (us)	Max Execution time (us)
Switching Device based PMSM model	2.5022	5.14
Switching Function based PMSM model	2.3867	4.92

Performance Comparison of Switching function at 20 us (real time) with Switching Device Model at 2 us (offline)

Real time performance of Switching Function based model simulated at 20 us (real time) is compared with Switching Device model at 2 us (offline). It can be seen that the results are very close. This demonstrates the effectiveness of using the switching function model at a much higher time step (of 20us) and get similar performance as a 2us model.