Comparing Methods to Mitigate the Effect of Grid Voltage Sag and

Frequency Variation on the Operation of Variable Speed Drives

MOHAMED IBRAHIM, MAGED N. F. NASHED, MONA N. ESKANDER

Power Electronics and Energy Conversion Department,

Electronics Research Institute,

Joseph Tito St., Huckstep, Qism El-Nozha, Cairo Governorate, Cairo,

EGYPT

Abstract: - In this paper, the effect of unsymmetrical grid voltage sag on the performance of a slip power

recovery drive is studied, and two strategies to mitigate its effects are investigated. The first strategy consists of

three phase parallel RL impedance connected to the rotor circuit. The second strategy consists of three phase

LCL filter connected to the rotor circuit. A PI controller is tuned to test the drive at constant speed. Also, the

effect of grid frequency drop on the drive performance is investigated with the same two protection strategies.

The proposed systems are modeled with Matlab/Simulink and the simulation results are compared. The drive

performance without any protection schemes are compared with its performance with the two proposed

strategies. The compared results include the stator voltage and current, the rotor voltage and current, the DC

link voltage and current, the electric torque, and the rotor speed.

Key-Words: - unsymmetrical voltage sag, three phase LCL filter, PI controller, grid frequency, three phase

parallel RL impedance, protection.

Received: February 12, 2023. Revised: November 29, 2023. Accepted: December 11, 2023. Published: March 1, 2024.

1 Introduction

The quality of electricity is an important issue for

electricity distribution companies and end-users

such as the industrial sector. Low power quality can

disturb the customer's production process, and this,

in turn, leads to loss of revenue. The variable speed

drive (VSD) technology is vastly used in various

industrial applications. The slip power recovery

drive (SPRD) is one of the employed VSDs in

industrial systems. It consists of a wound rotor

induction machine with its rotor circuit connected to

a 3-phase rectifier, a DC link, and a 3-phase inverter

to provide speed and torque control. It employs low

cost converters since the rotor power is a fraction of

stator power. These drives may be subjected to

power quality issues such as symmetrical and

unsymmetrical voltage sag, unrated supply

frequency, and harmonic distortion. Voltage sag

causes a high current peak, high torque peak at the

point of voltage recovery, and speed loss during the

sag. In addition, unsymmetrical voltage sag results

in increased losses, and therefore a rise in

temperature, a decrease in the motor's efficiency,

and a decrease in the lifetime of insulation, [1]

Several proposals for mitigating the effects of

voltage sag on the performance of VSD have been

proposed in the literature. Kinetic energy recovery

[2], increasing the DC bus capacitor size [3], adding

a boost converter [4], applying series dynamic

braking resistors [5], and applying different Flexible

AC Transmission System (FACTS) devices are

examples of these proposals, [6], [7], [8], [9].

Another power quality problem is the decrease

in supply frequency which causes a decrease in the

speed of the rotating field on the stator, affecting the

efficiency of the SPRD. Also, since the stator of the

SPRD is directly connected to the grid without an

electronic device, it is very sensitive to supply

frequency deviation. The influence of supply

frequency variation on the performance of induction

motor drives was studied without giving specific

solutions for such problems, [10], [11].

In this paper, the effect of unsymmetrical grid

voltage sag on the performance of the drive is

studied, and two strategies to mitigate their effects

are investigated. The first strategy consists of three

phase parallel RL impedance connected to the rotor

circuit. The second strategy consists of three phase

LCL filter connected to the rotor circuit. A PI

controller is tuned to test the drive at constant speed.

The proposed systems are modeled with

Matlab/Simulink and the simulation results are

compared. The performance without any protection

results are compared with the results of the two

WSEAS TRANSACTIONS on POWER SYSTEMS

DOI: 10.37394/232016.2024.19.2

Mohamed Ibrahim, Maged N. F. Nashed,

Mona N. Eskander

E-ISSN: 2224-350X

Volume 19, 2024

proposed strategies. The compared results include

the stator voltage and current, the rotor voltage and

current, the DC link voltage and current, the electric

torque, and the rotor speed. The decrease in

transients at the starting and end of the symmetrical

and unsymmetrical voltage sag in currents, voltages,

and electric torque are noticed with better

performance when using the LCL filter. Also, the

effect of grid frequency drop on the drive

performance is investigated with the same two

protection strategies. The compared results include

the stator voltage and current, the rotor voltage and

current, the DC link voltage and current, the electric

torque, and the rotor speed.

The paper’s contributions are summarized as

follows:

1. Investigating simple techniques for fault-ride-

through of slip energy recovery drives during

symmetrical and unsymmetrical voltage sags.

2. Investigating simple techniques to prevent the

diverse effects of frequency variation on the slip

energy recovery drives.

Hence improving the power quality of the grid-

connected drives can be achieved.

2 System Description

The block diagram of the investigated system is

shown in Figure 1, including the two proposed

protection schemes.

Fig. 1: System block diagram

2.1 Induction Machine Equations

The d-q axes equations describing the proposed

system are given as [8];

Vds = RsIds + pds - ω qs (1)

Vqs = RsIqs + pqs + ω ds (2)

Vdr = Rr Idr + pdr - ωrqr (3)

Vqr = RrIqr + pqr - ωrdr (4)

ds = LsIds + MIdr (5)

qs = Ls Iqs + M Iqr (6)

dr = Lr Idr + M Ids (7)

qr = Lr Iqr + M Iqs (8)

Electromagnetic torque equation:

Te = 1.5 P M (qs Idr – ds Iqr)/ Ls (9)

Where, sufixes s and r stand for stator and rotor

parameters respectively, V is the voltage, I is the

current,  is the flux, M is the mutual inductance, L

is the self-inductance, R is the resistance per phase,

ω is the synchronous speed, P number of pole pairs,

and "p" is the d/dt operator.

2.2 Rectifier

The three phase rectifier connected to the rotor

circuit consists of 6 diodes. It is described by:

  

 



 (10)

Where Vpeak is the rotor circuit voltage per phase.

2.3 The Three Phase Inverter

A three phase IGBT inverter is applied to change

the DC rectifier voltage to a 60 Hz AC voltage fed

back to the grid. It is controlled by a conventional PI

controller.

2.4 Three Phase Dynamic Rotor Impedance

It is applied during voltage sag only via bypass

switches, decreasing the rotor fault current and the

torque oscillations.

2.5 The LCL filter

LCL filters are specially designed to reduce the

harmonics of current absorbed by faults or power

converters, which can be useful in reducing the

effect of voltage sags. Mainly, they are made of a

parallel-series combination of reactors and

capacitors adapted to reduce the current harmonics.

In this paper, it is connected to the rotor circuit.

3 Simulation Results for Un-

Symmetrical Voltage Sag

A single phase unsymmetrical voltage sag (phase A

drops voltage drops from 460 t0 250 V) is assumed

for 0.2 seconds from (0.5- 0.7 sec.). The system is

modeled and simulated using Matlab/Simulink for

the three cases; namely, the SPRD without any

protection devices, the SPRD with the rotor circuit

added dynamic impedance, and the SPRD with the

LCL filter.

WSEAS TRANSACTIONS on POWER SYSTEMS

DOI: 10.37394/232016.2024.19.2

Mohamed Ibrahim, Maged N. F. Nashed,

Mona N. Eskander

E-ISSN: 2224-350X

Volume 19, 2024

3.1 SPRD Without any Protection Devices

Figure 2 and Figure 3 show the stator voltage and

current, the rotor voltage and current the DC link

voltage and current, the electric torque, and the rotor

speed, during voltage sag without protection

devices. The produced high current and voltage

peaks, and the torque oscillations can cause damage

to the machine and the power electronic devices.

Fig. 2: stator voltage and current, rotor voltage and

current during unsymmetrical voltage sag without

any protection devices

3.2 SPRD with Rotor Impedance and Un-

Symmetrical Voltage Sag

A 3-phase parallel RL impedance is connected to

the rotor circuit to damp the effect of unsymmetrical

voltage sag on the SPRD performance. The

magnitude of the stator current during the voltage

sag, shown in Figure 4, decreased to half of its value

compared to the case without any protection, while

the stator voltage profile was not affected. Also, a

decrease in the transient rotor voltage and current

peaks due to the unsymmetrical sag is obvious when

compared to their magnitude without protection

devices. The spike in Figure 4 is due to the sudden

change in grid voltage at the beginning of the

unsymmetrical fault. It takes a very short time,

hence it does not cause damage to the rotor circuit.

Fig. 3: DC voltage and current, the electric torque,

and the rotor speed during unsymmetrical voltage

sag without any protection devices

Fig. 4: Stator voltage and current, rotor voltage and

current during unsymmetrical voltage sag with rotor

impedance

Figure 5 shows a large drop in the magnitude of

the DC link peak voltage and current during the

voltage sag with fast recovery to their normal values

as the sag ends. Also, lower ripples are noticed

when compared to the case without a protection

scheme. However, ripples are noticed in the electric

torque profile due to the voltage sag but with

smaller magnitudes than the case without the added

RL impedance, while the drop in the rotor speed is

minimized.

WSEAS TRANSACTIONS on POWER SYSTEMS

DOI: 10.37394/232016.2024.19.2

Mohamed Ibrahim, Maged N. F. Nashed,

Mona N. Eskander

E-ISSN: 2224-350X

Volume 19, 2024

Fig. 5: DC voltage and current, electric torque, and

rotor speed during voltage sag with rotor impedance

3.3 SPRD with Rotor LCL and

Unsymmetrical Voltage Sag

A 3-phase LCL impedance is connected to the rotor

circuit to damp the effect of unsymmetrical voltage

sag on the SPRD performance. Results are shown in

Figure 6 and Figure 7. The magnitude of the stator

current during the voltage sag, shown in Figure 6,

decreased as compared to its value without rotor

LCL protection, but with a higher value when

compared to rotor RL protection scheme. The stator

voltage profile is not affected. Also, a decrease in

the transient rotor voltage and current peaks due to

the unsymmetrical sag is obvious when compared to

their magnitude without protection devices.

However, the rotor voltage and current during sag

are higher when compared to the rotor RL

protection scheme. The high frequency content in

Figure 6 means that the system with an LCL filter

needs a longer time to settle, which gives superiority

to the rotor RL damping scheme. Figure 7 shows a

large drop in the magnitude of the DC link peak

voltage and current during the voltage sag with fast

recovery to their normal values as the sag ends.

Also, lower ripples are noticed when compared to

the case without a protection scheme. Concerning

the electric torque, ripples are higher than the case

with the added RL impedance, while the rotor speed

variation during the voltage sag is minimized. It is

noticed that the DC voltage and current profiles as

well as the electric torque during the voltage sag are

similar to their profile when using the RL

impedance, while the stator current and the rotor

voltage and current are much better.

Fig. 6: Stator and rotor voltage and currents with

LCL filter during unsymmetrical voltage sag

Fig. 7: DC voltage and current, electric torque, and

rotor speed during voltage sag with rotor LCL filter

4 Effect of Supply Frequency Drop

on the SPRD Parameters

Since the stator of the SPRD is directly connected to

the grid without an electronic device, it is very

sensitive to supply frequency deviation. A drop in

the supply frequency from 60Hz to 50 Hz is

assumed from 0.5-0.7 sec, and the effect of the

proposed protection on the SPRD performance is

studied.

WSEAS TRANSACTIONS on POWER SYSTEMS

DOI: 10.37394/232016.2024.19.2

Mohamed Ibrahim, Maged N. F. Nashed,

Mona N. Eskander

E-ISSN: 2224-350X

Volume 19, 2024

4.1 SPRD during Supply Frequency Drop

without Protection

Figure 8 and Figure 9 display the stator voltage and

current, the rotor voltage and current, the DC

voltage and current, the electric torque, and the rotor

speed of the SPRD when the supply frequency

dropped from 60 to 50 Hz from 0.5sec to 0.7 sec

without protection scheme. It is noticed that the

frequency drop drastically affects the performance

of the SPRD.

Fig. 8: stator voltage and current, rotor voltage and

current due to frequency drop without any

protection

Fig. 9: DC link voltage and current, electric torque

and speed during grid frequency drop without

protection device

4.2 SPRD during Supply Frequency Drop

with Rotor RL Protection

Figure 10 and Figure 11 display the stator voltage

and current, the rotor voltage and current, the DC

voltage and current, the electric torque, and the rotor

speed of the SPRD when the supply frequency

dropped from 60 to 50 Hz from 0.5sec to 0.7 sec

with rotor RL protection. A drop in the stator

current transient during the fault is obvious. The

transient dropped from 40 amps without RL

impedance to approximately 20 amps. A small

decrease in the rotor voltage transient during the

fault took place, while a large decrease in the rotor

current transient is obvious. This behavior is due to

the added rotor inductance.

Figure 11 reveals the effect of the protection

scheme in decreasing the DC voltage and current

transients that occurred due to the frequency drop,

from 65 to 23 Volt. and from 35 Amp. without any

protection to 13 V with RL impedance.

Consequently, a large drop in the electric torque

transients is obvious, dropping from more than 150

Nm to 24 Nm, which is nearly its steady state value.

In addition, minimum ripples took place in the rotor

speed during the fault compared to the large speed

drop resulting from the frequency drop without

protection RL scheme.

Fig. 10: Stator and rotor voltages and currents

during frequency drop with added rotor RL

impedance

4.3 SPRD during Supply Frequency Drop

with Rotor LCL Protection

Figure 12 and Figure 13 display the stator voltage

and current, the rotor voltage and current, the DC

voltage and current, the electric torque, and the rotor

speed of the SPRD when the supply frequency

dropped from 60 to 50 Hz from 0.5sec to 0.7 sec

with rotor LCL protection. The stator current

WSEAS TRANSACTIONS on POWER SYSTEMS

DOI: 10.37394/232016.2024.19.2

Mohamed Ibrahim, Maged N. F. Nashed,

Mona N. Eskander

E-ISSN: 2224-350X

Volume 19, 2024

transient during the frequency drop slightly

decreased when compared to its value without

protection. Similarly, a small decrease in the rotor

current transients occurred, while the rotor voltage

transients are still large. The DC link voltage and

current, the electric torque, and the rotor speed

shown in Figure 12, improved as compared to the

case without any protection. However, these

transients are higher when compared to the added

RL impedance.

Fig. 11: DC voltages and currents, electric torque,

and speed during frequency drop with added rotor

RL impedance

Fig. 12: Stator and rotor voltages and currents

during frequency drop with added rotor LCL filter

Fig. 13: DC voltages and currents, electric torque,

and speed during frequency drop with added rotor

LCL filter

5 Conclusion

In this paper, the effect of unsymmetrical grid

voltage sag on the performance of a slip power

recovery drive (SPRD) is studied, and two strategies

to mitigate its effects are investigated. The first

strategy consists of a phase parallel RL impedance

connected to the rotor circuit. The second strategy

consists of three phase LCL filter connected to the

rotor circuit. A PI controller is tuned to test the drive

at constant speed. Also, the effect of grid frequency

drop on the drive performance is investigated with

the same two protection strategies. The proposed

systems are modeled with Matlab/ Simulink and the

simulation results are given. The drive stator voltage

and current, rotor voltage and current, DC link

voltage and current, electric torque, and the rotor

speed are calculated and their values are compared

for three cases, namely, without any protection

scheme, with the rotor added RL impedance, and

with the added LCL filter. It is concluded that the

added rotor RL impedance decreased the transients

that took place due to the unsymmetrical voltage sag

and those due to the frequency drop, drastically

while a moderate decrease in these transients

resulted when connecting the rotor LCL filter.

References:

[1] S. Ram, O. P. Rahi, and V. Sharmam, "A

comprehensive literature review on slip power

recovery drives", Renewable and Sustainable

Energy Reviews, Vol. 73, June 2017, pp: 922–

934,

WSEAS TRANSACTIONS on POWER SYSTEMS

DOI: 10.37394/232016.2024.19.2

Mohamed Ibrahim, Maged N. F. Nashed,

Mona N. Eskander

E-ISSN: 2224-350X

Volume 19, 2024

https://doi.org/10.1016/j.rser.2016.11.154.

[2] D. P. Panda, E. L. Benedict, G.

Venkataramanan, and T. A. Lipo, "A novel

control strategy for the rotor side control of a

doubly-fed induction machine", IEEE

Conference 36th Annual meeting of Industry

Applications, 30 Sept.-4 Oct. 2001, Chicago,

USA, pp: 1695–1702,

https://doi.org/10.1109/IAS.2001.955761.

[3] Alejandro Rolána, Joaquín Pedrab, and Felipe

Córcolesb, “Detailed study of DFIG-based

wind turbines to overcome the most severe

grid faults” Electrical Power and Energy

Systems (JEPE), Vol. 62, 2014. pp 868-878,

doi: 10.1016/j.ijepes.2014.05.031

[4] M. N. F. Nashed, A. M. Sharaf and M.

Eskander, "Fuzzy Logic Fault Ride-Through

Scheme for Electric Vehicles Using PMSM

AC Drives" International Journal on

Electrical Engineering and Informatics, Vol.

12, No 1, March 2020.

[5] K. Pietilainen, “Voltage sag ride through AC

Drives: Control and Analysis”, Final Thesis,

KTH, Royal Institute of Technology,

Stockholm, 2005.

[6] M. Sharanya, B. Basavaraja, and M. Sasikala,

"An Overview of Dynamic Voltage Restorer

for Voltage Profile Improvement",

International Journal of Engineering and

Advanced Technology (IJEAT), Vol. 2, No. 2,

Dec. 2012.

[7] R. Omar, N.A. Rahim, and M. Sulaiman,

“New control technique applied in dynamic

voltage restorer for voltage sag mitigation”,

American Journal of Engineering and Applied

Sciences, Vol. 3, No. 1, 2010, pp: 42-48.

[8] P. Jayanthi, and D. Devaraj, “Enhancement of

LVRT Capability in Grid Connected Wind

Energy Systems using Crowbar” 2019 IEEE

International Conference on Clean Energy

and Energy Efficient Electronics Circuit for

Sustainable Development (INCCES), 1 Dec.

2019,

https://doi.org/10.1109/INCCES47820.2019.9

167746.

[9] S. Shi, J. Xiao, S. Liu, L. Tu, Q. Cao, Y.

Liang, G. Zhang, and C. Mao, "A Scheme of

Dynamic Voltage Restorer to Improve Power

Quality in Large Scale Industrial Enterprises",

Journal of Power and Energy Engineering,

Vol. 4, No. 2, Feb. 2016, pp: 19-26.

[10] S. Rachev, L. Dimitrov, and D. Koeva, "Study

of the Influence of Supply Voltage on the

Dynamic Behavior of Induction Motor Low

Voltage Drive", International Scientific

Journal Machines. Technologies. Materials,

Vol. 11, No. 5, 2017, pp. 206-209.

[11] E. El-Kharashi, J. G. Massoud, and M. A. Al-

Ahmar, "The impact of the unbalance in both

the voltage and the frequency on the

performance of single and cascaded IM",

Energy, Vol. 181, August 2019, pp561-576,

https://doi.org/10.1016/j.energy.2019.05.169.

Contribution of Individual Authors to the

Creation of a Scientific Article (Ghostwriting

Policy)

The authors equally contributed in the present

research, at all stages from the formulation of the

problem to the final findings and solution.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

No funding was received for conducting this study.

Conflict of Interest

The authors have no conflicts of interest to declare.

Creative Commons Attribution License 4.0

(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en

_US

WSEAS TRANSACTIONS on POWER SYSTEMS

DOI: 10.37394/232016.2024.19.2

Mohamed Ibrahim, Maged N. F. Nashed,

Mona N. Eskander

E-ISSN: 2224-350X

Volume 19, 2024