Voltage Sag Effects on Energy-Optimal Controlled ... - IEEE Xplore

2012 IEEE International Conference on Power Electronics, Drives and Energy Systems December16-19, 2012, Bengaluru, India

Voltage Sag Effects on Energy-Optimal Controlled Induction Motor With Time-Varying loads Navneet Kumar, Thanga Raj Chelliah Water Resources Development & Management Department Indian Institute of Technology, Roorkee Roorkee, India [email protected], [email protected] Abstract— In this paper, the impact of most severe symmetrical voltage sag (type A) on an energy-optimal controlled induction motor (IM) is analyzed with time-varying load (TVL). The simulation results are provided to describe the dynamic performances (speed and torque profile) of the motor under voltage sag. The results are compared with IM connected to 3phase supply directly i.e without energy-optimal control (vector control). Model Based Control (MBC) is used to implement the said control. The effect of voltage sag in the operation of MBC is analyzed. To enhance ride through capability of IM and mitigate the effect of voltage sag, this paper investigates two techniques: (i) The Buck-Boost Converter, provides the desired DC link voltage (ii) Dynamic Voltage Restorer (DVR), provides the three phase voltage injection during voltage sag. Keywords— boost converter; dynamic voltage restorer; model based control; time-varying load; voltage sag

I.

INTRODUCTION

P

OWER-QUALITY (PQ) problem is an occurrence manifested in a nonstandard voltage, current, or frequency deviation that results in a failure or a miss operation of end-use equipment [1-2]. There are seven types of power quality problems such as [3]: 1. Transients (impulsive and oscillatory) 2. Short-duration root-mean-square (rms) variations (instantaneous, momentary and temporary) 3. Long duration rms variations (interruption, under voltages, over voltages, and current overload) 4. Imbalance (voltage and current) 5. Waveform distortion (DC offset, harmonics, interharmonics, notching, and noise broadband) 6. Voltage fluctuations 7. Power frequency variations Voltage sag is one of the most severe power quality disturbances to be dealt with by the industrial sector, as it can cause severe process disruptions and results are poor quality products and substantial economic loss.

A. Problem Description Three-phase induction motors (IMs) are the most frequently used machines in various electrical drives. About 70% of all industrial loads on a utility are represented by IMs [4]. Typical process like textile spinning is more sensitive to

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S.P. Srivastava Electrical Engineering Department Indian Institute of Technology, Roorkee Roorkee, India [email protected]

voltage sag and its financial losses are much higher. Because textile industries have high technology machines including electronic control cards and driver controlled motors, poor power quality may damage the system and cause production failure [5]. From a measurement taken from an establishment, the 6-month costs for poor electrical quality is calculated as $149000 due to voltage sag [5]. Research on energy-optimal control on induction motor, particularly, at time-varying load is rapidly increasing in these decades [6]-[8], where flux level of the motor is adjusted in accordance with the load-torque/speed variations. During this operation motors operate very close to its rated. Studies on voltage sag effects on energy-optimal controlled induction motor are essential at present. B. Contribution Model-Based Control is implemented on a 30-hp induction motor operating with textile mill load diagram. Comprehensive analysis is carried out on the motor (operating with and without energy-optimal control) during voltage sag. Two voltage mitigation topologies, namely Buck-Boost converter and Dynamic Voltage Restorer, are applied and the results are compared. C. Paper organization The organization of this paper is as follows. Section II briefly explain the types of voltage sags, section III explain industrial time-varying load, section IV comprises an energy optimal control namely MBC for energy optimization, section V various methods for voltage sag mitigation, section VI presents the simulation results of 30-hp IM motor and the paper concludes at section VII. II.

VOLTAGE SAG TYPES

Table I classifies categories of voltage sags according to IEEE standard [3]. Voltage sag (dip) is basically the decrease in rms voltage between 0.1 pu and 0.9 pu, with duration between 0.5 cycles and 1 min. A recent survey attributes that 92% of all disturbances in power system is caused by voltage sags. The electrical sensitive load often trips or shuts down when voltage sag occur. It’s very important to know how these sensitive equipments works when the voltage sag occurs and what are

the methods for mitigation the effects of voltage sag. The various classification of three-phase voltage sag is discussed in [9]. TABLE I. Categories Instantaneous sag Momentary sag Temporary sag

TYPE OF VOLTAGE SAG Typical Duration (50 Hz) 0.5 - 30 cycles (10 - 600 msec) 30 cycles – 3 sec (600 msec – 3 sec) 3 sec – 1 min

Typical Voltage Magnitude 0.1 – 0.9 pu 0.1 – 0.9 pu

III.

TIME VARING LOAD

A ring spinning frame in textile industry manufactures the cotton into yarn that winded in spindles (Fig. 2) and used to feed cone winding machine. After that it can be used to make end products such as clothing with the help of weaving machine. The quantity of the yarn in the spindles varies from zero (when the process starts) to full (when process completes), hence the motor shaft load varies from zero to rated [6].

0.1 – 0.9 pu

Voltage sags can be either symmetrical or unsymmetrical, if the individual phase voltages are equal and the phase relationship is 120°, the sag is symmetrical otherwise, the sag is unsymmetrical [11]. Voltage sags can be grouped into seven types, denoted as A, B, C, D, E, F and G. Table II shows their expressions (where h is the sag depth) and Fig. 1 shows their phasor diagrams [9]. TABLE II.

MATHEMATICAL EXPRESSION FOR VARIOUS VOLTAGE SAGS

Figure 2. Textile spinning ring frame

In this paper, 30 hp IM has been considered to drive this type of TVL and analyze its operation with voltage sag. In order to illustrate the importance of efficient controllers in the industrial processes and sag mitigation techniques, considered the load diagram of ring spinning frame in a textile industry (Fig. 9). ‘T’ is the time consumption for the completion of one process. Although load diagram is continuous but we have considered the discrete load diagram for easy analysis. IV.

EFFICIENCY OPTIMAL CONTROLLER

The induction machine should operate with the rated flux for the rated value of load torque, where as for load torque less than rated, the reduction of flux causes a reduction in magnetizing current and iron losses. For a very low load torque (upto about 15% of the rated value), energy saving work can reduce power loss by even 70-90% [10]. The present as a variable work considers motor flux producing current and finds its optimum through MBC. Particle swarm optimization (PSO) algorithm is used by offline to calculate the optimal value of flux and is stored in look-up tables. Different energy-optimal controllers (both scalar and vector) are discussed in [10], [7].

Figure 1. Types of voltage sag

A. Model Based Controller The Model Based Controller is a feed-forward approach, which calculates the optimum set of variables of the machine, depending on the optimization (maximize or minimize) of an objective function, defined using the machine parameters [11]. The objective function used in the present work is the total loss of induction motor drive. The drive carrying MBC shown in Fig. 4, measures stator current and rotor speed of the motor and through the loss model it determines the optimal value of (variable). The flow of model based controller is shown in Fig. 3. The approach requires knowledge of machine parameters which include core losses and main inductance flux saturation.

K e - Eddy current coefficient, K h - Hysteresis coefficient,

Where

C fw - Mechanical loss coefficients,

C str - Stray loss

coefficient.

V. VOLTAGE SAG MITIGATION METHODS Methods for voltage sag mitigation are explored, discussed and their feasibility for the PQ problem is summarized in [13]. Buck-Boost converter and DVR are:

Figure 3. Flow chart of model based control

The flux producing current command for vector control of induction motor can be calculated from the optimal flux obtained from scalar model as shown in Eqn. (1). Maximum levels of flux and stator currents are forced as constraints in the algorithm. It is noted that the motor’s optimum operation is normally below the rated flux and hence flux constraint is not very important in the algorithm. Once optimal flux producing current ( ) is calculated as in Eqn. (1), it is given to the stator reference current generation block as shown in Fig. 4 and generates appropriate pulses for PWM inverter which results optimal operation of induction motor in terms of minimum loss or maximum efficiency.

(1 + τ r s) * φm Lm

Figure 5. Buck-Boost converter circuit [14] DC link Voltage (V)

τ r -Rotor time constant.

DC link current (Amp)

Where

(1) 900 850 750 650 550 450 350 250 150 200 100 0 -100 -200 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Time (sec)

(a) Voltage sag condition

1.8

2

compensated by compensated by Buck-Boost converter Buck-Boost converter

* = ids

A. Buck-Boost Converter Buck-Boost converter for voltage sag mitigation is considered with sag and provide in block diagram [14] shown in Fig. 5. It is connected across DC link to improve the voltage profile under power quality disturbance such as sags and swells. In case of sag, Buck-Boost converter increases the DC link voltage (boost operation) while in case of swells decrease the DC link voltage (buck operation). Hence the motor performance does not get affected due to voltage sag and swells [14], [15]. Fig. 6(a) shows the effect of voltage sag on DC link voltage and current and Fig. 6(b) shows the DC link voltage and current compensated by Buck-Boost converter. In BuckBoost Converter operation DC link voltage and current are smooth as shown in Fig. 6(b).

900 850 750 650 550 450 350 250 150 200 100 0 -100 -200 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Time (sec)

(b) After boost operation

Figure 4. Efficiency optimization controllers Figure 6. DC link voltage and current

The individual loss equations in the IM are given by [12]: 2 ' '2 Copper losses Pc = Rs I s + Rr I r

{ (

)

(2)

}

2 2 2 Iron losses Pi = K e 1 + s a + K h (1 + s )a ψ m

Stray losses

Pstr = Cstrω 2 I r'2

(3) (4)

Mechanical losses Pm = C fω ω 2

(5)

The total losses in IM drive system is given by Ploss = Pc + Pi + Pstr + Pm (6) From equations (2) - (5), the total losses can be rewritten as [12]:

{ (

)

}

Ploss = Rs I s2 + Rr' I r'2 + K e 1 + s 2 a 2 + K h (1 + s )a φ m2

+ C str ω 2 I r'2

+ C fω ω

2

(7)

B. Dynamic Voltage Restorer DVR is known as an effective device to mitigate voltage sag. The DVR consists of injection transformer, AC harmonic filter, high speed switching pulse width modulation (PWM) inverter and DC energy storage unit as shown in Fig. 7. A description of the basic design of DVR has been presented in [16] and sag detection technique is discussed in [17]. The DVR injects three single-phase voltages in series with the load voltage by synchronizing with the incoming supply voltage. The phase angle and magnitude of the injected voltage varies as a result of variable real and reactive power exchange between the DVR and the distribution system. The amount of real and reactive power supplied by the DVR depends on the type of voltage disturbance.

* As indicated in the Fig. 7, VS* , VL* and VDVR are the fault supply voltage vector, the restored load voltage vector, and the DVR injection voltage vector, respectively. Then the DVR injection voltage can be obtained as Eqn (8). * VDVR = VL* − VS* (8) = the pre-fault load voltage vector Where

Figure 7. Dynamic voltage Restorer (DVR) topology [16]

500 0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0

0.2

0.4

0.6

0.8

1 1.2 Time (sec)

1.4

1.6

1.8

2

1 0.5 0 500 0

Figure 8. Load voltage compensated by DVR when voltage sag occurs

SIMULATION RESULTS AND DISSCUSSION

Speed (rad/sec)

Load Torque (Nm)

150 100 50 0

150 100

0.4 0.8 1.2 1.6 Time (sec)

2

0

0

0.4 0.8 1.2 1.6

200 0

-200

-400 0.2 0.4 0.6 0.8

1

1.2 1.4 1.6 1.8

-400 0.2 0.4 0.6 0.8

2

160

1

1.2 1.4 1.6 1.8

2

1

1.2 1.4 1.6 1.8

2

-400 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Time (sec)

2

160

140 120 100 80 0.2 0.4 0.6 0.8

1

1.2 1.4 1.6 1.8

140 120 100 80 0.2 0.4 0.6 0.8

2

400

400

200 0 -200 -400 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Time (sec)

50 0

0

400

200

-200

Torque (Nm)

200

200

400 3 Phase supply Voltage (V)

Simulations are carried out for 415V, 50 Hz, 4 poles, 30-hp IM whose equivalent circuit parameters are Rs=0.251ohms, Rr=0.249ohms, Ls=0.004H, Lr=0.004H, Lm=0.00416H, Moment of inertia=0.305kg-m2, Rated torque=150 Nm, Rated speed=150 rad/s. The time-varying load considered for this study is shown in Fig. 9. The simulation of vector controlled 30-hp squirrel-cage induction motor (IM) under voltage sag condition with TVL is carried out and the simulation results are presented below and comparison is performed with IM connected to 3-phase supply directly. To illustrate the effectiveness of the voltage sag mitigation techniques namely (i) Buck-Boost converter and (ii) DVR, simulation results are presented.

Speed (rad/sec)

VI.

Torque (Nm)

-500

3 Phase supply Voltage (V)

-500

Speed (rad/sec)

Voltage (Compensated by 3 Phase Voltage sag DVR) Supply voltage (V) Dectection

Fig. 8 shows the 3-phase supply voltage having 60% voltage sag in three segments. The voltage sag detection is responsible for DVR operation. Once the sag is detected DVR inject the appropriate magnitude in supply voltage, and load voltage is become compensated as shown in Fig. 8.

Voltage sag (60%) with duration of 0.2 sec is considered in three regions of operation. Light load region (0.6sec to 0.8sec), medium load region (1.1sec to 1.3sec), and full load region (1.6sec to 1.8sec) shown in Fig. 8 are considered for analysis. Fig. 10 shows the speed and torque profile when IM is directly connected to the supply. Fig. 10(a) represents the speed and torque for normal supply voltage whereas the Fig. 10(b) represents the variations in speed and torque during sag. Fig. 11 shows flux, speed, torque and DC link power for vector controlled IM for constant flux operation and MBC operation. Although MBC offers ripples in speed and torque profile but helpful for energy conservation at light load condition. Fig. 12 shows flux, speed, torque and DC link power for vector controlled IM connected to 3 phase supply having voltage sag for constant flux operation and MBC operation. At light loads, effect of voltage sag on the motor by the conventional VC is almost negligible whereas its effects on torque and speed of the motor are significant (particularly at voltage sag with high magnitude) when the motor is connected directly to the supply. It is because of the motor requires less torque at light load and hence the energy stored in the DC link capacitor is sufficient to drive the less torque for small duration. VC with MBC is capable to remove the effects of voltage sag for light and medium load conditions in addition with saving of energy. The detailed effect of various magnitude of sag on speed and torque is concluded in Table 3 and Table 4 for vector controlled IM and comparison is given to IM connected to supply directly. Fig. 13 shows that the voltage sag mitigation techniques buck-boost converter and DVR are capable to nullify the effect of voltage sag on speed and torque. Although the speed and torque profile are more or less same but DVR offers less ripples in torque profile as compared to buck-boost converter.

2

200 0 -200

2

Time (sec)

Figure 9. Load diagram considered for study

(a) Normal supply

(b) Supply with voltage sag

Figure 10. IM connected to 3-phase supply

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2



400 200 0 -200 -400 0.2

400 200 0 -200 -400 0.2

1

1.2

1.4

1.6

1.8

2

Flux 0.4

0.4

0.6

0.6

0.6

0.8

0.8

0.8

1

1

1

1.2

1.2

1.2

1.4

1.6

1.4

1.6

1.4

1.6

1.8

1.8

1.8

2

0 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Speed (rad/sec)

0.4

0.5

2

152 151 150 149 148 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2

200 150 100 50 0 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.4

0.6

0.8

1 1.2 Time (sec)

1.4

1.6

1.8

2

4

x 10 4 3 2 1 0 0.2

DC Link Power (W)

4

DC Link Power (W)

0.8

Torque (Nm)

Flux Speed (rad/sec) Torque (Nm)

0 0.2

200 150 100 50 0 0.2

0.6

1

1 0.5

152 151 150 149 148 0.2

0.4

0.4

0.6

0.8

(a)

1 1.2 Time (sec)

1.4

1.6

1.8

2

x 10 4 3 2 1 0 0.2

Constant flux operation

(b) MBC operation

400 200 0 -200 -400 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2



Figure 11. Vector controlled IM connected to normal 3 phase normal supply

0.5 0.4

0.8

1

1.2

1.4

1.6

1.8

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.4

250 200 150 100 50 0 0.2

0.6

0.8

1

1.2

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0.4

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1

1.2

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0.5

2

0 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2

155 150 145 140 135 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2

250 200 150 100 50 0 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.4

0.6

0.8

1 1.2 Time (sec)

1.4

1.6

1.8

2

Speed (rad/sec)

155 150 145 140 135 0.2

0.6

Torque (Nm)

Torque (Nm)

Speed (rad/sec)

0 0.2

4

x 10 10 7.5 5 2.5 0 0.2

4

x 10

DC Link Power (W)

DC Link Power (W)

0.4

1 Flux

Flux

1

400 200 0 -200 -400 0.2

0.4

(a)

0.6

0.8

1 1.2 Time (sec)

1.4

1.6

1.8

2

10 7.5 5 2.5 0 0.2

Constant flux operation with voltage sag

(b) MBC operation with voltage sag

150 148 0.2 200

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

100 0 0.2

Speed (red/sec)

152

Torque (Nm)

Torque (Nm)

Speed (red/sec)

Figure 12. Vector controlled IM connected to 3 phase supply having voltage sag

0.4

0.6

0.8

(a)

1 1.2 Time (sec)

1.4

1.6

Buck-Boost operation

1.8

2

152 150 148 0.2 200

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.4

0.6

0.8

1 1.2 Time (sec)

1.4

1.6

1.8

2

100 0 0.2

(b) DVR operation

Figure 13. Speed and torque after mitigation voltage sag

EFFECT ON SPEED FOR VARIOUS MAGNITUDE OF VOLTAGE SAG

TABLE IV.

Post sag Settling time (sec)

Dip (rad/sec)


Dip (rad/sec)


0.07 0.08

24 130

0.15 0.20

2.5 10 40 87 117

0.11 0.12 0.14 0.16 0.20

50 140

0.12 0.13

57 194

0.17 0.21

Medium load

Dip (Nm)

8 60 96

0.06 0.14 0.14 0.17 0.17

59 90

0.09 0.10

68 100

The effect of voltage sag (type-A) on a MBC operated induction motor with textile-mill load is analyzed. The results are compared with the motor connected directly to the supply (without vector control). The effects of voltage sag are slightly higher with the motor under MBC due to the reduction in flux level at below rated loads. Vector control with constant flux operation is also offered good dynamic performances of the motor, due to DC link, in comparison with the direct supply. Two voltage sag mitigation topologies namely DVR and Buck-boost converter have applied to nullify the effects of voltage sag on the motor.

[8]

[9] [10]

[11]

[12]

REFERENCES

[2] [3] [4] [5]

[6]

[7]

G.T. Heydt, Electric Power Quality, 2nd ed. WestLafayette, Stars in a Circle, 1994. A. Ghosh and G. Ledwich, Power Quality Enhancement Using Custom Power Devices. Kulwer Academic, 2002. “IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std 1159-2009, June 2009. Z. Maljkovic, M. Cettolo, et.al, 2001. The impact of the induction motor on short-circuits current. IEEE Ind. Application Magazine, 11-17. F. Koçyi_it, E. Yanıko_lu, A. S. Yilmaz and M. Bayrak, “Effects of power quality on manufacturing costs in textile industry,” Scientific Research and Essay, Vol.4 (10), pp. 1085-1099, October 2009. C. Thanga Raj, S. P. Srivastava, and Pramod Agarwal, “Differential Evolution based Optimal control of Induction Motor Serving to Textile Industry”, Int. J. of Computer Science, Vol. 35, No. 2, 2008. C. Thanga Raj, S. P. Srivastava, Pramod Agarwal, “Particle Swarm and Fuzzy Logic Based Optimal Energy Control of Induction Motor for a

[13]

[14]

[15]

[16]

[17]

0.17 0.20

40 110 146

0.07 0.15 0.18 0.19 0.21

Dip (Nm)


0.16

IM vector Controlled with MBC


Dip (Nm)

43

IM vector controlled

Dip (Nm)


-


Dip (Nm)

-

IM without vector controlled

Dip (Nm)


0.08 0.11 0.12 0.17 0.18


Dip (Nm)

2 10 46

VII. CONCLUSION

[1]




Dip (Nm)




Full load

Dip (Nm)

No sag 10% 30% 50% 70% 90%


20 72

EFFECT ON TORQUE FOR VARIOUS MAGNITUDE OF VOLTAGE SAG

Light load IM without vector controlled

Dip (rad/sec)

0.12 0.12 0.12 0.13 0.13



2 5.5 18 53 83.5


Dip (rad/sec)

0.15



36


Dip (rad/sec)


-

Full load



Dip (rad/sec)

-

0.11 0.11 0.12 0.12 0.15

IM without vector controlled Dip (rad/sec)


1.5 5 10 24 50.5


Dip (rad/sec)

No sag 10% 30% 50% 70% 90%



Dip (rad/sec)


Medium load

99 130

0.08 0.20

110 150


TABLE III. Light load

0.18 0.21

Mine Hoist Load Diagram,” Int. Journal of Computer Science, Vol.36, No.1, pp. 17-25, 2009. Navneet Kumar, Thanga Raj Chelliah,Radha Thangaraj, S.P. Srivastava, “Economical analysis of induction motor for a mine hoist load diagram,”World Congress on ICT, Uni. Of Mumbai, IEEE Computer Society Press, pp. 1310-1315, 2011. M.H.J. Bollen, “Understanding Power Quality Problems Voltage Sags and Interruption”, IEEE Press, 2000. Bazzi A. M., Krein P. T., “Review of Methods for Real-Time Loss Minimization in Induction Machine,” IEEE Trans on industry application, Vol 46, No. 6, pp. 2319-2328, Nov-Dec 2010. P. Gnacinski, “Energy saving work of frequency controlled induction cage machine,” Energy Conversion and Management, Vol. 48, pp. 919926, 2007. I. Kioskesidis, N. Margaris, “Loss minimization in scalar controlled induction motor drives with search controller,” IEEE Trans. Power Electronics, Vol. 11, No. 2, pp. 213-220, 1996. A. Felce, G. Matas, Y. Da Silva, “Voltage Sag Analysis and Solution for an Industrial Plant with Embedded Induction Motors,” 39th IAS annual meeting, Vol 4, pp. 2573 – 2578, Oct 2004. Kurt Stockman, Frederik D’hulster, Kevin Verhaege, Marcel Didden, Ronnie Belmans, “Ride-through of adjustable speed drives during voltage dips,” Electric Power Systems Research 66 (2003) 49-58. S. S. Deswal, Ratna Dahiya, and D. K. Jain, “Application of Boost Converter for Ride-through Capability of Adjustable Speed Drives during Sag and Swell Conditions,” World Academy of Science, Engineering and Technology 47, 2008. RAI. Amrita and Nadir .A.K "Modeling and simulation of dynamic voltage restorer (DVR) for enhancing voltage Sag,"Senors and transducers Journal, Vol.87, IssueI, pp.85-93, January 2008. Y. Sillapawicharn, Y. KumsuwanAn, “ Improvement of Synchronously Rotating Reference Frame Based Voltage Sag Detection for Voltage Sag Compensation Applications under Distorted Grid Voltages,” IEEE PEDES 2011, pp. 100-103, Dec 2011.