NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Variable Frequency Operations 

of an Offshore Wind Power 

Plant with HVDC-VSC 

Preprint

V. Gevorgian, M. Singh, and E. Muljadi

To be presented at the IEEE Power and Energy Society General 

Meeting

San Diego, California

July 22-26, 2012

Conference Paper

NREL/CP-5500-53464

December 2011
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1 

Abstract—Based on DOE study, wind power generation may 

reach 330 GW by 2030 at the level of penetration of 20% of the 

total energy production. From this amount of wind power, 54 

GW of wind power will be generated at offshore wind power 

plants. The deployment of offshore wind power plants requires 

power transmission from the plant to the load center inland. 

Since this power transmission requires submarine cable, there is 

a need to use High-Voltage Direct Current (HVDC) transmission. 

Otherwise, if the power is transmitted via alternating current, the 

reactive power generated by the cable capacitance may cause an 

excessive over voltage in the middle of the transmission distance 

which requires unnecessary oversized cable voltage breakdown 

capability.

The use of HVDC is usually required for transmission 

distance longer than 50 kilometers of submarine cables to be 

economical. The use of HVDC brings another advantage; it is 

capable of operating at variable frequency. The inland 

substation will be operated to 60 Hz synched with the grid, the 

offshore substation can be operated at variable frequency, thus 

allowing the wind power plant to be operated at constant 

Volt/Hz.

In this paper, a constant Volt/Hz operation applied to the 

Type 1 wind turbine generator. Various control aspects of Type 

1 generators at the plant level and at the turbine level will be 

investigated. 

Index Terms—wind turbine generator, offshore wind power 

plant, variable speed, induction generator, HVDC, HVAC, and 

renewable energy.

I. INTRODUCTION

ASED on the “20% Wind Energy by 2030” DOE study, 

wind power generation could reach 330 GW by 2030 at 

the level of penetration of 20% of the total energy production. 

From this amount of wind power, 54 GW of wind power will 

be generated at offshore wind power plants [1]. The 

deployment of offshore wind power plants requires power 

transmission from the plant to the load centers inland. 

According to [2], the U.S. gross offshore wind resource 

prevails for the areas of transitional depths (30-60 m) or 

deepwater areas (>60 m) that are generally located at farther 

distances from the shore.

The offshore wind power plants (WPPs) can be connected to 

the grid using either AC or DC transmission. The use of highvoltage direct-current (HVDC) is justified for submarine 

transmission distances longer than 50-60 kilometers [3]. The use 

of HVDC brings many advantages that are well described in 

technical literature. One advantage that may be of additional 

benefit to offshore wind power is its capability to operate at 

variable frequency. While the onshore converter substation will 

be operated at 60 Hz synchronized with the grid, the offshore 

converter station can be operated at variable frequency.

In this paper, a constant Volt/Hz operation of offshore 

voltage source converter (VSC) based HVDC terminal applied 

to the offshore WPP that consists of Type 1 (squirrel-cage 

induction generator/SCIG) wind turbine generators. Similar 

single or multi-terminal offshore system topologies were 

previously analyzed in [4], [5], and [6] with the focus on 

dynamic and transient simulations for various events, 

including changes in wind speeds and voltage faults [8], [9]. 

Here we focus on steady-state performance of constant 

Volt/Hz operation of Type 1 offshore- based WPP.

A general layout diagram of VSC-HVDC interconnected 

offshore WPP is shown in Figure 1. In the case of Type 1 

wind turbine generators (WTG), under steady-state conditions 

all units operate at constant rotational speeds that depend on a 

frequency set by VSC and wind speeds at each individual 

WTG. The variable frequency operation that is well known to 

improve energy capture on a single turbine level can also be 

applied on a group level. It is important to keep in mind that 

conventional induction generators and transformers are 

designed to operate most efficiently at a constant Volt/Hz 

ratio. This means that voltage of the collector system must 

change with the frequency to maintain this ratio constant. 

Below, we investigate issues of variable frequency, - constant 

Volt/Hz steady-state operation of Type 1 WTG-based WPPs 

with VSC-HVDC terminal, and estimate gains in energy

capture associated with such operation.

In this paper, the following sequence will be followed. In 

section II, we discuss constant Volt/Hz operation of VSCHVDC interconnected WPP. In section III, the operation of a 

single Type 1 WTG is described. In section IV, the effects of 

aggregation and wind speed diversity on constant Volt/Hz 

operation is investigated. In section V, we perform a 

comparison between constant Volt/Hz and conventional 

variable speed operation in terms of energy production.

Figure 1: Offshore WPP connected via HVDC to the grid

II. WIND POWER PLANT CONTROL

In an offshore wind power plant, the wind flow is assumed 

to be more uniform compared to onshore. The surface 

roughness is very smooth compared to inland wind power 

plants where the wind flow may be diverted by different 

obstacles (hills, rocks, vegetation, etc.), thus creating nonuniform wind speeds among the turbines. Based on these

uniform wind speeds, we operate the entire WPP at variable 

Variable Frequency Operation of an Offshore 

Wind Power Plant with HVDC-VSC

V. Gevorgian, Member, IEEE, M. Singh, Member, IEEE, and E. Muljadi, Fellow, IEEE

B
2 

uniform wind speeds, we operate the entire WPP at variable 

frequency to match the WPP average wind speed Vw.ave .Thus, 

the operation of individual wind turbines can be optimized. 

A. Variable speed operation at constant V/Hz 

 1) Choosing the rated V/Hz 

The V/Hz is a quantity that is proportional to the rated flux 

operating on the electromagnetic devices connected to the 

alternating currents within the collector systems. This includes 

the transformer and the electric machines. 

We can choose the nameplate rating of the electric machine 

as the starting point. For an induction generator operating at 

constant voltage (i.e. 4160 V) and 60 Hz, the operation at 

constant V/Hz requires that the induction generator must be 

connected to a variable–voltage, variable-frequency voltage 

source which can deliver the rated value of V/Hz. The rated 

V/Hz ratio (ܭ (௥of the generator can be determined as: 

ܭ ௥ൌ ௏೒೐೙.ಽಽ.ೝೌ೟೐೏

௙ೝೌ೟೐೏

 (1) 

Where ܸ௚௘௡.௅௅.௥௔௧௘ௗ is the rated line to line voltage of the 

generator, and ݂௥ is the rated frequency. 

Thus, for the example given, the ௏ಽಽ

ு௭ ൌ ସଵ଺଴ ௏

଺଴ ு௭

ൌ 69.33 ௏

ு௭. 

This value is set by the power converter at the HVDC 

connection at the offshore substation. 

 2) Tracking the Vw.ave 

An example of the typical dependence of wind rotor power 

coefficient Cp on tip-speed ratio (TSR) is illustrated in Figure 2 

shown below for different blade pitch angles [7]. 

Figure 2: Cp versus TSR for a typical wind turbine 

For an induction generator, the operating slip is usually 

very small (1% or less) in a high-power, high-efficiency 

induction generator. Thus, the operating rotational speed is 

practically proportional to the frequency of the generator. 

The operation of a wind turbine is optimized so that we can 

operate the wind turbine at its Cpmax corresponding to the tipspeed ratio of TSRtgt. For the Cp-TSR curve presented in Figure 

2, the value of TSRtgt is close to 8. 

The operating frequency of the wind turbine generator can 

be computed as: 

݂ ൌ ்ௌோ೟೒೟௏ೢ.ೌೡ೐

ோ್೗ೌ೏೐௄೑.ഘ

 (2) 

and 

ܭ௙.ఠ ൌ ସగ

௣ே೒೐ೌೝ

 (3) 

Where: 

f = frequency to be used (Hz) by the power converter of 

the offshore-side power converter. 

kf.ω = conversion factor from Hz to rad/sec 

p = number of poles of the generator 

Ngear = gear ratio of the gearbox used 

The operating voltage of the output of the power converter 

at the offshore side of HVDC can be computed as: 

ܸ௅௅ିு௏஽஼ ൌ ܭ௥݂݊ௌ்݊ (4) 

where 

݊ௌ = substation transformer winding ratio 

்݊ = turbine (pad-mounted) transformer winding ratio 

Thus, the voltage and frequency of the HVDC at the 

offshore must be commanded to follow the values VLL-HVDC

and f. 

Figure 3: WPP control block diagram for V/Hz operation.

In Figure 3, an overall control diagram example is shown. 

The wind speed average (Vw.ave) is computed from the number 

of sensors located at different locations in WPP. By using at 

least three input sensors, the magnitude and direction of the 

Vw-ave can be found. The operating frequency and its 

corresponding voltage commands can be computed and fed to 

the power converter offshore to set the voltage and frequency 

in the collector systems of the WPP. The constant Volt/Hz 

operation enables all transformers and induction generators to 

operate at rated flux to avoid magnetic saturation. 

In the offshore WPP, the wind speed within the plant is 

more uniform than in a land-based WPP. The minor variation 

of wind speed at individual turbine with respect to the Vw.ave

will be minor. If a wind turbine received a significantly higher 

wind speed than the Vw.ave used to determine the frequency and 

voltage, this wind turbine will generate higher output power 

and the output current may exceed the rated current. To 
3 

protect individual wind turbines from operating at an overload 

conditions, the pitch control must be deployed. 

III. TYPE I WIND TURBINE – INDUCTION GENERATOR

Induction machines have been the workhorse of modern 

industry. Ever since its invention in 1883 by Nikola Tesla, 

this electric machine has been the favorite among other 

electric machines. A squirrel-cage induction machine operates 

without brushes; thus, the operation and maintenance are very 

simple. They have been used in WTG applications for years. 

The concept of constant Volt/Hz control for a Type 1 wind 

turbine is illustrated in Figure 4. In this example, the turbine 

rated power is 2.5 MW. The target (or optimum) power is 

determined by the rotor maximum power coefficient (Cpmax). It 

is possible to operate the single turbine along the target power 

line by controlling the frequency and amplitude of the AC 

voltage on turbine terminals while maintaining a constant ratio 

between voltage and frequency. 

Since the rated operating slip of a high-efficiency induction 

generator is very small (about 1%), the operating rotational 

speed is very close to the synchronous rotational speed. For 

wind turbine generators, this operating frequency can be 

computed from the average wind speed at the WPP as shown 

in equation 2. To operate the induction generator at rated 

V/Hz, the collector system voltage must be adjusted to operate 

according to equation 4. 

Figure 4: Constant V/Hz control 

The turbine operates with zero pitch angle while following 

the target power line. The pitch control becomes active to limit 

the output current and to limit the turbine RPM at maximum 

value (about 24 RPM as shown in Figure 4), thus limiting the 

turbine power at the rated value of 2.5 MW. In this particular 

example, a 20-60 Hz frequency range covers the overall 

operational limits of the Type 1 wind turbine. The frequency 

must change linearly as a function of wind rotor RPM to 

provide such operation. Therefore, the frequency also changes 

linearly with wind speed according to Figure 5. 

Figure 5: Frequency vs. wind speed at constant V/Hz operation

The power characteristics of the same example for a 2.5 MW 

wind turbine is shown in Figure 6 where the meshed surface 

represents the operational envelope of the generator, and the 

dotted lines represent the target power achieved at constant 

Volt/Hz control as a function of both electrical frequency and 

wind speed. 

Figure 6: Power characteristics of Type 1 wind turbines 

Figure 7: Current characteristics of Type 1 wind turbines 
4 

The flat portion of the surface represents the generator current 

limit at its rated value. It is identical to limiting the 

electromagnetic torque of ge nerator at rated level as well to 

ensure the integrity of both the electrical generator and 

mechanical components of the wind turbine. A care needs to 

be taken to operate the Type 1 wind turbine generator under 

the surface shown in Figure 6 by combining both pitch and 

Volt/Hz controls. In a similar manner, the operational 

envelope of Type 1 generator is shown in Figure 7, where the 

horizontal portion of the surface represents the generator 

operating at rated current (or rated torque). 

IV. COMPARISON BETWEEN CONSTANT

FREQUENCY AND CONSTANT V/HZ OPERATION 

A. Single turbine operation 

To demonstrate the advantage of the variable Volt/Hz system, 

the power curves of a single 2.5-MW wind turbine were 

calculated for the cases of variable and fixed frequency 

operations as shown in Figure 8. 

Figure 8: Power curve comparisons 

As it can be observed from Figure 8, the variable Volt/Hz 

operation gives superior performance for the whole range of 

operational wind speeds, whereas fixed frequency operation 

can be optimized only for a narrower range of wind speeds. 

B. Impact of non-uniformity of wind speeds within wind 

power plant 

WPPs cover a very large area. Although the surface 

roughness across the sea water is much smoother than onshore 

WPPs, nevertheless, the wind speed within the WPP is not 

totally uniform. In this subsection, the impact of wind power 

variation from one turbine to another will be explored. The 

impact on the energy yield and the possible control strategy 

will be explored. 

The power curves shown in Figure 8 are used to determine 

the total power output of the large offshore WPP that consists 

of 100 turbines (250-MW total power rating). A single VSCbased HVDC converter sets both AC voltage and frequency 

that is the same for all Type 1 WTGs. If the wind speed was 

uniformly similar across the whole WPP, then each individual 

WTG would be operating along the solid-line power curve 

shown in Figure 8. In reality, there is always some wind speed 

diversity across the WPP. The wind speed at each individual 

wind turbine will be distributed randomly around some 

average value. We assume for simplicity that at any given 

instance in time, the wind speed across the whole WPP is 

distributed according to normal distribution, as shown in the 

Figure 9 example for an average wind speed ܸ௔௩௘ ൌ 8 ௠

௦ and 

standard deviation ߪൌ1. 

Figure 9: Wind speed diversity across WPP (N=100)

The normal probability density function for wind speed 

deviation ∆ܸ[m/s] from the mean value ܸ௔௩௘[m/s] for the 

whole WPP: 

ࢌሺ∆ࢂሻ ൌ ૚

૛ࢂ∆ିࢋ ࣊૛√࣌

 (5) ૛࣌૛

Where ߪ]m/s] is the standard deviation of wind speed from its 

mean value. 

The wind speed at each individual wind turbine then can be 

calculated using a cumulative distribution function 

corresponding to normal probability density: 

ࡺ ൌ ሻࢂ∆ሺࡲ

૛ ሾ૚ ൅ ࢌ࢘ࢋ ቀ√૛

૛

ࢂ∆

 (6) ሻሿ࣌ሺ࢔ࢍ࢙ࢉ ቁ ࣌

Where ݁ݎ݂and ܿݏ݃݊ are Gauss error and signum functions 

respectively, and N is total number of wind turbines connected 

to the same frequency bus. 

The probability density function for wind speed deviations 

from the mean across the WPP is shown in Figure 10 for 

different values of standard deviation wind speed (σ=0, 1, 2 

and 3 m/s). An ideal, uniform, WPP will have σ=0m/s, while 

a very diverse onshore WPP covering a very large area may 

have σ=3m/s. Larger standard deviation ߪ indicates a large 

diversity of wind speeds at the WPP. 
5 

Figure 10. Normal probability density of wind speed deviations 

Figure 11: Distribution of wind speed deviations 

This approach assumes symmetrical distribution of wind 

speeds above and below the average value, as shown in the 

Figure 11 example for a WPP consisting of 100 wind turbines 

(N=100). Thus, WTG #50 is at its average wind speed ܸ௔௩௘. 

The wind speed for each individual turbine can then be 

determined from Figure 10 and Figure 11: 

 (7) ሻ࢑ ,࣌ ,࢑ࢂ∆ ൅ ࢋ࢜ࢇࢂሺࢌ ൌ ࢑ࢂ

Where ݇ is the turbine index number, and Δܸ࢑ is the deviation 

of wind speed from the mean at turbine #k.

Power production of each individual wind turbine depends on 

its wind speed and is determined from power curves shown in 

Figure 8 for each operational frequency. The total power 

output of the WPP can then be calculated as: 

૙૙૚ ሻ࢑ࢂ ,ࢌሺࡼ ∑ ൌ ሻࢌ .࣌ .ࢋ࢜ࢇࢂሺ࢒ࢇ࢚࢕࢚ࡼ

(8) ૚ୀ࢑

The total power output of the WPP is then a function of 

average wind speed for the whole WPP, wind speed standard 

deviation ߪ, and electrical frequency ݂. The collector system 

frequency ݂ is controlled to be proportional to the average 

wind speed according to equation 2, and the voltage is 

controlled to maintain the rated V/Hz according to equation 4. 

Figure 12: Total WPP power at 6 m/s average wind speed 

Figure 13: Total WPP power at 8 m/s average wind speed 

Figure 14: Total WPP power at 10 m/s average wind speed 
6 

Figure 15: Total WPP power at 12 m/s average wind speed 

Some calculation results are shown in Figure 12 through 

Figure 15 (A, B, C and D) for four different average wind 

speeds (ܸ௔௩௘= 6, 8, 10, and 12 m/s) and four standard 

deviations (ߪ=0, 1, 2, and 3 m/sec) respectively. It can be seen 

from these figures that for every combination of ܸ௔௩௘ and ߪ, 

there is an optimum electrical frequency that produces 

maximum total WPP power. The only exception is in Figure 

15 when all turbines operate at the flat portion of the power 

curve. It is important to note that cases where ߪൌ0 are not 

realistic since there is always wind speed diversity from 

turbine to turbine in the same WPP. The ߪൌ0 cases are 

shown here for reference and comparison purposes only. 

Figure 16: Optimum frequency control 

C. Energy production comparison 

The aggregate power curves for the whole WPP are needed 

for annual energy production estimation purposes. Such 

calculated power curves are shown in Figure 17 and Figure 18 

for the cases of fixed 60 Hz and variable frequency (constant 

V/Hz) respectively. These curves were calculated for the same 

example WPP consisting of N=100 Type 1 WTGs for different 

values of ߪ. As can be seen in both figures, the larger values 

of ߪ cause larger deviations of aggregate WPP power from the 

single turbine power curve. 

Figure 17: Aggregate WPP power at fixed 60 Hz 

Figure 18: Aggregate WPP power at Volt/Hz control 

The annual energy production can be calculated using 

power curves shown in Figure 17 and Figure 18, and a 

Weibull wind speed probability density function: 

ܹሺܸሻ ൌ ௞

௖ ሺ

௏

௖

ሻ௞ିଵ݁ିሺೇ

೎ሻೖ

 (10) 

Where k and c are the Weibull shape and scale parameters 

respectively. The annual energy production for each value of 

ߪ can then be calculated as: 

ܧሺߪሻ ൌ 8760݄ݎ ׬ ܲሺߪ, ܸሻܹሺܸሻܸ݀ ௏௠௔௫

଴ (11) 

Where ܲሺߪ, ܸሻ is the aggregate WPP power curve 

determined from Figure 17 and Figure 18. 

The resulting annual energy production values are shown in 

Figure 18 for the cases of 60 Hz and variable frequency 

operation for different values of standard deviation of wind 

speeds (the values of Weibull factors k=2 and c=9 m/sec were 

used). The gains in energy production are higher for smaller 
7 

values of ߪ. For example, the variable frequency operation 

produces about 12-13% more energy compared to 60 Hz 

operation for a case when ߪ ൌ 1.

Figure 19: Energy production comparison 

V. COMPARISON WITH VARIABLE SPEED

OPERATION 

We conducted the analysis to compare the annual energy 

production between a Type 1 WPP with V/Hz control and 

constant 60 Hz WPP using Type 3 or Type 4 variable-speed 

wind turbines. The layout of offshore VSC-HVDC 

interconnected WPPs using Type 4 topology is shown in 

Figure 20 below. 

Figure 20: A WPP with variable-speed Type 4 wind turbines 

The aggregate power curves for different values of ߪare 

shown in Figure 21. The annual energy production was 

calculated using the same Weibull parameters. A comparison 

of annual energy outputs with Type 1 WPPs operating at fixed 

60 Hz and variable frequency (constant V/Hz ratio) is shown 

in Figure 22. As expected, the conventional variable-speed 

operation offers higher energy production. This is more 

obvious for the sites with higher wind speed diversity (or 

higher values of ߪ(. However, for lower ߪ, the difference 

between V/Hz and variable-speed options becomes small (less 

than 3% for ߪ ൌ 1 ݉/sec ሻ. This fact creates a promising 

opportunity for the proposed V/Hz control for Type 1 offshore 

WPPs at considerable distances from the shore. 

Figure 21: Aggregate power curves for Type 4 WPPs 

Figure 22: Energy production comparison with variable speed 

option 

VI. ONSHORE WPP DATA ANALYSIS 

Wind speed data from large onshore WPP consisting of 100 

wind turbines have been analyzed to determine the wind speed 

distributions among individual wind turbines. 

Figure 23: 10-min wind speed data for an onshore WPP 

A nearly 1-month time series of 10-min average wind speed 

data for individual wind turbines in large onshore WPP 

consisting of 100 units is shown in Figure 23. The analysis 

showed that normal distribution fits the distribution of wind 

speeds among individual wind turbines for each 10-min 
8 

interval. The mean σ over the period of observation is close to 

1 m/s as can be seen in Figure 24. 

Figure 24: Std. deviation of wind speeds for 100 wind turbines

A sensitivity of the mean value of σ to the number of wind 

turbines for the same WPP is shown in Figure 25. It is 

expected that the standard deviation in an offshore WPP will 

be smaller than the standard deviation of the onshore WPP 

presented in this section.

Figure 25: Std. deviation vs. number of wind turbines

VII. CONCLUSIONS AND FUTURE PLANS

In this paper, we have investigated the operation of an 

offshore WPP connected to the grid via an HVDC-VSC 

system. The offshore substation is operated in variable 

frequency mode in a rated V/Hz ratio to allow operation of the 

WPP in variable speed and near its optimum value of 

maximum Cp. In this paper, the diversity of WPPs is analyzed 

and a comparison of annual energy production of various WPP 

control options is performed. It was shown that estimated 

annual energy production of Type 1-based WPPs in constant 

V/Hz operation is close to the output of Type 3 or 4 WPPs in 

conventional variable-speed operation. It is expected that the 

standard deviation in an offshore WPP will be smaller than the 

std dev of the onshore WPP presented in section VI.

Future plans include detailed analysis of the proposed 

system for capital cost and cost of energy (COE) in offshore 

applications. Also, the same authors are working on a similar 

constant V/Hz operation concept for Types 2, 3, and 4 wind 

turbines. The benefits may include simpler turbine topologies, 

smaller power converters, and better electrical efficiencies, 

etc. 

VIII. REFERENCES

[1] “20% Energy by 2030”, DOE/GO-102008-2567, U.S. Department of 

Energy Report, July 2008

[2] “Large-scale offshore wind power in the U.S”. NREL report, 

NREL/TP-500-49229, September, 2010. 

[3] N.M. Kirby, M.J. Luckett, L. Xu., W. Siepmann, “HVDC transmission 

for large offshore wind farms” IEEE AC-DC Power transaction, 

November 2001, London, Conference publication No485, pp 162-168

[4] L. Trilla, O. Gomis-Bellmunt, A. Junyent-Ferre, A.E. Alvarez, A. 

Sudria-Andreu, "Control of a squirrel cage induction generator wind 

farm connected to a single power converter”, UPEC 45th Conference, 

2010. 

[5] D. Jovcic, "Interconnecting offshore wind farms using multiterminal 

VSC-based HVDC," IEEE PES meeting, 2006. 

[6] L. Trilla, O. Gomis-Bellmunt, A. Sudria-Andreu, Jun Liang . "Control 

of SCIG wind farm using a single VSC”, EPE 2011 proceedings

[7] N. Miller, W. Price, J. Snachez-Gasca, “ Dynamic modeling of GE 1.5 

and 3.3 wind turbine generators”, Version 3.0, October 27, 2003

[8] Arulampalam G. Atputharajah, J. B. Caliao, N. Ramtharan, J. B. 

Ekanayake, and N. Jenkins. “Simulated onshore fault-ride-through of 

offshore wind farm connected through VSC HVDC”, Wind 

Engineering, 32(2):103-113, 2008.

[9] T.D. Vrionis, X.I. Koutiva, N.A. Vovos, and G.B. Giannakopoulos.

“Control of a HVDC link connecting a wind farm to the grid for fault 

ride-through enhancements”, Power Systems, IEEE transactions, 22(4), 

2007

IX. BIOGRAPHIES

Vahan Gevorgian (M’97) graduated from the 

Yerevan Polytechnic Institute (Armenia) in 1986. 

During his studies he concentrated on electrical 

machines. His thesis research dealt with doubly-fed 

induction generators for stand-alone power Systems. He 

obtained his Ph.D. degree in Electrical Engineering 

Dept. from the State Engineering University of Armenia 

in 1993. His dissertation was devoted to a modeling of 

electrical transients in large wind turbine generators.

Dr. Gevorgian is currently working at the National 

Wind Technology Center (NWTC) of National Renewable Energy Laboratory 

(NREL) in Golden, Colorado, USA, as a research engineer. His current 

interests include modeling and testing of various applications of wind turbine 

based power systems. He is a member of IEC61400-21 wind turbine power 

quality standard development group.

Mohit Singh (M’2011) received his M.S. and Ph.D. 

in Electrical Engineering from the University of Texas, 

Austin in 2007 and 2011 respectively. His research is 

focused on dynamic modeling of wind turbine 

generators.

Dr. Singh is currently working at the National 

Renewable Energy Laboratory (NREL) in Golden, 

Colorado, USA, as a post-doctoral researcher in 

transmission and grid integration of renewable energy. 

His current interests include modeling and testing of various applications of 

wind turbine generators and other renewable energy resources. He is s 

member of the IEEE and is involved in the activities of the IEEE Power and 

Energy Society (PES).

Eduard Muljadi (M’82-SM’94-F’10) received his 

Ph. D. (in Electrical Engineering) from the University 

of Wisconsin, Madison. From 1988 to 1992, he taught 

at California State University, Fresno, CA. In June 

1992, he joined the National Renewable Energy 

Laboratory in Golden, Colorado. His current research 

interests are in the fields of electric machines, power 

electronics, and power systems in general with 

emphasis on renewable energy applications. He is member of Eta Kappa Nu, 

Sigma Xi and a Fellow of the IEEE. He is involved in the activities of the 

IEEE Industry Application Society (IAS), Power Electronics Society, and 

Power and Energy Society (PES).

He is currently a member of various committees of the IAS, and a member 

of Working Group on Renewable Technologies and Dynamic Performance 

Wind Generation Task Force of the PES. He holds two patents in power 

conversion for renewable energy.