Activation Schemes of Synthetic Inertia Controller on Full

Activation Schemes of Synthetic Inertia Controller on

Full Converter Wind Turbine (Type 4)

Francisco M. Gonzalez-Longatt

School of Electrical, Electronic and System Engineering

Loughborough University

Loughborough, United Kingdom

[email protected]

Abstract—One of the challenges in future energy systems is the

massive use of high power converters that decouple new energy

sources from the AC power grid, disabling natural frequency

response. This situation decreases the total system inertia

affecting the ability of power system to overcome system

frequency's disturbances. It has been established by the wind

power industry a controller to enable inertial response on wind

turbines generators (WTG) enabling the frequency response:

Artificial, Emulated, Simulated, or Synthetic Inertia. However,

there is a clear lack of knowledge about activation scheme used

for those controllers and how they work in practical manner.

This paper proposes three activation schemes for synthetic

inertia on WTG based on full converters: (i) Continuously

operating triggering, (ii) Under-frequency trigger and (iii)

Maximum-Frequency gradient trigger. Simulations over a test

system are used for a preliminary evaluation of the proposed

activation schemes. The main contribution of this paper is the

three schemes to activate the synthetic inertia controller and the

simulations results that demonstrate under-frequency trigger

provides good dynamic response.

Index Terms-- Frequency controller, frequency stability, power

system, protection scheme, wind turbine generator.

I. INTRODUCTION

Future energy systems will look completely different to

the power systems on nowadays [1]. High and low power

converters will be massively deployed almost everywhere

into on the electric network [2], [3] and for very different use:

(i) high power interfaces of the renewable energy produced

by highly variable generators, (ii) interface of several

technologies for energy storage, each one with very different

time constants, and (iii) interconnecting several synchronized

power systems, creating an Pan-European transmission

network which facilitate the massive integration of large-

scale renewable energy sources and the balancing and

transportation of electricity markets. The high/low power

converters typically tend to decouple energy sources from the

pre-existent AC power systems [3]. During a system

frequency disturbance (SFD) the generation/demand power

balance is lost, the system frequency will change at a rate

initially determined by the total system inertia (HT).

However, future power systems will increase the installed

power capacity (MVA) but the effective system inertial

response will stay the same nowadays [4]. The result is

deeper frequency excursions of system disturbances.

There are several good papers [1]-[3], [5], and technical

reports [6]-[7] dealing with theory [8]-[9], modelling [10] and

simulation [11] of inertial response of wind turbine

generators (WTG) and some of them provide general ideas

about possible impacts on power systems and there effects on

transient under-frequency response [12]-[13]. Even some

controls strategies have been proposed to mitigate the impact

of reduced inertia [14]. However, there is lack of knowledge

about control schemes used to activate the synthetic inertia.

This aim of this paper is to propose and to evaluate

activation schemes of synthetic inertia controller on full

converter wind turbine (FCWT) –Type 4. The paper is

organized as follows. Section II introduces the concept of

synthetic inertia and presents releasing “hidden” inertia

controller. Section III proposes three activation schemes for

the synthetic inertia: (i) Continuously Operating, (ii) Under-

frequency Trigger and (iii) Maximum-Frequency Gradient

Trigger. Section III the simulations results are used to assess

the impact of the proposed activation schemes on the system

frequency response and electro-mechanical variables on the

WTG. For illustrative purposes, the test system used in this

paper considers a large synchronous generator representing a

reduced model for a traditional power system and an

equivalent wind turbine representing the reduced equivalent

model of a small-size lossless wind farm. The main

contributions of this paper are: (a) proposing three schemes to

activate the synthetic inertia controller and (b) a preliminary

assessment of these schemes. Simulations results on a test

system demonstrates under-frequency trigger provide good

dynamic response. Finally, the advantages/disadvantages of

the activation schemes are discussed in Section IV.

978-1-4673-8040-9/15/$31.00 ©2015 IEEE

mailto:[email protected]

II. SYNTHETIC INERTIA

Modern WTGs use power electronics converters to enable

variable speed operation in order to capture wind energy over

a wide range of speeds. However, power converter isolates

the rotational speed from the system frequency so WTG

based on back-to-back AC/DC/AC converters offer no

natural response to system frequency [15], [10]. The WT

industry has created several controllers for modern WTG’s in

order to provide inertial response (and governor response on

some cases) for large frequency deviation for, short-duration:

Artificial, Emulated, Simulated, or Synthetic Inertial.

Examples of synthetic inertia controlled commercially

available for WTG are: General Electric WindINERTIA™

[16], ENERCON Inertia Emulation [17].

The objective of the synthetic inertia control is “to extract

the stored inertial energy from the moving part on WTGs”

[18]. There are several versions of synthetic inertia

controllers; however they can be classified in two main

approaches: (a) Releasing “hidden” inertia and (b) Reserve

capacity in pitch. In this paper the hidden inertia approach is

considered and it is named synthetic inertia from here.

Synthetic inertia concept allows a controller to the take the

kinetic energy from a WT rotating mass. This controller is

well-explained in several publications [11], [8]. It is control

loop that increases electric power output during the initial

stages of a significant downward frequency event. The active

power (inertial power, P) of the control is achieved by:

2 sys

syn sys

dfP H f

dt (1)

where Hsyn express the synthetic inertia (sec) and fsys system

frequency (p.u). Implementation of synthetic inertia

controller is depicted on Figure 1.

d

dK

dtsysf 2Hsyn

measP,r ref

,r meas

rPI

MPPTP

Conterter

Synthetic Inertia Controller

r

P

MPPT

refP

HsynP

System

Frequency

[p.u]

P

1 2

f

f f

K

s T s T

min

max

min

max

Figure 1. Representative diagram of Maximum Power Point Tracking

(MPPT) controller and Synthetic Inertia Controller (shadowed) [1].

Several publications relate the main aspects about

synthetic inertia [19], [18]; however, there is not a paper that

formally discusses the trigger mechanism to activate the

synthetic inertia controller. Three activation schemes are

presented and discussed in this paper:

A. Scheme I: Continuously Operating

This is the approach assumed in several publications [9],

[5], however is unrealistic. Many publications assume there is

not a triggering mechanism for the synthetic inertia

controller, in fact, it means the inertia controller receives

continuously a system frequency measurement (fmeas) signal

from the AC system and it is used to derivate the inertial

power (P) using (1) –see Figure 1. This is an unrealistic

control scheme because kinetic energy is taken from rotating

mass continuously and wind turbine is not allow the recover

its kinetic inertia in typical normal operation. However, this

scheme is included in this paper only for comparison

purposes.

B. Scheme II: Under-frequency Trigger

This activation scheme uses a trigger controller that

produces a trigger signal (ts) based on a comparator. The

controller compares the system frequency measurement (fmeas)

with a frequency threshold (fact), the output signal is

generated to activate the synthetic inertia controller if system

frequency measured is below the action frequency (fact). The

activation function of this controller is as follow:

0Trigger Signal:

1

meas act

meas act

ts f f

ts f f

(2)

C. Scheme III: Maximum-Frequency Gradient Trigger

This activation scheme uses a controller that is similar to

the typical logic control observed in ROCOF relays. It

measures the frequency and calculates df/dt, once the rate of

change of frequency exceeds the pre-determined setting

(df/dtact), a trip signal is initiated. The activation function of

the maximum-frequency gradient trigger for synthetic inertia

is defined by:

0

Trigger Signal:

1

meas

act

meas

act

df dfts

dt dt

df dfts

dt dt

(3)

The df/dtact is threshold that activates the synthetic inertia

controller. This approach has been used for years on ROCOF

relays and it is used on [20] to activate the synthetic inertia,

there is not discussion about its implication on that reference.

III. SIMULATIONS AND RESULTS

This section presents simulations and results over a Test

System. An equivalent synchronous generator (GS) and loads

are used as representative equivalent model of a traditional

power system and a small transmission system is included

considering two voltage levels. VSWT using an Electrically

Excited synchronous generator (EESG), Type 4-C, is used on

the simulation for demonstrative purposes. The output of the

generator is passed through the full rated power converter to

the grid. In this paper, an equivalent model of a cluster of

304.5MW direct-drive EESG is considered (similar

characteristic of the Enercon E-112). Figure 2 depicts the

general structure of a VSWT the model for the direct-drive

EESG. This model uses a back-to-back converter, details of

all models used can be found on [21]-[22]. The parameters

used for these models are escalated to simulate an equivalent

10 4.5 MW wind farm. DIgSILENT PowerFactoryTM [23]

is used for time-domain simulations and DIgSILENT

Simulation Language (DSL) is used for dynamic modelling

[24]. Figure 3 to 5 show the DSL models created for the

activation schemes considered in this paper.

Equivalent Windfarm10x4.5MW

Event

Hwt = 2.33 sec

Hgs = 10.0 sec

Rec Gen3.29 kV1.00 p.u.-5.18 deg

DC Bus7.26 kV1.10 p.u.0.00 deg

HV110.00 kV1.00 p.u.-0.00 deg

Gen3.30 kV1.00 p.u.-0.00 deg

MV20.00 kV1.00 p.u.

-150.63 deg

LV 3.31 kV1.00 p.u.

-147.54 deg

G~

WT

45.00 MW4.08 Mvar

7.91 kA

Tr1

-5.00 MW0.05 Mvar

0.14 kA

5.00 MW0.00 Mvar

0.03 kA

Load 50 MW

50.00 MW-0.00 Mvar

1.44 kA

Series Reactor

-45.00 MW0.00 Mvar

7.91 kA

45.00 MW4.08 Mvar

7.91 kA

Load 40 MW

40.00 MW-0.00 Mvar

0.21 kA

G~

GS

45.00 MW0.00 Mvar

0.24 kA

DC Capacitor

0.00 MW0.00 Mvar0.00 kA

Tr2

-45.00 MW-0.05 Mvar

1.30 kA

45.00 MW2.48 Mvar

7.87 kA

PWM Generator-Side

-45.00 MW0.00 Mvar7.91 kA

45.00 MW0.00 Mvar6.20 kA

PWM Grid-Side

45.00 MW2.48 Mvar

7.87 kA

-45.00 MW0.00 Mvar-6.20 kA

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Figure 2. Test System.

A SFD is applied to the test system and the response of the

main electromechanical variables on the power system

(equivalent synchronous generator -GS) side and wind farm

(WF) are shown on Figure 6. Simulations include three

activation schemes and base case (No control label on Figure

6) where no frequency support is provided by the wind farm.

The main impact of all considered activation schemes is

momentary reduce the active power on the synchronous

generator (GS).

Hidden Inertia: Continuous Operation

Inertia1st Order FilterDerivate

Synthetic Inertia Control

-

{Kp+Ki/s}Kp,Ki

Lmax

Lmin

-

1/(1+sT)Tfilter

Sink

2HH

1/Kfn

s

Hidden Inertia: Continuous Operation

0

1

2

3

fglongatt

PowerFactory 14.1.3

Evaluation of Trip Schemes for Synthetic Inertia

Francisco M. Gonzalez-Longatt Hidden Inertia:Continuous Operation

December 2012

Project:

Graphic: Graphic

Date: 12/6/2012

Annex:

Dwr

dwsys_dtFmeas wsys dwdt

Pin

P

wr

Delta_P

wr_ref

Pref

P_

mp

pt

DIg

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Figure 3. DSL Model Scheme I Hidden Inertia Trigger f: Trigger by Under Frequency

InertiaDerivate


1st Order Filter

Detectorfmin

1/(1+sT)Tfilter

Limit..

y_max

y_min

s 2HH

{Kp+Ki/s}Kp,Ki

Lmax

Lmin

-

Sink

1/Kfn

Hidden Inertia Trigger f: Trigger by Under Frequency

0

1

2

3

fglongatt

PowerFactory 14.1.3


Francisco M. Gonzalez-Longatt Hidden Inertia Controller. Trigger: dfdt

December 2012

Project:

Graphic: Graphic

Date: 12/6/2012

Annex:

DELTA_..

Trip_Signal

dwdtLI..dwdt

Pin

Pref

P_

mp

pt

Fmeas

Dwr

P

wr

wr_ref

DIg

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Figure 4. DSL Model Scheme II

Hidden Inertia Trigger dfdt: Trigger by Rate of Change of Frequency

Inertia


Derivate 1st Order Filter

s

Detectormax_dfdt

-

{Kp+Ki/s}Kp,Ki

Lmax

Lmin

-

1/(1+sT)Tfilter

Sink

2HH

1/Kfn

Hidden Inertia Trigger dfdt: Trigger by Rate of Change of Frequency

0

1

2

3

fglongatt

PowerFactory 14.1.3


Francisco M. Gonzalez-Longatt Hidden Inertia Controller. Trigger: dfdt

December 2012

Project:

Graphic: Graphic

Date: 12/6/2012

Annex:

dwdt

FILdwdt

Dwr

Fmeas

Pin

P

wr

wr_ref

Pref

P_m

ppt

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Figure 5. DSL Model Scheme III

9.99987.97995.95993.93991.9200-0.1000 [s]

1.0061

0.9996

0.9931

0.9866

0.9800

0.9735

[-]

No Control

Scheme II: Red

Scheme III: Pink

Seheme I: Blue

9.99987.97995.95993.93991.9200-0.1000 [s]

0.8803

0.7902

0.7000

0.6098

0.5197

0.4295

[p.u.]

No Control

Scheme I: Blue

Scheme II: Red

Scheme III: Pink

GS: Active Power (p.u.)

System Frequency (p.u.)

fglongatt Evaluation of Trip Schemes for Synthetic Inertia Results(1)

Francisco M. Gonzalez-Longatt, PhD Comparison

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9.99987.97995.95993.93991.9200-0.1000 [s]

1.0005

0.9990

0.9975

0.9960

0.9945

0.9930

[p.u.]

Scheme I

Scheme III

Scheme II

No Control

3.492 s 0.994 p.u.

3.580 s 0.995 p.u.

3.939 s 0.996 p.u.

9.99987.97995.95993.93991.9200-0.1000 [s]

0.5145

0.4986

0.4828

0.4669

0.4510

0.4352

[p.u.]

2.014 s 0.486 p.u.

1.726 s 0.482 p.u.

1.375 s 0.474 p.u.

6.277 s 0.440 p.u.

6.284 s 0.443 p.u.

6.284 s 0.442 p.u.

No Control Scheme I

Scheme III

Scheme II

WT: Rotor Speed (p.u.)

WT: Active Power (p.u.)

fglongatt Evaluation of Trip Schemes for Synthetic Inertia Results WT


Date: 2/7/2015

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Figure 6. Simulation Results: (a) Grid Side and (b) Wind farm Side.

The activation scheme based on continuously merriment

signal (Scheme I) is the only scheme that includes a

negligible time delay on the activation of synthetic inertia, it

is an expected result because the control scheme is acting

continuously to any frequency change. The small time delay

found on the response is provided by the first order filter used

to remove the noise amplification on the derivate of the

measured frequency. Activation Scheme II and III require

activation condition must be satisfied before activate the

synthetic inertia controller, and it depends on the whole

system frequency response where the characteristics of the

traditional power system (equivalent synchronous generator)

has impact on the dynamic performance. Simulation results

show the under-frequency trigger activation (Scheme II)

produces the second faster activation time (ta ~ 2.0 ms) after

(a) Grid Side Results

(b) Wind Farm Results

the frequency disturbance detection (fact = 0.998 p.u.). The

activation time for maximum-frequency gradient trigger is the

longest (ta ~ 450 ms), it is because the df/dt depends on the

total system inertia. In this case the system’s inertia is large

compared with the system inertia provided by the controller.

Details of the wind turbine frequency response are shown

on Figure 7. The active power (PTW) values of the secondary

peak following the activation action are shown on Figure 7

and numeric values of the primary peak are presented on

Table I. 3.92643.31482.70332.09171.48020.8686 [s]

1.0062

0.9996

0.9931

0.9866

0.9800

0.9735

[-]

3.92643.31482.70332.09171.48020.8686 [s]

0.8597

0.8489

0.8380

0.8272

0.8164

0.8055

[p.u.]

3.854 s 0.850 p.u.

2.015 s 0.814 p.u.

1.369 s 0.826 p.u.

1.739 s 0.818 p.u.

1.144 s 0.847 p.u.

3.92643.31482.70332.09171.48020.8686 [s]

1.0062

0.9995

0.9927

0.9860

0.9792

0.9725

[-]

2.084 s 0.974

2.122 s 0.975

2.039 s 0.975

2.071 s 0.976

2.038 s 0.975 2.125 s

0.975

3.92643.31482.70332.09171.48020.8686 [s]

0.4955

0.4840

0.4726

0.4611

0.4496

0.4381

[p.u.] 2.007 s 0.486 p.u.

1.713 s 0.482 p.u.

1.362 s 0.474 p.u.



WT: Active Power (p.u.) System Frequency (p.u.)

Scheme III

Scheme II

Scheme I



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2.57902.38432.18961.99491.80021.6055 [s]

1.0061

0.9996

0.9931

0.9866

0.9800

0.9735

[-]

2.57902.38432.18961.99491.80021.6055 [s]

0.8803

0.7902

0.7000

0.6098

0.5197

0.4295

[p.u.]

2.57902.38432.18961.99491.80021.6055 [s]

0.9844

0.9823

0.9801

0.9780

0.9759

0.9737

[-]

2.084 s 0.974

2.071 s 0.976

2.038 s 0.975

2.125 s 0.975

2.57902.38432.18961.99491.80021.6055 [s]

0.4955

0.4839

0.4723

0.4607

0.4491

0.4375

[p.u.]




Scheme III

Scheme II

Scheme I



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Figure 7. Details of wind farm active power production and system

frequency.

Inertial power provided by the activation Scheme III is the

largest and it is intrinsically related with the threshold df/dtact.

The initial peak of inertia power (PTW,max) after the activation

process is large in all cases and it is caused by the df/dt,

however, Scheme II exhibit a larger peak than Scheme I. This

initial peak indicates a quick response on releasing large

amount of kinetic energy in the rotating masses on the WT

caused by the power electronic converter. However,

discharging that kinetic energy to the grid is only for a short

period available, and potential dangerous consequences on

the mechanical parts must be seriously evaluated. A more

retailed impact of the activation schemes on the system

frequency response is evaluated considering the changes on

the frequency and power indicators shown on Table I.

TABLE I. MAIN INDICATORS OF FREQUENCY RESPONSE

No Control Scheme I Scheme II Scheme III

PWT,max (p.u) - 0.48172 0.48940 0.49292

tmax - 1.71217 1.01842 1.65683

fmin (p.u) 0.97404 0.97581 0.97500 0.97533

tmin (s) 2.08617 2.08417 2.12142 2.03583

df/dt (p.u/s) -0.02924 -0.03962 -0.03330 -0.03181

The effect of the activation scheme on the rate-of-change-

of-frequency and frequency nadir (fmin) is very important on

the system frequency stability. The positive effect of all

schemes is shown on Figure 8. The synthetic inertia modifies

the df/dt, however, activation Scheme II causes the slower

change (fmin ~ 0.975 p.u @ 2.1214 s) and Scheme I and

Scheme III produces almost the same change on fmin but

Scheme III reach the frequency nadir first (tmin = 2.035 s)

compared with Scheme II. The activation Scheme II, under-

frequency trigger, the best dynamic response in terms of

lowest frequency nadir and delaying the time where the nadir

is reached.

An important aspect about the impact of activation

schemes is the active power production of the traditional

synchronous generator. The long activation time on the

synthetic inertia controller caused by the Scheme III imply

traditional generator must quickly react to cope with the

system frequency disturbance, that situation makes this

activation scheme unpractical in a future electricity network

with low inertia. Scheme II produce a fast response and

initially reduce the active power solicitation form the

traditional generator, however, Scheme II provide the best

performance in term of release the requirements of active

power from the traditional generators.

3.92643.31482.70332.09171.48020.8686 [s]

1.0062

0.9996

0.9931

0.9866

0.9800

0.9735

[-]

3.92643.31482.70332.09171.48020.8686 [s]

0.8597

0.8489

0.8380

0.8272

0.8164

0.8055

[p.u.]

3.854 s 0.850 p.u.

2.015 s 0.814 p.u.

1.369 s 0.826 p.u.

1.739 s 0.818 p.u.

1.144 s 0.847 p.u.

3.92643.31482.70332.09171.48020.8686 [s]

1.0062

0.9995

0.9927

0.9860

0.9792

0.9725

[-]

2.084 s 0.974

2.122 s 0.975

2.039 s 0.975

2.071 s 0.976

2.038 s 0.975 2.125 s

0.975

3.92643.31482.70332.09171.48020.8686 [s]

0.4955

0.4840

0.4726

0.4611

0.4496

0.4381

[p.u.]




Scheme III

Scheme II

Scheme I



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Figure 8. Details of frequency response of the traditional power system side

considering the activation schemes.

The changes on the response of the inertial power and the

system frequency for several Hsyn is depicted in Figure 9.

Increased values of Hsyn increase inertial power contribution

and delay and reduce fmin. However, Scheme II is highly

affected by changes in Hsys and high values can cause loss of

synchronism of the EESG.

3.96413.34502.72592.10681.48770.8685 [s]

0.9786

0.9775

0.9765

0.9754

0.9743

0.9733

[-]

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0.4592

0.4553

0.4515

0.4476

0.4437

0.4398

[p.u.]

1.702 s 0.458 p.u.

1.683 s 0.457 p.u.

1.708 s 0.453 p.u.

1.701 s 0.452 p.u.

1.659 s 0.450 p.u.

1.424 s 0.450 p.u.

3.96413.34502.72592.10681.48770.8685 [s]

1.00

0.80

0.60

0.40

0.20

0.003.96413.34502.72592.10681.48770.8685 [s]

1.00

0.80

0.60

0.40

0.20

0.00

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2.41382.29882.18382.06881.95381.8388 [s]

0.9760

0.9756

0.9752

0.9748

0.9744

0.9740

[-]

2.087 s 0.97403

2.086 s 0.97405

2.082 s 0.97413

2.082 s 0.97422

2.087 s 0.97441

2.082 s 0.97545

2.41382.29882.18382.06881.95381.8388 [s]

0.4592

0.4553

0.4515

0.4476

0.4437

0.4398

[p.u.]

2.41382.29882.18382.06881.95381.8388 [s]

1.00

0.80

0.60

0.40

0.20

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1.00

0.80

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0.00

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9.99947.97955.95973.93981.9199-0.1000 [s]

1.0113

1.0034

0.9954

0.9874

0.9795

0.9715

[-]

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0.5204

0.5017

0.4830

0.4642

0.4455

0.4268

[p.u.]

1.38042 s0.48901 p.u.

1.38042 s0.47925 p.u.

1.35542 s0.46951 p.u.

1.40142 s0.45969 p.u.

Unstable

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1.00

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0.40

0.20

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0.80

0.60

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0.00

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3.03382.70832.38272.05721.73161.4061 [s]

0.9863

0.9834

0.9804

0.9774

0.9745

0.9715

[-]

2.13542 s0.97334

2.09017 s0.97404

2.09642 s0.97443

2.11642 s0.97481

2.12942 s0.97519

2.14542 s0.97556

3.03382.70832.38272.05721.73161.4061 [s]

0.5204

0.5017

0.4830

0.4642

0.4455

0.4268

[p.u.]

1.38042 s0.48901 p.u.

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1.00

0.80

0.60

0.40

0.20

0.003.03382.70832.38272.05721.73161.4061 [s]

1.00

0.80

0.60

0.40

0.20

0.00

DIg

SIL

EN

T

3.56893.00452.44011.87571.31130.7469 [s]

1.0013

0.9956

0.9899

0.9842

0.9785

0.9727

[-]

3.56893.00452.44011.87571.31130.7469 [s]

0.4622

0.4577

0.4532

0.4487

0.4441

0.4396

[p.u.]2.02283 s0.45931 p.u.

2.01883 s0.45746 p.u.

2.02483 s0.45374 p.u.

2.03483 s0.45187 p.u.

1.96383 s0.45037 p.u.

1.65483 s0.46100 p.u.

1.01717 s0.45379 p.u.

3.56893.00452.44011.87571.31130.7469 [s]

1.00

0.80

0.60

0.40

0.20

0.003.56893.00452.44011.87571.31130.7469 [s]

1.00

0.80

0.60

0.40

0.20

0.00

DIg

SIL

EN

T

2.32342.22292.12232.02171.92121.8206 [s]

0.9757

0.9754

0.9750

0.9747

0.9743

0.9740

[-]

2.07283 s0.97439

2.07683 s0.97432

2.08183 s0.97418

2.08083 s0.97411

2.08283 s0.97405 2.08417 s

0.97404

2.32342.22292.12232.02171.92121.8206 [s]

0.4622

0.4577

0.4532

0.4487

0.4441

0.4396

[p.u.]2.02283 s0.45931 p.u.

2.32342.22292.12232.02171.92121.8206 [s]

1.00

0.80

0.60

0.40

0.20

0.002.32342.22292.12232.02171.92121.8206 [s]

1.00

0.80

0.60

0.40

0.20

0.00

DIg

SIL

EN

T

Figure 9. Details of frequency response of the inertial power and system’s

frequency considering the activation schemes and changes on Hsyn.

IV. CONCLUSION

This paper proposes three activation schemes for synthetic

inertia controller on WTG based on full rated power

converters: (i) continuously operating triggering, (ii) under-

Scheme I

Scheme II

Scheme III

frequency trigger and (iii) maximum-frequency gradient

trigger. Time-domain simulations over a simple test system

are used to evaluate the system frequency response

considering the activations schemes proposed. The main

electromechanical variables related to the frequency response

on the power system side and wind farm side have been

evaluated. Simulation results demonstrate the activation

scheme based on under-frequency trigger, provide the best

dynamic response in terms of lowest frequency nadir and

delaying the time where the nadir is reached. The inertial

power provided by the under-frequency scheme is lower than

using ROCOF, however, the most beneficial point is delay

the point of minimum frequency allowing a to governors in

traditional synchronous generators activate its response and

recover the system’s frequency. The value of the synthetic

inertia (Hsyn) must be carefully selected, beyond the physical

value, because it has important impact on the electro-

mechanical dynamic of the electrically excited synchronous

generator and the system frequency response.

The main contribution of this paper is the development of

three schemes to activate the synthetic inertia controller and

its assessment. Results demonstrate an outstanding system

frequency response when the synthetic inertia is activated

using the under-frequency trigger. The author is proposing

the under-frequency trigger and maximum-frequency gradient

trigger as main activation scheme considering the inertia

values on the system to be used. However, further evaluations

are required: (i) defining the optimal value of the trigger

frequency as a function of total system inertia, (ii)

determining the impact of this activation scheme on

frequency control and protection schemes, (iii) determining

the impact of activation schemes of synthetic inertia in a

detailed model of wind farms on system frequency response

considering

V. REFERENCES

[1] F. Gonzalez-Longatt, "Frequency Control and Inertial Response

Schemes for the Future Power Networks," in Large Scale Renewable

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Singapore, 2014, pp. 193-231.

[2] F. M. Gonzalez-Longatt, "Effects of the synthetic inertia from wind

power on the total system inertia: simulation study," in Environment

Friendly Energies and Applications (EFEA), 2012 2nd International

Symposium on, 2012, pp. 389-395.

[3] F. Gonzalez-Longatt, "Impact of synthetic inertia from wind power on

the protection/control schemes of future power systems: Simulation

study," in Developments in Power Systems Protection, 2012. DPSP

2012. 11th International Conference on, 2012, pp. 1-6.

[4] F. Gonzalez-Longatt, "Frequency Control and Inertial Response

Schemes for the Future Power Networks," in Advances in Technologies

for Generation, Transmission and Storage, Green Energy and

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[7] GE. (2009). WindINERTIA Control fact sheet. Available:

http://www.ge-

energy.com/products_and_services/products/wind_turbines/index.jsp

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[15] G. Lalor, A. Mullane, and M. O'Malley, "Frequency control and wind

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[16] N. Miller, K. Clark, and R. Walling, "WindINERTIA: Controlled

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[17] S. Wachtel and A. Beekmann, "Contribution of Wind Energy

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[18] F. Gonzalez-Longatt, "Impact of synthetic inertia from wind power on

the protection/control schemes of future power systems: Simulation

study," in 11th International Conference on Developments in Power

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[19] F. Gonzalez-Longatt, "Effects of the synthetic inertia from wind power

on the total system inertia: simulation study," in 2nd International

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2012), , Northumbria University in Newcastle upon Tyne, UK, 2012,

pp. 389 – 395.

[20] J. Björnstedt, "Integration of Non-synchronous Generation. Frequency

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[23] DIgSILENT, "DIgSILENT PowerFactory," 14.0.524.2 ed. Gomaringen,

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[24] F. Gonzalez-Longatt and J. Rueda, PowerFactory Applications for

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Activation Schemes of Synthetic Inertia Controller on Full