1SINTEF Energy Research

Modelling and simulation of grid connected

power electronic converters

2SINTEF Energy Research

Simulations can typically be used to identify:

Required converter and device characteristics (power circuit and control)Resonance and stability problemsPower quality issuesPower efficiency and energy saving feasibilityFault handling capabilitiesPerformance in weak power systems

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Required skills and experience

The use of adequate level of modellingsimplifications

Limitation and validity of modelsLimitation of numerical simulations (general and tool specific)Validation of simulation resultsSelecting appropriate time stepsImplementation of new models / control blocks

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Advantages compared to laboratory measurement

Cheaper and fasterResponse to abnormal situations and “destructive” tests at no cost or dangerWaveforms from all places in the circuit can easily be monitored (no measurement noise and no need for special probes)Topology changes at small costEffect of parameter variation are easily testedComponent ratings need not to be known in advanceSimplification of non-important parts are much easier

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PSCAD / EMTDC simulation tool

Commercial available (Developed in Canada)Our preferred simulation tool for time domain analysisSpecially suited for simulation of power electronicsIncludes

component models needed for simulation of converters in power systemscontrol blocks suitable for controller modelling

New models (electric or control blocks) can be implemented by the user

6SINTEF Energy Research

Example: Active front-end rectifierModel includes (next slide):

Converter power circuit with LC filter on AC-sideDC-link capacitor and DC power source / loadAC-grid modelConverter control

phase locked loopDC-link voltage controllerCurrent controller and pulse width modulation

Model typically used for:Investigation of stability and resonance problemsComponent ratingController designPower quality analysis

7SINTEF Energy Research

Simulation model (top level)

g_Rp

g_Rm

g_Sp

g_Sm

g_Tp

g_Tm

g_Sp

g_Tp

g_Tm

g_Sm

g_Rp

I_T

I_S

I_T

I_S

0.0014

0.0014

I_R

I_S

I_T0.0014

VT

*1E3

DC-link Active front end PWM converter

VR

Load / Source

g_Rm

123

Scale factor (from kA to per unit (pu))(1.0 pu = 28 A = 0.028kA)Note: Peak values !

0.053

0.053

A

B

C

A

B

C

66.0

I_mains_R

I_mains_S

I_mains_T

2T3

2T6

2T5

2T2

D3 D5

D4 D6 D2

I_mains_T

I_C_R

I_C_T

*

I_DC

V_DC

V_DC

V_DC

I_DC *1E3

P_DC

66.0

I_C_R 66.00.001

0.001

I_DC

I_R

I_PWM

I_PWM

I_PWM

ConverterDC_ref (kV)

0.1

0.9

0.45

I_react (pu)

-1

1

0

PWM controlOFF ON

1

IR Fund. rms

A rms0 0.04

0

VST

VRS

VRS

VTR

Power

AB

PQ

*2.5

I_R

Fault detectionShort cir. PWM

0 1

0

Power flowP

kW-10 10

-10

Q

kW-10 10

-10

MainsMains Volt

0.2

0.3

0.23

Frequency

40

60

50

Phase

-360

360

0

*25.2

*25.2

*25.2

2T4

VS

VT

LoadLoad (kA)

-0.05

0.05

0.001

V_DC

0 1

0

P_DC

-20 20

-20

0.053

Active front end converterWed Dec 12 09:13:34 2001

ny_frontend_11FileDir J:\DOK\12\MO\Prosjekt\Sip 12X127\EMTDC\Frontend

2T1 D1

Short cir. PWM

I_R

0.0

0.0

| X |*1E3

Power calculation

0.020.02

6600.06600.0 V_DC_P

VR

Short circuit detection

V_DC_P

0.0

0.0

0.001

VS

I_mains_S0.05

A

B

C

VFPh

0.0003

0.0003

0.0003

0.0003

0.0003

0.0003

VCC

_R

VCC

_S

VCC

_TFilter Point of common

connection

FFT-analysis

Result to fileWrite

-- Trigg

0

Store resultsOFF ON

0

V_R_0 V_R_G

V_DC

V_DC_N

V_R_G

V_R_0

Save result to file forMatlab analysis

V_R_0

V_R_G I_R

VCC_R

VRS

I_mains_R

I_mains_R

VR

In5

fileWrite Store

resulttrigger trigger

In4

In3

In2

In1

resultto file

In7

In6

lagreres.f

*2.5

VTR

VST

VRS

123

Scale factor (from kV to per unit (pu))(1.0 pu = 400 V DC = 0.4kV)

Scale factor (from kV to per unit (pu))(1.0 pu = 230 V AC = 0.23kV)Note: Peak values

DCVoltage

Controller

I_ampVDC_ref

VDC_mea Generation of

I_ref

I_act

I_react

Theta

curr. references Current V_refI_ref

I_mea

Controller

g_Rp

g_Rm

g_Sp

g_Sm

g_Tp

g_Tm

On

PWM

V_ref

I_C_S

Power Grid

FFT

In1

In2

In3

In4

In5

Plot

*3.074

*3.074

*3.074 Phase

loop (PLL)

VAC_mea Thetalocked

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Example: Step reversal of reactive current/power flow

Untitled

Time (sec)

no name

0.05 0.07 0.09 0.11 0.13 0.15-0.05

-0.03

-0.01

+0.01

+0.03

+0.05IR IS IT

no name

0.05 0.07 0.09 0.11 0.13 0.15-0.05

-0.03

-0.01

+0.01

+0.03

+0.05I mains R I mains S I mains T

Convert.currents before filtering

Convert.currents after filtering

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0.17 0.175 0.18 0.185 0.19 0.195-0.04

-0.02

0

0.02

0.04

Time (500 ks /s ) (RMS :0.023189 )

File:./ex03.emt/res ultat

I R

0 0.5 1 1.5 2 2.5

x 104

0

2

4

6

Frequency (Hz) (500 ks /s Delta freq. :50 Hz ) (%THD=6.9015 RMS =0.023189 )

File:./ex03.emt/res ultat

I RExample: Harmonic content in

converter current (Matlab)

Current in filter inductor

Harmonics in % of fundamental

x 104 Hz onx-axis

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Example: Converter connected distributed production

Active front-end converter model with:Voltage control loopFrequency controller

Model typically suited for investigation of:Stability

connected to AC main gridisolated mode

Performance during transition from grid connected to isolated mode of operationVoltage quality aspectsPerformance during and after faults

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From grid connected to stand-alone (loss of AC network, resistive load)

Converteroutput voltage(kV)

Current supplied from converter to load(kA)

Current supplied from AC-grid to load(kA)

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Three phase terminal short circuit (Illustartion of current limiter)

Converteroutput voltage(kV)

Converter output current (before filtering)(kA)

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Example: Analysis of inter-harmonic in a large industrial plant

Determination of generator and compressor shaft torque oscillations due to inter-harmonic currents from grid connected convertersDetailed models of mechanical system was interfaced to the electrical system modelChallenges:

Modelling variable speed drive controllersGetting the necessary input dataModelling the mechanical systems (generator trains and compressors) Interfacing the mechanical system model

(complete modelled system shown on next slide)

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Analysis of inter-harmonic in a large industrial plant (also island op. mode)

G G G G G

=∼

=∼

=∼

=∼

M

=∼

=∼

=∼

=∼

M

=∼

=∼

=∼

=∼

M

M

Agregated 11 kV load Agregated

LV load

AC -grid

132 kV

=∼

=∼

=∼

=∼

M

11 kV

132 kV

11 kV

18 MW VSDincl. model of shaft dynamic

32 MW VSDincl. model of shaft dynamics

65 MW VSDincl. model of shaft dynamics

50 MW 50 MW 50 MW 50 MW 50 MW

M

∼

cable

132 kV

Agregated 6.6 kV load

M M

Agregated 11 kV load Agregated

LV load

M

Agregated 6.6 kV load

M

65 MW VSDincl. model of shaft dynamics

Generator train models incl. model of shaft dynamics of each

plant

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Model development and implementation

SINTEF has the necessary experience and competence for model development and implementationExamples of models developed and implemented in the PSCAD/EMTDC simulation software

Averaging model of PWM converter that simplifies simulations of large systems with many convertersPSCAD/EMTDC model for calculation of converter losses and temperature stress in hard-switched semiconductorsModel for inclusion of dead-time in combination with the interpolated switch model in PSCAD/EMTDC

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Example of developed model :PWM averaging model

Purpose: less complexity and faster time domain simulation studies of grid connected converters, while still maintaining sufficient converter dynamic accuracy.The model has the same average V-I terminal relationship on AC and DC side as a full switched model (averaged over one switching period).Simulation time steps larger than the switching period can be used since the switches are not modelled.The control circuit is modelled in the same way as if a full switched model were used except that modelling of gate-pulse generation is not needed.Over-modulation, saturation effects and other non-linearity's are also modelled correctly

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Illustration: PWM averaging model

Untitled

Time (sec)

no name

0.158 0.16 0.162 0.164 0.166 0.168-0.05

-0.028

-0.006

+0.016

+0.038

+0.06IR IS I_R_AV I_S_AV IT I_T_AV

Filter inductor currents (kA) for switched model and for average model just before and after the reactive power reversal

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Model developed:Converter loss calculation model

Losses and temperature conditions for semiconductor switching devices are important issues of a converter design (e.g. maximum temperature and temperature cycling). Proper simulation models for loss calculation is therefore needed.

A semiconductor loss calculation model has therefore been implemented in the PSCAD/EMTDC simulation software.

The new implemented model can be used for estimation of losses and temperature cycling in hard-switched semiconductors (thyristors, transistors and diodes).

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Verification of simulations

SINTEF has the necessary equipment, laboratories and simulation tools verify simulation models The converter model has been verified against measurements on the laboratory prototype (project memo

Time (sec)0.23 0.234 0.238 0.242 0.246 0.25-0.05

-0.03

-0.01

+0.01

+0.03

+0.05mea_cur IR

Comparison between measured (red) and simulated (black) converter output current (kA) before and after a step in the current phase angle