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Aboutaleb Haddadi, Ph.D.Sr. Engineer Scientistahaddadi@epri.com

Evangelos Farantatos, Ph.D.Sr. Project ManagerGrid Operations & Planning R&D GroupEPRIefarantatos@epri.com

IEEE PSRC MeetingSep 2021

IBR Short Circuit Model -

Considerations for VCCS

Tabular Model

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IBR Short Circuit Model

C24 –recommended table for WTG short circuit model in commercial platforms

▪ IBR is represented by a voltage-dependent current source.

▪ It is possible to have different tables for different time frames.

▪ The table format enables IBR plant owner to provide short circuit

data without revealing proprietary inverter control.

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Negative Sequence Representation of Type IV Wind Turbine

Generator/Solar Photovoltaic Inverter

▪How should the table look like when inverter injects negative sequence current?

–Challenge: Separate tables for +seq and -seq may not provide sufficient accuracy

▪ Example:

– +Seq and –Seq tables produced using

an EMT IBR model (PSCAD, EMTP, etc)V1 (pu) I1 (pu)

Angle(I1/V1)(degrees)

1.00 1.00 0

0.90 0.99 -17

0.72 0.74 -50

0.70 0.62 -60

+Seq table

V2 (pu) I2 (pu)Angle(I2/V2)

(degrees)

0.00 0.00 0

0.11 0.22 -90

0.21 0.41 -90

0.28 0.53 -90

-Seq table

Fault with

varying

resistance

If the table is used to represent the inverter under a different

fault with a different combination of V1 and V2, the table

may not give correct I1 and I2 values.

Example: For a fault with V1=0.72 pu and V2=0.28 pu, the

table gives I1 = 0.74@-50° and I2=0.53@-90°;

I1+I2 > 1.1 pu which is not realistic!

BUT

A joint +seq and –seq table is needed.

I_lim=1.1pu

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IBR Fault Ride Through (FRT) Requirement

VDE-AR-N 4120

Example: For V2=0.25 p.u. and k=2 I2=2*0.25=0.5 p.u.

I2=k*(V2)

Example: For V1=0.5 p.u. and kFRT=2 I1=2*(1-0.5)=1 p.u.

I1=kFRT*(1-V1)

+Seq dynamic reactive current injection -Seq dynamic reactive current injection

But the converter current is limited (e.g., 1.1 pu)In reality, I1 and I2 get limited by a converter current limiter logic.

I1 & I2 are coupled.

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Negative Sequence Representation – Cont’d

▪To account for the coupling between I1 and I2,

a joint table is needed.

▪What is the format (what quantities does I2

depend on)?

–V1, I1, angle(I1/V1)

–V2, I2, angle(I2/V2)

–angle(V2/V1)

–etc?

Time since initiation of a fault (cycles)

V1 (pu) V2 (pu)

3 cycles

I1 (pu)Angle(I1/V1)

(Degrees)I2 (pu)

Angle(I2/V2)

(Degrees)

0.9 0.0

0.1

0.2

0.3

0.4

0.5

0.8 0.0

0.1

0.2

0.3

0.4

0.5

0.7 0.0

0.1

0.2

0.3

0.4

0.5

….. ….. ….. ….. ….. …..

?

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▪There is no standardization for current limiter logic implementation.

▪There are several options:

Current Limiter Logic

Current limiter logic implementation

impacts I2 characteristics.

Active and reactive

components of I1 and I2

(I1r, I2r, I1p, I2p)

d- and q-axis components

of I1 and I2 (I1q, I2q, I2d,

I1d)

Phase currents

(Ia, Ib, Ic)

And other options...

I2 depends on {V1, V2, I1}

I2 depends on {V1, V2, I1, angle(V2/V1)}

I2 depends on {V1, V2, I1, angle(V2/V1)}

Current limiter logic

implementation options

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Current Limiter Logic Implementation Options

▪ In one implementation, current limit may be

applied to active and reactive currents:

(I1 = I1p + j*I1r, I2 = I2r)

1. ABS(I1r)+ABS(I2r) < reactive current limit

2. I2 leads V2 by 90 degrees

3. ABS(I1p) < active current limit

▪ I2 becomes dependent on V1, V2, and I1

Suggested table formatTime since initiation of a fault (cycles)

V1 (pu) V2 (pu)

3 cycles

I1 (pu)Angle(I1/V1)

(Degrees)I2 (pu)

Angle(I2/V2)

(Degrees)

0.9 0.0

0.1

0.2

0.3

0.4

0.5

0.8 0.0

0.1

0.2

0.3

0.4

0.5

0.7 0.0

0.1

0.2

0.3

0.4

0.5

….. ….. ….. ….. ….. …..

I2=I2r

V2

V1

I1

I1p

I1r

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Current Limiter Logic Implementation Options – Cont’d

▪ In another implementation, current limit may be applied to d-

and q-axis current components:

(I1 = I1d + j*I1q, I2 = I2d + j*I2q)

1. ABS(I1q)+ABS(I2q) < q-axis current limit

2. I2 leads V2 by 90 degrees

3. ABS(I1d)+ABS(I2d) < d-axis current limit

▪ dq frame is fixed; it is established by a phase locked loop

based on V1.

▪ I2 becomes dependent on V1, V2, I1,angle(V2/V1)

Suggested table formatTime since initiation of a fault (cycles)

V1 (pu) V2 (pu) Angle(V2/V1)

3 cycles

I1 (pu)Angle(I1/V1)

(Degrees)I2 (pu)

Angle(I2/V2)

(Degrees)

0.9 0.0 0

60

120

180

240

300

0.1 0

60

120

180

240

300

0.2 0

60

120

180

240

300

….. ….. ….. ….. ….. ….. …..

I2

V2

V1

I1

I1d

I1q

I2q

I2d

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Demonstrating ResultsExample

Iq_lim=1.1 pu

Id_lim=1.1 puItot_lim = 1.1 pu

V1(pu)

V2(pu)

I1q(pu)

I1d(pu)

I2q(pu)

I2d (pu)

Case 2: angle(V2/V1) = -90 deg

0.5 0.5 0.73 0 0.37 0

Case 1: angle(V2/V1)= 0 deg

0.5 0.5 1 0 0 0.47

• Dependence of I2 on angle(V2/V1)

when current limit is applied to d-

and q-axis current components.

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Other Implications

▪ If I2 is dependent on angle(V2/V1), the phase current of IBR may be different under

different faulted phases.

FaultAngle

(V2/V1)V1 (pu) V2 (pu) Ia (pu) Ib (pu) Ic (pu)

AG 165° 0.837@21.1° 0.131@-176.9° 0.773@-3.3° 0.670@-104.9° 0.915@130.9°

BG -75° 0.842@17° 0.123@-58° 0.814@8.2° 0.680@-131.6° 0.529@132.1°

CG 45° 0.840@17° 0.123@62° 0.534@12.6° 0.815@-111.8° 0.676@108.9°

Example:

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Main Question

▪Depending on current limiter logic, I2 may depend on various quantities.

What quantities need to be included in the table

for adequate representation of I1 and I2?

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Suggested Table

▪ A table based on V1, V2, angle(V2/V1)

may be impractical; for 10 values of V1,

V2, angle(V2/V1) the table will haveto

have 1000 rows!

▪ One option is to generate the table only

based on V1 & V2 to have a more

reasonably sized table, but there will be

some error depending on the current

limiter logic implementation.

▪ A simulation case study suggested:

– ~20% error in amplitude of I1 and I2 (on

average)

– Up to 30° error in phase angle

angle(V2/V1)=0V1

(pu)V2

(pu)I1

(pu)Angle

(I1/V1)I2

(pu)Angle

(I2/V2)0.9 0.0 1.00 -10.52 0.00 0.00

0.1 1.04 -11.09 0.20 90.000.2 0.94 -12.29 0.40 90.000.3 0.78 -14.89 0.60 90.000.4 0.50 -23.66 0.80 90.000.5 0.18 -90.00 0.92 90.000.6 0.16 -90.00 0.94 90.000.7 0.14 -90.00 0.96 90.000.8 0.12 -90.00 0.98 90.000.9 0.11 -90.00 0.99 90.00

0.8 0.0 1.10 -21.36 0.00 0.000.1 1.01 -23.49 0.20 90.000.2 0.85 -27.96 0.40 90.000.3 0.61 -41.17 0.60 90.000.4 0.37 -90.00 0.73 90.000.5 0.32 -90.00 0.79 90.000.6 0.28 -90.00 0.82 90.000.7 0.25 -90.00 0.85 90.000.8 0.22 -90.00 0.88 90.000.9 0.20 -90.00 0.90 90.00

0.6 0.0 1.10 -46.68 0.00 0.000.1 0.92 -60.21 0.20 90.000.2 0.73 -90.00 0.37 90.000.3 0.63 -90.00 0.47 90.000.4 0.55 -90.00 0.55 90.000.5 0.49 -90.00 0.61 90.000.6 0.44 -90.00 0.66 90.000.7 0.40 -90.00 0.70 90.000.8 0.37 -90.00 0.73 90.000.9 0.34 -90.00 0.76 90.00

… … … … … …

Demonstrating result:

Consider an IBR using current limiter

logic based on dq limits. The table

needs to account for angle(V2/V1).

For simplicity, we represent the IBR

using only one table with V1&V2

assuming angle(V2/V1)=0°

Assume a fault with

V1 = 0.6 pu, V2 =0.5 pu

angle(V2/V1)=90°. The IBR

contributes

I1=0.69@-90° and I2=0.86@90°

(correct values).

However, the table gives

I1=0.49@-90° and I2=0.61@90°

(estimated table values) which

means ~40% error in the amplitude

of I1&I2.

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Another Suggestion: User-Defined IBR Short Circuit Model

▪ Given the complication of VCCS table, an option is to use

a User-Defined Model (UDM) of inverter in short circuit

programs

– I1 and I2 are calculated based on generic equations

representing IBR control and current limiter scheme.

– An example is the generic equation-based model proposed

by EPRI implemented within PSS®CAPE and ASPEN

OneLiner (v15).

▪ The UDM could be a black-box

DLL-based OEM model:

– The concept is similar

to OEM DLL inverter

models used in EMT tools

(PSCAD, EMTP, etc)

and stability tools

(PSS/E, PSLF, etc).

13

FRT Current Limiter & PQ Priority Neg. Seq. Current Control

Reference: Short-Circuit Phasor Models of Converter-Based Renewable Energy Resources for Fault Studies, EPRI, Palo Alto, CA: 2017. 3002010936.

EPRI proposed IBR

short circuit model

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Type III Wind Turbine Generator

▪ VCCS tabular model of Type III is not as

complex as Type IV; Separate +seq and -seq

tables could provide sufficient accuracy.

▪ Reason:

– Under traditional coupled control, the control scheme

of Rotor-Side Converter (RSC) & Grid Side

Converter (GSC) eliminates I2.

– I2 is only contributed by the stator and can be

estimated & represented using the -seq impedance

of the machine.

– In practice, a small coupling between I1 and I2 does

exist due to the negative sequence active power

flowing from RSC to rotor; however, this coupling

can be reasonably ignored.

– Compliance with the German code may require

additional I2 injection by GSC, in which case the

table will face the same complexity issue as Type IV.

I2 = V2 / Z2,machine

I2 = 0

V2

V1 (pu) I1 (pu)Angle(I1/V1)

(degrees)

1.00 1.00 -0.05

0.90 1.10 -10.41

0.80 1.04 -22.7

0.70 0.96 -38.77

0.60 0.87 -66.63

… … …

+Seq table -Seq table

V2 (pu) I2 (pu)Angle(I2/V2)

(degrees)

0.10 0.29 99.85

0.20 0.58 99.85

0.30 0.87 99.85

0.40 1.16 99.85

0.50 1.45 99.85

… … …

I2 = 0

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