Adil Abdalrahman, ABB Power Grids, HVDC, 2016-11-24

Experiences from SSTI-studies in

HVDC projects

© ABB Group
December 3, 2016 | Slide 1
HVDC2014-009991
Contents

§ General
§ Approach for SSTI Studies
§ SSTI Screening Study
§ SSTI Detailed Study
§ Examples from HVDC projects (SSTI and SSCI)
§ Ferroresonance

HVDC2014-009991
 § General
General § Approach for SSTI
Studies
§ Screening study
§ Detailed study
§ Examples from HVDC
projects (SSTI and
SSCI)
§ Ferroresonance

HVDC2014-009991
Steam Turbines

Steam turbine-generator shafts are

more susceptible to torsional interaction
due mainly to the fact that they have
several torsional frequencies in the
subsynchronous range.

HVDC2014-009991

|
Definition of SSO, SSR, SSTI
§ SSO – Subsynchronous Oscillation
Problems related to SSO:
§ SSR – Subsynchronous Resonance:
passive elements (series
compensated lines)
§ SSTI - Subsynchronous Torsional
Interaction: active elements (power
system controls e.g. HVDC,
SVC/STACOM, high speed governor,
PSS)
§ SSCI - Subsynchronous Control
Interaction: interaction between power
electronic control systems, e.g. of
HVDC or DFIG WTG, and series
compensated line. Purely electrical
phenomenon (not related to
mechanical shaft system)

HVDC2014-009991
SSTI with LCC HVDC

HVDC
Controls
α
Idc

Ig Vg

ω Rotating
HP-IP Exciter
Turbine LP Generator
Turbine

HVDC2014-009991
SSTI with VSC HVDC

HVDC2014-009991
Torsional stresses and fatigue

§ The increasing speed perturbations cause torsional

vibrations in the shaft structure of the generator

§ Torsional stresses and fatigue of the shaft when

the fatigue limit of the shaft material is exceeded.

Fatigue limit: limitng value of the cyclic stress to

which shaft
can be subjected such that practically no
cumulative fatigue damage.
Cycles: cycle to crack initiation

HVDC2014-009991
 Approach for § General

SSTI Studies § Approach for SSTI

Studies
§ Screening study
§ Detailed study
§ Examples from HVDC
projects (SSTI and
SSCI)
§ Ferroresonance

HVDC2014-009991
Approach for SSTI Studies

§ SSTI Screening Study based on UIF factor

§ Detailed SSTI Studies based on damping torque analysis
§ Design Subsynchronous Damping Controller (SSDC), if necessary
§Verification of SSDC by time domain simulation, verification in
Factory System Test and during commissioning

HVDC2014-009991
 § General
§ Approach for SSTI Studies
§ Screening study
SSTI Screening
§ Detailed study
Study
§ Examples from HVDC projects
(SSTI and SSCI)
§ Ferroresonance

HVDC2014-009991
Screening study
Unit interaction factor, UIF:

2
MW HVDC  SC 
UIF =  1 − Gout

MVA Gen  SC Gin 

MW HVDC = the rating of the HVDC system

MVAGen = the rating of the generator under study
SCGin= the system short circuit capacity at the HVDC commutating bus with the generator
under study being in service
SCGout = the system short circuit capacity at the HVDC commutating bus without the
generator under study being in service

§Low possibility of interaction if UIF < 0.1

§For a purely radial case SCGout is zero and thus UIF is equal to 1 times the ratio of HVDC to
generator rating (e.g. when the first generator has just been synchronized during Black Start
of an AC grid via HVDC-Light link)=>UIF=1 if the generator is of the same size as the HVDC

HVDC2014-009991
Screening Study, example of scenario

HVDC2014-009991
Screening Study, typical result data

Contingency
Machine
Level Outage ID UIF

N-0 0.0081
Line 1
N-1 1 0.0092
Unit 1 Line 2
N-2 2 0.0875
Line 3,
N-3 1 0.2274
N-0 0.0000
Line 1
N-1 1 0.0000
Line 2
N-2 2 0.0002
Line 3
N-3 2 0.0013
Line 4
N-4 1 0.0999
Unit2 Line 5
N-5 1 0.2804
Line 6
N-5 1 0.2804
Line 7
N-5 1 0.2804
Line 8
N-5 1 0.2804
Line 9
N-5 1 0.2279
N-0 0.0000
Line 1
N-1 1 0.0000
Line 2
N-2 2 0.0002
Line 3
N-3 2 0.0012
Line 4
N-4 1 0.0955
Unit 3 Line 5
N-5 1 0.2682
Line 6
N-5 1 0.2682
Line 7
N-5 1 0.2682
Line 8
N-5 1 0.2682
Line 9
N-5 1 0.2270

HVDC2014-009991
 § General
§ Approach for SSTI Studies
§ Screening study
Detailed SSTI
§ Detailed study
Study § Examples from HVDC projects
(SSTI and SSCI)
§ Ferroresonance

HVDC2014-009991
Damping torque analysis: total (net) damping
§Impact of the turbine control loop (governor) is neglected, hence
∆ =0 ∆
= = − ∆
§Thissystem is asymptotically stable as long as the Nyquist ∆
∆
curve for the open-loop frequency function ( ) ( ) does = ( ) = { }
not encircle the point -1 ∆
For stable operation: + ≥0
§Mechanical dynamics are passive for all frequencies:
Re ( ) = ≥0 − 90 ≤arg ( ) ≤ 90 ≥0 ( ℎ )
§A sufficient criterion for stability is that the electrical dynamics
also are passive: −180 ≤arg ( ) ( ) ≤ 180, which
implies that the Nyquist curve cannot encircle -1
§Thus,it’s desirable that Re ( ) = ≥ 0 for all all
frequencies

∆Tm Rotor Mechanical

§ The mechanical damping
positive and can compensate a
is
System, Gm ∆ω
negative electrical damping in a
certain frequency range
+
_
§D is the electrical damping and may
not be positive for all frequencies.
§Stabilitycan be guaranteed if ≥0 Electrical Model of
at and in the neighbourhood of the
Machine and
torsional frequencies irrespective of
of the mechanical damping.
∆Te Transmission System
(including HVDC),
HVDC2014-009991
Ge
Mechanical damping

§ Low at low torsional frequencies and high toward

synchronous frequency
Sources
§ Dissipation forces of windage
§ Bearing friction
§ Material hysteresis damping
§ Steam/gas/water forces on the turbine blades
§ Steam damping increases with load
§ Gas damping is a more complicated function

HVDC2014-009991
Strategy for SSTI study using damping torque analysis

§ Mechanical damping is independent of the network

conditions
§ If the level of electrical damping with HVDC is as good as
or better than the electrical damping without HVDC, then
torsional stability is assured
§ Add SSDC, if necessary

HVDC2014-009991
Small signal analysis: Model for calculation of electrical
damping
 ∆Te 
De = Re 
 ∆ω 
Electrical Model of
Machine and
Transmission System
(including HVDC)

_ Te + ∆Te
δ
Rotor Mechanicalω + ∆ ω ωo
System s
+

Obtaining an analytical expression for transfer function of De is complicated as it shall include all dynamics from
the generator, a.c. network and HVDC as well as its feedback control system. Therefore, De computation is
performed using a time-domain simulation with programs such as PSCAD where the complete system with the
generator, a.c. network and HVDC is modelled.

HVDC2014-009991
SSDC

§ SSDC integrated with the HVDC control

§ Controls may be located far away from the machines
§ Transfer function: typically HP + lead/lag + LP and a
negative gain
§ Input: Typically frequency of the ac commutating
voltage/rotor speed (LCC) or Upcc (VSC)
§ Output: modulation of the control angle, α (in LCC)
: modulation of the voltage reference (in VSC)

HVDC2014-009991
Typical for tubine-generator model

§Required for multi-mass

model

HVDC2014-009991
 § General
§ Approach for SSTI Studies
§ Screening study
Examples from
§ Detailed study
HVDC projects
§ Examples from HVDC projects
(SSTI and SCCI) (SSTI and SCCI)
§ Ferroresonance

HVDC2014-009991
Example 1

220 MW LCC HVDC Link

§ 800 MVA generator unit, radially

Electrical Damping
1
 ∆Te Reference
 C1: withCase without HVDC connected to the HVDC link follwing
De = Re 
Case HVDC

 ∆ ω 
N-3 contingency (UIF=0.23), rectifier
0.5 side
Negative damping around the
De (pu/pu)

0
torsional frequency 19.6 Hz
§ Subsynchronous damping controller
(SSDC) shall be be added.
-0.5
5 10 15 20 25 30 35 40 45 50
Freq (Hz)

HVDC2014-009991
Example 1: SSDC action

Electrical Damping
1
Reference Case without HVDC
Case C1: HVDC Without SSDC
Case C1: HVDC with SSDC

0.5
De (pu/pu)

-0.5
5 10 15 20 25 30 35 40 45 50
Freq (Hz)

§SSDC increases the electrical damping by adding a

contribution to the firing angle α

HVDC2014-009991
 Example 1: SSTI time domain verification

SSDC Reg: OFF ON

HVDC2014-009991
Example 2

700 MW VSC HVDC transmission link

§ 375 MVA thermal generator radially
connected to the HVDC link during
black start (active power is approx. 0.53
pu) e.g. when the black starting
converter is in Frequency& Voltage
control
§ The subsynchronous torsional
frequencies for the unit are; 16.4Hz,
24.9Hz and 33.2Hz.
§SSDC was needed in order to
maintain the necessary electrical
Electrical damping for 375 MVA generator, black starting damping margin for frequencies above
converter operates as rectifier
approximately 25Hz

HVDC2014-009991
Example 2

Electrical damping verification by time domain simulation

Electrical Damping Verification in Time Domain Electrical Damping Verification in Time Domain
for the torsional frequency 16.4 Hz without and for the torsional frequency 24.9 without and with
with the SSDC activated. the SSDC activated.

HVDC2014-009991
Example 2
Electrical damping verification by time domain simulation

Electrical Damping Verification in Time Domain for the torsional

frequency 33.2 Hz without and with the SSDC activated.

HVDC2014-009991
Fenno-Skan 2

§ 800 MW and 500kV HVDC interconnector between Finland (Rauma) and

Sweden (Dannebo). Total length of DC OHL+cableof 303 km, Pole 2 of
Fenno-Skan HVDC transimission link.

§ Converter stations are located near Nuclear power

plants Olkiluoto in Finland and Forsmark in Sweden.
§ During FST SSTI the electrical damping was verified
for Olkiluoto 1/2-3 and Forsmark 1/2-3 generator
units using one mass model models (frequency
domain) and multi-mass models (time domain)=>
quite challenging FST set-up due to the complexity of
Generator rated power: Forsmark 1 (990 MW),
machine models, total four turbine generators. Forsmark 2 (1,120 MW) and Forsmark3 (1,170 MW)
Olkiluoto 1 (860 MW), Olkiluoto 2 (860 MW).
§ SSTI also verified during system tests at site

Ref: New Fenno-Skan 2 HVDC pole with an upgrade of the existing Fenno-Skan 1 pole. Cigre 2012. Available [online]:
https://library.e.abb.com/public/a0f7529d389aef26c1257a86002783d6/New%20Fenno-Skan%202%20HVDC%20pole%20with%20an%20upgrade%20of%20the%20existing%20Fenno-Skan%201%20.p
HVDC2014-009991
Fenno-Skan 2 (SSTI verification at site)

§ Two tests were performed at site for SSTI verification in both

sides:
§ Step in current order, 0,1 pu
§ Simulated DC-line faults, with re-start of the DC-link. DC fault
executed close to nominal power level, starting with tests at
minimum and 50% power. The DC-line faults at high power were
chosen in order to get enough power changes in the network to
initiate SSTI
§ Oscillations at the torsional mode frequencies were measured at
the converter stations by the SSTI supervision based on deviation
in period time of the AC-voltage

Ref: New Fenno-Skan 2 HVDC pole with an upgrade of the existing Fenno-Skan 1 pole. Cigre 2012. Available
[online]:
https://library.e.abb.com/public/a0f7529d389aef26c1257a86002783d6/New%20Fenno-Skan%202%20HVDC%20pole%20with%20an%20upgrade%20of%20the%20existing%20Fenno-Skan%201%20.p

HVDC2014-009991
Fenno-Skan 2 (SSTI verification at site)

§ Disturbance caused by the tests was to moderate to initiate any

significant SSTI since:
§ Network configurations during the system tests were fairly
strong, and the corresponding SSDC was activated
§ The damping of the shaft contains both a mechanical part and
an electrical part, where the mechanical is the most important
component, and the surrounding network configuration
contributes to the electrical part
§ Conclusion from the tests:
§ The link with its SSTI regulator did not deteriorate the damping
of the torsional frequencies.

Ref: New Fenno-Skan 2 HVDC pole with an upgrade of the existing Fenno-Skan 1 pole. Cigre 2012. Available [online]:
https://library.e.abb.com/public/a0f7529d389aef26c1257a86002783d6/New%20Fenno-
Skan%202%20HVDC%20pole%20with%20an%20upgrade%20of%20the%20existing%20Fenno-Skan%201%20.pdf

HVDC2014-009991
Example 4 (SSCI)

350 MW Back- to- Back LCC link

A B

Two main issues:

§ Sub-Synchronous Control Interaction (SSCI) can occur between series
compensated lines and the converter control system.
§ Ferroresonance oscillations can occur between series compensated
lines and the saturable magnetic core of a converter transformer.
Transformer saturation occurs due to voltage recovery after a.c. faults
are cleared (Pseudo-inrush)=>inrush currents rich in harmonic content
produce harmonic voltage amplification through the resonant
impedance of the system.
HVDC2014-009991
Example 4 (SSCI assessment)

§ Control instability due to SSCI occurs when the a.c. network

resonance frequency matches with converter resonance frequency
§ Assessment approach:
§ Impedance scanning of AC network (including AC filters) for
frequency sweep from 1 Hz to 65 Hz. Impedance was measured on
the converter bus (bus A in the diagram) for different AC network
configurations.
§ Driving point impedance of HVDC system as a function of sub-
synchronous frequency was measured at the converter bus on side,
both for export and import scenarios.
§Time domain simulation: AC faults and step changes in active
power and dc current

HVDC2014-009991
Example 4 (SSCI assessment)

§ AC network configurations likely to happen in reality:

§ Zero Capacitor Bypassed
§ One Capacitor Bypassed in One Double Circuit
§ One line out in one double circuit

HVDC2014-009991
Example 4 (SSCI assessment)

Results: AC network impedance

Zero Capacitors Bypassed

One Capacitor Bypassed in One Double Circuit

Series resonance when the system

reactance changes from negative to
positive. The resonance frequencies for
the three configurations are 30Hz,
34.5Hz and 35.2Hz respectively
One series compensation line out
HVDC2014-009991
Example 4 (SSCI assessment)

Results: HVDC system impedance (rectifier)

Normal Power Direction (from A to B), 350 MW Normal Power direction (from A to B), 435 MW

Normal Power Direction (from A to B), 35 MW

HVDC2014-009991
Example 4 (SSCI assessment)
Results: HVDC system impedance (inverter)

Reverse Power Direction (from B to A), 350 MW Reverse Power Direction (from B to A), 435 MW

Reverse Power Direction (from B to A), 35 MW

HVDC2014-009991
Example 4 (SSCI assessment)
Results: 3Ph-G. 100ms, <10% rem.Volt. Bus of series compensated lines.
(435 MW from A to B, zero cap bypassed).
1000 500
Econv_S1P1:1 [kV]
Econv_S1P1:2 [kV]
Econv_S1P1:3 [kV]

Econv_S2P1:1 [kV]
Econv_S2P1:2 [kV]
Econv_S2P1:3 [kV]
0 0

-1000 -500
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
2 2
P_S1P1 [pu]

P_S2P1 [pu]
1 1

0 0

-1 -1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
2 2
UD_P1 [pu]

UD_P1 [pu]
1 1

0 0

-1 -1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
4 4
IORD_LIM_S1P1

IORD_LIM_S2P1
ID_P1 [pu]

ID_P1 [pu]
2 2

0 0

-2 -2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
ALPHA_MEAS_S1P1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

ALPHA_MEAS_S2P1
ALPHA_ORD_S1P1

ALPHA_ORD_S2P1
100 150

50 100

0 50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
150
GAMMA_S1P1

150 GAMMA_S2P1
100
100
50
50
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time [s] Time [s]

rectifier inverter
HVDC2014-009991
 § General
§ Approach for SSTI Studies
§ Screening study
Ferroresonance
§ Detailed study
§ Examples from HVDC projects
(SSTI and SCCI)
§ Ferroresonance

HVDC2014-009991
Ferroresonance

§ An oscillating phenomenon between a non-linear inductance and a

capacitor.
§ Elements needed for a circuit to exhibit Ferroresonance:
§ A non-linear inductance e.g. transformer magnetic core
§ A Capacitance:
§ Voltage grading capacitors in HV circuit breakers
§ Conductor interphase capacitance
§ Capacitance to ground of cables or long lines
§ Series capacitor or shunt capacitor banks
§ A voltage or a current source
§ Low losses (less damping)
§ It is not so easy to predict the occurrence of ferroresonance.
§ Ferroresonance results in high voltages and currents, however, the
waveforms are usually irregular in shape.

Introduction
HVDC2014-009991
 Study System

HVDC back-to-back System

BUS 1 (400 kV) BUS 2 (400 kV)
Power flow

Filter Banks Filter Banks

System Parameters AC System 1 AC System 2
Pole I Pole I
Pole I (P1) AC Voltage 400kV 400 kV
DC Voltage 200 kV
AC System 1 AC System 2 DC Power Pole I rated for 500 MW
Pole II rated for 500 MW
Zs Pole II (P2) Zs Short Circuit MVAmin 1 200 MVA 2 500 MVA
Series compensated line Short Circuit MVAmax 3 000 MVA 8 000 MVA

Filter Banks Filter Banks

Pole II Pole II

§ Each pole has two 6-pulse converters connected in series on rectifier and inverter side.
§ At the rectifier side, both the poles are connected by 400 kV double-circuit transmission
lines (nearly 225 km in length) having 50% series compensation.

System parameters
HVDC2014-009991
Fault at Rectifier Side AC Bus
For 3-phase to ground fault
§(a) Line to neutral ac voltage
500
U -A C 1
U -A C 2
U -A C 3

0 100
§(a) Rectifier side α in degrees
-500

A LP H A- P 1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 50
1
§(b) Dc power (p.u.)
P D _ P 1 [p u ]

-1 0
0.5 1 1.5 2
-2 Time [s]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
150 15

S pe ctra of ALPHA - P1
§(c) Rectifier side α in degrees
A L P H A-P 1

100 §(b) FFT of α shown in plot ‘a’

10
50 FFT
0
5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
80 §(d) Inverter side ɣ in degrees
G A M M A- P 1

0
60 20 30 40 50 60 70 80
Frequency [Hz]
40

20 1.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time [s] 1

ID_P 1 [pu]
§(a) Dc current through Pole I
0.5

-0.5
0.5 1 1.5 2
Time [s]
0.3
S pectra of ID_P 1 [pu]

§ Post fault recovery is not stable 0.2

§(b) FFT of dc current shown in plot
‘a’
§ Observed 42 Hz frequency component on dc side 0.1

0
20 30 40 50 60 70 80 90 100
§© ABB Group Frequency [Hz]
December 3, 2016 | Slide
§HVDC2014-009991 43
Fault at Rectifier Side AC Bus
For 3-phase to ground fault

File: 3phg_Rect_WOctrl
500

U -A C 1
U -A C 2
U -A C 3
-500
§(a) Rectifier side ac bus voltage
§(a) Line to neutral ac
500 -1000
voltage 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7
U -A C 1
U -A C 2
U -A C 3

0 §FFT Time [s]

-500 400

S pe ctra o f U -A C 1
S pe ctra o f U -A C 2
S pe ctra o f U -A C 3
§(b) FFT of ac voltage shown in plot
300
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ‘a’
1
§(b) Dc power (p.u.)
P D _ P 1 [p u ]

200
0
100
-1
0
-2 0 10 20 30 40 50 60
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency [Hz]
150
§(c) Rectifier side α in degrees
A L P H A -P 1

100
50
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
80
§(d) Inverter side ɣ in degrees
G A M M A- P 1

60

40

20
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time [s]
§ Post fault recovery is not stable

§ Observed 42 Hz frequency component on dc side

§ Observed 8 Hz frequency component on ac side

§© ABB Group
December 3, 2016 | Slide
§HVDC2014-009991 44
Ferroresonance

§ Possible options to mitigate ferroresonance

§ Bypass the series capacitor upon detection of ferroresonance
§ It limits the power transfer capability of the system
§ It is not a complete solution just a remedy
§ To install a reactor across the series capacitor
§ Extra cost
ü Best way is to have a supplementary control to damp the ferroresonance.
ü No extra cost for the additional equipment
ü Fast control

Possible solutions
HVDC2014-009991
Ferroresonance Damping Controller (FDC)
Ferroresonance damping controller principle

Ferroresonance Damping
Controller (FDC)

Idc αmod
αcontrol
HVDC Control ∑ αorder

§ The output of the controller is αorder which is generated by adding the

alpha modulation signal αmod from the FDC to αcontrol calculated by the
existing HVDC current control

§© ABB Group
December 3, 2016 | Slide
§HVDC2014-009991 46
 Performance of Damping Controller
Three phase to ground fault at rectifier side

§(a)Line to neutral ac §(a)Line to neutral ac

500 voltage 500 voltage
U -A C 1
U -A C 2
U -A C 3

U -A C 1
U -A C 2
U -A C 3
0 0
-500 -500

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
1 1
§(b) Dc power (p.u.) §(b) Dc power (p.u.)
P D _ P 1 [p u ]

P D _ P 1 [p u ]
0 0

-1 -1

-2 -2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
150 150
§(c) Rectifier side α in degrees §(c) Rectifier side α in degrees
A LP H A-P 1

A L P H A-P 1
100 100
50 50
0 0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
80 80
§(d) Inverter side ɣ in degrees §(d) Inverter side ɣ in degrees
G A M M A- P 1

G A M M A- P 1
60 60

40 40

20 20
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time [s] Time [s]

Plots for Pole I without damping controller Plots for Pole I with damping controller
§© ABB Group
December 3, 2016 | Slide
§HVDC2014-009991 47
Performance of Damping Controller
Three phase to ground fault at rectifier side

S 1: ID _P 1 [ pu]
S 2: ID _P 1 [ pu]
0
§(a) Dc current through Pole I
-2
1.5 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65

S pe ctra of S1 :ID _P 1 [pu]

S pe ctra of S2 :ID _P 1 [pu]
§(a) Dc current through Pole I Time [s]
S1:ID_P1 [pu]
S2:ID_P1 [pu]

0.5 §(b) FFT of dc current shown in plot ‘a’

0.2
0
0.1
-0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0
150
§(b) Rectifier side α in degrees 20 30 40 50 60 70 80 90 100
100 Frequency [Hz]
ALPHA- P1
ALPHA P1
-

50
100
0
§(a) Rectifier side α in degrees

ALPHA--WO--CNTRL
ALPHA W CNTRL
80
-50
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
20 60

--
§(c) Modulation signal from the damping controller
10

--
40
Alpha-Mod

0
20
-10
-20 0
0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65
-30 Time [s]

Spectra of ALPHA--WO --CNTRL

Spectra of ALPHA W CNTRL
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 15
Time [s] §(b) FFT of α shown in plot ‘a’

--
10

--
5

§© ABB Group
0
December 3, 2016 | Slide
§HVDC2014-009991 48 20 30 40 50 60 70 80 90 100
Frequency [Hz]