The document discusses insulation coordination design details for HVDC converter stations. It provides definitions for various impulse withstand levels needed, including switching impulse withstand level (SIWL), lightning impulse withstand level (LIWL), and front of wave (FOW) impulse. It discusses the reasons for these different impulse levels and provides the design criteria. It also summarizes the different types of arresters used on the AC and DC sides of converter stations, providing their ratings and maximum voltages. Coordination is discussed between the AC line and station arresters to ensure adequate margins.

1. Insulation co-ordination design
details
S. Naresh Ram
NRLDC
POSOCO

2. Why we need Arrestors ?
The arresters are designed for the worst case of 3-phase ground fault followed by the recovery saturation over
voltages.
This fault case gives the highest energy duty for the arrester
• MCOV (Maximum Continuous Operating Voltage)
The MCOV for equipment on the ac side of the converter station is the highest r.m.s. value, including harmonics, of the
sinusoidal voltage applied at the terminals of the equipment.
• CCOV (Crest value of the Continuous Operating Voltage)
The CCOV for equipment on the dc side of the converter station, stressed by a voltage with or without commutation
overshoots, is the highest continuous crest value of the voltage excluding commutation overshoots.
Both designations MCOV and CCOV will be used in order to describe the arrester operating voltage.
Why we need SIWL ?
• Ans Load Rejection / Re-energization 1kA<Icharg<3kA; Convertor commutation overshoots during TOV conditions
The SIWL(Switching Impulse Withstand Level) shall, in any case, not be below 1050 kV for all AC-side equipment( Voltage
wave front 50/2500 micro sec; Current wave front 36/90 micro sec)
Why we need LIWL ?
Ans Since Switchyard shielding limits direct strikes <20kA; Back flashover on first tower outside switchyard
The LIWL(Lightning Impulse withstand level) for circuit breakers, bushings and other equipment shall not be below
1425 kV (Voltage wave front 1.2/50 micro se; Current wave front 8/20 micro sec)
Basic Definitions

3. • Reasons for Switching Impulse:-
Circuit breaker operation; Protective Switching Equipment Energisation; Load Rejections;
Convertor switching transients
Reasons for Lightning Impulse:-
• Direct lightning stroke to line - rare on shielded systems
• Back flashover - lightning strikes pylon shield wires
Reasons FOW (Front of Wave) Impulse
(Current wave front: 1/2 micro sec )
Bushing flashovers within valve hall, causing discharge of stray capacitances through relatively
short lengths
of conductor.
LIWL= 1.4 times SIPL (Protection Level)
= 1.2 times LIPL (Protection Level)
• The SIWL shall not be less than 0.83 times the LIWL, nor below 1050 kV for
equipment connected to the 400 kV AC-bus

4. AC side fault of PMU plot
R phase to Earth fault; A/R attempted
but Un successfull
Over shoot in other phases

5. Commutation failure PMU plot
Transient Over Voltage , Due
to HVDC restart attempts

6. AC and DC side Arrestors
Arresters on AC side are
usually specified by their rated
voltage and maximum
continuous operating voltage
Peak value of voltage between
phase conductor &
earth or between phase conductors having
highest system voltage peak.
Arrestors on DC side rated voltage is
not defined and continuous
operating voltage is defined
differently.
PCOV (Peak Continuous Operating Voltage)

7. Typical Voltage Wave shapes at
various Locations
Voltage 12 pulse
The starting point of the insulation co-
ordination process within the valve hall
is the rating of the valve surge arrester
(V) by balancing continuous energy
absorption against protective level.

8. Arrestors
discharging
currents
Ground fault between valve and HV-bushing of
converter transformer during rectifier operation.
In this case the valve arresters type 'V1' protecting the 3-
pulse commutation group on the HV side,
could be stressed with high currents and energies.
This is due to the fact that the DC- current
on the faulted pole will go to zero, which corresponds to
a load rejection.
During this time these arresters will discharge the DC-
line through the smoothing reactor and converter
transformer.
This fault will be cleared by tripping the ac circuit
breakersCCOV= Transformer secondary
voltage * 1.414
For HVDC 500kV 1000MW ;
Type of
Arrestors
Max. voltage
across arrester
(SIPL)
SIWL Max.
energy
dissipated
in
arrester
V1 491 kV at 1.2
kA
1.15*491 2.12MJ
V2 495 at 1.05 1.15*495 0.54MJ
The highest stresses are expected if the transferred
switching surge appears between the phases
(e.g. R & Y), where only one valve of the involved phases is
conducting
R
Y
B
Designing criteria of Valve hall Surge arrestor

9. Arrestors
discharging
currents
Simulated result of fault For
HVDC 500kV 1000MW ;
R
Y
B
Stresses of Valve Arrester "V1" during
Ground Fault at Transformer Bushing,
rectifier
operation
1.2KA
Around 2MJ
Around 500KV

10. AC Side Arrestors
AC System consists of parallel connected
circuits and apart from some special cases the
requirement is to establish the insulation level
bet. phase to earth and phase to phase level
• SIWL at least 1.15 times the SIPL
• LIWL at least 1.25 times the SIPL
• FWWL at least 1.25 times the FWPL
Converter transformer:-A,A2 ( Max energy ,
generally used at Bus; whose fault current heavy)
valve side of the Yy transformer windings:- A2
• Max. voltage across arrester „A“ = 610 kV at 2 kA
• Max. energy dissipated in arrester „A“ = 3.3 MJ
• AC Filter Banks:- AA (For slower front time
surges the arrester type 'AA' energy is sharedwith
the converter transformer arrester type ‘A’)
AC-filters:- F ac
DC Side Arrestors
HVDC converter stations on the other hand consist of
series connected bridges, each bridge requiring a
different insulation strength to earth and within each
bridge the electric strength is different for the
various components
• SIWL at least 1.20 times the SIPL.
• LIWL at least 1.25 times the SIPL.
• FWWL at least 1.25 times the FWPL
12-pulse valve group:- C
Dc smoothing reactor :- D
DC-line :- D
DC-filters:- Fdc
12-pulse group and on the neutral line :- E
Thyristor valves connected to HV-side of the smoothing
reactor:- V1
Other Thyristors:- V2
Different types of Arrestors for different application

11. Type of
Arrestors
MCOV (in KV) CCOV SIPL (KV at
KA)
LIPL (KV at
KA)
Energy (KJ)
A 440/ 3 - 610 @2 714@20 6000
AA
(Filter)
440/ 3
- 610 @1.3 1 714@13.3
A2( 500KV
HVDC)
Transformer
secondary
voltage YY
(valve side)
530 946@1 1065@5 3200
A3 (500KV
HVDC)
Transformer
secondary
voltage YD
(valve side)
308 527@1 595@4 1800
For 400kV AC Station
(1) The coordinating current for switching surges for the AA arrester, which has the same protective level as the A arrester
is chosen to 2kA x 2/3 » 1.3kA, because the AA arrester consists of only 2 columns per arrester housing

12. Coordination with AC Line Arresters
The characteristics of the surge arresters connected to the 400 kV ac lines
• Rated arrester voltage: 390 kV
• Min. Switching Surge residual voltage (at 1 kA): 730 kV
• Max. Switching Surge residual voltage (at 1 kA): 780 kV
• Max. residual voltage / lightning surge (at 10 kA): 900 kV
• Max. residual voltage / lightning surge (at 20 kA): 975 kV
Switching Surge Protective Level Type 'A'/'AA': 610 kV at 2/1.3 kA.
Assuming minimum arrester characteristic of the ac line arresters and maximum
characteristic of the type 'A'/'AA' arrester it can be concluded that a minimum margin
(730 kV - 610 kV) / 610 kV = 19.7 % exists.
This margin is fully adequate to avoid any overstressing of the parallel connected 400 kV ac
line arresters.
Typically, a margin of >10% can be assumed to be sufficient

13. 4m
3.5m
Minimum Air clearance; Phase to Ground=3.5m; Phase to phase= 4m.