Metal Oxide Surge Arresters

Metal Oxide Surge Arresters Prof. Dr.-Ing. Volker Hinrichsen Darmstadt University of Technology High Voltage Laboratories [email protected]

Contents • Arre Arrest ster er ap appl plic icat atio ionn in in gen gener eral al • cons considera iderations tions on prot protectiv ective e chara characteri cteristics stics • Ar Arre rest ster er des desig ignn (stat (statio ionn arre arrest ster ers) s) • por orce cela lain in ho hous used ed • polymer ho housed • Con onffig igur urin ingg arre arresster erss • ele lect ctrric ical al dat ataa • mec echa hannic ical al dat ataa

Contents • Arre Arrest ster er ap appl plic icat atio ionn in in gen gener eral al • cons considera iderations tions on prot protectiv ective e chara characteri cteristics stics • Ar Arre rest ster er des desig ignn (stat (statio ionn arre arrest ster ers) s) • por orce cela lain in ho hous used ed • polymer ho housed • Con onffig igur urin ingg arre arresster erss • ele lect ctrric ical al dat ataa • mec echa hannic ical al dat ataa

Development of Surge Arresters over the past 25 years Internally gapped SiC arresters with porcelain housings 1980

MO arresters without gaps with porcelain housings Æ "state of the art" latest by 1990 MO arresters without gaps with polymeric housings (mv; distribution class)

1990

MO arresters without gaps with polymeric housings (hv; station class)

Technology

2000

Development of Surge Arresters over the past 25 years

1980

Failure Failure Failu re rates rates of of MO MO arresters: arresters: arrest ers: Distribution: 0.1 %/a %/a ... ... 11 %/a %/a (with Distribution: 0.1 (with geographical geographical variations) variations) High-voltage: virtually zero zero High-voltage: virtually Expected Expected Expec ted life life time life time of time of MO MO arresters: arresters: arrest ers: >> 30 30 years? years?

(no (no indication indication for for any any severe severe degradation degradation of of MO MO material material so so far) far) 1990

2000 2003

Market Market share share of of polymer polymerr housed polyme housedMO MO arresters: arresters: arresters:

… 80 % %… … >> 90 90 % % Distribution: 80 n Distribution: o Reason Rea sons: s: par poo pe rmance nce pporc elain in hhous ed ttype ss i Reason Rea sons: s: -- partly partly partly tly poor poorrr perfo poo performa perfo rforma rmance nce of of porc porcela orcela elain in hous housed oused ed type types ypes t a -- benefits benefi ts of polyme ric designs: desig ns: sealing, seal hand benefits benef its of polymeric polymeric polymeric designs: designs: sealing, sealing, ing, handling, handling, handling, ling, u t i overload overload performance performance s s -- cost/price! cost/price! ' y a High-voltage: 30 %, with increasing tendency d Reaso eason ns: - hv user userss mor more e con conse serv rva ative tive o T - higher higher requir requireme ements nts - cost/price!

Typical Arrester Application: Transformer Protection U s = 420 kV

Siemens / VEAG

Special Arrester Application: Protection of an SC Capacitor Bank U s = 550 kV

Special Arrester Application: Line Arresters U s = 245 kV

U s = 800 kV

ABB / AEP

Special Arrester Application: HVDC Valve Protection Arresters U DC = 600 kV

Arrester Application

Fundamentals of Insulation Coordination 5 . u . p / 4 e g a t l o v 3 ) r e v o ( f o 2 e d u t i n g a 1 M

0

Possible voltages without arresters

Withstand voltage of equipment

Voltages limited by arresters Lightning overvoltages (Microseconds)

Switching overvoltages (Milliseconds)

Temporary overvoltages Highest voltage of equipment (Seconds) (Continuously)

Time duration of (over-)voltage

Voltage-Current Characteristic of an MO Arrester ( U s = 420 kV) α ≤

5

I = k ·U α with

α values

α ≤

up to 50

5

1200 1100 1000

V k / e g a t l o v f o e u l a v k a e P

10-kA residual voltage = lightning impulse protection level = 823 kV

900 800 700 600 500 400

4 . 2 r o t c a F

Peak value of rated voltage: √2·U r = √2·336 kV = 475 kV

Peak value of continuous operating voltage: √2·U c = √2·268 kV = 379 kV

300

Peak value of line-to-earth voltage: √2·U s /√3 = √2·242 kV = 343 kV

200 100

Leakage current î res ≈ 100 µA

Nominal discharge current I n = 10 kA

0 10-4

10-2

1 Peak value of current / A

8 decades of magnitude

10 2

10 4

Voltage-Current Characteristic of an MO Arrester ( U s = 420 kV) 400

1,00

300 200

0,50

V 100 k / e 0 g a t l o V-100

0,25

.

Simplified circuit diagram

0,75

Voltage

0,00 -0,25 Current

-200

-0,75

-400

-1,00 0

C V k / e g a t l o V

at U U = U U cc: I I total ≈ 1 mA total ≈

-0,50

-300

R = f(u)

A m / t n e r r u C

5

10 Time / ms

900 800 700 600 500 400 300

15

20

18 16 14 12 10 8 6

Voltage

Current

200 100

4 2

0 -100

0 -2 0

5

10

15

20

25

30

35

A k / t n e r r u C

at î î = I I nn: ûû ≈≈ 825 kV

Voltage-Current Characteristic of an MO Arrester Voltage

Simplified circuit diagram

R = f(u)

C

“Resistive component“

t n e r r u C , e g a t l o V

Total leakage current Time

at U U = U U cc: I I total ≈ 1 mA total ≈ I I capacitive ≈ 1 mA capacitive ≈ î î resistive ≈ 10 µA … 100 µA resistive ≈

MO Resistors Ø 70 mm

Ø 100 mm Ø 58 mm

Ø 48 mm

Ø 78 mm

Ø 41 mm

Ø 32 mm

U -I -

vs. E -J -Characteristics

U U --I I --characteristics characteristics for for different different MO MO resistors resistors

common common E characteristics E --J J --characteristics

LI Protection Characteristics 2 T r av eling w a

v e ef f ec ts

4 m

3

Inductivity of current path ≈1 µH/m (here: L = 10 µH)

1 Currents exceeding I n

Doubling of voltage due to full reflection at "open" end of line

!!!!! •• Voltage Voltageatatarrester arresterterminal terminal might higherthan thanthe theLI LI mightbe behigher protection protectionlevel level

m 5 . 3

•• Voltage Voltageatatterminals terminalsof of equipment equipmenttotobe beprotected protectedare are higher higherthan thanvoltage voltageat atthe the arrester arresterterminal terminal

m 5 . 2

Protective Distance – Model Calculation 1 (U m = 420 kV) Overvoltage surge of s = 800 kV/ µ s

Arrester u pl = 800 kV = const. x = 0

Transformer LIW = 1425 kV ℓ ?==

300 m x =

ℓ

Protective Distance – Model Calculation 1 (U m = 420 kV) t t = = 00 µs µs

2000 1600

1600

u Arr (x = 0)

kV 1200

1200 800

800 400

400

0 0

-400

0 kV

-1200 x = 0

uu Arr = 0 kV Arr 0 kV

x = ℓ

1200 800 400

x = ℓℓ:: x =

1

1600

-800

x = 0: 0: x =

0,5

uuTr = 0 kV Tr = 0 kV 0

u Tr (x = ℓ)

1,5

2 µs

2,5

Protective Distance – Model Calculation 1 (U m = 420 kV) t t = = 0,5 0,5 µs µs

2000 1600

1600

u Arr (x = 0)

kV 1200

1200 800

800 400

400

u 1v

0 0

-400

0 kV

-1200 x = 0

uu Arr = u 1v = 400 400 kV kV Arr u1v

x = ℓ

1200 800 400

x = ℓℓ:: x =

1

1600

-800

x = 0: 0: x =

0,5

uuTr = u 1v == 00 kV kV Tr = u1v 0

u Tr (x = ℓ)

1,5

2 µs

2,5


2000 1600

1600

u Arr (x = 0)

kV 1200

1200 800

800

u 1v

400

400

0 0

-400

0 kV

-1200 x = 0

uu Arr = u 1v = 800 800 kV kV Arr u1v

x = ℓ

1200 800 400

x = ℓℓ:: x =

1

1600

-800

x = 0: 0: x =

0,5

uuTr = u 1v == 00 kV kV Tr = u1v 0

u Tr (x = ℓ)

1,5

2 µs

2,5


2000 1600

1600

u Arr (x = 0)

kV 1200

1200

u 1v

800 400

800 400

u 1r

0

u 2v

0

-400

0

0,5

1

1,5

1600

-800

kV

-1200 x = 0

uu Arr = u 1v + uu2v = Arr u1v 2v (1200 – V == 800 800 kV kV (1200 – 400) 400) kkV x = 0: 0: x =

x = ℓ

1200 800 400

uuTr = u 1v ++ uu1r = Tr = u1v 1r (400 800 kV kV (400 ++ 400) 400) kV kV == 800 x = ℓℓ:: x =

0

u Tr (x = ℓ) Increase at double steepness!

2 µs

2,5


2000 1600

1600

u Arr (x = 0)

kV 1200

u 1v

1200

800

800

u 1r

400

400

0 0

-400

0

u 2v

-800

kV

uu Arr = u 1v + uu2v = Arr u1v 2v (1600 – V == 800 800 kV kV (1600 – 800) 800) kkV x = 0: 0: x =

x = ℓ

1200 800 400

uuTr = u 1v ++ uu1r = Tr = u1v 1r (800 1600 kV kV (800 ++ 800) 800) kV kV == 1600

1

1600

-1200 x = 0

0,5

x = ℓℓ:: x =

0

u Tr (x = ℓ)

1,5

2 µs

2,5


2000 1600

u 1v

1600

u Arr (x = 0)

kV 1200

1200 u 1r

800

800

400

400

0 u 3v

-400 -800

u 2r

0 0 kV

-1200 x = ℓ

uu Arr = u 1v + uu1r + u 2v + uu3v = Arr u1v 1r + u2v 3v (2000 V == 800 800 kV kV (2000 ++ 400 – 400 – 1200 – 1200 – 400) 400)kkV x = 0: 0: x =

1200 800 400

x = ℓℓ:: x =

1

1600

u 2v

x = 0

0,5

uuTr = u 1v ++ uu1r + u 2v + uu2r = Tr = u1v 1r u2v 2r

(1200 1600 kV kV (1200 ++ 1200 – 1200 – 400 – 400 – 400) 400) kV kV == 1600

0

u Tr (x = ℓ)

1,5

2 µs

2,5

Protective Distance – Model Calculation 2 (U m = 24 kV) Assumptions: • overvoltage surge as a voltage ramp 1 000 kV/µs (1 kV/ns) • arrester limits voltage to 80 kV at its terminals

100

100 Voltage at transformer

90

] V k [ u

80

80

70

70

60 50

Voltage at transformer

90

Voltage at arrester

40

] V k [ u

60 50 40

30

30

20

20

10

10

0

Voltage at arrester

0 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 t [ns]

a) Distance arrester - transformer: 1.5 m (propagation time 5 ns)

0

10 20

30 40

50 60

70 80

90 100 110 120

t [ns]

b) Distance arrester - transformer: 3 m (propagation time 10 ns)

Protective Distance – Estimation (Rule of Thumb) Due to traveling wave effects on the line the protection of the equipment by an arrester can be guaranteed only for short distances between arrester and equipment. Simplified estimation of the protective distance *):

x s

LIWV U pl

(LIWV / 1.15) - U pl x s = · v tw 2·s

s v tw

*) For more detailed information see IEC 60099-5, IEC 60071-1 and IEC 60071-2

protective distance [m] standard rated lightning impulse withstand voltage [kV] LI protection level of the arrester [kV] front steepness of the overvoltage [kV/µs] (in the range of 1000 kV/µs) propagation speed of traveling wave: - 300 m/µs (overhead line) (equals " c 0") - (150 ... 200) m/µs (cable)

Example 1: Distribution network, U m = 24 kV, insulated neutral, arrester of U r = 30 kV: x s =

(125 / 1.15) - 80 2·1000

· 300 =

4.3 m

!!!

Example 2: Transmission network, U m = 420 kV, effectively earthed, arrester of U r = 336 kV: x s =

(1425 / 1.15) - 823 2·1000

· 300 =

62.4 m !!!

Representative Overvoltage (acc. to IEC 60071-2) U rp

= U pl +

adopted return rate 1/a shielding failure rate + back flashover rate 1/a ⋅m

A

L

Lt =

n ( Lsp

+ Lt )

Lsp ... span length in m L ... distances a1 + a2 + a3 + a4 in m n ... number of connected lines A ... factor describing the lightning performance

of the OHL in kV (see next slide)

Note: Note: nn should reasonably be set to n = 11 (if only one one line is = 22 (if two two is connected) connected) or or n n = n = or or more more lines are connected). Assuming nn > 2 could yield too optimistic results that are

not valid in a real failure scenario (e.g. possible loss of lines).

Representative Overvoltage (acc. to IEC 60071-2) Factor A describing the lightning performance of an OHL

[IEC 60071-2]

Representative Overvoltage (acc. to IEC 60071-2) Example: U s = 420 kV

• • • • • • •

U pl = 825 kV; A = 11000 kV (four conductor bundle) L = 30 m Lsp = 400 m

2 lines connected; Shielding failure rate (typ. for Germany; one OHGW): 2.5 per 100 km and year = 2.5·10-5 (a·m)-1 Adopted return rate: 1·10-3 a-1 ≥

= 40 m 2.5 ⋅10−5 A L 11000 kV 30 m U rp = U pl + = 825 kV + ⋅ 2 (400+40) m n Lsp + Lt

Lt

Note Note 1: 1:

=

1 ⋅ 10−3

These equations yield representative representative overvoltages, overvoltages, which are not implicitly implicitly the the real real overvoltages! are not

= 1200 kV Note Note 2: 2: No No effect effect of the the lightning overvoltage amplitude amplitude!

Increase of Protection Voltage by Inductive Voltage Drops Example: outdoor arrester U s = 420 kV U r = 336 kV

4 m

u10kA, 8/20 µs = 823 kV (= U pl) u10kA, 1/2 µs = 872 kV m 5 , 3

Specific inductance of surge current path ≈ 1 µH/m Length of surge current path ≈ 10 m

⇒ Inductance of surge current path ≈ 10 µH m 5 , 2

Steepness of surge current impulse ≈ 10 kA/µs

⇒ Additional inductive voltage drop 100 kV

Arrester Design

Examples of High-Voltage Arresters

Grading Rings – Corona Rings Corona rings

• Beginning with a height of about 1.5 m to 2 m arresters need grading rings for control of voltage distribution along the arrester axis. • Corona rings serve to reduce RIV, usually applied in system voltages of 550 kV and higher.

Grading rings

Examples of Medium-Voltage Arresters

Design of a Porcelain Housed High-Voltage Arrester

Pressure relief vent

O-ring Sulfur cement bonding

Pressure relief diaphragm

MO column

Compression spring Supporting rod (FRP) Fixing plate (FRP) Porcelain housing Aluminum flange

Basic Designs of Polymer Housed High-Voltage Arresters

Porcelain/Type A MO column

Type B1a

Gas

Type B1b

FRP supporting structure

Type B2

Type A "Tube Design" Type Type A A

"tube "tube design" design"

• "conventional" approach (like porcelain type) • gas volume included • separate sealing system • pressure relief vents • outer housing: silicone silicone rubber rubber (SR) (all types: HTV, RTV, LR/LSR)

Porcelain/Type A MO column

Gas


Type A "Tube Design" Top cover plate Flange with venting outlet Sealing ring Pressure relief membrane Compression spring MO resistor column Composite hollow core insulator (FRP tube/ rubber sheds)

Type A "Tube Design" Nearly any desired mechanical strength and energy absorption capability (separate housing, multi-column possible) Safest possible short-circuit performance (closed tube) Single unit arrester up to U m = 300 kV (control of radial fields) Most expensive design Internal partial discharges possible (depending on design) Separate sealing system – risk of sealing deficiencies Æ

The Type A arrester is the typical "special feature" arrester.

Basic designs of polymer housed high-voltage arresters Type B

• no (intentional) gas volume included Type B1 Type Type B1a B1a

"wrapped design" "wrapped "wrapped design" design"

• FRP material directly directly wrapped onto MO stack stack • outer housing slipped over or molded on on (SR, (SR, EPDM, EPDM/SR EPDM/SR blends blends …) Type B1a

MO column

Gas


Type B1a "Wrapped Design" Implementation example 1:

• Fiber glass rovings soaked in uncured epoxy resin or pre-impregnated ribbons are wound crosswise around the MO stack. • They do not fully overlap and form rhombic "windows". • Best compromise between mechanical strength and short-circuit performance must be found. MO column

FRP wrap

main orientation of glass fibers


• Full overlapping of the ribbons or pre-impregnated FRP mats with appropriate (crosswise) orientation of the glass fibers • Forms a closed tube (good for mechanical strength, bad for short-circuit performance) • Slots as pre-determined weakened breaking areas

MO column

FRP wrap


Example: Ohio Brass


• Pre-impregnated FRP mats with axial orientation of the glass fibers • Forms a closed tube, which however easily tears open by the arc in case of short-circuit

MO column

FRP wrap



• no (intentional) gas volume included Type B1 Type Type B1b B1b

"wrapped design" "wrapped "wrapped design" design"

• FRP material with with distance distance to MO stack • gap filled by other material (solid/semi-solid) • outer housing slipped over or molded molded on (SR, EPDM, EPDM/SR blends …) Type B1b

MO column

Gas


Type B1 "Wrapped Design" Most economical design; lowest market prices Short-circuit performance better than for porcelain Lightweight; easy to handle Limited mechanical strength (diameter of housing, wall thickness) Big differences in performance (e.g. with regard to moisture ingress, short-circuit performance) depending on design variants and implementation Multi-unit arresters even for lower system voltages (radial fields) Æ

The Type B1 arrester is the typical "low cost" arrester.


• no (intentional) gas volume included Type Type B2 B2

"cage "cage design" design"

• FRP rods or loops form an open open cage cage around the MO stack • outer housing directly molded onto onto the the MO MO stack stack (silicone rubber)

Type B2

MO column

Gas


Type B2 "Cage Design" 1st sub-variant

Loops

Example: ABB Switzerland

Type B2 "Cage Design" 1st sub-variant

Loops

Type B2 "Cage Design" 2nd sub-variant

Loops + bondage

Example: ABB Sweden

Type B2 "Cage Design" 3rd sub-variant

Rods

Example: Siemens

Type B2 "Cage Design" 3rd sub-variant

Rods

Example: Siemens

Type B2 "Cage Design" Economical design; low market prices Short-circuit performance better than for porcelain Mechanical strength usually higher than for B1 design Lightweight; easy to handle Limited mechanical strength (diameter; mechanical strength of MO blocks) Multi-unit arresters even for lower system voltages (radial fields) ÆThe

Type B2 arrester is a higher performance "low cost" arrester.

Configuring Arresters

System Highest voltage of the system U s Grounding l a c i r t c e l e

Temporary overvoltages (TOV)

Arrester

Environment

Min. MCOV, U c,min → rated voltage U r1 Rated voltage U r MCOV, U c Rated voltage U r2

Lightning current stress

Nominal discharge current

Energy (line discharge, switching overvoltages)

Line discharge class

LIWV, safety margin, distance (protection zone)

LI protection level, SI protection level

Density of lightning strikes, magnitude of lightning strikes

Active part specified

l a c i n a h c e m

Length of housing, number of units, flashover distance (withstand voltages)

Short-circuit current

Mechanical stress (short-circuit current, tensile loads)

Creepage, sheds Diameter, material, length of units (number of units)

Height of erection

Pollution

Seismic stress

Choice of Continuous Operating and Rated Voltage System

(phase-to-phase)

1 3

System

(phase-to-earth)

Arrester

TOV (1 s) TOV (10 s)

Highest system voltage

1-s-voltage

Nominal system voltage

Rated voltage ≥ + 5 % Cont. operating voltage

Note: Nominal system voltage of no interest for configuring an arrester!

1.25

Choice of Continuous Operating and Rated Voltage U r1

= 1.25 · U c,min = 1.25 · (1.05 · U m/√3) U TOV

U r2

= _______ =

1,3

f(t TOV)

Power-frequency vs. time (U -t -) characteristics

1,25

k TOV

1,2 1,15 r

U 1,1 / U = v o

t

k

1,05 1 0,95 0,9 0,85 0,8 0,1

1

10 t

U U

100

1000

/s

is the higher value of U U and U U , rounded up to a multiple of three three

Calculation Example 1 (U m = 550 kV) U m = 550 kV

U 10sec = 1.4 · U m / 3 = 445 kV

Rated Voltage: U c, min = 1.05 · U m/√3 = 333 kV U r1 = 1.25 · U c, min = 416 kV U TOV

445

U r2 = _______ = _______ = 414 kV k TOV 1.075

⇒ U r, min = 417 kV

U r, typ = 420 kV

LIWV = 1550 kV

LD-class = 5

Calculation Example 2 (U m = 24 kV; isolated neutral) U m = 24 kV

U 10sec…1h = U m = 24 kV

Rated Voltage: U c, min = U m = 24 kV U r1 = 1.25 · U c, min = 30 kV U TOV

19.4

U r2 = _______ = _______ = 18.1 kV k TOV 1.075

⇒ U r, min = 30 kV

U r, typ = 30 kV

LIWV = 125 kV

LD-class = ---



Arrester

Environment










l a c i n a h c e m





Height of erection

Pollution

Seismic stress

Direct Lightning Strokes to Overhead Line Conductors The nominal discharge current I n is a coordination current on which the protective characteristics and thus insulation coordination are based. Question: What is a reasonable value for I n?

To answer this question: what are the highest possible currents of a lightning stroke directly into the overhead line conductor?

Direct Lightning Strokes to Overhead Line Conductors CIGRÉ electro-geometrical model

and r g are the maximum striking distances of a return stroke to the stepped leader. • The higher the stroke current, the higher r c and r g. • I m is the maximum current at and above which no strokes will terminate on the phase conductor:

• r c

1

h+ y ⎡ ⎤ 0.75 ⎢ ⎥ 2 I m ≈ ⎢ 7.1 ⋅ (1 − sin α ) ⎥ ⎢ ⎥ ⎣ ⎦

Examples:

α α = shielding angle

Strokes Strokes between between A A and and B B

h = 60 m, y = 45 m, phase phase conductor conductor

Strokes Strokes between between BB and and C CÆ ground wire wire Æ ground

⇒

I m

α =

30 °

α =

15 °

36 kA

h = 30 m, y = 25 m,

Lightning Stroke and Surge Propagation on a Transmission Line 1

2

1 Lightning stroke: two traveling waves of û = ½ ·Z · î (Example: û = ½ · 350 Ω · 20 kA = 3.5 MV) 2 1st insulator: flashover Example:

• 100 % flashover voltage (negative polarity*)): ud100 ≈ 2100 kV for U m = 420 kV • max. current of propagating wave: î = 2100 kV / 350 Ω = 6 kA

Surge currents are limited to values below 10 kA!

Lightning Impulse Current Stress of Station Arresters Usually, Usually, no no direct direct lightning lightning strokes strokes of of discharge discharge currents currents higher higher than than ≈≈ 20 20 kA kA on on shielded shielded transmission transmission lines lines (all (all other other strokes strokes will will hit hit the the shield shield wire wire or or directly directly the the ground) ground) Currents Currents limited limited by by flashover flashover voltage voltage of of line line insulators insulators and and surge surge impedance impedance of of the the line: line:

î î = ûûflashover Z flashover /Z Examples: Examples: U U mm == 123 Z == 450 ≈ 600 Ω 123 kV, kV, ûûflashover 600 kV, kV, Z 450 Ω flashover ≈

î î = = 1.3 1.3 kA kA

U U mm == 420 Z == 350 ≈ 22 100 Ω 420 kV, kV, ûûflashover 100 kV, kV, Z 350 Ω flashover ≈

î î = = 66 kA kA

LI LIcurrents currentsin inthe thesubstation substationusually usuallybelow below10 10kA kA



Arrester

Environment










l a c i n a h c e m





Height of erection

Pollution

Seismic stress

Energy Requirements

Two aims:

1) 1) Mechanical Mechanical integrity integrity of of the the MO MO blocks blocks 2) 2) Thermal Thermal stability stability

Energy Requirements

Single impulse energy absorption capability • Energetic overloading (puncture or thermo-mechanical cracking of one or more MO-resistors)

Energy Requirements

Thermal energy absorption capability

Electrical power losses

s e , s n s o o i t l a r p e i s w s o i p d l t a c a i r e t H c e l e

Limit of thermal stability (unstable operating point)

Heat dissipation Normal operation (stable operating point)

Temperature

"Thermal" and "Impulse" Energy Values "Thermal"

"Impulse"

1)

2 long duration current impulses, 1 minute apart Ur for 10 seconds

1 long duration current impulse Uc for 30 minutes

Time

Time

Preheat to 60 ºC max. 100 ms

Energy input by two long duration current impulses 1 minute apart (each impulse 50% of the injected energy); thermal stability required 1)

Note: If no thermal stability has to be guaranteed after

energy injection (i.e. arrester de-energized afterwards) higher energy values are allowed.

Energy input by one long duration current impulse t 4 ms; thermal stability not critical

Thermally Equivalent Prorated Section for Operating Duty Test Current supply Gripping Hard tissue

Gripping

Cork

Test sample

Porcelain

Temperature measurement

Current supply

Choice of Line Discharge Class acc. to IEC 60099-5 IEC 60099-5, 1996-02 Surge Arresters - Part 5: Selection and application recommendations Table 1: L D c la s s

a p p ro x . U m kV

a p p r o x . lin e le n g t h km

app rox. Z Ohm

a p p r o x . o v e r v o l ta g e factor *) p.u.

1

≤ 245

300

450

3 ,0

2

≤ 300

300

400

2 ,6

3

≤ 420

360

350

2 ,6

4

≤ 550

420

325

2 ,4

5

≤ 800

480

300

2 ,4 *)

1 p.u. =√2 x Um/ √3

Line Discharge Class (IEC 60099-4) – Problem of Definition W · (U – U res)) ·· 1/ = U 1/Z W = U res Z ·· T T res · (U LL – U res

Arrester classification


Surge impedance of the line Z (Ω)

Virtual duration of peak T (µs)

Charging voltage U L (kV d.c.)

10 000 A

1

4,9 U r

2 000

3,2 U r

10 000 A

2

2,4 U r

2 000

3,2 U r

10 000 A

3

1,3 U r

2 400

2,8 U r

20 000 A

4

0,8 U r

2 800

2,6 U r

20 000 A

5

0,5 U r

3 200

2,4 U r

Example: Example:

A kJ/kV (2·2 A MO MO arrester arrester with with resistors resistors of of 44 kJ/kV (2·2 kJ/kV) kJ/kV) energy energy absorption absorption capability capability may may be be specified U res U r r == 2, specified as as aa Class Class 22 arrester arrester if if U 2, res//U but U res U r r == 2.4. but as as aa Class Class 33 arrester arrester if if U 2.4. res//U A U res U r r == 22 needs A Class Class 33 arrester arrester of of U needs res//U resistors kJ/kV (2·3 resistors of of 66 kJ/kV (2·3 kJ/kV). kJ/kV).



Arrester

Environment










l a c i n a h c e m





Height of erection

Pollution

Seismic stress

LI protection level from U -I -Characteristics U U --I I --characteristics characteristics for for

different different MO MO resistors resistors

= (2.8 ... 3.4)·U U U pl pl = (2.8 ... 3.4)·U cc

û r û c

Calculation Examples 1 + 2 (U m = 550 kV and 24 kV) 1) U m = 550 kV

U 10sec = 1.4 · U m / 3 = 445 kV

LIWV = 1550 kV

LD-class = 5

U r, typ = 420 kV

Rated Voltage: Protection Level:

1550 kV

U 10kA = 420 kV · 2.3 *) = 966 kV

<

__________

1.4

1550 kV ( __________ = 1107 kV ) 1.4

;

*) Typical value for LD class 5, but manufacturer dependant

2) U m = 24 kV

Rated Voltage:

U 10sec…1h = U m = 24 kV

LIWV = 125 kV

LD-class = ---

U r, typ = 30 kV

Protection Level:

125 kV U 10kA = 30 kV · 2.67 *) = 80 kV

<

_________

1.4

125 kV ( _________ = 89 kV ) 1.4

;

Typical Values of Protection Level Standard lightning Lightning impulse impulse withstand protective level upl voltage LIWV kV kV kV 24 Resonant earthed 125 80 123 Resonant earthed 550 370 145 Solidly earthed 650 295 245 Solidly earthed 950 485 420 Solidly earthed 1425 825 550 Solidly earthed 1550 960 U s

Neutral earthing

As a rule of thumb, a factor LIWV/upl ≥ 1.4 offers sufficient protection against lightning overvoltage:

Factor LIWV/upl

1.56 1.49 2.2 1.96 1.73 1.61

LIWV u u pl pl

________ ________

1.4 The voltage at the terminals of the equipment to be protected must not reach values above LIWV/1.15 (K s = 1.15 = safety factor for non-self restoring insulation, acc. to IEC 60071-2)

Protection Level and Stability against Power-Frequency Stress 600

Example: U m = 245 kV, neutral effectively earthed U r = 224 kV u10kA=538 kV

500 400

U r = 198 kV u10kA= 475 kV

] V k [ 300 u

200 1.4 times line-to-earth-voltage 100 Line-to-earth-voltage 0 0,0001

0,001

0,01

0,1

1

10 i [A]

100

1000

10000

100000

Lower LI protection level higher specific power-frequency stress Protection level should be set to reasonable (not necessarily the lowest) values!



Arrester

Environment










l a c i n a h c e m





Height of erection

Pollution

Seismic stress

Housing Requirements

• Mechanical strength • static and dynamic loads by connected conductors • strength against seismic events • Dielectric strength • Short-circuit performance • Performance under polluted conditions • shed profile • creepage distance • flashover distance • partial heating of active parts • internal partial discharges • hydrophobicity (incl. dynamics of hydrophobicity)

Mechanical Strength of Housing Minimum recommended strength if there are no further requirements (given by conductor loads ÅÆ wind, vibration, short-circuit current forces): System voltage Um

Fmin static

Fmin dynamic min. breaking value

(kV)

(N / lbf)

(N / lbf)

(N / lbf)

≤ 420

400 / 90

1000 / 225

1200 / 270

550

600 / 135

1500 / 337

1800 / 405

800

800 / 180

2000 / 450

2400 / 540

(Table valid for porcelain housed arresters)

• Ratio F dyn / F stat = 2.5 for porcelain housings • Ratio F dyn / F stat for polymer housings not yet definitely established

Pressure Relief of a Porcelain Housed Arrester Unit

1) Puncture and flashover of individual MO resistor(s)

2) Internal arc along the full length of the unit

3) Opening of pressure relief devices and venting of the unit

Pressure Relief of a Cage Design Polymer Housed Arrester Unit 1.

Arrester has failed and gas begins to be expelled through the housing.

2.

The gas streams trigger an external flashover and the internal arc is commutated to the outside

Pressure Relief Test according to IEC 60099-4 Ed. 2 (inf.) Test with high current (rated short-circuit current):

5

10

16

20

31.5

40

50

63

80 kA

(additionally: tests with ≈50% and ≈ 25% of rated short-circuit current)

Basic idea:

Ød

• Explosion not allowed • Thermal breaking allowed (definition: all parts within the circular enclosure)

Arrester unit H Circular enclosure

Test with low current:

600 A ± 200 A

Factors that improve pressure relief performance:

• Short housings • Large gas volume • Fast opening pressure relief devices

h >= 0,4 m

• High mechanical strength (porcelain quality, thickness)

• Favorable short circuit

Ø D = (d + 2H) >= 1,8 m Problems: - how to initiate the failure

current loop

see 37/317/CDV

Influence of the Short Circuit Current Loop i

i

i

i

i i

Porcelain Polymer (with gas volume included) Polymer (without gas volume included)

i

i

worst case

most favorable case

neutral case

favorable case

most favorable case

worst case

most favorable cases

worst case

Examples for Successful Pressure Relief Tests with High Current Test with 63 kA/0,2 s on porcelain housed arrester

Examples for Successful Pressure Relief Tests at High Current Test with 63 kA/0,2 s on polymer housed high-voltage arrester (FRP hollow insulator)

Before test

After test

Examples for Successful Pressure Relief Tests at High Current Test with 63 kA/0,2 s on polymer housed high-voltage arrester (cage design)

Examples for Successful Pressure Relief Tests at High Current Test with 20 kA/0,2 s on polymer housed distribution arrester (cage design)

Short-Circuit Performance – Wrap-Up

• In general, polymer housed arresters tend to offer a "safer" short-circuit performance.

• But not all polymer housed arresters are intrinsically "safe". • Design differences must still be regarded.

Performance under Pollution Conditions Measures against: 1 • Long creepage distance • Optimized shed profile 2 • Few number of units (best: single-unit arrester) Radial filed stress:

3

2

- risk of "internal" partial discharges, degradation of the MO resistors and deterioration of the supporting structure or - risk of puncture in case of the housing directly applied to the MO column Risk of partial heating of the active parts (see Annex F of IEC 60099-4)

1 Risk of external flashover

3 In case of gas volume included: • large distance MO column - housing • no sharp edges at the MO column • MO blocks with stable aging characteristics • internal gas volume free of oxygen • high tracking resistance of supporting structure • use of desiccants In case of no gas volume included: • sufficient thickness of housing • optimized shed profile • high tracking resistance of materials

General: Highrated ratedand andcontinuous continuousoperating operatingvoltage voltage General:High

rather ratherthan thanlow lowprotection protectionlevel levelififpossible possible(leads (leadstoto moderate moderateelectrical electricalstress stressunder undercontinuous continuousoperating operating conditions and improves long term stability) conditions and improves long term stability)

Performance under Pollution Conditions – Radial Field Stress MO column Gas or solid

Conductive layer

Solid t n i , l a i x a

U

Uradial

Summary - Characteristic Values (1): Main Data • Continuous operating voltage (U c/MCOV) • Rated voltage (U r ) • Rated frequency • Rated short-circuit current (I s) • Line discharge class (LD-Class) • Nominal discharge current (I n) (8/20 µs) • LI protection level (U pl) (= residual voltage at I n) Additional: residual voltages for different current shapes and amplitudes

Summary - Characteristic Values (2): Additional Data • Long duration current impulse withstand capability (amplitude, time) • Energy absorption capability (in kJ/kV of U r or U c) • High current impulse capability (4/10 µs) (for distribution arresters) • Temporary overvoltage (TOV) capability (1 s, 10 s, 100 s) • Creepage distance • Dielectric withstand values of the housing • Permissible mechanical headloads (static, dynamic)

Metal Oxide Surge Arresters

Voltage and Temperature Distribution of MO Arresters 1800 1600 1400 1200 ] 1000 m m [ H 800

H= 1200 mm H= 1465 mm H= 1805 mm

600 400 200 0 0,7

0,8

0,9

1

1,1

1,2

1,3

1,4

1,5

U/Umittel U/Umean

⇒ Grading rings necessary for arrester heights > 1.5 m ... 2 m

1,6

Arrester U r = 224 kV, Voltage Distribution, Equivalent Circuit 2200 2000 1800 1600 1400 ] m1200 m [ t h g i e 1000 H

800 600 400 200 0 0,7

0,8

0,9

1

1,1

1,2

1,3

Specified Currents for Residual Voltage Tests on MO Arresters Double exponential current impulses: definition by T / 1 T 2 and î T 1 T 2 î

Front time [µs] Time to half value [µs] Amplitude [kA] from IEC 60060-1

FS UL

T 1 = 1.25 · T

L

R PO

C

Typical test circuit

Specified Currents for Residual Voltage Tests on MO Arresters • Switching current impulse: (30…100)/(2·T 1) µs, î <= 2 kA • Lightning current impulse: 8/20 µs, î <= 40 kA (nominal discharge current I n usually 5, 10 or 20 kA) • High current impulse: 4/10 µs, î <= 100 kA (typical values 65 and 100 kA) • Steep current impulse: 1/<20 µs, î <= 20 kA

Specified Currents for Energy Tests on MO Arresters Long duration current impulse: î <= 2 kA T d

Virtual duration of the peak

T t

Virtual total duration

Standard values of T d: 500, 1000, 2000, 2400, 2800, 3200 µs

from IEC 60060-1 Ln UL

Cn

L1 C1

Typical test circuit

FS PO

Accelerated Aging Procedure Accelerated Aging Test:

• Part of Operating Duty Test acc. to IEC 60099-4 • Quality assurance for running MO production

Test Conditions:

• ϑ = 115 ºC, U = 1,05 ·U c, t = 1000 h (6 weeks) • "actual surrounding medium": Air, N2, SF6, (CO2, N2H2) Accelerated Ag ing T es t ac c. to IEC 99-4 2 1,8 Normal Behaviour

1,6

Aging Resistor

1,4 1,2 o P / P

1 0,8 0,6 0,4 0,2 0 0

5

10

15

20 sqrt (t) [s qrt (h)]

25

30

35

40

Accelerated Aging Tests

Switching Surge Operating Duty Test (IEC 60099-4) 2 long duration current impulses, 1 minute apart

Conditioning 4 groups of 5 impulses at In 8/20 µs, superimposed on 1,2 x Uc

2 high current impulses 100 kA, 4/10 µs

1 minute apart

Ur for 10 seconds

Uc for 30 minutes

Preheat to 60 °C

Cool down to ambient temperature

Time interval not specified

Test evaluation (pass criteria):

max. 100 ms

• Decrease of temperature, power loss, resistive component of the leakage current • No significant change of residual voltage (max. 5%) • No puncture, flashover or cracking of MO blocks

Time

High Current Impulse Operating Duty Test (IEC 60099-4) Conditioning 4 groups of 5 impulses at In 8/20 µs, superimposed on 1,2 x Uc

1 high current 1 high current impulse 4/10 µs impulse 4/10 µs

Ur for 10 seconds Uc for 30 minutes

Time 1 minute apart

Preheat to 60 °C

Cool down to ambient temperature

Time interval not specified

Test evaluation (pass criteria):

max. 100 ms

• Decrease of temperature, power loss, resistive component of the leakage current • No significant change of residual voltage (max. 5%) • No puncture, flashover or cracking of MO blocks

Metal Oxide Surge Arresters

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