Distance And Earth Fault Protection[1].pdf

Distance Protection and

Earth-Fault Protection in

Medium-Voltage Distribution Networks ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F1 of 63

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Content Introduction Distance Protection Limits of overcurrent protection Where is distance protection preferred ? Basic principle of distance protection Limits of the distance protection Special features for application in networks with isolated or resonant grounded neutrals Preferred starter mode Phase-preference-logic Pros and Cons compared with overcurrent protection

Earth-fault protection in networks with isolated neutrals Earth-fault protection in resonant grounded networks ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F2 of 63

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Introduction Types of neutral grounding CE

CE

RN 3kV - 24kV - small rural networks - city networks - industry

8kV - 24kV - rural networks - big city networks

» 1500 A

5 ....300 A

3kV - 33kV - Generators - Industry - small networks

10 ... 64 kA

XN 33kV - 132kV Limited step- and touch-voltages.

33 kV - 800 kV

Different designations are used: - Petersen coil compensated - compensated - resonant grounded

Topics of this course ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F3 of 63

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Introduction Types of neutral grounding CE

CE

RN

3 ... 24 kV (110 kV) IEmax = 3I0max

Z0source

of total network (except prot. line)

Umax Ph-E U0

very high

3 ... 24 kV

T2403 / T2403_2.ppt / C11e / F4 of 63

33 ... 132 kV

IE limited, with trip

» 3·RN

» Un ph-ph » Unphase -N

Topics of this course ABB Power Automation Ltd

10 ... 64 kA

XN

IE <30 A without trip IE >30 A rather with trip

XCE

» 1500 A

5 ....300 A

» 3·XN

33 ... 800 kV IE >10 kA with trip

(2 ... 4)·Z1source < 0.8·Un ph-ph < 0.73 Unphase -N

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Distance Protection in medium voltage networks

Distance Protection Limits of overcurrent protection Where is distance protection preferred ? ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F5 of 63

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Distance Protection in medium voltage networks Open Ring I>

non-directional

The direction of the short-circuit current is toward the fault. No return current is possible

0.6s

0.8s

IKS 0.6s

0.4s

0.4s

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0.2s

0.2s

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Distance Protection in medium voltage networks Open ring with two parallel lines I>

non-directional

The direction of the short-circuit current leaving the bus is toward the fault. No return current is possible

0.6s

0.8s

0.1s 0.6s

0.4s

0.8s

0.1s

I> directional Two directions are possible, due to parallel lines

0.4s

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0.2s

0.2s

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Distance Protection in medium voltage networks Closed ring departing from one common bus I>

non-directional

The direction of the short-circuit current leaving the bus is toward the fault. No return current is possible

I> directional Two directions are possible, due to the closed ring

1.0s

1.0s

0.2s

0.1s

0.8s

0.8s

1.0s

0.1s

0.4s 0.6s

0.4s ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F8 of 63

0.6s

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Distance Protection in medium voltage networks Directional overcurrent is not applicable in meshed networks 0.4 s 0.2 s 0.2 s

TRIP

0.4 s

Infeed right

Infeed left 0.4 s 0.2 s 0.2 s

TRIP

0.4 s

TRIP

No selectivity possible ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F9 of 63

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Distance Protection in medium voltage networks Distance protection solves the problem in the meshed network Zone 3, t3 » 0.6 .. 0.9 s Zone 2, t2 » 0.4 .. 0.5 s Zone 1, t1 = 0

Infeed right Infeed left

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TRIP

TRIP

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Distance Protection in medium voltage networks Distance-zone grading time [sec.]

The two conditions can be contradictory. Compromises are often necessary. Considering infeed-factors may help.

approx. 1.8·a

Z3 = 0.85 (a +b2) 1.2 a < Z2 < 0.85 (a + b1)

b2

Z1 = 0.85 • a b1 a

A ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F11 of 63

Z [W/phase]

B

C

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Distance Protection in medium voltage networks Principle of the distance function (simplified) I

Relay

ZS UG »

UG I= ZS + ZL

ZL U

Uph - ph ZL = IR - IS for 2- and 3-phase faults

ZL =

Uph - ground Iph + 3I0 × k0

for single-phase to ground faults

Relay will trip for:

I > Iset AND ZL < Zset AND Fault forward ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F12 of 63

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Distance Protection in medium voltage networks Distance protection consists of two main functions Starter Measurement • General fault criteria

• Accurate impedance measurement

• Phase-selection

• Directional

• Non-directional minimum impedance

• Several zones and timers for impedance/time grading

or overcurrent - used for switch-on-to-fault function - back-up trip

j·X

• Start of the zone-timers

Reactance Direction

j·X

Resistance

R R ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F13 of 63

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Distance Protection in medium voltage networks Distance protection with overcurrent starter 30

[W/phase] Zone 4

20

j·X

Zone 3 10 Zone 2 0

Zone 1

R -10

-30

-20

-10

0

10

Distance-zones ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F14 of 63

20

30

[W/phase]

Ohm/Phase = positive sequence Ohms

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Distance Protection in medium voltage networks Distance protection with minimum-impedance starter 30 Starter

[W/phase] 20

j·X

Zone 3 10 Zone 2 0

Zone 1

R -10 -30

-20

-10

0

10

20

Distance-zones ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F15 of 63

30

[W/phase]

Ohm/phase = positive sequence Ohms

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Distance Protection in medium voltage networks Polygonal distance protection 5 directional zones 1 non-directional zone

j·X1 Starter Zone 3

REL316

Overreachingzone

Medium voltage distance protections require many zones. Remote back-up has to be provided for the adjacent lines, since redundant protection is not usual in such networks. Reverse directed zone

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Zone 2 Zone 1

R1

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Distance Protection in medium voltage networks Limits of the distance protection On very short overhead-lines and on most cable lines a reasonable zone grading is not possible • Several sections have to be protected together • A directional comparison scheme gets imperative Distance protection requires powerful CT’s If communication links are available between the two line ends, the following alternatives are possible: • Line-differential protection • Comparison schemes with directional overcurrent relays

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Distance Protection in medium voltage networks Single-phase to ground fault in networks with directly grounded neutrals or neutrals grounded over current limiting reactors

Inductive zero sequence source, the short-circuit current is high

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Distance Protection in medium voltage networks In Networks with direct grounding or grounding over current limiting reactors (some kA) the distance relays are able to detect phase-to-phase faults and phase-to-ground faults A separate ground fault protection is an option To detect phase-to-ground faults the protection shall be able to work even for fault currents below max. load current. The starter shall use a minimal impedance characteristic or other voltage dependent characteristics.

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Distance Protection in medium voltage networks Distance protection in networks with isolated or resonant grounded neutrals cannot detect single-phase-to-ground faults In networks with isolated or resonant grounded neutrals the fault current for a single-phase-to-ground fault amounts from some 10 up to just over 100 Amps In such networks the distance protection operates for faults between phases only For ground faults the small fault current does in most cases not allow a distance measurement and the capacitive source impedance might fool the distance measurement

If single-phase to ground shall be detected, a separate protection becomes necessary ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F20 of 63

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Distance Protection in medium voltage networks Requirements to distance protection in isolated and compensated networks As starter criteria overcurrent-functions are preferred. This will avoid non-desired starts for single ground faults The overcurrent starter is set above the maximal load current and well above the maximal capacitive earth-fault current Other advantages of the overcurrent-starter : • No fast fuse-failure blocking-function is required • In case of double-buses and VT’s on the buses only, no blocking of the distance function is required during the switching over procedure from one bus to another

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Distance Protection in medium voltage networks

Cross-country faults in networks with neutrals isolated or resonant grounded

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Distance Protection in medium voltage networks

“Cross-country” faults in networks with isolated or resonant grounded neutrals are short circuits and have to be tripped by the distance protection

Two subsequent single-phase to ground faults with footing points in different locations ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F23 of 63

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Distance Protection in medium voltage networks Cross - country fault on radial lines R S T Ground

Infeed

Line1

Ground

Line 2

Ground

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Distance Protection in medium voltage networks Phase preference logic for distance protection In networks with isolated neutrals or with Petersen coil grounding some clients specify a so called “phase preference logic”. In case of cross-country faults in a meshed network, it is the aim of this logic to trip only one line. To reach this goal, “phase-switched” measurement is used (in case of “full-scheme” relays the 5 non-relevant loop-measurements are blocked)

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Distance Protection in medium voltage networks Cross - country fault in a meshed network Currents used for distance measurement if phase preference is R - T - S

R S T Ground

Ground R S T

Line1

Ground

Line 2

Ground

To trip only one line, all distance protections have to measure in one and the same type of fault loop (R-E in above example). The protection needs a Phase-Preference Logic. All distance protections in the network shall use identical logic's. ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F26 of 63

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Distance Protection in medium voltage networks Phase preferences Phase preferences R - T - S acyclic means :

R

R has preference over T For a fault R - Ground - T the line with the fault R to ground will trip

acyclic

T has preference over S For a fault S - Ground - T the line with the fault T to ground will trip

R has preference over S

T

S

For a fault R - Ground - S the line with the fault R to ground will trip

R

Phase preferences R - T - S - (R) cyclic means : R has preference over T For a fault R - Ground - T the line with the fault R to ground will trip

cyclic

T has preference over S For a fault S - Ground - T the line with the fault T to ground will trip

S has preference over R For a fault R - Ground - S the line with the fault S to ground will trip ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F27 of 63

T

S

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Distance Protection in medium voltage networks Phase preferences Modes with acyclic phase-preference R-T-S R-S-T T-S-R T-R-S S-R-T S-T-R

[mechanical relay LI4]

Modes with cyclic phase-preference R - T - S - (R) T - R - S - (T)

[mechanical relays LH1, L3, LZ3]

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Earth-fault protection in medium voltage networks

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Earth-fault protection in medium voltage networks Measurement of 3I0 is not influenced by phase-currents

3I0>

IT>

IS>

IR>

3 phase-currents + earth-current 3I0

3I0>

IT>

IR>

2 phase-currents + earth-current 3I0

Measurement of 3I0 allows a low setting for ground-faults The setting is independent on load current ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F30 of 63

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Earth-fault protection in medium voltage networks Earth-fault protection with definite time overcurrent relays

Radial network, neutral grounding in infeed station only 110 kV

16 kV

On consumer ends, neutrals on MV-level are NOT grounded Note: Every grounded neutral behaves like an earth-fault current source !! ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F31 of 63

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Earth-fault protection in medium voltage networks Earth-fault protection with non-directional IDMTL overcurrent relays

Example: Normal inverse BS 142

t[s] =

0.14s × k 0.02

æ IE ö ç ÷ I set è ø

Tripping time in seconds

Radial network, neutral grounding in infeed station only 100

10

t-min

k is settable (typically 0.05 .... 1.1) The t-min setting allows time-grading with the main short-circuit protection, i.e. overcurrent, distance, line-differential.

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1

1

10 IE / Iset

100

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Earth-fault protection in medium voltage networks CT in Holmgreen connection, measurement of 3I0 Ratio depends on load current

Ratio depends on load current

2000/1A

Ratio Independent of load current

65/1 (=195/1 related to 3I0)

2 secondary windings

3 I0 (IR + IS + IT) + (DIR + DIS + DIT) ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F33 of 63

Error due to load current cancelled out, in case all CT's produce identical errors.

I0 Same errors as std. Holmgreen

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Earth-fault protection in medium voltage networks Toroidal CT's are used to achieve a correct measurement of low ground currents. Accuracy and CT ratio are independent of load current

Grounding cable routed back through the CT, since the shield current may not influence the measurement.

isolated

3 I0

3 I0 = IR + IS + IT Load current has no influence on measurement ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F34 of 63

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Earth-fault protection in medium voltage networks

Phase - CT‘s

CT for measurement of 3·I0 = earth-fault current by use of a toroidal cable-CT

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Earth-fault protection in medium voltage networks Required blocking of sensitive earth-fault protection, in case phaseCT’s are used for ground-current measurement (Holmgreen) Since the the current-setting is low, the earth-fault protection should be blocked by the ph-ph start of the short-circuit protection (overcurrent, distance or line-differential) or alternatively be delayed for a couple of seconds. Reason: During high current phase-to-phase faults, secondary spill-currents in the neutral of the CT’s can be produced, due to unequal CT errors in the individual phases. Such currents might fool the sensitive ground fault protection.

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Earth-fault protection in medium voltage networks

Earth-fault detection in networks with isolated or compensated neutrals

C0

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3 C0

High impedance zero sequence source, the short-circuit current is small

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Earth-fault protection in medium voltage networks Phase-to-ground voltages in isolated or compensated networks RF = 0 W During a single-phase to ground fault N

R

USN

T

E

UTN

USN

U0

UTE

USE

URE

U0 = UNE = - URN

S

UTN

UNE = U0

URN

U0

UTE

U0 UTE

USE

E

The positive sequence voltage remains unchanged The negative sequence voltage = 0 The zero sequence voltage U0 = - pos. seq. voltage of the grounded phase

URN = URE

USE + UTE = 3 U0

URN

USE

U0 U0

USE No-fault condition

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UTN = UTE USN = USE

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Earth-fault protection in medium voltage networks VT‘s in not effectively grounded systems, i.e.isolated neutral, Petersen coil compensated or grounding over resistance

Un 3

100V 3 100V 3

2 secondary windings ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F39 of 63

n e

Rated voltage factor = 1.9 for (8) hours. Specify VT for not effectively grounded network. Un-e = U0 = voltage neutral to earth = neutral displacement voltage For a metallic single-phase to ground fault in a not effectively grounded system U0max = UnPhase -Neutral U0 Un - e = 3 = U0 = 100V 3 Other standard secondary voltages are: 110 V, 115 V, 125 V

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I [A] at 1.6 Ohm station ground resistance

U [V]

Earth-fault protection in medium voltage networks Admissible step- and touch-voltages as specified by the Swiss-authorities (SEV)

Fault-duration 0 .... 0.9 s

Fault-duration over 0.9 s

1000

80

U I

100

U

70 60 50 40

I

30 20

10

10 0

1 0,0

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0,1

1

0,5

Time [s]

0.9

0.9

2

3

4

5

6

7

Time [s]

RE = 1.6 Ohm = measured value

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Earth-fault protection in medium voltage networks

Earth-fault detection in networks with isolated neutrals

C0

3 C0 Capacitive source impedance, the short-circuit current is small

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Earth-fault protection in medium voltage networks Radial lines are preferred, I> directional required for lines

110 kV

16 kV

I> nondirectional

I>

I>

I>

I>

I>

directional

directional

directional

directional

directional

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Earth-fault protection in medium voltage networks Earth-fault

R S T Ground

Line with ground-fault

3·I0 = IE » 0

Ground

IT URE=0 3·I0 = IE

Line(s) without fault

IS USE

UTE

3·U0 ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F43 of 63

Ground

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Earth-fault protection in medium voltage networks Single-line diagram

Network with isolated neutrals Distribution of capacitive ground currents

DEF

Line # 1

IC1 DEF

IF = S IC

Line # 2

IC2 DEF

Use functions Isinj or I0*U0*sinj

Line # 3

I0> U0

IC3

DEF Line # 4

C = 3 C Phase to ground

IC4 ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F44 of 63

C

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Earth-fault protection in medium voltage networks Equivalent diagram in symmetrical Neutrals isolated components

G

Single line diagram

Positive sequence

U1 = URN

Line #1 Negative sequence C01

Line #2

U2 » 0

Line #3 I01

C02

C03

UF RF

isolated

Ground-fault R-E

I02

C01

I03

C03 U0 IF/3

C02 Zero sequence

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3 RF

UR

U0 UT

US

Phase to ground voltages [p.u.]

Earth-fault protection in medium voltage networks Phase-to-ground voltages in isolated or compensated networks at different neutral displacement voltages U0 = f(RF) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

UT US

UR

Earth-fault indication only Phase indication possible

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Neutral displacement voltage U0 [p.u.]

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Earth-fault protection in medium voltage networks VT’s with damping resistor against ferro-resonance Specify also the long-time permitted current in the open delta winding.

Un 3

n

100V 3

e

R

In small a network exists the danger of ferro-resonance due to resonance between the network capacity phase to ground and the VT reactance. Generator buses or other buses not yet loaded with lines, are likely to produce ferro-resonance. R will damp ferro-resonance.

100V 3

Note:

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In Petersen compensated networks the angular error of the VT, loaded with R, meight harm the wattmetric protection. Use non-linear device or feed relays from another VT.

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Earth-fault protection in medium voltage networks

Earth-fault in networks with resonant grounded neutrals

L

C0

L

3 C0

1 wL » 3wC0

High impedance zero sequence source, the ground current is small ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F48 of 63

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Earth-fault protection in medium voltage networks Single-line diagram

Network with resonant grounded neutrals Distribution of capacitive ground currents

Line # 1

IC1 Line # 2

IF << S IC

IC2 Line # 3

IF

Petersen coil

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IC3

S IC Line # 4

IC4

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Earth-fault protection in medium voltage networks Petersen-coil compensated network (resonant grounding) Equivalent diagram in symmetrical components

Single line diagram Line 1 C01

G Positive sequence system

Line 2

Line 3

C02

C03 Negative sequence system

RF

R

U1 = URN

L

U2 » 0

I01 UF

Ground fault R-E I02

C01

C02

3 RF

I03

3R

3L

C03

U0

IF/3

Zero sequence system

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Earth-fault protection in medium voltage networks Amplitude-error Fi and angular-error di of CT‘s Ohmic burden

Ip

Reactive burden

S1

P1

Is

Rw

Im P2

Ip Rb

Zb

S2

S1

P1 Rw

Im P2

Is Xb

S2

Im

Us = Ip (Rw+Rb) Us = Ip (Rw+j Xb)

Im Fi » 0

di

Is

Fi di

Is

Ip

Ip

Is leads Ip

The angular error di might fool the earth-fault protection in a resonant grounded network ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F51 of 63

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Earth-fault protection in medium voltage networks Compensated network with ohmic neutral resistor Distribution of the ground currents Line # 1

IF »I resistive

IC1

Use functions Icosj or I0*U0*cosj

Line # 2

IC2 Line # 3

IF

R

S IC Line # 4

Iresist. ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F52 of 63

IC3

IC4

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Earth-fault protection in medium voltage networks

3 single-phase transformers

Injection of ohmic current by means of three single phase transformers

I0Watt

I0Watt

IWatt = 3 I0Watt primary

U0>

T2403 / T2403_2.ppt / C11e / F53 of 63

t1

t2

R I0Watt secondary

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I0Watt

t1 avoids pick-up in case of self-extinguishing faults t2 limits the time the resistor is loaded

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Earth-fault protection in medium voltage networks Other principles for directional earth-fault functions Functions using 5th harmonics The ground currents of the 5th harmonics are not compensated by the Petersencoil. Directional functions measuring U0250Hz·I0250Hz·sin j can be used. Condition which allow application: The amount of 5th harmonics must be relevant and the capacity phase-to ground of the total network shall be relatively high.

Transient measurement (Wischer-Relay) At fault inception the polarities of the rate of change of zero-sequence current and voltage is compared, this comparison is used to determine the direction. Disadvantages:

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Only suitable for alarm, not for tripping Only one measurement at fault inception is possible

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Earth-fault protection in medium voltage networks Industrial plant with high-voltage motors In industrial networks with high-voltage motors a minimal value of ground-fault current is welcomed to realise a stator-earthfault protection by simple I0> functions. However, the earth-fault current is limited to avoid high damages in the faulted motor. A protection range of 70 - 80% of full winding is aimed for i.e. 20 - 30% of the winding, located towards the neutral, are not protected. The ground fault current for the not protected part is 70 - 80% reduced.

To achieve the necessary current, the neutrals of the infeedtransformers are grounded over resistors.

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Earth-fault protection in medium voltage networks Damages on high-voltage motors due to stator ground-faults (Source : Leroy Sommer) 80

Fault-current [A]

70 60 50 40

high damages

considerable damages

30 20

small damages

10 0 0

1

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2

3

4

5

6

Fault time [seconds]

7

8

9

10

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Earth-fault protection in medium voltage networks Earth-faults protection in network with isolated neutrals: Earth-fault directional relays for lines and sensitive zerosequence overcurrent relays for machines and transformers. Earth-faults protection in network with resonant grounding : The protection system has to be carefully planned and set. For measurements based on fundamental frequency, toroidal cable CT‘s are required with low angular errors.

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Earth-fault protection in medium voltage networks

Special cases and explanation of some not obvious phenomena

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Earth-fault protection in medium voltage networks Not-permitted scheme !!! Never ground two neutrals of a transformer in a compensated network

Netz L

CE

Single line diagram

Equivalent diagram for the zero-sequence system

X0Source

XTransfo H-L

+j 3wL

>5 XTransfo H-L

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X»0

3 I0 >> IEnormal U0 >> U0normal -j X0C = -j / (wCE )

Series - resonance !!! Þ very high U0

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Earth-fault protection in medium voltage networks Isolated neutrals, only one outgoing line I00°

R

IW

I090°

Single line diagram

IC

Use directional I0> functions with 45° characteristic measuring angle

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Earth-fault protection in medium voltage networks High capacitive ground-current in a network with isolated neutrals might produce extensive U0-voltage for a fault at end of a long line Single line diagram

Netz G

Equivalent diagram in the zero-sequence system

3·I0 > IEnormal U0 > U0normal

j X0L

-j X0C

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U0normal

ABB

Earth-fault protection in medium voltage networks Compensated network, wrong watt-metric zero-sequence currents due to different zero-sequence impedance’s of parallel lines IE 1 IE

Overhead line Cable

IE

IE 2 URN IE 2 IE

Overhead line

IE

Iw’

Iw’

-Iw’ IE 1

IE

Cable

U0

ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F62 of 63

ABB

Earth-fault protection in medium voltage networks Never use auto-transformers in a network with isolated or compensated neutrals 6 kV

3 kV R

UNE = U0 U0 = - URN(6)

URE(3) USE(3) UTE(3)

URN(3) USN(3) UTN(3)

URN(6)

USN(6) UTN(6)

USE(6) UTE(6)

URE(6)

S T

N

Ground U0 URN(6) U0

URN(3)

The zero-sequence voltage is not transformed, because the transformer-neutral is not grounded. The zero-sequence voltage of the 6kV side will be fully effective in the 3kV side. Motors might be damaged.

URE(6) =0

U0

USN(6)

UTE(6)

U0

UTN(3)

U0

USN(3)

URE(3)

UTN(6)

U0

USE(6) UTE(3)

ABB Power Automation Ltd T2403 / T2403_2.ppt / C11e / F63 of 63

USE(3)

ABB