1
Power System Protection
Ground faults in resonance
grounded systems
2
Briefly about the speaker
• Professor at Norwegian Univ. Science and
Technology – Dept. Electrical Engineering
– Power system transients and protection
– High
Hi h voltage
lt
engineering,
i
i
stress
t
calculations
l l ti
– Recent focus on Power Transformers
• Developer of ATPDraw
• Sabbatical at MTU
– Room 628
628, phone 487
487-2910
2910
– hhoidale@mtu.edu
3
Solid vs. resonance neutral
+ No over-voltages in fault
situations
- High
Hi h ffault
lt currentt
+ Easy fault detection
+ G
Ground
ou d faults
au s pe
persist
ss
• Fast trip and reclosing
- Poor power quality
- Extra stress
-
Undefined voltages to ground.
High at resonance.
+ Ideally zero fault current. Arc
self-extinguish.
- Difficult fault detection/location.
Requires special competence
and maintenance.
- In dynamic systems the coil
must adapt. Expensive
equipment
equipment.
+ Can continue to operate during
ground fault. Increased power
q
quality.
y
• Safety issues: down/broken
conductors
4
How can be reduce fault current further
from isolated neutral systems?
• Compensate capacitive current with inductive.
• Resonance principles:
I
C
L
+
V
-
 1

V
I  V 
 j C  
 1  2 LC
 j L
 j L


At resonance (2LC=1) the current is zero
L=
(3Cg)-11
Cg
5
Usage of resonance grounding
Dy
Dy
• In Norway
– MV system
y
12-24 kV
– Distribution level 66-132 kV
• Self-tuning Peterson coils used
• Weather conditions result in a lot of temporary
ground fault
• Power quality is very important to the industry
– Fast trip & reclosing not acceptable
• Increasingly a preferred solution in Europe
6
Zero sequence measurements
• Current I0: Sum Ia, Ib, Ic

– Summing the current numerically
– Residual connection
– Summation transformer
(Toroidal/Rogowski coil)
3I0
3I0
I0
Lower
accuracy
due to CT
differences
• Voltage V0: Sum Va, Vb, Vc
– Open delta
3V0
– Neutral point (isolated or resonance earth)
V0
7
Resonance grounding
• Inductance added in the neutral point
• Compensate for the capacitive current through Cg; help
the arc self extinguish
• The coil can be automatically tuned when the system
changes (Petersen-coil (patent from around 1920)
– Adjust air gap
– The coil changes it value incrementally and notice how the
neutral voltage changes. From a theoretical curve the resonance
point is estimated.
• Over-compensation preferred to limit VN
– Avoid resonance also when parts of the system become
disconnected
• The unit Ampere is used to quantify the coil; IL=Vp/L
8
Variable inductance
N
• Adjust air gap
hVpa
h2Vpa
Vpa
TRENCH
L
C
Increases with
asymmetry and
resistance
VN
Normal
point of
operation
R
Resonance
IL~1/L
1/L
No-fault situation
9
First attempt to analyze the
neutral voltage
• Need to know: Maximum neutral voltage in
normal operation. Set U0 pick-up above this limit
N
hVpa
h2Vpa
Vpa
L
3I0
I a  (V N  V
pa
)  j C
I b  (V N  h 2  V
I c  (V N  h  V
pa
pa
)  j C
)  j C
3 I 0  ( I a  I b  I c )  V N  j 3C 
C
 VN 
0
??
j 3C  1 / j L
• At resonance j3C+1/jL=0 and the neutral
voltage becomes undefined
undefined… (→ ∞))
• Need to add sophistication here…
V N
j L
10
Effect of asymmetry and
conductive line charging
N
Increases with
asymmetry and
resistance
hVpa
h2Vpa
VN
Vpa
L
G
C
Normal
point of
operation
C+C
Under-comp. Over-comp.
I a  (V N  V
pa
I b  (V N  h 2  V
I c  (V N  h  V
Resonance
)  (G  j C )
pa
pa
No-fault situation
)  (G  j (C   C ))
)  (G  j C )
3 I 0  ( I a  I b  I c )  3V N  ( G  j  C )  (V N  h 2  V
 VN 
 h 2 V
pa
 j  C
3G  j 3C  j  C  1 / j L
 VN 
pa
)  j  C 
Vp
 VN
s
d
1
 j
u
u
V N
j L
,m ax

V p C
3G
IL~1/L
11
Resonance grounded system
d i fault
during
f l ((L=constant))
• Equivalent circuit (fault phase A)
Vpa
N
+
3G
L
3Cg
VN  V0 
A
If
Rf
V pa  jL
jL  R f (1   L  3C g )
2
If 
V ppa  (1  2 L  3C g )
jL  R f (1  2 L  3C g )
Vpa at resonance,
also for large Rf
• In reality asymmetry (C) and conductive
elements (G||1/Rf) affect fault current and VN
• A resistance across L is often actively connected
to “find” the fault (identify faulty feeder)
12
Example of neutral voltage
• From ATP simulations
– In reality the resonance peek is typically lower
– …and measurements are required
35
Stay away from this
resonance area
Neu
utral voltage [kkV]rms
30
LCC
100 k
50. km
LCC
LCC
LCC
50. km
50. km
20
Fault 0k
15
Fault 3k
10
No‐fault
1 pu
5
U
Y
25
Normal point
of operation
V
SAT
50. km
1
M
WRITE
max
min
0
0
0.1 nF
0-3k
2
4
6
8
10
12
Coil "current" [A]
14
16
18
20
13
Directional over-current relay –
resonance grounded
d d systems
• Start when V0 goes above a fixed limit; premeasure the coil response max(VN,healty);
• After a delay connect resistor R (|| to L)
• Trip if current I0 enters trip zone (2nd quadrant)
• Trip zone set based on natural conductive
current and asymmetry
I01
-I0
I02
I0CG
I03
R
L
For each feeder
I0L
V0
V0
G
VN
C
I0R
Trip zone
H lth
Healthy
IL
14
Trip zone
• The resistive component of I0 (in phase
with V0) is used
-II0
-II0
Trip zone
I0CG
V0
For each feeder
I0L
V0
R
Healty feeder
R
Faultyy feeder
I0CG
 3 I 0 i  ( I ai  I bi  I ci )
 V 0  (3G i  j 3C i )
Angle quite close to 90°
I0R
T i zone
Trip
 3 I 0 i   ( I ai  I bi  I ci )
 V 0  ( 3 G i  j  3 C i )  (V 0  V
pa
)/ R
f

1 
 V 0   3(G t  G i )  j 3(C t  C i ) 

j L 

15
Relaying logic
• Is zero sequence voltage above threshold?
– V0>Vlim (type 59G over-voltage relay)
• Is zero sequence current above threshold
and in correct quadrant (4th)?
– I0>Ilim
I0 Ili (type
(t
67N di
directional
ti
l over-currentt relay)
l )
+
59G (V0>Vlim)
-
67N (I0>Ilim)
52 trip
16
Wave/Wischer relays
• Used in the 132 kV resonance earthed system
y
• Measure the transient surge and use time of
arrival principles
• Communication required
• Trip the two relays that first identify the fault
• Sensitive
S
i i to di
disturbances
b
lik
like lilightning
h i