Protection Systems
D i F
Design
Fundamentals
d
t l
Transmission Line Protection
FRCC System Operator Subcommittee
- Spring Seminars 2011 -
Objectives
• Review Definitions
• Review Purpose of Relays (Protection
Systems)
• Review 5 Design Attributes protection
systems
• Review Types of transmission line
protection
• Review Communications Based Schemes
• Review Protection Redundancy vs Backup
2
1
Purpose of Relays (what ?)
• NERC Definition of “Protection System”:
“Protective relays, associated communication systems,
voltage and current sensing devices,
devices station batteries and DC
control circuitry.
• Merriam- Webster definition of “Relay”:
“an electromagnetic device for remote or automatic control
that is actuated by variation in conditions of an electric circuit
and that operates in turn other devices (as switches) in the
same or a different circuit.”
• Power
P
System
S t
Relaying
R l i
textbook
t tb k definition
d fi iti
off “Relay”:
“R l ”
“equipment that detect abnormal power system conditions,
and initiate corrective action as quickly as possible in order to
return the power system to its normal state”.
3
Purpose of Relays (why ?)
• Silent sentinels that “watch” the power system - Relays are
continuously monitoring the system to detect abnormal condition on Power
System, and initiate corrective actions as quickly as possible in order to return
the power system to normal state
state.
• Minimize power system equipment damage
• Minimize danger to people
• Provide automatic response (no human intervention)
• Remove electrical stress from other equipment
• The power system – one big machine electrically
connected
• Provide a quick response to remove faulted equipment
• Restore the power system to a “normal state”
• Increase reliability of the overall power system
4
2
Minimize the damage
Minimize the effects of faults
2003
Northeast
Blackout
2/26 UFLS Event
- 3 phase fault
- Failed 138 kV switch in Miami
- Disabling of all local protective relay
equipment while troubleshooting
- Led to Delayed clearing of a fault
-Fault remained on the system for:
approximately 1.7 seconds
Resulting effect on frequency
3
Design Attributes of Relays
• 5 Attributes inherent to protection system design
• Reliability
li bili (2
( components - dependability
d
d bili and
d security)
i )
• Selectivity of Relays (zones of protection)
• Relay Speed
• Simplicity
• Economics/Cost
7
Design Attributes of Relays
(cont)
‰ Reliability contains two components:
‰ Dependability – “the degree of certainty that a relay or relay
system will operate correctly” when called upon to trip
– Easy to ascertain dependability by testing
‰ Security – “relates to the degree of certainty that a relay or relay
system will not operate incorrectly” say when there is no fault or
fault is outside the zone.
– Not easy to ascertain, due to wide range of conditions that might
be presented to the relay and control systems. Can’t be
predicted.
– How does FRCC track and learn from occurrences ?
Misoperations !
8
4
Reliability of Relays
‰ Reliability’s 2 components:
‰ Dependability – Two independent redundant systems increase certainty, backup, secondary, multiple redundant
schemes. Redundancy will be discussed later.
‰ Security Two independent series systems - will not operate
incorrectly
9
IMPROVE DEPENDABILITY
• Primary Relay scheme is dedicated to a
specific zone of protection. A Primary relay
scheme operates with no intentional time
d l
delay.
• Redundant Relay scheme does the exact
same function as the primary and provides
redundant protection
• Back-up Relay Scheme is usually time
delayed. It will typically remove more of the
system elements than required by operation
of the primary relay scheme.
5
Dependability: Backup, primary,
secondary, redundancy
Security
6
Design Attributes of Relays
(cont)
‰ Selectivity – Minimum system disconnection for
isolation of faulted equipment - Zone of Protection
‰ Generally a zone of protection is designed for each
system element. The faulted element is isolated. Some
cases more than one elements is combined in one zone
of protection.
DEDICATED RELAYS FOR EACH ZONE
Transmission line Generators
Substation Bus
Capacitor bank
Reactor
Circuit Breaker
Transformers
Dist. Feeder
Combined Elements: Two or more of the above elements.
13
7
ZONES OF PROTECTION
Note: Not to
be confused
with
“Zone 1” ,
“Zone 2”,
“Zone 3”, to
be discussed
later
15
SPEED: minimize the damage
8
Design Attributes of Relays
(cont)
‰ Speed – Attribute to remove a fault as quickly as
possible
‰ 2/26/08 – FRCC UFLS Event
The initiating event was a three phase fault on a failed 138 kV switch at a
transmission substation located West of Miami, Florida. The disabling of all
local protective relay equipment while troubleshooting a transmission
switch led to delayed clearing of a fault that developed on the switch.
The fault remained on the system for approximately 1.7 seconds.
• “delayed clearing”
– Typical clearing time - primary protection – 0.067
0 067 seconds
– Typical local backup clearing time – 0.255 seconds
The depressed voltages in the area of the fault led to protective equipment trips
of the two Nuclear generating units at Turkey Point as well as the loss of
additional fossil generation at that and other sites in the Region .
17
Design Attributes of Relays
(cont)
‰ Speed – definitions
‰ “Instantaneous” – relay operates without
intentional time delay (inverse time delay
characteristic)
‰ “Time Delay” – an intentional time delay is
inserted between relay decision and operation
‰ “High-Speed” – relay operates in less than 50
mS
S typical
i l (3 cycles)
l )
‰ “Ultra High Speed” – relay operates in 4 mS or
less (non-standard but currently typical
18
9
What design
attribute is
this ?
Design Attributes of Relays
(cont)
Simplicity – Fewer components and attributes in a design result in:
• A system with fewer potential Human and Equipment Performance
issues
• Usually more cost effective, both immediate and life-cycle
• Easier to troubleshoot when issues occur
20
10
Design Attributes of Relays
(cont)
Economics - Protection & Control systems typically range
from 10 – 15% of the cost of the equipment they protect,
down from 15 – 20% jjust ten – fifteen yyears ago
g p
prior to
uprocessor technology coming to market.
The more equipment you install the more money is required
– cost benefit vs dependability and security equation –
You use overlapping
pp g zones /
backup protection etc.
Critical equipment you install Fully redundant systems
21
Are we done yet ?
We’re just getting started !
Any Questions on Design Attributes
22
11
Transmission Line Protection
Road Map to the End
• TLP Options:
• Overcurrent relays
– Instanstaneous
– Time delay
• Directional Overcurrent
• Non-pilot
– Step
Step-Distance
Distance Relays
– Zones 1, 2 & 3
• Pilot Protection
• Carrier types and
communication at a high
level
• Cover some relay types
• Define
– “Primary Protection”
– “Backup Protection”
– “Redundancy
• “Redundancy”
• Communication Outages
23
The Goal of Line Protection
To detect transmission line faults and
initiate isolation of that line fault 3-5
cycles.
cyc
es A fault
au t du
duration
at o o
of longer
o ge time
t e (8
plus cycles) can cause instability.
Typical Transmission line
Substation A
Substation B
12
Overcurrent Protection
•
•
•
•
Only looks at line current
Simple – setting (normal load +25%)
Does not see faults “behind” - no fault current
C
Can
coordinate
di t with
ith relays/fuses
l
/f
downstream
d
t
– Add time delay
– Coordination for all types (10 types) of faults
becomes difficult (phase to phase, phase to
ground, 3 phase etc.)
Radial Load
Substation A
Ohm’s Law:
V=IxZ
I = V/Z
Relay trip when
I = set current flow
Distance or Impedance Relay
• As the protected system gets more
complex
• Available changes in configuration
(network)
• Changes in generating patterns
• Wider variation in fault current than
radial
• Can’t just use current monitoring for
reliable protection
• We need to look at current AND voltage
13
Distance or Impedance Relay
Look at Current and
Voltage to detect fault
V = 100 V
Conclusion: if we know voltage and
current we can estimate the
impedance
If we know the conductor and
construction (impedance per mile
of transmission line is fairly
constant)
t t)
Ohm’s Law:
V=IxZ
Z = V/I
THE LINE CONDUCTOR AND
CONSTRUCTION IS SAME,
DISTANCE IS PROPORTIONAL TO
IMPEDANCE
We can estimate a where a fault is
based on measured voltage and
current on the line
Distance or Impedance Relay
How to know if line has a FAULT??
• We know the properties of the transmission line
– Length
– Impedance
• We know the current on the line from the
substation
• We know the voltage on the line bus at the
substation
• Ohms Law
– V = I x R or V = I x Z (for Impedance)
• We can identify a set of “Z”s for faults and set
the relay to pick-up
14
Step Distance Relaying
Its hard to pinpoint the “end” of the line “Z”
– use “zones”……
The “art”
art of Relaying –
Company PHILOSPHIES may vary
Zone 1
Zone 2
80%
120%
Zone 3
Typical setting
of zones
120% of combined
29
Step Distance Line Protection
why 80%, 120% etc
• Create Zones of Protection
• Z1 = 80%
line impedance no time delay
dela
• Z2 = 120% line impedance time delay• Z3 = 120% line impedance and 120% of
next zone; with a time delay – backup next
zone
15
Limitations of Step Distance
Line Protection
• Problem: can’t clear both ends of the line
instantaneously for faults near one end
• As loading increases the “Z” viewed by the
relay may cross into the “Z” trip area of the
relay setting
• Harder to coordinate multi-terminal and
parallel line
• So far – all information for “tripping decision”
has come from one substation
Pilot Protection
• Provides instantaneous protection over
entire line section
• 2 modes “blocking mode” (integral to line)
or “tripping
tripping mode”
mode (independent comm
channel)
• Communicate information from one
terminal to the other terminal (in the zone
of protection)
• Communication methods:
–
–
–
–
Power line carrier
Microwave
Fiber optics
Pilot cable
• Speed up remote end clearing time
16
Pilot Protection
• Schemes can be classified as:
– Directional Comparison
– Phase comparison
– Pilot wire
Depending on type of sensing used
• Further described as :
– Blocking
– Unblocking
– Transfer trip
Depending on how transmitted signal is used
• Transfer trip is further categorized as:
– Direct
– Permissive underreaching
– Permissive overreaching
Types of Communication Channels
1. Power line carrier – On/Off, FSK
ƒ Reliable, expensive, noise, freq. availability
2. Pilot wire – dedicated 2 wire copper circuit
ƒ Expensive
3. Leased line – digital or audio tone
ƒ Dedicated channel, low installation cost
ƒ Reliability, ongoing costs
4. Microwave – digital or analog
ƒ Capacity,
C
it low
l
noise/location,
i /l
ti
weather
th
5. Fiber optic – digital
ƒ Noise immunity/network delays, cost
34
17
Current Differential
87L
87L
Communication channel
Basic End to End
Communication Scheme
21
21
18
Permissive Overreaching
Transfer Trip
1. LOCAL END DETECTS FAULT, SEND PERMISSIVE TRIP
2. REMOTE END RECEIVES PT, IF REMOTE SEES FAULT, IT TRIPS
AND SENDS PT TO OTHER END
3. CONSIDER FAULT INSIDE AND OUTSIDE
Directional Comparison
Blocking
1. LOCAL END DETECTS FAULT, DOES NOT SEND BLOCK TRIP
2. REMOTE END SEES FAULT AND IT DID NOT RECEIVE BLOC FROM REMOTE, IT
TRIPS. ALSO, IT DOES NOT SEND BLOCK. PASSIVE PERMISSION
3. FAULT CLOSE BY AND OUTSIDE, BLOCK IS SENT AND TRIP IS BLOCKED.
4. CONSIDER FAULT INSIDE AND OUTSIDE
19
Loss of Blocking Communication
Example #1 Adjacent line fault with Blocking scheme Channel
To Ranch
1) No communications on Cedar- Corbett 230 kV line
Possible temporary
2) Fault on Cedar – Deltrail 230 kV line (close to Cedar) remedy: Study opening line
3) Corbett terminal sees fault on Deltrail line (zone 2) at a distribution sub, which
should only allow
4) Corbett terminal does not receive blocking signal
instantaneous zone 1 trips.
5) Corbett breakers 8W45 and 8W99 trip high speed
6) Both Cedar and Corbett will trip for any zone 2 fault
Loss of Communication
Example # 2 close in fault on line with blocking scheme Channel
To Ranch
1) No communications on Cedar- Corbett 230 kV line
2) Fault on Cedar – Corbett 230 kV line (close to Cedar)
3) Cedar sees zone 1 fault - trips 8W62 & 8W48 instantaneously
4) Corbett terminal sees fault on zone 2 and trips high speed
Internal – NO issue
5) Breakers 8W45 and 8W99 at Corbett trip
6) All faults on Cedar-Corbett line will still be cleared high speed
20
Communication Based Schemes
Pilot Protection,
Protection Differential,
Differential and Directional comparison
comparison. DCB
DCB, POTT
cover in a little bit detail, Mention DUCB, PUTT, DUTT.
Discussion / Simulation
41
Road Map Continued
• Pilot Protection Schemes
• Directional Comparison,
– POTT
– PUTT/DUTT
– DCB
– DCUB
CURRENT DIFF
POTT- next
21
FRCC Handbook Procedure
43
REDUNDANCY
Protection system components (below) make up a
protection system. In some cases, for redundancy
two system are provided. A complete “Chain” of
components make up a system.
AC Current Source
AC Voltage Source
Protective Relay
Communication Channel -
DC Control Circuitry
Auxiliary Relay
DC Source
Trip Coil
44
22
REDUNDANCY
Non-redundant system
Single DC
P
Power
supply
l
45
REDUNDANCY
Fully-redundant system
Diverse DC
power supply
46
23
Typical DC Control Circuit
47
Protection System Outages
If a protection system (chain of components)
does not meet all of the requirements
described above
above, then the protection system
is not redundant.
From an operations standpoint –
- is the remaining protection “redundant”
back up
- Is the remaining protection “back-up”
- Understand what “secondary” means
- What elements will be tripped for a fault in
the affected protection zone
48
24
49
• Questions ?
25