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Topic 7: Pilot Protection
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EE785EE785--Advanced Power System ProtectionAdvanced Power System Protection
Dr. E. A. Dr. E. A. FeilatFeilatElectrical Engineering DepartmentElectrical Engineering DepartmentSchool of EngineeringSchool of EngineeringUniversity of JordanUniversity of Jordan
Introduction
Alternating-current lines are commonly classified by function, which is related to voltage level. Although there are no utility- wide standards, typical classifications are as follows:
Distribution (0.4 to 34.5 kV). Circuits transmitting power to the final users.
Sub transmission (13.8 to 138 kV). Circuits transmitting power to distribution substations and to bulk loads.
Transmission (69 to 765 kV). Circuits transmitting power between major substations or interconnecting systems, and to wholesale outlets. Transmission lines are further divided into:
• Low Voltage (LV):≤
1 kV, 1kV < (MV) ≤
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• High-voltage (HV): 69 to 230 kV
• Extra-high-voltage (EHV): 345 to 765 kV
• Ultra-high-voltage (UHV): greater than 765 kV2
Transmission lines Protection must be compatible with the Protection of all the equipment they connect.
Requires co-ordination of settings, operation times, etc.
Directionality associated with the design of the power system
A radial system can have fault current flowing only one direction
A network can have fault current flowing in either direction
Length of the line has direct effect on the setting of the relays
Relays are applied to protect a given line segment and back up adjacent line segments
Difficult to distinguish between a fault at the end of a line and the beginning of the next
Voltage class must be also considered when applying a relay system
The higher voltage levels would have more complex relay systems3
Introduction
Most faults experienced in a power system occur on the lines connecting generating sources with usage points. Just as these circuits vary widely in their characteristics, configurations, length, and relative importance, so do their Protection and techniques.
There are several protective techniques commonly used for line Protection:
Instantaneous over current
Time over current
Step time over current
Inverse time distance
Directional instantaneous and / or time over current
Zone distance
Pilot relaying4
Protection Type Selection
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Line Classification
Radial Lines or Feeders
Only one positive-sequence source.
Possible zero-sequence contribution to ground faults from both ends.
Distribution lines without synchronous motor load.
Looped Lines
Positive-sequence sources at both ends.
Trip both ends.
Transmission and some distribution lines
Protection Types of Distribution and Transmission lines
Over current Protection:
the simplest, most economical protection type
has found widespread use in distribution utility and industrial systems
limited to radial lines
the addition of directionality extends the application of over current Protection to looped lines.
Distance Protection:
used in many transmission lines.
to increase operating speed, a communications channel can be used to exchange information between directional or distance elements.
this type of arrangement is directional comparison pilot Protection.
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Protection Types
Differential Protection:
can be applied to transmission lines over a communications channel. This arrangement offers the best Protection.
Historically, the current-balance principle served to protect parallel transmission lines. This principle involved comparing the magnitudes of the currents of both lines. A fault at one of the lines created a difference between these currents.
A variant of the differential principle is the phase comparison principle, in which we compare the phase angles of the currents at both line ends.
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Distance Protection
• For the radial system, disregarding the influence of load, the fault current in each phase is balanced and is equal to the phase current measured by the relays at the substation.
This current depends on the following parameters:• System voltage• Line impedance• Distance to the fault• Thevenin impedance equivalent to the system “behind” the
substation bus
The Thevenin impedance depends on the conditions of the system, such as the topology and system loading.
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Distance Protection
This relay is called impedance or “under-impedance” relay because the relay design is such that the relay operates for an impedance condition. The relay measures or “sees” a given impedance, equal to the ratio of the applied sinusoidal voltage and the applied sinusoidal current.
The advantages of the application of a distance relay in comparison to that of an overcurrent relay are:
Greater instantaneous trip coverage
Greater sensitivity (overcurrent relays have to be set above twice load current)
Easier setting calculation and coordination
Fixed zone of Protection, relatively independent of system changes, requiring less setting maintenance
Higher independence of load
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Distance Protection
• This example shows the calculations involved in the determination of a simple impedance relay setting.
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Distance Protection
• This example shows the calculations involved in the determination of a simple impedance relay setting referred to the secondary circuit.
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Distance Protection
This form of polygonal impedance characteristic is provided with forward reach and resistive reach settings that are independently adjustable. It therefore provides better resistive coverage than any mho-type characteristic for short lines.
This is especially true for earth fault impedance measurement, where the arc resistances and fault resistance to earth contribute to the highest values of fault resistance. To avoid excessive errors in the zone reach accuracy, it is common to impose a maximum resistive reach in terms of the zone impedance reach.
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Distance Protection
Under-Reach - Effect of Remote Infeed• A distance relay is said to under-reach when the impedance presented to it
is apparently greater than the impedance to the fault.
Percentage under-reach is defined as:
ZR = intended relay reach (relay reach setting)ZF = effective reach
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Distance ProtectionUnder-Reach - Effect of Remote InfeedThe main cause of underreaching is the effect of fault current infeed at remote
busbars. This is best illustrated by an example.
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Distance ProtectionUnder-Reach - Effect of Remote InfeedThe main cause of underreaching is the effect of fault current infeed at remote busbars.
This is best illustrated by an example.
The relay at A will not measure theCorrect impedance for a fault on line section ZC due to current infeed IB .
Consider a relay setting of ZA +ZC .For a fault at point f, the relay is presented with an Impedance:
So for relay balance:Therefore the effective reach is:
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Distance Protection
Under-Reach - Effect of Remote Infeed
• It is clear from the Equations that the relay will underreach.
• It is relatively easy to compensate for this by increasing the reach setting of the relay, but care has to be taken. Should there be a possibility of the remote infeed being reduced or zero, the relay will then reach further than intended.
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Distance Protection
Over-Reach• A distance relay is said to over-reach when the apparent
impedance presented to it is less than the impedance to the fault.
Percentage over-reach is defined by the equation:
ZR = intended relay reach (relay reach setting)ZF = effective reach
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Pilot Protection
Pilot Protection (or teleProtection) is a generic name for the design of different transmission line Protection alternatives that use a communications channel.
• The most important advantage of pilot Protection is the provision of high speed tripping (permission) or to prevent high speed tripping (blocking) at all terminals for faults anywhere on the line.
• Without pilot Protection, high-speed tripping for all terminals will only occur for faults that are within the area where the zone 1 elements overlap.
• Pilot protection allows over-reaching zones of protection to ensure full protection of the line as well as high speed tripping.
Pilot Protection is typically applied to transmission lines with nominal voltage levels of 115 kV and greater.
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Pilot Protection
It is an adaptation of the principles of differential relaying that prevents the use of control cable between terminals.
Pilot refers to a communication channel between the ends of a line to provide instantaneous clearing over 100% of the line.
The communication channels generally used are:
Power line carrier,
Microwave
Communication cable
Fiber optics.
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Pilot Protection
For comparison purposes, Pilot Protection can be divided into two groups,
• Directional comparison systems• Current-only systems.1. Directional comparison Protection uses the channel to exchange
information on the status of directional or distance elements at both terminals. If both elements operate, there is an internal fault.
2. If one of the elements operates and the other restrains, the fault is outside the protected line.
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Directional comparison Pilot Protection schemes are designed around sending one bit of data across the teleProtection channel at very high speed.
In some schemes, this one bit tells the other end that it has permission to trip (permissive).
In other schemes, the bit represents a signal to tell the other end not to trip (block).
There are many variations but the most prevalent Intertripping Schemes are the following:
Direct Underreaching Transfer Trip (DUTT)
Permissive Overreaching Transfer Trip (POTT)
Permissive Underreaching Transfer Trip (PUTT)
Directional Comparison Blocking (DCB)
Directional Comparison Unblocking (DCUB)
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Pilot Protection (Directional comparison)
1. Direct Underreaching Transfer Trip (DUTT)• When the zone 1 unit of the relays operate, they initiate a signal that is sent
along the communications link to trigger an immediate tripping at the other end of the line.
• The scheme is simple and has the advantage of being extremely fast; however, it has the disadvantage that it may set off undesirable circuit breaker tripping if there is any maloperation of the communication equipment.
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Pilot Protection (Directional comparison)
Bus
Line
Bus
Zone 1
Zone 1
2. Permissive Underreaching Transfer Trip (PUTT)• This scheme is similar to the DUTT scheme, but differs in that
the zone 2 unit at the receiving end has to detect the fault as well before the trip signal is initiated.
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Pilot Protection (Directional comparison)
Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
To protect end ofline
3. Permissive Overreaching Transfer Trip (POTT)
At the minimum, a POTT scheme requires a forward overreaching element at each end of the line. This is typically provided by a Zone 2 element set to reach about 120%-150% of the line length. If each relay sees the fault in the forward direction, then the fault can be determined to be internal to the protected line.
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Pilot Protection (Directional comparison)
Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
Local Local RelayRelay Remote Remote RelayRelay
Remote Remote Relay FWD Relay FWD II GNDGNDLocal Relay Local Relay –– Z2Z2
ZONE 2ZONE 2
Local Relay Local Relay FWD IFWD IGNDGND
TRIPTRIP
Remote Relay Remote Relay –– Z2Z2
POTT TX
ZONE 2ZONE 2
POTT RX
CommunicatioCommunicatio n Channeln Channel
POTT Scheme
4. Directional Comparison Blocking (DCB)
In a directional comparison blocking scheme, each line terminal has reverse looking elements (Zone 3) and forward overreaching elements (Zone 2). The relay will send a block signal to the remote end if it sees the fault in the reverse direction.
Relay detection of a fault in the reverse direction indicates that the fault is outside of the protected zone. The logic allows the relay to trip if it sees the fault in the forward direction and does not receive a blocking signal from the remote end.
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Pilot Protection (Directional comparison)
5. Directional Comparison Un Blocking (DCUB)• Same Basic Logic as POTT Scheme
Allows tripping for short time when channel fails
May Overtrip for External Fault
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Pilot Protection (Directional comparison)
Phase-comparison and current-differential systems only use current information. The figure depicts a schematic diagram of current-only systems.
Phase-comparison systems compare the phase of the currents at the line terminals. For internal faults, these currents are in phase. For external faults, the currents are approximately 180o out of phase
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Pilot Protection (Current based comparison)
OO
Pilot wires
SUBSTATION 1 SUBSTATION 2
O: Operating unitsFault current
Internal fault = Current flow through the operating units= Trip
Load current
Load current = No current through the operating units = No trip
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Pilot Protection (Differential)
The differential current is not exactly zero for external faults.
The most common causes of false differential current in transmission line differential relays are the following:
Line charging current
Tapped load
Channel time-delay compensation errors
Current transformer saturation
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Pilot Protection (Differential)