Showing posts with label Transmission Lines. Show all posts
Showing posts with label Transmission Lines. Show all posts
TRANSMISSION LINE PROTECTION LENGTH CONSIDERATIONS BASICS
Short lines (SIRs > 4)
Typical protection schemes and communication channels used are as follows:
— Current differential (FO, PW, AT, MW)
— Phase comparison (FO, PW, AT, PLC, MW, R)
— POTT (FO, PW, AT, PLC, MW, R)
— Directional comparison blocking (FO, PW, AT, PLC, MW, R)
Short lines result in a small current magnitude difference and minimal voltage drop difference between close-in and remote faults. Non-pilot overcurrent relays usually cannot be set to discriminate between internal and external line faults. Non-pilot distance relays may be able to discriminate between internal and external line faults for lines with an SIR as high as 20.
However, it may not be possible to coordinate non-pilot distance relays on adjacent long lines with the short line distance relay zones. Therefore, pilot relay schemes utilizing communication channels are used. The most effective forms of pilot protection for short lines include current differential, phase comparison, POTT, and directional comparison blocking schemes.
None of these schemes require distance elements to be set for less than the line impedance. Schemes utilizing Zone 1 distance elements should provide the capability to handle arc resistance or fault impedance, which can be significant compared to the line impedance.
Current differential and phase comparison systems do not provide remote backup for adjacent system elements. However, non-pilot overcurrent and distance relay schemes can be applied as backup protection, provided time delays are adjusted to provide coordination.
Medium lines (SIRs < 4, but > 0.5)
Typical protection schemes used are as follows:
— Phase comparison (AT, PLC, MW)
— Directional comparison blocking (AT, PLC, MW)
— Permissive underreaching transfer trip (FO, MW)
— POTT or unblocking (FO, MW, PLC)
— Step distance
— Step or coordinated overcurrent
— Inverse time overcurrent
— Current differential
Lines with SIRs less than four allow more effective relay discrimination. Relay schemes utilizing under reaching elements can now be set. Zone 1 can be set to underreach the remote end and still protect 80 90% of the line.
The last 10–20% of the line can be protected by a Zone 2 overreaching element. Since selectivity and sensitivity can usually be met, the speed with which a fault needs to be cleared may require the application of pilot relaying. If slow fault clearing is acceptable, simple step distance or overcurrent relays can be applied.
Long lines (SIRs < 0.5)
Typical protection schemes used are as follows:
— Phase comparison (PLC, MW)
— Directional comparison blocking (PLC, MW)
— PUTT, POTT, or unblock (AT, PLC, MW)
— Step distance
— Step or coordinated overcurrent
Long lines have small SIRs, and most long lines are extra high voltage (EHV) or ultra high voltage (UHV) lines. EHV and UHV lines almost always require high-speed tripping of all terminals for stability purposes and to minimize damage caused by the fault.
Long lines can also have other system elements included in the line, such as series capacitors. This makes the total line impedance variable under certain conditions and introduces transient behavior that makes selectivity difficult. Phase and directional comparison protection schemes over PLC or MW are well suited to this type of application.d to provide coordination.
TRANSMISSION LINE RELAYING SELECTION TUTORIALS
The selection of line protection
requires the consideration of several factors, some of which are
mutually exclusive. Knowledge of the most probable failures,
recommendations of equipment suppliers, and good practical judgment
can assist the protection engineer in determining which of the
factors deserve the most emphasis.
One of the most important design
considerations of relaying is reliability. Relaying reliability is
separated into two aspects: dependability and security. Dependability
is defined as “the degree of certainty that a relay or relay system
will operate correctly.
” Security is defined as “the
degree of certainty that a relay or relay system will not operate
incorrectly.” In other words, dependability is a measure of the
relay’s ability to operate when it is supposed to operate. Security
is a measure of the relay’s ability to avoid operation for all
other conditions for which tripping is not desired.
Dependability is relatively easy to
obtain in relay design or in the application of a number of relays.
Testing using operating conditions, fail-safe designs, and redundancy
are methods to ensure dependability. Security is harder to attain; an
almost infinite variety of tests would be needed to simulate all
possible conditions to which a relay may be exposed.
Various engineering practices can
enhance dependability. These include independence of design,
different operating principles, redundancy within the relay systems,
local backup methods, remote backup methods, and application of
relays and relay systems that cause undesirable trips upon failure.
Security can be enhanced by using
relays that fail into a “disarmed” mode, series connected
protection, improved monitoring and self-checking, and emphasis on
high-quality components. Another important design consideration is
protection selectivity or coordination.
Selectivity is the ability of relays
and relaying systems to cooperate with each other to minimize the
outaged area resulting from a fault. Coordination refers to the
process of applying relays to operate as fast as possible for
conditions with their primary zone, but to have delayed, or
coordinated operations for conditions within an extended backup zone.
Selectivity and coordination must be
achieved to ensure maximum service continuity. Fault clearing time is
an important consideration in the selection of line relaying.
Requirements for relaying speed must be carefully determined. If the
relaying is too slow, system instability, excessive equipment damage,
and adverse effects on customer service may result.
However, faster protection tends to
compromise relay system security and selectivity. There is a limit to
the speed with which a relay can correctly respond due to the
transients present in the power system itself.
Sensitivity of protection refers to the
minimum operating quantities that must be available for the relays to
detect an abnormal condition. While this factor is still important,
most modern relays are using solid state or microprocessor
technologies that are many times more sensitive than their
electromechanical predecessors.
Certain problems, such as
high-impedance ground faults, inherent system voltage unbalances, and
high source-to-line impedance ratios (SIRs) still challenge the
sensitivity of relays and should be considered in relay selection.
The line protection design may often
fail to recognize one of the more important design factors
simplicity. The multifunction and programmable capabilities of modern
relays have created an abundance of special solutions to possible
system problems.
The implementation of these solutions
challenges the application engineer, those responsible for setting
the relays, and operations and maintenance personnel. The problems
caused by incorrect or incomplete implementation of overly complex
protection may create more serious consequences than not providing
special solutions.
The protection engineer should
carefully weigh the consequences and probability of each problem to
determine if it justifies using complex special solutions. Economic
evaluations of protection options will continue to be necessary.
Protection engineers have long pointed to the relatively low cost and
high importance of relays compared with the equipment they protect.
However, it is fundamental to attempt
to achieve the required protection at the lowest cost. In recent
years, more importance has been placed on economic analysis that
considers more than just the lowest initial cost. Installation and
maintenance costs, as well as the cost of unreliable protection, are
sometimes considered.
In addition, modern protection usually
offers many features not previously available that may result in
improvements in operations, restoration of the system, and post-fault
analysis. The value of these improvements should be considered in a
complete economic evaluation of alternatives.
Certain transmission system
configurations and characteristics require protective relaying with
communication systems to provide high-speed clearing for all faults
within the zone of protection. Pilot relaying schemes are employed to
provide this high-speed fault clearing.
These pilot relaying systems require
the transmission of information during a fault between relays that
are located in different substations to determine if the fault is
internal or external to the zone of protection. This is accomplished
using several different methods, ranging from direct hardwire
communications to fiber optic communications systems.
Different protection schemes and
communication channels have different degrees of reliability.
Knowledge of these different communication options is necessary to
determine the reliability of the protection scheme being considered.
The network configuration and local system loading requirements may
also affect the type of pilot protection scheme chosen.
The protection scheme required will
often dictate the type of relaying communications that will be used.
On the other hand, the relaying communications may limit the types of
relay schemes that can be used. Obviously, the requirements of the
relaying scheme must be considered, along with the types of
communications available.
NON DIRECTIONAL OVERCURRENT RELAY SCHEMES FOR TRANSMISSION LINES
Overcurrent protection is the simplest and least expensive form of fault protection that can be placed on transmission lines. The operating principles depend only on current magnitude. The ac connections for three-phase and one ground time overcurrent (TOC) relays and instantaneous overcurrent (IOC) relays are shown in below.
 |
| Connections for overcurrent phase and ground relay |
Negative and zero-sequence overcurrent relays do not operate for balanced loads or for three-phase faults, but can have pickup settings well below the expected load. Tripping may be instantaneous, delayed for a fixed time, or delayed for a time inversely proportional to the current magnitude.
Figure below shows some of the various shapes of time/current characteristics that may be used.
TYPICAL TOC CURVES
Time Dial 5
Multiples of pu
 |
| Time overcurrent curve shape comparison |
To protect an entire nonradial line, time delays are generally required to achieve coordination with downstream protective devices.
Figure below shows how coordination is achieved between a relay with a time and instantaneous element (the primary relay), and an upstream (backup relay) with only a time element.
 |
| Coordination of time overcurrent relays |
The pickup value of the time element should be set to prevent tripping for the maximum load current that can flow in either direction on the line. The time adjustment (i.e., dial) should generally be set to produce the fastest operating time that will not result in miscoordination with other protection behind or in front of the terminal.
The effect of varying the time adjustment is illustrated in Figure below for a typical time overcurrent relay.
Very Inverse
Effect of Time Dial
 |
| Varying time adjustment on overcurrent relay characteristic curves |
This makes the coordination of nondirectional relays more difficult, and sometimes impossible. In some applications, these relays may be applied only at the terminal with the higher fault current source.
SYMMETRICAL COMPONENTS CALCULATION FOR TRANSMISSION LINES
Determining impedances for transmission lines is more challenging and generally involves making calculations from the physical parameters of the line and its conductors. The algorithm and equations given in this subclause describe the procedure, and experienced protection engineers find understanding the theoretical basis for this procedure helpful.
All the equations given in this subclause are for 60 Hz systems; impedances for systems at other frequencies can be determined by ratio or by modifying the formulae. Alternatively, computer programs are available to calculate line impedances.
The first consideration is that the positive- and negative-sequence impedances of transmission lines are equal. A transmission line is a passive component that responds in the same way to positive- and negative-sequence excitation.
Because sequence impedances are the relationships between respective sequence voltages and currents, calculation of one impedance suffices for both needs.
The positive-sequence reactance of a transmission line can be thought of as the impedance that would relate voltage and current when the three conductors or a transmission line are shorted together at one end, while excited by a positive-sequence source of voltages at the other end. This impedance can be calculated using the following equation:
where
GMD is the geometric mean spacing between phase conductors (e.g., the cube root of the product of the three-phase spacings) (m),
GMR is the geometric mean radius of the phase conductor (m).
GMD should be calculated for the specific spacings of the array of conductors making up the transmission line, while GMR is a parameter for the conductor that is available from the conductor manufacturer.
Positive-sequence resistance can be read directly from conductor tables. Calculating the zero-sequence impedance of a transmission line is more challenging. The concept can be viewed as follows:
All three phases of a transmission line are shorted together to ground at the source end, while all three conductors are shorted together and to both ground and the overhead ground wire (OHGW) at the other end.
When a single phase source of voltage is then applied at the source end, a current flows. The ratio of the single-phase driving voltage to the resulting current flow is the zero-sequence impedance of the line.
Physically, current flows from the faulted conductor into both ground and the OHGW as depicted in Figure 2-6a; the current flows from the source out through the phase conductors and returns through a complex path consisting of the OHGW and the earth.
Figure 2-6a—Illustration of insulation flashover on open wire line showing return current flowing through OHGW of transmission line and through earth
From this physical picture, it is apparent that the zero-sequence impedance should, therefore, consist of three branches as indicated in Figure 2-6b: the zero-sequence impedance of the phase conductors, the zero-sequence impedance of the static wire return (OHGW), and the zero-sequence impedance of the earth return.
Figure 2-6b—Zero-sequence equivalent circuit that accounts for self impedance of transmission line and the impedances of earth and OHGW return paths
Values can be calculated for the various branches of Figure 2-6b using the following equations:
where
Ra is the resistance of the phase conductor (Ω/km),
GMD2 is the geometric mean spacing of all conductors—phase and static (OHGW) wires (m),
GMR2 is the geometric mean radius of k static (OHGW) wires (m),
k is the number of static (OHGW) wires,
r is the earth resistivity (typically 100) (Ω⋅m),
Rgw is the resistance of one ground wire (Ω/km),
f is the system frequency.
HIGH-VOLTAGE (HV) TRANSMISSION LINE PROTECTION SCHEMES
Most transmission lines are protected by
directional distance relays. These may serve as backup protection to other
schemes in service, or these may be the sensing components in various forms of
differential protection.
Figure below shows the basic elements of a
PLC (power line carrier) system extensively used for protection of HV
transmission lines.
An HV transmission line is capable of
simultaneous functions of communications and electrical energy transmission.
PLC equipment consists of three distinct parts: terminal assemblies consisting
of transmitters, receivers, and protective relays; the coupling and tuning
equipment, which connects the terminals to selected points on the transmission
line; and the transmission line itself, which provides a suitable channel for
the transmission of carrier energy in PLC bands of frequencies between
terminals.
Coupling to lines is accomplished by means
of high-voltage capacitors, which provide a low-loss path to carrier signals
and block 60 Hz power frequency energy from the carrier equipment. Line traps
minimize the loss of carrier power into adjacent lines and prevent external
ground fault currents from short-circuiting the carrier signal of the unfaulted
line.
Carrier Frequencies.
Frequencies in the range of 30 kHz to 500
kHz have been employed for PLC relaying and other communication purposes. The
range is high enough to be isolated from the transmission line and the noise it
creates and yet not so high as to give rise to excessive attenuation.
There are two basic types of signals used
for teleprotection channels. Keyed carriers are sometimes referred to as AM,
amplitude modulation: It is normally off and intelligence is transmitted by
turning the carrier on and off.
This type of signal is normally used in
blocking-type relaying systems. The frequency may be in range from 29 kHz to 31
kHz, and the signal could be applied to a single sideband (SSB) PLC channel.
Frequency Shift Keyed Carrier.
This signal is always on, which provides a
means of continuously monitoring the channel. The frequency shift keyed (FSK)
carrier is less susceptible to noise and has a greater operating range. FSK
channels have been available with two-frequency operations, high and low shift
frequencies for additional security.
Blocking Schemes.
Transmission line faults are detected using
either high-speed phase comparison, in which the phase of the currents at the
two terminals are compared, or direction comparison relaying. The scheme
operates in a trip permissive mode.
A received signal is used to block tripping
of the protected line for external faults. The blocking scheme is biased toward
dependability because channel or remote relay failure will result in operation
of the local blocking relay.
Tripping Schemes.
A phase comparison tripping scheme channel
is keyed to the trip condition every half cycle during the fault. The scheme is
biased toward security so that a failure of the channels or relays would result
in non operation of the local relay for external and internal faults.
Directional distance relays can be used both for phase as well as ground fault
conditions.
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