Transmission line
protection
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| Multi Bus Transmission Line |
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| Multi Terminal Transmission Line |
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| Zone Protection |
As you already know, the real
purpose of transmission line protection is to detect faults or abnormal
operating conditions and to initiate corrective action. The most common
parameters which reflect the presence of a fault are the voltages and currents
at the terminals of the protected apparatus or at the appropriate zone
boundaries.
The fundamental problem in
power system protection is to define the quantities that can differentiate
between normal and abnormal conditions
Relaying Features
1. Reliability
Reliability, in system
protection parlance, has special definitions which differ from the usual
planning or operating usage. A relay can misoperate in two ways: it can fail to
operate when it is required to do so, or it can operate when it is not required
or desirable for it to do so.
To cover both situations, there
are two components in defining reliability:
1.
Dependability –
which refers to the certainty that a relay will respond correctly for all
faults for which it is designed and applied to operate.
2.
Security –
which is the measure that a relay will not operate incorrectly for any fault.
Most relays and
relay schemes are designed to be dependable since the system itself is
robust enough to withstand an incorrect trip-out (loss of security),
whereas a failure to trip (loss of dependability) may be catastrophic in terms
of system performance.
2. Zones of
Protection
The property of security is
defined in terms of regions of a power system – called zones of
protection for which a given relay or protective system is
responsible. The relay will be considered secure if it responds only to faults
within its zone of protection.
Figure 1 shows typical zones of
protection with transmission lines, buses, and transformers, each residing in
its own zone. Also shown are ‘‘closed zones’’ in which all
power apparatus entering the zone is monitored, and ‘‘open’’ zones, the limit
of which varies with the fault current.
Closed zones are also known as ‘‘differential,’’
‘‘unit,’’ or ‘‘absolutely selective,’’ and open
zones are ‘‘non-unit,’’ ‘‘unrestricted,’’ or ‘‘relatively selective.’’
3. Relay Speed
It is, of course, desirable to
remove a fault from the power system as quickly as possible.
However, the relay must make its decision based upon voltage and current waveforms,
which are severely distorted due to transient phenomena that follow the
occurrence of a fault.
The relay must separate the
meaningful and significant information contained in these waveforms upon which
a secure relaying decision must be based. These considerations demand that the
relay take a certain amount of time to arrive at a decision with the necessary
degree of certainty.
The relationship between the relay response time
and its degree of certainty is an inverse one and is one of
the most basic properties of all protection systems.
Although the operating time of
relays often varies between wide limits, relays are generally classified by
their speed of operation as follows:
1.
Instantaneous —
These relays operate as soon as a secure decision is made. No intentional time
delay is introduced to slow down the relay response.
2.
Time-delay —
An intentional time delay is inserted between the relay decision time and the
initiation of the trip action.
3.
High-speed —
A relay that operates in less than a specified time. The specified time in
present practice is 50 milliseconds (3 cycles on a 50 Hz
system).
4.
Ultra high-speed —
This term is not included in the Relay Standards but is commonly considered to
be operation in 4 milliseconds or less.
4. Primary and
Backup Protection
The main protection system for
a given zone of protection is called the primary protection system.
It operates in the fastest time possible and removes the least amount of
equipment from service.
On Extra
High Voltage (EHV) systems 345kV and above, it is common to
use duplicate primary protection systems in case a component in one primary
protection chain fails to operate. This duplication is therefore intended to
cover the failure of the relays themselves. One may use relays from a different
manufacturer, or relays based on a different principle of operation to avoid
common-mode failures.
The operating time and the
tripping logic of both the primary and its duplicate system are the same.
It is not always practical to duplicate every
element of the protection chain. On High
Voltage (HV) and EHV systems, the costs of transducers and circuit breakers are
very expensive and the cost of duplicate equipment may not be justified.
On lower voltage systems, even
the relays themselves may not be duplicated. In such situations, a backup set
of relays will be used. Backup relays are slower than the primary relays and
may remove more of the system elements than is necessary to clear the fault.
5. System
Configuration
Although the fundamentals of
transmission line protection apply in almost all system configurations, there
are different applications that are more or less dependent upon specific
situations.
5.1 Operating
Voltages
As voltage ratings in Nigeria, Transmission
lines will be those lines operating at 132 kV and above, subtransmission lines
are 33 kV to 132 kV, and distribution lines are below 33 kV. These are not
rigid definitions and are only used to generically identify a transmission
system and connote the type of protection usually provided.
The higher voltage systems would normally be
expected to have more complex, hence more
expensive, relay systems. This is so because higher voltages have more
expensive equipment associated with them and one would expect that this voltage
class is more important to the security of the power system.
The higher relay costs,
therefore, are more easily justified.
5.2 Line Length
The length of a line has a
direct effect on the type of protection, the relays applied, and the settings.
It is helpful to categorize the line length as ‘‘short,’’ ‘‘medium,’’
or ‘‘long’’ as this helps establish the general relaying applications
although the definition of ‘‘short,’’ ‘‘medium,’’ and ‘‘long’’ is not precise.
A short line is one in which
the ratio of the source to the line impedance (SIR) is large (>4 e.g.), the
SIR of a long line is 0.5 or less and a medium line’s SIR is between 4 and 0.5.
It must be noted, however, that the per-unit
impedance of a line varies more with the nominal voltage of the line than with
its physical length or impedance. So a ‘‘short’’ line at one voltage level
may be a ‘‘medium’’ or ‘‘long’’ line at another.
5.3 Multiterminal
Lines
Occasionally, transmission
lines may be tapped to provide intermediate connections to additional sources
without the expense of a circuit breaker or other switching device.
Such a configuration is known as a multi terminal
line and, although it is an inexpensive measure for strengthening the
power system, it presents special problems for the protection engineer.
The difficulty arises from the
fact that a relay receives its input from the local transducers, i.e., the
current and voltage at the relay location.
