Introduction
Siemens is one of the world’s leading sup-pliers of protective equipment for power systems.
Thousands of our relays ensure first-class performance in transmission and distribu-tion networks of all voltage levels, all over the world, in countries of tropical heat or arctic frost.
For many years, Siemens has also signifi-cantly influenced the development of pro-tection technology.
s In 1976, the first minicomputer (process computer) based protection system was commissioned: A total of 10 systems
for 110/20 kV substations were supplied and are still operating satisfactorily today. s Since 1985 we have been the first to manufacture a range of fully numerical
relays with standardized communication interfaces.
Today, Siemens offers a complete pro-
gram of protective relays for all applica-tions including numerical busbar protec-tion.
To date (1996), more than 50,000 numer-ical protection relays from Siemens are
providing successful service, as stand-
alone devices in traditional systems or
as components of coordinated protec-
tion and substation control.
Meanwhile, a second-generation inno-
vative series has been launched, incor-
porating the many years of operational
experience with thousands of relays,
together with users’ requirements,
(power authority reommendations).
State of the art
Mechanical and solid-state (static) relays have been almost completely phased out of our production because numerical relays are now preferred by the users due to their decisive advantages:
s Compact design and lower cost due to integration of many functions into one
relay
s High availability even with less mainte-nance due to integral self-monitoring
s No drift (aging) of measuring characteris-tics due to fully numerical processing
s High measuring accuracy due to digital filtering and optimized measuring algo-
rithms s Many integrated add-on functions, for example, for load-monitoring and
event/fault recording
s Easy and secure read-out of information via serial interfaces with a PC, locally or remotely
s Possibility to communicate with higher-level control systems
Fig. 12: Numerical relay range of Siemens
Power System Protection
service faultModern protection management All the functions, for example, of a line pro-tection scheme can be incorporated in one unit:
s Distance protection with associated
add-on and monitoring functions
s Universal teleprotection interface s Autoreclose and synchronism check
Protection-related information, can be called up on-line or off-line such as:s Distance to fault
s Fault currents and voltages
s Relay operation data (fault detector pick-up, operating times etc.)s Set values
s Line load data (kV, A, MW, kVAr)
To fulfill vital protection redundancy require-ments, only those functions which are in-terdependent and directly associated with each other are integrated in the same unit.For back-up protection, one or more addi-tional units have to be provided.
All relays can stand fully alone. Thus the traditional protection concept of separate main and alternate protection as well as the external connection to the switchyard remain unchanged.
”One feeder, one relay“ concept Analog protection schemes have been en-gineered and assembled from individual relays. Interwiring between these relays and scheme testing has been carried out manually in the workshop.
Data sharing now allows for the integration of several protection and protection related tasks into one single numerical relay. Only a few external devices may be required for completion of the total scheme. This has significantly lowered the costs of engineer-ing, assembly, panel wiring, testing and commissioning. Scheme failure probability has also been lowered.
Engineering has moved from schematic diagrams towards a parameter definition procedure. The documentation is provided by the relay itself. Free allocation of LED operation indicators and output contacts provides more application design flexibility.
Metering included
For many applications, the protective-cur-rent transformer accuracy is sufficient for operational meter
ing. The additional was more for protection of meters under system fault conditions. Due to the low thermal withstand ability of the me-ters, they could not be connected to the Consequently, additional s and meters are now only necessary where high accuracy is for revenue metering.
Fig. 13: Numerical relays, increased information availability
On-line remote data exchange
A powerful serial data link provides for interrogation of digitized measured values and other information, stored in the pro-tection units, for printout and further processing at the substation or system control level.
In the opposite direction, settings may be altered or test routines initiated from a re-mote control center.
For greater distances, especially in outdoor switchyards, fiber-optic cables are prefera-bly used. This technique has the advantage that it is totally unaffected by electromag-netic interference.
Off-line dialog with numerical relays
A simple built-in operator panel which requires no special software knowledge or codeword tables is used for parameter input and readout.
This allows operator dialog with the protec-tion relay. Answers appear largely in plain-text on the display of the operator panel. Dialog is divided into three main phases:
s Input, alternation and readout of settings s Testing the functions of the protection device and
s Readout of relay operation data for the three last system faults and the autore-close counter.
Modern system protection management
A more versatile notebook computer may be used for upgraded protection manage-ment.
The relays may be set in 2 steps. First, all relay settings are prepared in the office with the aid of a PC and stored on a floppy or the hard disk. At site, the settings can then be transferred from a portable PC into the relay. The relay confirms the settings and thus provides an unquestionable record.
Vice versa, after a system fault, the relay memory can be uploaded to a PC and comprehensive fault analysis can then take place in the engineer’s office.
Fig. 14: PC-aided setting procedure Fig. 15: Communication options
Parameter
Line data O/C Phase settings O/C Earth settings Fault Recording Breaker Fall
1000
11001200150028003900D
Parameter
Line data O/C Phase settings O/C Earth settings Fault Recording Breaker Fall 1000
11001200150028003900C
Parameter
Line data O/C Phase settings O/C Earth settings Fault Recording Breaker Fall 1000
11001200150028003900B
Parameter Line data O/C Phase settings O/C Ground settings Fault recording Breaker fail
100011001200150028003900
A
Relay data management
Analog-distribution-type relays have some 20–30 setpoints. If we consider a power system with about 500 relays, then  the number adds up to 10,000 settings. This required considerable expenditure in set-ting the relays and filing retrieval setpoints.A personal computer-aided man-machine dialog and archiving program assists the relay engineer in data filing and retrieval.The program files all settings systemati-cally in substation-feeder-relay order.Corrective rather than preventive maintenance
Numerical relays monitor their own hard-ware and software. Exhaustive self-moni-toring and failure diagnostic routines are not restricted to the protective relay inself,but are methodically carried through from current transformer circuits to tripping re-lay coils.
Equipment failures and faults in cir-cuits are immediately reported and the pro-tective relay bl
ocked.
Thus the service personnel is now able to correct the failure upon occurrence, result-ing in a significantly upgraded availability of the protection system.Adaptive relaying
Numerical relays now offer secure, con-venient and comprehensive matching to changing conditions. Matching may be initi-ated either by the relay’s own intelligence or from the outside world via contacts or serial telegrams. Modern numerical relays contain a number of parameter sets that can be pretested during commissioning of the scheme (Fig. 17). One set is normally operative. Transfer to the other sets can be controlled via binary inputs or serial data link. There are a number of applications for which multiple setting groups can upgrade the scheme performance, e.g.
a) for use as a voltage-dependent control of o/c relay pick-up values to overcome alternator fault current decrement to be-low normal load current when the AVR is not in automatic operation.
b) for maintaining short operation times with lower fault currents, e.g. automatic change of settings if one supply trans-former is taken out of service.
c) for “switch-onto-fault” protection to pro-vide shorter time settings when energiz-ing a circuit after maintenance.
The normal settings can be restored automatically after a time delay.
Fig. 16: System-wide setting and relay operation library
Fig. 17: Alternate parameter groups
10 000setpoints
system ca. 500relays
200
setpoints
sub
bay
20
setpoints
bay 4flags OH-Line
1200flags p. a.system
Relay operations
Setpoints
111300 faults p. a.ca. 6,000 km OHL (fault rate:
5 p. a. and 100 km)
d) for autoreclose programs, i.e. instanta-neous operation for first trip and delayed
operation after unsuccessful reclosure.e) for cold load pick-up problems where high starting currents may cause relay operation.
f) for ”ring open“ or ”ring closed“ oper-ation.
Mode of operation
Numerical protection relays operate on the basis of numerical measuring principles.The analog measured values of current and voltage are decoupled galvanically from the plant secondary circuits via input transduc-ers (Fig. 18). After analog filtering, the sampling and the analog-to-digital conver-sion take place. The sampling rate is, de-pending on the different protection princi-ples, between 12 and 20 samples per
period. With certain devices (e.g. generator protection) a continuous adjustment of the sampling rate takes place depending on the actual system frequency.
The protection principle is based on a cy-clic calculation algorithm, utilizing the sam-pled current and voltage analog measured values. The fault detections determined by this process must be established in several sequential calculations before protection reactions can follow.
A trip command is transferred to the com-mand relay by the processor, utilizing a dual channel control.
The numerical protection concept offers a variety of advantages, especially with re-gard to higher security, reliability and user friendliness, such as:
s High measurement accuracy:
The high ultilization of adaptive algo-rithms produce accurate results even during, problematic conditions s Good long-term stability:
Due to the digital mode of operation,drift phenomena at components due to ageing do not lead to changes in accura-cy of measurement or time delays
s Security against over- and underfunction With this concept the danger of an unde-tected error in the device causing protec-tion failure in the case of a network fault is clearly reduced when compared to con-ventional protection technology. Cyclical and preventive maintenance services have therefore become largely obsolete.The integrated self-monitoring system
(Fig. 19) encompasses the following areas:–Analog inputs
–Microprocessor system –Command relays.
Setting of protection relays
Numerical protection devices are able to handle a number of additional protection related functions, for which additional de-vices were required in the past.
A compact numerical protection device can replace a number of complicated conven-tional single devices.
Protection functions, configurations  and marshalling data are selected by parameter setting. Functions can be activated or de-activated by configuration.
By marshalling internal logic alarms (which are produced by certain device functions on the software side) to light-emitting diodes or to alarm relays, an allocation between these can be made (Fig. 20).The same also applies to the input con-tacts.
A flexible application according to the
specific requirements of the plant configu-ration is possible thanks to the extensive marshalling and configuration options.All set values are stored in E 2PROMS.In this way the settings cannot be lost as a result of supply failure.
The setting values are accessed via 4-digit addresses.
Each parameter can be accessed and al-tered via the integrated operator panel or an externally connected operator terminal.
Fig. 18: Block diagram of numerical protection

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