{"id":600,"date":"2019-04-18T16:56:28","date_gmt":"2019-04-18T15:56:28","guid":{"rendered":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/?post_type=chapter&p=600"},"modified":"2024-03-12T14:36:56","modified_gmt":"2024-03-12T14:36:56","slug":"power-system-protection","status":"publish","type":"chapter","link":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/chapter\/power-system-protection\/","title":{"raw":"Power System Protection","rendered":"Power System Protection"},"content":{"raw":"

Power System Protection<\/h1>\r\nThe power system protection (PSP) comprises the electric power protective relays and the switchgear automatics, which have to:\r\n\u2981 protect power system elements against the effects of short-circuit overcurrents, operational overcurrents, voltage loss,\r\n\u2981 to increase reliability of energy supply.\r\n\r\nProtecive relays are the main elements of the power system protection. The relays measure either directly or with the help of current and voltage transformers chosen electric quantities and after eventual overrun of their starting values, they may either signal the fault or switch off the device acting on the final control element, i.e. on circuit breaker.\r\n\r\nDenotations and principles of operation of basic relays applied in the schemes of the PSP are presented in table 8.1.\r\n

\"\"<\/p>\r\n

\"\"<\/p>\r\n \r\n\r\nProtective devices can be divided into the following functional groups:\r\n\u2981 relay protection system \u2013 its main objective is to switch off a faulty element,\r\n\u2981 automatic reclosing equipment \u2013 restores network configuration to the configuration as similar as possible to orginal design,\r\n\u2981 preventive power system protection \u2013 its objecive is to prevent system malfunction through signaling emergency states, automatic load shedding, switching off, etc.\r\n\r\nThose functions are often combined in a single device, which is called the power system protection set.\r\nProtection equipment operational reliability is gained through its redundancy.\r\nOperational selectivity is obtained through the graduation of the operation time of the time-delay protection device, or proper selection of the starting values of the instantaneous protection device.\r\nNow, beside still exploited analogous protection equipment, digital protection systems are introduced to the exploitation.\r\nIn this lecture we will discuss the basic principles of protection devices operation: electric power lines, transformers, generators and motors.\r\n

\"\"<\/p>\r\n

\"\"<\/p>\r\n \r\n

Power lines<\/h1>\r\nPOWER LINES \u2013 Preventive and automatic reclosing\r\nThe following solutions are applied in electric power networks:\r\n\u2981 Automatic stand-by switching-on \u2013 When the undervoltage relay detects the loss of supply in the main circuit the back-up supply circuit is closing.\r\n\u2981 Automatic reclosing is used in the overhead lines. It recloses the line after its emergency outage, after the time of the current break, suitably selected for the possibilities of the line and circuit breakers, to restore normal work, if the short-circuit is temporary. Twofold automatic reclosing is usually used in the MV networks with the times of the breaks: first 0.5-1 s, second about 10 s. Single-phase and three-phase automatic reclosing is the most frequently applied in HV and EHV lines. Usually it is fast, single reclosing. The automatic reclosing systems cooperate with line protection devices against short-circuits.\r\n\u2981 Under-frequency load shedding. It is automatic switching-off the receiving substation, when the deep decrease of frequency exists in the EPS. It corrects the power balance in the power system and increases the frequency, reducing the degree of danger. Loads expected to be switched-off as first are the less important loads in the sense of social losses. Under-frequency load shedding is carried in a few stages, for example: I stage \u2013 48,5 Hz, II \u2013 48 Hz, III \u2013 47,5 Hz, IV \u2013 47 Hz.\r\n\u2981 Automatic load restoration (after under-frequency load shedding) has a goal to restore the supply to the receivers after their disconnection by the under-frequency load shedding.\r\n\r\n \r\n

POWER LINES \u2013 protection against short-circuits<\/h3>\r\nProtection of MV lines against multi-phase faults\r\n\u2981 Instantaneous overcurrent relay is the basic protection device of the line segment. It operates when the current in the line rises above the starting value of the line protection and is installed in two phases. The relay is backed-up by time-delay overcurrent relays with lower starting currents than the instantaneous ones. Those relays enable reserving of protection, because they have low starting current and the time delay adjusted in such a way, that the line segments, when approaching to the supply point, have delays lengthened by Dt = 0,3-0,7 s .\r\n\u2981 The protection device cooperate with automatic reclosing systems. Residual (current) protection devices and distance protection devices are also applied in short, important MV line segments.\r\n

\"\"<\/p>\r\nFig.8.1. Overcurrent protection of power lines:\r\na) two-phase time-delay and instantaneous protection \u2013 principle of operation; b) time graduation in branched radial network (relays 3 and 6 have settings resulting from farther line segments, which haven\u2019t been shown)\r\nI>, I>> - overcurrent relays of time-delay and instantaneous systems; t \u2013 time relay; PP \u2013 current transformers; W \u2013 tripping device of the circuit-breaker\r\n\r\nProtection of MV lines against phase-to-earth faults\r\n\u2981 Protection device reacts on the rise of zero sequence current or power component above the starting value. The zero sequence current can be measured by parallel connection of secondary sides of current transformers in three-phase circuits (Holmgreen\u2019s filter) (Fig.8.2) or by enfolding three-phase circuits by a core of a current transformer (Ferranti\u2019s transformer (Fig. 8.3))\r\n

\"\"<\/p>\r\nProtection of MV lines against phase-to-earth faults\r\n\r\n\u2981 The zero sequence voltages may be measured by connection of secondary windings of voltage transformer individual phases in open delta (Fig. 8.4). Relays measuring zero sequence active power component are applied in overhead lines with the insulated neutral; they signal the short-circuit.\r\n

\"\"<\/p>\r\nProtection of MV lines against phase-to-earth faults\r\n\r\n\u2981 Relays measuring zero sequence reactive power component are applied in compensated lines; they signal the short-circuit.\r\n\r\n\u2981 Relays measuring zero sequence current component are used in the networks with neutral grounded through a resistor; they act on switching-off the line with the short-circuit.\r\n\r\nProtection of HV and EHV lines\r\n\u2981 This part will be limited to the discussion about the most commonly used distance protection systems, sending the persons interested in other kinds of protection to the literature.\r\n\u2981 Distance protection is a protection measuring impedance from a place of short-circuit, it has directional properties and it acts with a delay depending on the distance from the place of short-circuit according to the given impedance-time characteristics (Fig. 8.5).\r\n\r\n[caption id=\"attachment_616\" align=\"aligncenter\" width=\"425\"]\"\" Fig. 8.5.Time zones of a distance protection[\/caption]\r\n\r\nProtection of HV and EHV lines\r\n\r\n\u2981 Triggering set of the protection device detect the fault existence, trigger the time set and, after determining the type of fault, trigger corresponding measuring set.\r\n\u2981 The measuring element set switch off line:\r\n- in time t1<\/sub> if the fault is in the first zone having 0,85 of the protected line segment,\r\n- in time t2<\/sub> for the faults in the II zone, which reaches the half of the shortest line segment outgoing from the substation B (Fig. 8.5)\r\n- in time t3<\/sub> for the faults in wider the range.\r\n\u2981 The protection has usually 3-5 time zones.\r\n\r\n \r\n

Transformers<\/h1>\r\n

TRANSFORMER PROTECTION<\/h3>\r\n\u2981 Particular role belongs to protection against internal faults in the transformer. In smaller units these are fuses or instantaneous overcurrent protection devices. In the units of power greater than 5 MVA differential protection devices with blocking of its operation under the influence of magnetizing inrush, when the transformer is switched on, is applied. The differential protection device measures current on both sides of the transformer. In case of the fault inside the transformer those currents have different values and this triggers protective device. The protection devices against external faults are, at the same time, back-up protection for lines outgoing from the transformer. Time-delay overcurrent relays or distance relays are used for this purpose in the units with upper voltage 220 kV and 400 kV.\r\nTransformers\r\n\u2981 Gas bubble protection device is installed in the connector between the transformer tank and the oil conservator (Fig. 8.7).\r\n\u2981 This relay is triggered in case of winding insulation damage, which cause the oil gassing, or when the level of oil decreases because of the tank leakage.\r\n\u2981 Less important damages are only signalised, while at more hazardous situations device switches off the transformer.\r\n\u2981 Autotransformers and transformers with upper voltage 400 kV, 220 kV, should have two sets of basic protection devicess, backing-up each other, transformers with upper voltage less or equal to 110 kV \u2013 single set. An example of a transformer protection device has been shown in Fig. 8.8.\r\nTransformers\r\n\r\n[caption id=\"attachment_617\" align=\"aligncenter\" width=\"604\"]\"\" Fig. 8.7. Gas-detector relay (Bucholtz) (taken from [1]): 1 \u2013 float, 2 \u2013 flow plate, 3 \u2013 mercury switch, 4 \u2013 relief vent valve, 5 \u2013 release valve, W \u2013 switching off, S \u2013 signaling[\/caption] \r\n\r\n[caption id=\"attachment_619\" align=\"aligncenter\" width=\"908\"]\"\" Fig. 8.8. 110 kV\/MV, 16 MVA transformer protection: 1 \u2013 time-delay overcurrent protection against external faults (in 3 phases), 2 \u2013 against overloads (in 1 phase), 3 \u2013 biased differential protection against internal faults, 4 \u2013 gas and gas bubble protection of the transformer tap changer; 5 \u2013 two-stage temperature protection, W1, W2 \u2013 circuit-breakers, SO \u2013 warning signalization, W \u2013 outage (W1, W2)[\/caption]\r\n

Generators<\/h1>\r\n

GENERATOR PROTECTION<\/h3>\r\nGenerator protection devices are adjusted to the generator rated power. Protection devices dedicated to generators (not used for lines or transformers protection) are:\r\n\u2981 Protection against the effects of the phase currents asymmetry, reacting on negative sequence current component,\r\n\u2981 Protection against single short-circuits in the excitation winding, signaling the faults,\r\n\u2981 Protection against double short-circuits in the excitation winding, based on the bridge systems,\r\n\u2981 Protection against voltage rise in hydrogenerators, caused by the increase of the rotational speed, realized with the help of the overvoltage relay.\r\n\u2981 Protection against excitation loss, which leads to generator asynchronous work with shorted or open excitation circuit, realized by under-impedance protection. It measures internal reactance of the generator, which at asynchronous work is much less than at synchronous operation.\r\n

Motors<\/h1>\r\n

PROTECTION OF ASYNCHRONOUS MOTORS<\/h3>\r\nProtection devices for motors are adjusted the nominal voltage and the rated power of the asynchronous motor .\r\n\r\n\u2981 Basic requirement of short-circuit protection devices and overload protection devicses are to not operate during motor start-ups.\r\n\u2981 Protection device against voltage reduction (0,6-0,7 Un) should act with long time delay (6-10 s), if the motor is intended for the self-start. Long delay prevents from motor outages during the breaks when automatic reclosing operates.","rendered":"

Power System Protection<\/h1>\n

The power system protection (PSP) comprises the electric power protective relays and the switchgear automatics, which have to:
\n\u2981 protect power system elements against the effects of short-circuit overcurrents, operational overcurrents, voltage loss,
\n\u2981 to increase reliability of energy supply.<\/p>\n

Protecive relays are the main elements of the power system protection. The relays measure either directly or with the help of current and voltage transformers chosen electric quantities and after eventual overrun of their starting values, they may either signal the fault or switch off the device acting on the final control element, i.e. on circuit breaker.<\/p>\n

Denotations and principles of operation of basic relays applied in the schemes of the PSP are presented in table 8.1.<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

 <\/p>\n

Protective devices can be divided into the following functional groups:
\n\u2981 relay protection system \u2013 its main objective is to switch off a faulty element,
\n\u2981 automatic reclosing equipment \u2013 restores network configuration to the configuration as similar as possible to orginal design,
\n\u2981 preventive power system protection \u2013 its objecive is to prevent system malfunction through signaling emergency states, automatic load shedding, switching off, etc.<\/p>\n

Those functions are often combined in a single device, which is called the power system protection set.
\nProtection equipment operational reliability is gained through its redundancy.
\nOperational selectivity is obtained through the graduation of the operation time of the time-delay protection device, or proper selection of the starting values of the instantaneous protection device.
\nNow, beside still exploited analogous protection equipment, digital protection systems are introduced to the exploitation.
\nIn this lecture we will discuss the basic principles of protection devices operation: electric power lines, transformers, generators and motors.<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

 <\/p>\n

Power lines<\/h1>\n

POWER LINES \u2013 Preventive and automatic reclosing
\nThe following solutions are applied in electric power networks:
\n\u2981 Automatic stand-by switching-on \u2013 When the undervoltage relay detects the loss of supply in the main circuit the back-up supply circuit is closing.
\n\u2981 Automatic reclosing is used in the overhead lines. It recloses the line after its emergency outage, after the time of the current break, suitably selected for the possibilities of the line and circuit breakers, to restore normal work, if the short-circuit is temporary. Twofold automatic reclosing is usually used in the MV networks with the times of the breaks: first 0.5-1 s, second about 10 s. Single-phase and three-phase automatic reclosing is the most frequently applied in HV and EHV lines. Usually it is fast, single reclosing. The automatic reclosing systems cooperate with line protection devices against short-circuits.
\n\u2981 Under-frequency load shedding. It is automatic switching-off the receiving substation, when the deep decrease of frequency exists in the EPS. It corrects the power balance in the power system and increases the frequency, reducing the degree of danger. Loads expected to be switched-off as first are the less important loads in the sense of social losses. Under-frequency load shedding is carried in a few stages, for example: I stage \u2013 48,5 Hz, II \u2013 48 Hz, III \u2013 47,5 Hz, IV \u2013 47 Hz.
\n\u2981 Automatic load restoration (after under-frequency load shedding) has a goal to restore the supply to the receivers after their disconnection by the under-frequency load shedding.<\/p>\n

 <\/p>\n

POWER LINES \u2013 protection against short-circuits<\/h3>\n

Protection of MV lines against multi-phase faults
\n\u2981 Instantaneous overcurrent relay is the basic protection device of the line segment. It operates when the current in the line rises above the starting value of the line protection and is installed in two phases. The relay is backed-up by time-delay overcurrent relays with lower starting currents than the instantaneous ones. Those relays enable reserving of protection, because they have low starting current and the time delay adjusted in such a way, that the line segments, when approaching to the supply point, have delays lengthened by Dt = 0,3-0,7 s .
\n\u2981 The protection device cooperate with automatic reclosing systems. Residual (current) protection devices and distance protection devices are also applied in short, important MV line segments.<\/p>\n

\"\"<\/p>\n

Fig.8.1. Overcurrent protection of power lines:
\na) two-phase time-delay and instantaneous protection \u2013 principle of operation; b) time graduation in branched radial network (relays 3 and 6 have settings resulting from farther line segments, which haven\u2019t been shown)
\nI>, I>> – overcurrent relays of time-delay and instantaneous systems; t \u2013 time relay; PP \u2013 current transformers; W \u2013 tripping device of the circuit-breaker<\/p>\n

Protection of MV lines against phase-to-earth faults
\n\u2981 Protection device reacts on the rise of zero sequence current or power component above the starting value. The zero sequence current can be measured by parallel connection of secondary sides of current transformers in three-phase circuits (Holmgreen\u2019s filter) (Fig.8.2) or by enfolding three-phase circuits by a core of a current transformer (Ferranti\u2019s transformer (Fig. 8.3))<\/p>\n

\"\"<\/p>\n

Protection of MV lines against phase-to-earth faults<\/p>\n

\u2981 The zero sequence voltages may be measured by connection of secondary windings of voltage transformer individual phases in open delta (Fig. 8.4). Relays measuring zero sequence active power component are applied in overhead lines with the insulated neutral; they signal the short-circuit.<\/p>\n

\"\"<\/p>\n

Protection of MV lines against phase-to-earth faults<\/p>\n

\u2981 Relays measuring zero sequence reactive power component are applied in compensated lines; they signal the short-circuit.<\/p>\n

\u2981 Relays measuring zero sequence current component are used in the networks with neutral grounded through a resistor; they act on switching-off the line with the short-circuit.<\/p>\n

Protection of HV and EHV lines
\n\u2981 This part will be limited to the discussion about the most commonly used distance protection systems, sending the persons interested in other kinds of protection to the literature.
\n\u2981 Distance protection is a protection measuring impedance from a place of short-circuit, it has directional properties and it acts with a delay depending on the distance from the place of short-circuit according to the given impedance-time characteristics (Fig. 8.5).<\/p>\n

\"\"
Fig. 8.5.Time zones of a distance protection<\/figcaption><\/figure>\n

Protection of HV and EHV lines<\/p>\n

\u2981 Triggering set of the protection device detect the fault existence, trigger the time set and, after determining the type of fault, trigger corresponding measuring set.
\n\u2981 The measuring element set switch off line:
\n– in time t1<\/sub> if the fault is in the first zone having 0,85 of the protected line segment,
\n– in time t2<\/sub> for the faults in the II zone, which reaches the half of the shortest line segment outgoing from the substation B (Fig. 8.5)
\n– in time t3<\/sub> for the faults in wider the range.
\n\u2981 The protection has usually 3-5 time zones.<\/p>\n

 <\/p>\n

Transformers<\/h1>\n

TRANSFORMER PROTECTION<\/h3>\n

\u2981 Particular role belongs to protection against internal faults in the transformer. In smaller units these are fuses or instantaneous overcurrent protection devices. In the units of power greater than 5 MVA differential protection devices with blocking of its operation under the influence of magnetizing inrush, when the transformer is switched on, is applied. The differential protection device measures current on both sides of the transformer. In case of the fault inside the transformer those currents have different values and this triggers protective device. The protection devices against external faults are, at the same time, back-up protection for lines outgoing from the transformer. Time-delay overcurrent relays or distance relays are used for this purpose in the units with upper voltage 220 kV and 400 kV.
\nTransformers
\n\u2981 Gas bubble protection device is installed in the connector between the transformer tank and the oil conservator (Fig. 8.7).
\n\u2981 This relay is triggered in case of winding insulation damage, which cause the oil gassing, or when the level of oil decreases because of the tank leakage.
\n\u2981 Less important damages are only signalised, while at more hazardous situations device switches off the transformer.
\n\u2981 Autotransformers and transformers with upper voltage 400 kV, 220 kV, should have two sets of basic protection devicess, backing-up each other, transformers with upper voltage less or equal to 110 kV \u2013 single set. An example of a transformer protection device has been shown in Fig. 8.8.
\nTransformers<\/p>\n

\"\"
Fig. 8.7. Gas-detector relay (Bucholtz) (taken from [1]): 1 \u2013 float, 2 \u2013 flow plate, 3 \u2013 mercury switch, 4 \u2013 relief vent valve, 5 \u2013 release valve, W \u2013 switching off, S \u2013 signaling<\/figcaption><\/figure>\n

 <\/p>\n

\"\"
Fig. 8.8. 110 kV\/MV, 16 MVA transformer protection: 1 \u2013 time-delay overcurrent protection against external faults (in 3 phases), 2 \u2013 against overloads (in 1 phase), 3 \u2013 biased differential protection against internal faults, 4 \u2013 gas and gas bubble protection of the transformer tap changer; 5 \u2013 two-stage temperature protection, W1, W2 \u2013 circuit-breakers, SO \u2013 warning signalization, W \u2013 outage (W1, W2)<\/figcaption><\/figure>\n

Generators<\/h1>\n

GENERATOR PROTECTION<\/h3>\n

Generator protection devices are adjusted to the generator rated power. Protection devices dedicated to generators (not used for lines or transformers protection) are:
\n\u2981 Protection against the effects of the phase currents asymmetry, reacting on negative sequence current component,
\n\u2981 Protection against single short-circuits in the excitation winding, signaling the faults,
\n\u2981 Protection against double short-circuits in the excitation winding, based on the bridge systems,
\n\u2981 Protection against voltage rise in hydrogenerators, caused by the increase of the rotational speed, realized with the help of the overvoltage relay.
\n\u2981 Protection against excitation loss, which leads to generator asynchronous work with shorted or open excitation circuit, realized by under-impedance protection. It measures internal reactance of the generator, which at asynchronous work is much less than at synchronous operation.<\/p>\n

Motors<\/h1>\n

PROTECTION OF ASYNCHRONOUS MOTORS<\/h3>\n

Protection devices for motors are adjusted the nominal voltage and the rated power of the asynchronous motor .<\/p>\n

\u2981 Basic requirement of short-circuit protection devices and overload protection devicses are to not operate during motor start-ups.
\n\u2981 Protection device against voltage reduction (0,6-0,7 Un) should act with long time delay (6-10 s), if the motor is intended for the self-start. Long delay prevents from motor outages during the breaks when automatic reclosing operates.<\/p>\n","protected":false},"author":7,"menu_order":9,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"part":20,"_links":{"self":[{"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/pressbooks\/v2\/chapters\/600"}],"collection":[{"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/wp\/v2\/users\/7"}],"version-history":[{"count":6,"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/pressbooks\/v2\/chapters\/600\/revisions"}],"predecessor-version":[{"id":620,"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/pressbooks\/v2\/chapters\/600\/revisions\/620"}],"part":[{"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/pressbooks\/v2\/parts\/20"}],"metadata":[{"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/pressbooks\/v2\/chapters\/600\/metadata\/"}],"wp:attachment":[{"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/wp\/v2\/media?parent=600"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/pressbooks\/v2\/chapter-type?post=600"},{"taxonomy":"contributor","embeddable":true,"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/wp\/v2\/contributor?post=600"},{"taxonomy":"license","embeddable":true,"href":"http:\/\/pb.ee.pw.edu.pl\/pb\/iepe\/wp-json\/wp\/v2\/license?post=600"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}