UNBALANCED (NEGATIVE SEQUENCE) CURRENTS
Unbalanced loads, unbalanced system faults, open conductors, or other unsymmetrical Unbalanced loads, unbalanced system faults, open conductors, or other unsymmetrical operating conditions result in an unbalance of the generator phase voltages. The resulting operating conditions result in an unbalance of the generator phase voltages. The resulting unbalanced (negative sequence) currents induce double system frequency currents in the unbalanced (negative sequence) currents induce double system frequency currents in the rotor that quickly cause rotor overheating. Serious damage to the generator will occur if the rotor that quickly cause rotor overheating. Serious damage to the generator will occur if the unbalanced condition
unbalanced condition is allowed is allowed to persist to persist indefinitely. indefinitely. The ability The ability of a of a generator togenerator to withstand these negative sequence currents is defined by ANSI C 50.13 - 1977 as I
withstand these negative sequence currents is defined by ANSI C 50.13 - 1977 as I 2222t = k,t = k, where the negative sequence current is expressed in per unit of the full load current and where the negative sequence current is expressed in per unit of the full load current and the time is given in seconds.
the time is given in seconds.
Negative sequence stator currents, caused by fault or load unbalance, induce Negative sequence stator currents, caused by fault or load unbalance, induce doublefrequency currents into the rotor that may eventually overheat elements not doublefrequency currents into the rotor that may eventually overheat elements not designed to be subjected to such currents. Series unbalances, such as untransposed designed to be subjected to such currents. Series unbalances, such as untransposed transmission lines, produce some negative-sequence current (I2) flow. The most serious transmission lines, produce some negative-sequence current (I2) flow. The most serious series unbalance is an open phase, such as an open breaker pole. ANSI C50.13-1977 series unbalance is an open phase, such as an open breaker pole. ANSI C50.13-1977 specifies a continuous I2 withstand of 5 to 10% of rated current, depending upon the size specifies a continuous I2 withstand of 5 to 10% of rated current, depending upon the size and design of the generator. These values can be exceeded with an open phase on a relay will alert the operator to the existence of a dangerous condition.
relay will alert the operator to the existence of a dangerous condition.
Figure 13-17 Negative Sequence Current Relay (46) protects against rotor Figure 13-17 Negative Sequence Current Relay (46) protects against rotor overheating due to a series unbalance or protracted external fault. Negative overheating due to a series unbalance or protracted external fault. Negative
sequence voltage relays (47) (less commonly applied) also responds sequence voltage relays (47) (less commonly applied) also responds
Negative sequence voltage (47) protection, while not as commonly used, is an available Negative sequence voltage (47) protection, while not as commonly used, is an available means to sense system imbalance as well as, in some situations, a misconnection of the means to sense system imbalance as well as, in some situations, a misconnection of the generator to a system to which it is being paralleled.
generator to a system to which it is being paralleled.
UNBALANCED (NEGATIVE SEQUENCE) CURRENTS UNBALANCED (NEGATIVE SEQUENCE) CURRENTS Table 13-1 lists the typical k-values
Table 13-1 lists the typical k-values
Table 13-1. Generator k-Values Table 13-1. Generator k-Values
A negative sequence overcurrent relay (ANSI Device No. 46) is the recommended A negative sequence overcurrent relay (ANSI Device No. 46) is the recommended protection for this unbalanced condition.
protection for this unbalanced condition.
Figure 13-18. Current Unbalance Relay Time Current Characteristics Figure 13-18. Current Unbalance Relay Time Current Characteristics
LOSS OF PRIME MOVER (MOTORING) LOSS OF PRIME MOVER (MOTORING)
Generator anti-motoring protection is designed for protection of the prime mover, or the Generator anti-motoring protection is designed for protection of the prime mover, or the system, rather than for
system, rather than for protection of the generator protection of the generator itself. itself. Motoring results from low Motoring results from low primeprime mover input to the generator, such as would occur if the steam supply to the turbine or the mover input to the generator, such as would occur if the steam supply to the turbine or the oil supply to the
oil supply to the diesel were lost. diesel were lost. When the prime mover input When the prime mover input to the generator cannotto the generator cannot meet all the losses, the deficiency is supplied by the system -- the generator absorbs real meet all the losses, the deficiency is supplied by the system -- the generator absorbs real power and reactive power flow (not relevant at this point) may be in or out depending on power and reactive power flow (not relevant at this point) may be in or out depending on the voltage
the voltage (system excitation). (system excitation). Under Under motoring conditions, motoring conditions, steam turbine steam turbine blades canblades can overheat, water wheel turbine blades can cavitate, and fire or possible explosion can result overheat, water wheel turbine blades can cavitate, and fire or possible explosion can result in a diesel unit.
in a diesel unit.
When the prime mover spins at synchronous speed with no power input, the approximate When the prime mover spins at synchronous speed with no power input, the approximate reverse power that is required to motor a generator, as a percentage of the nameplate kW reverse power that is required to motor a generator, as a percentage of the nameplate kW rating, is listed in Table 13-2.
rating, is listed in Table 13-2.
Table
Table 13-2. 13-2. Maximum Motoring Maximum Motoring PowerPower
Although there are a number of non-electrical (mechanical) protection schemes for the Although there are a number of non-electrical (mechanical) protection schemes for the generator prime mover, a reverse power relay (ANSI Device No. 32) is used to provide generator prime mover, a reverse power relay (ANSI Device No. 32) is used to provide supplemental protection.
supplemental protection.
The reverse power relay should have sufficient sensitivity such that motoring power The reverse power relay should have sufficient sensitivity such that motoring power provides 5-10 times the minimum pickup power of the relay. An induction disc directional provides 5-10 times the minimum pickup power of the relay. An induction disc directional power relay is frequently used to introduce sufficient time delay necessary to override power relay is frequently used to introduce sufficient time delay necessary to override momentary power surges
momentary power surges that might occur that might occur during synchronizing. during synchronizing. A time A time delay of delay of 10-1510-15 seconds is typical.
seconds is typical.
The reverse-power feature (32) in Fig. 13-19 senses real power flow into the generator, The reverse-power feature (32) in Fig. 13-19 senses real power flow into the generator, which will occur if the generator loses its prime-mover input. Since the generator is not which will occur if the generator loses its prime-mover input. Since the generator is not faulted, CTs on either side of the generator would provide the same measured current.
faulted, CTs on either side of the generator would provide the same measured current.
Figure 13-19. Anti-motoring (32) Loss-of-Field (40), Protection Figure 13-19. Anti-motoring (32) Loss-of-Field (40), Protection
In a steam-turbine, the low pressure blades will overheat with the lack of steam flow.
In a steam-turbine, the low pressure blades will overheat with the lack of steam flow.
Diesel and gas-turbine units draw large amounts of motoring power, with possible Diesel and gas-turbine units draw large amounts of motoring power, with possible mechanical problems. In the case of diesels, the hazard of a fire and/or explosion may mechanical problems. In the case of diesels, the hazard of a fire and/or explosion may occur due to unburnt fuel. Therefore, anti-motoring protection is recommended whenever occur due to unburnt fuel. Therefore, anti-motoring protection is recommended whenever the unit may be connected to a source of motoring power.
the unit may be connected to a source of motoring power.
Where a non-electrical type of protection is in use, as may be the case with a steam Where a non-electrical type of protection is in use, as may be the case with a steam turbine unit, the 32 relay provides a means of supervising this condition to prevent opening turbine unit, the 32 relay provides a means of supervising this condition to prevent opening the generator breaker before the prime mover has shut down. Time delay should be set for the generator breaker before the prime mover has shut down. Time delay should be set for about 5-30 seconds, providing enough time for the controls to pick up load upon about 5-30 seconds, providing enough time for the controls to pick up load upon synchronizing when the generator is initially slower than the system.
synchronizing when the generator is initially slower than the system.
Since motoring can occur during a large reactive-power flow, the real power component Since motoring can occur during a large reactive-power flow, the real power component needs to be measured at low power factors.
needs to be measured at low power factors.
Fig. 13-20 shows the use of two reverse-power relays: 32-1 and 32-2. The 32-1 relay Fig. 13-20 shows the use of two reverse-power relays: 32-1 and 32-2. The 32-1 relay supervises the generator tripping of devices that can wait until the unit begins to motor.
supervises the generator tripping of devices that can wait until the unit begins to motor.
Overspeeding on large steam-turbine units can be prevented by delaying main and field Overspeeding on large steam-turbine units can be prevented by delaying main and field breaker tripping until motoring occurs for non-electrical and selected electrical conditions breaker tripping until motoring occurs for non-electrical and selected electrical conditions (e.g., loss-of-field and overtemperature). Relay 32-1 should be delayed maybe 3 seconds, (e.g., loss-of-field and overtemperature). Relay 32-1 should be delayed maybe 3 seconds, while relay 32-2 should be delayed by maybe 20 seconds. Time delay would be based on while relay 32-2 should be delayed by maybe 20 seconds. Time delay would be based on generator response during generator power swings. Relay 32-2 trips directly for cases of generator response during generator power swings. Relay 32-2 trips directly for cases of motoring that were not initiated by lockout relay 86NE — e.g., governor control motoring that were not initiated by lockout relay 86NE — e.g., governor control malfunction.
malfunction.
Figure 13-20. Reverse-power relay 32-1 prevents load rejection before prime mover Figure 13-20. Reverse-power relay 32-1 prevents load rejection before prime mover shutdown for selected trips; relay 32-2 operates if motoring is not accompanied by shutdown for selected trips; relay 32-2 operates if motoring is not accompanied by
LOSS OF EXCITATION (FIELD) LOSS OF EXCITATION (FIELD)
Protection to avoid unstable operation, potential loss of synchronism, and possible Protection to avoid unstable operation, potential loss of synchronism, and possible damage is important and is typically applied for all synchronous machines. Such protection damage is important and is typically applied for all synchronous machines. Such protection is included in the excitation system supplied with the machine, but additional protection is is included in the excitation system supplied with the machine, but additional protection is recommended to operate independently both as supplemental and backup protection.
recommended to operate independently both as supplemental and backup protection.
Generators have characteristics known as capability curves. Typical curves are shown in Generators have characteristics known as capability curves. Typical curves are shown in Figure 13-21. Temperature limits are basically zones, so these curves are designer’s Figure 13-21. Temperature limits are basically zones, so these curves are designer’s thermal limits.
thermal limits. As overheating varies wAs overheating varies with operation, three arcs ith operation, three arcs of circles define the of circles define the limits.limits.
In one area of operation the limit is the overheating of the rotor windings, in another, in the In one area of operation the limit is the overheating of the rotor windings, in another, in the stator windings; and in the third, in the stator end iron.
stator windings; and in the third, in the stator end iron.
Loss of excitation can, to some extent, be sensed within the excitation system itself by Loss of excitation can, to some extent, be sensed within the excitation system itself by monitoring for loss of field voltage or current. For generators that are paralleled to a power monitoring for loss of field voltage or current. For generators that are paralleled to a power system, the preferred method is to monitor for loss of field at the generator terminals.
system, the preferred method is to monitor for loss of field at the generator terminals.
When a generator loses excitation power, it appears to the system as an inductive load, When a generator loses excitation power, it appears to the system as an inductive load, and the machine begins to absorb a large amount of VARs. Loss of field may be detected and the machine begins to absorb a large amount of VARs. Loss of field may be detected by monitoring for VAR flow or apparent impedance at the generator terminals.
by monitoring for VAR flow or apparent impedance at the generator terminals.
The power diagram (P-Q plane) of Fig. 13-22 shows 40Q characteristic of a typical loss of The power diagram (P-Q plane) of Fig. 13-22 shows 40Q characteristic of a typical loss of field relay with a representative setting, a representative generator thermal capability field relay with a representative setting, a representative generator thermal capability curve, and an example of the trajectory following a loss of excitation. The first quadrant of curve, and an example of the trajectory following a loss of excitation. The first quadrant of the diagram applies for lagging power factor operation (generator supplies VARs). The the diagram applies for lagging power factor operation (generator supplies VARs). The trajectory starts at point A and moves into the leading power factor zone (4th quadrant) trajectory starts at point A and moves into the leading power factor zone (4th quadrant) and can readily exceed the thermal capability of the unit. A trip delay of about 0.2-0.3 and can readily exceed the thermal capability of the unit. A trip delay of about 0.2-0.3 seconds is recommended to prevent unwanted operation due to other transient conditions.
seconds is recommended to prevent unwanted operation due to other transient conditions.
A second high speed trip zone might be included for severe underexcitation conditions.
A second high speed trip zone might be included for severe underexcitation conditions.
Figure 13-21. Typical generator Capability Curve (10 MVA) Figure 13-21. Typical generator Capability Curve (10 MVA)
Figure 13-22. For loss of Field, the Power Trajectory moves from Point A into the Figure 13-22. For loss of Field, the Power Trajectory moves from Point A into the
Fourth Quadrant Fourth Quadrant
When impedance relaying is used to sense loss of excitation, the trip zone typically is When impedance relaying is used to sense loss of excitation, the trip zone typically is marked by a mho circle centered about the X axis, offset from the R axis by X'd/2. Two marked by a mho circle centered about the X axis, offset from the R axis by X'd/2. Two zones sometimes are used: a high speed zone and a time delayed zone (Figure 13-23) zones sometimes are used: a high speed zone and a time delayed zone (Figure 13-23)
Figure 13-23.
Figure 13-23. Loss of Loss of Excitation using Impedance Excitation using Impedance RelayRelay
With complete loss of excitation, the unit will eventually operate as an induction generator With complete loss of excitation, the unit will eventually operate as an induction generator with a positive slip. Because the unit is running above synchronous speed, excessive with a positive slip. Because the unit is running above synchronous speed, excessive currents can flow in the rotor, resulting in overheating of elements not designed for such currents can flow in the rotor, resulting in overheating of elements not designed for such conditions. This heating cannot be detected by thermal relay 49, which is used to detect conditions. This heating cannot be detected by thermal relay 49, which is used to detect stator overloads.
stator overloads.
Rotor thermal capability can also be exceeded for a partial reduction in excitation due to an Rotor thermal capability can also be exceeded for a partial reduction in excitation due to an operator error or regulator malfunction. If a unit is initially generating reactive power and operator error or regulator malfunction. If a unit is initially generating reactive power and then draws reactive power upon loss of excitation, the reactive swings can significantly then draws reactive power upon loss of excitation, the reactive swings can significantly depress the voltage. In addition, the voltage will oscillate and adversely impact sensitive depress the voltage. In addition, the voltage will oscillate and adversely impact sensitive loads. If the unit is large compared to the external reactive sources, system instability can loads. If the unit is large compared to the external reactive sources, system instability can result.
result.
INTEGRATED APPLICATION EXAMPLES INTEGRATED APPLICATION EXAMPLES
Figs. 13-24 through 13-28 show examples of protection packages.
Figs. 13-24 through 13-28 show examples of protection packages.
Fig. 13-24 represents bare-minimum protection, with only overcurrent protection.
Fig. 13-24 represents bare-minimum protection, with only overcurrent protection.
Generators with such minimum protection are uncommon in an era of Generators with such minimum protection are uncommon in an era of microprocessor-based multifunction relays. Such protection likely would be seen only on very small based multifunction relays. Such protection likely would be seen only on very small (<50kVA) generators used for standby power that is never paralleled with the utility grid or (<50kVA) generators used for standby power that is never paralleled with the utility grid or other generators. It may appear to be a disadvantage to use CTs on the neutral side as other generators. It may appear to be a disadvantage to use CTs on the neutral side as shown, since the relays may operate faster with CTs on the terminal side. The increase in shown, since the relays may operate faster with CTs on the terminal side. The increase in speed would be the result of a larger current contribution from external sources. However, speed would be the result of a larger current contribution from external sources. However, if the CTs are located on the terminal side of the generator, there will be no protection prior if the CTs are located on the terminal side of the generator, there will be no protection prior to putting the machine on line. This is not recommended, because a generator with an to putting the machine on line. This is not recommended, because a generator with an internal fault could be destroyed when the field is applied.
internal fault could be destroyed when the field is applied.
Figure 13-24. Exaple of Bare-minimum Protection (Low–impedance Grounding Figure 13-24. Exaple of Bare-minimum Protection (Low–impedance Grounding ))
Fig. 13-25 shows the suggested minimum protection with low-resistance grounding. It Fig. 13-25 shows the suggested minimum protection with low-resistance grounding. It includes differential protection, which provides fast, selective response, but differential includes differential protection, which provides fast, selective response, but differential protection becomes less common as generator size decreases below 2MVA, on 480V protection becomes less common as generator size decreases below 2MVA, on 480V units and below, and on generators that are never paralleled with other generation.
Fig. 13-25 shows the suggested minimum protection with low-resistance grounding. It Fig. 13-25 shows the suggested minimum protection with low-resistance grounding. It includes differential protection, which provides fast, selective response, but differential includes differential protection, which provides fast, selective response, but differential protection becomes less common as generator size decreases below 2MVA, on 480V protection becomes less common as generator size decreases below 2MVA, on 480V units and below, and on generators that are never paralleled with other generation.