Universidad Carlos III de Madrid
2. CONTENIDO DEL MANIFIESTO: IDEAS ESENCIALES
Figure 2-67 shows, more detailed, the power swing detection characteristic. The inclination angleϕPis assumed to be 90°. The setting parameters Za, Zb, Zcand (Zd– Zc) determine the rectangle. It is symmetrical as to its vertical axis. The limit or Zb reaches in reverse direction into the generator. The forward reaches are Zc) into the unit transformer, and the second stage Zd) into the network system. Two
characteristics are available. The lower area, characteristic 1 (i.e. the non-hatched area in Figure 2-67), covers the electrical centre being in the generator block until the unit transformer, the hatched area, characteristic 2, discriminates the electrical centre being in the network system. The point of crossing of the symmetry axis is decisive for the assignment to the characteristic.
Power swings are three-phase symmetrical occurrences. The first prerequisite is therefore the symmetry of the currents which is verified by evaluation of the negative sequence current. A condition for power swing detection is that the positive sequence component of the current exceeds an adjustable limit I1and the negative sequence current remains below an adjustable value I2.
δ=0°
(1–m)Ztot UN UG --- = 0.7
UN UG ---= 0.8
UN UG ---= 1.0
UN UG ---= 1.2 UN
UG ---= 1.3
δ=0°
δ=180°
δ=180°
(0.5–m)Ztot ϕP
–mZtot Z(m)
Im(Z)
Re(Z)
Figure 2-67 Polygonal Out-of-Step Characteristic and Typical Power Swing Occurrences
An out-of-step condition requires, additionally, that the impedance vector enters a power swing characteristic at one side and leaves it at the other side (loss of synchronism, casesandin Figure 2-67). This is characterized in that the real component of the impedance (or its component rectangular to the symmetrical axisϕP has changed its sign while passing through the characteristic.
It is also possible for the impedance vector to enter and leave the power swing polygon at the same side. In this case, power swing tends to be stabilized (casesandin Figure 2-67).
When an out-of-step condition is recognized, i.e. when the impedance vector has passed through a power swing characteristic, an annunciation is issued which also identifies the characteristic. Additionally, a counter n1 (for characteristic 1) or n2 (for characteristic 2) is incremented.
Out-of-step protection pick-up is indicated when a counter is set to 1. Another out-of-step indication is given, for an adjustable time period, each time a counter is
incremented. After an adjustable holding time pick-up resets unless a new power swing condition has been recognized. The holding time is triggered each time a counter is incremented.
Trip command is given when the number of out-of-step periods, i.e. one of the counters, has reached a selectable number. The trip command is held for at least the set timeT-HOLDING. The minimum trip command durationTMin TRIP CMDdoes not start until the pickup has reset.
Zd–Zc
Zc
Zb
Za
Z=R +jX Characteristic 2
Zd
Characteristic 1 Re(Z)
Im(Z)
ϕP
Out-of-Step Protection (ANSI 78)
Figure 2-68 shows the logic diagram of the out-of-step protection. The feature has two stages and can be blocked by a binary input.
Figure 2-68 Logic Diagram of the Out-of-Step Protection
3512 T-SIGNAL 3509 REP. CHAR. 1 Reset 3510 REP. CHAR. 2 Reset
>BLOCK O/S O/S BLOCKED
UL1
O/S – Out-of-step signal for the duration ofT-SIGNAL Ch. 1 – Characteristic 1
Ch. 2 – Characteristic 2
U1 – Complex positive sequence component of the voltages I1 – Complex positive sequence component of the currents I2 – Complex negative sequence component of the currents ^
FNo. 05053 FNo. 05062
TMinTRIP CMD
2.18.2 Setting Hints
General The out-of-step protection is only effective and accessible if this function has been set during the configuration of the protective functions (Section 2.2, address0135, OUT-OF-STEP=Enabled. SetDisabledif the function is not required. Address3501 OUT-OF-STEPis used to switch the functionONorOFF, or to block only the trip command (Block Relay).
Pickup The out-of-step protection operates only when the positive sequence component of the currents has exceeded a minimum threshold3502 I1> RELEASE(overvoltage pickup). As out-of-step conditions are symmetrical occurrences, a maximum value of negative sequence current3503 I2< RELEASEmust not be exceeded.
The determining factor for overcurrent settingI1> RELEASEis the maximum possible operating current. Pickup under conditions of permissible overload should be ex-cluded. The setting should therefore be set above the maximum anticipated (over-) load current (at least 120 % IN. Depending on network conditions, smaller pickup thresholds can be chosen, so that the measurement (see Figure 2-68) may be released all the time. As out-of-step conditions are symmetrical occurrences, the pickup threshold of the negative sequence component of the currentI2< RELEASE should be set to approx. 20 % IN.
Impedance Settings
The measured impedances during power swing conditions are decisive for the settings. For the direction to the machine (as viewed from the location of the voltage transformers), the power swing reactance of the machine must be considered, which is approximately the transient reactance Xd' of the machine. Consequently, you will calculate the secondary transient reactance and set it for Zb≈ Xd' (see Figure 2-69).
Figure 2-69 Power Swing Polygon Zd–Zc
Zc
Zb
Za Characteristic 2
Zd
Re(Z) Im(Z)
ϕP
Characteristic 1 ý þ ü
ü
ý
þ
≈Xd’
≈(0.7 to 0.9) ZK Transf Locus diagram
of power swing impedance
Out-of-Step Protection (ANSI 78)
Xd' can be calculated from the per unit reactance xd' as follows:
where Xd' – Transient reactance of the machine xd' – Transient per unit reactance UN, Gen – Primary rated machine voltage IN, Gen – Primary rated machine current üct – Current transformer ratio üvt – Voltage transformer ratio
Dependent on the type of generator and on the secondary current, a secondary voltage UN= 100 V or 120 V leads to approximately the reactance ranges listed in Table 2-10.
As it is presupposed that the generator is connected with the network via a unit transformer, the setting in the network direction is chosen such that the reactance reach of characteristic 1 is approximately 70 % to 90 % of the transformer impedance, and the reach of characteristic 2 is into the network. Thus,Zcat address3506is set to 70 % to 90 % of the transformer impedance XSC. For characteristic 2, the remaining portion of the transformer impedance is set at3507 Zd - Zc, if necessary
complemented by the impedance of the additional line section to be monitored.
Table 2-11 shows typical values of the impedances of unit transformers XSCfor secondary rated currents IN= 1 A and IN= 5 A, the relationship of the values is according to the following equation:
.
Table 2-10 Transient Machine Reactances (Referred to Secondary Side)
Generator Type Xd Xd
Table 2-11 Per Unit Impedance Voltages and Impedances of Transformers
Transformer Type uSC XSC
UN= 100 V/ IN= 1 A
The setting Zais decisive for the width of the power swing polygon. This setting value 3504 Za is determined by the total impedance Ztotand can be derived from the equation in Figure 2-70. Ztotcan be calculated from the sum of Zband Zd; then the power swing angle is valid between the machine e.m.f. and the network. Optionally, Ztotcan be calculated from Zb+ Zc; in this case the power swing angle is valid between the machine e.m.f. and the unit transformer. The default setting of address3504 Za corresponds to the latter case. Usually, a power swing angleδ= 120° is chosen, since the generator voltage UGand the system voltage UNequal the voltage difference:
Figure 2-70 Power Swing Polygon and Impedance Vectors with Power Swing Angleδ
Maximum Power Swing Frequency
The polygon width Zadetermines also the maximum detectable power swing frequency. The consideration that, at maximum power swing frequency, at least two impedance measurements must have been carried out within the power swing polygon, leads to the following approximative formula for the maximum detectable power swing frequency fP:
For a rated frequency of 50 Hz (i.e. T = 20 ms), for example, the above formula delivers:
as the maximum detectable power swing frequency.
Za Ztot⁄2 generator and unit transformer (as illustrated) or
Out-of-Step Protection (ANSI 78)
The inclination angleϕof the power swing polygon can be set at address3508 PHI POLYGONand thus matched to the conditions.
Calculation Example:
Current transformers üct= 500 A/1 A
Thus, the secondary transient reactance of the generator:
Zb≈Xd' thus determines the setting of address3505 Zb.
The secondary reactance of the unit transformer is derived from the primary reactance by considering the transformer ratios:
Assuming that the characteristic should cover 85 % of the transformer reactance, the setting of Zcresults in≈ 0.85⋅ 4.2Ω ≈ 3.6Ω.
Assuming that the remaining transformer reactance (0.15⋅4.2Ω) and the covered system reactance should be 10Ω, the setting of3507results in:
Zd - Zc=6.4 Ω.
The width Zaof the polygon is determined by the total impedance Ztot. In this calculation example, the total impedance Ztotis that of characteristic 1, i.e. the sum of generator reactance and one portion of the unit transformer reactance; that is the sum of the setting values for Zband Zc= 12Ω+ 3.6Ω = 15.6 Ω):
Number of Power Swings
Address3509 REP. CHAR. 1determines the number of out-of-step periods for characteristic 1 which will lead to trip, i.e.the number of times this characteristic must have been passed through. For characteristic 1, 1 or 2 passes are normally adequate as out-of-step conditions with the electrical centre within the power station unit should not be tolerated too long, and the power swing frequency tends to accelerate during an out-of-step condition, so that the electrical and dynamic stress of the machine increases. On the other hand, for out-of-step conditions with the electrical centre being in the network system a higher number of slip periods can be tolerated, so that address3510 REP. CHAR. 2can usually be set to 4.
Each time characteristic 1 or 2 is passed through, a holding time set at address3511 T-HOLDING) is started. A detected out-of-step condition is maintained for that time so that the counter is incremented with the next passing through. When no renewed pickup occurs within this time, the counters n1or n2respectively are reset, i.e. the out-of-step condition is “forgotten”. This time should be set higher that the longest expected slip period (i.e. lowest slip frequency). Settings between 20 s and 30 s are good values.
Each time one of the counters n1or n2is incremented, the holding time is restarted, and an annunciation “Out-of-step characteristic 1" or “Out-of-step characteristic 2" is issued. These annunciations disappear after the time which is set at address3512 T-SIGNAL. If this time is set higher that the time between two power swings, the annunciation “Out-of-step characteristic 1(2)” begins with the first out-of-step
detection and ends after the last detected out-of-step detection, prolonged wit the set timeT-SIGNAL.