Universidad Nacional de Educación a Distancia
4. CONCLUSIONES
General Dynamic occurrences such as sudden load changes, short-circuits, automatic reclo-sures or switching operations within the power system may cause power swings.
Therefore, the impedance protection is complemented by a power swing blocking function to avoid spurious tripping.
Power swings are three-phase symmetrical occurrences. The first prerequisite is therefore the symmetry of the phase currents, which is verified by evaluation of the negative sequence current. This means that asymmetrical (all single-phase and two-phase) short-circuits cannot cause the power swing blocking to pick up. Even if a pow-er swing has been detected, the asymmetrical short-circuits following it quickly deac-tivate the power swing blocking and make a trip by the impedance protection possible.
Since a power swing happens much more slowly than a short-circuit, the rate of change of the impedance is a reliable criterion for its identification. Because of its sym-metrical nature, the positive sequence impedance obtained from the positive se-quence components of the currents and voltages is evaluated.
Logic Figure 2-63 shows the logic diagram of the power swing blocking. In the upper section, the current symmetry monitoring can be seen. A release signal is given if there is a three-pole pickup with no negative sequence system current. For detection of power swings, a power swing polygon (P/SPOL) is used which is greater than the trip polygon (TPOL). The distance between the two polygons can be set (common setting for R and X direction). The user can choose for each setting parameter whether the trip polygon refers only to characteristic Z1 or to characteristics Z1 & Z2. In the latter case, the trip polygon is the maximum impedance value.
Measuring Principle
The criterion for power swing blocking is composed of the power swing polygon, its distance to the trip polygon, the trip polygon itself and the rate of change of the imped-ance. The protection compares the first impedance value after entering the power swing polygon (at a moment Tent) with the last impedance value outside the polygon (at a moment Tent-∆t). The time∆t is determined by the measuring interval, which is one cycle. If the rate of change of the impedance vector thus determined is less than a set value∆Z/∆t, a power swing is detected. The impedance stage is not blocked, however, until the impedance vector enters the trip polygon TPOL.
If the first impedance value is both inside the P/SPOL and the TPOL, the protection detects immediately a short-circuit, because there must be at least one impedance value between the P/SPOL and the TPOL. The distance between the power swing
polygon P/SPOL and the trip polygon TPOL, and the rate of change∆Z/∆t are matched to one another in such a way that power swings are reliably detected and the desired impedance zone (Z1 or Z1 & Z2) of the impedance protection is blocked. The blocking remains effective until the measured impedance vector has left again the trip polygon / power swing polygon, the impedance changes faster than the change rate, or asymmetrical power conditions rule out the possibility of a power swing. The power swing blocking time is also limited by a parameter setting
(T-ACTION P/S).
Blocking of the Impedance Stages
Power swing blocking is mostly used for impedance stage Z1, because the delay time T1 for this stage is set low. Accordingly, a high delay time T2 must be set for zone Z2.
In the overreach zone Z1B no power swings can occur by definition, since the network breaker is open and there is thus no second machine for power swings. Likewise, the power swing blocking does not block the non-directional overcurrent stage (T3).
Figure 2-63 Logic Diagram for the Power Swing Blocking of the Impedance Protection
2.17.3.1 Setting Hints
The power swing blocking is only effective if address3313 POWER SWINGhas been set to ON.
A sensible compromise has to be found for the distance between the power swing polygon and the trip polygon (parameter:P/SPOL-TPOL(address3314) and for the rate of change (parameter:dZ/dt(address3315)). It must be kept in mind that the rate of change is not constant and decreases with increasing proximity to the origin of the coordinate system.The rate of change is also determined by power system condi-tions, such as the impedance between the systems involved in the power swing, and by the power swing frequency (see also Section 2.18 Out-of-Step Protection).
3315 dZ/dt
Z(Tent) First value within P/S polygon (at Tent)
Z(Tent-∆t) Last value outside P/S polygon P/SPOL Power swing polygon TPOL Trip polygon
∆Z /∆t Rate of change of the impedance vector
<
>
∆Z /∆t
see Figure 2-61
Impedance Protection (ANSI 21)
The following relation allows to estimate the rate of change:
Meaning:
X Reactance between the sources of the power swing fp Power swing frequency
δ Power swing angle
Figure 2-64 shows an example of how the rate of change evolves as a function of the power swing angle. The rate of change is smallest at an angle of 180°, but increases proportionally with the angle considered.
Figure 2-64 Evolution of the Rate of Change (fp= 1 Hz; X = 10Ω)
For this reason, the setting value dZ/dt must also be coordinated with the impedance jump occurring at the start of a short-circuit.
To do so, you determine the minimum operating impedance (ZL, min), form the differ-ence to the setting of the impedance zone (e.g. Z1) and calculate the impedance gra-dient, taking into account the one-cycle measuring interval.
Example:
Umin= 0.9 UN, Imax= 1.1 IN, uK= 10 %,∆t = 20 ms UN= 100 V, IN= 1 A
dZ t( )
---dt dR t( ) ---dt
≈ Xπfp
2sin2(πfpt)
--- Xπfp 2sin2 δ
2 ---è øæ ö --- inΩ
----s
= =
50
0 100 150 200 250 300 350 400
50 100 150 200 250 300
δ Degrees
dZ(δ) Ω/s
Z1 0 7, ⋅uK⋅UN 100 %⋅ 3 I⋅ N
--- 4 04, Ω
= =
If safety factor 4 is chosen, dZ/dt should never be set higher than 500Ω/s (or 100Ω/s for 5 A transformers).
The default setting for dZ/dt is 300Ω/s, which should be adequate for most applica-tions. This is also the basis for the minimum distance P/SPOL - TPOL, assuming that for detection of a power swing there must be one impedance value between P/SPOL and TPOL.
P/SPOL – TPOL > dZ/dt⋅∆t = 300Ω/s⋅0.02 s = 6Ω(selected setting value: 8Ω) All other settable parameters are advanced parameters which need not normally be modified.
Whether a power swing can cause an overfunctioning of the impedance protection de-pends mainly on the time the impedance vector remains inside the trip polygon. This time can only be reliably determined by „transient“ calculations.
If the rate of change in the proximity of 180° is known, it can be the basis for a rough estimation of the time.
T = 2⋅Zcharacteristic/dZ/dt (180°)
The above data yield the following value:
Zcharacteristic= Z1 = 4Ω
dZ/dt (180°) = 20Ω/s (from Figure 2-64) T = 2⋅ 4Ω/20Ω/s = 0.4 s
This means that for delay times of more than 0.4 s no power swing blocking is needed.
Table 2-9 Parameters
Address Parameter Remark
3316A BLOCKING OF Default setting is Z1, as there is little or no delay for this stage. The delay time of Z2 is determined by the power system protection, and greater (see also the hints below).
3317A T-ACTION P/S The default setting is 3 s. This time depends on the minimum possible power swing frequency.
ZL min, 0 9, ⋅UN 3 1 1⋅ , ⋅IN
--- 47 24, Ω
= =
dZ
---dt = (ZL min, –Z1)∆/t = 43 20, Ω/20 ms = 2160 Ω/s
Impedance Protection (ANSI 21)
2.17.3.2 Settings of the Impedance Protection
2.17.3.3 Information from the Impedance Protection
Addr. Setting Title Setting Options Default Setting Comments
3301 IMPEDANCE
PROT.
OFF ON
Block relay for trip com-mands
OFF Impedance Protection
3302 IMP I> 0.10..20.00 A 1.35 A Fault Detection I> Pickup
3303 U< SEAL-IN ON
OFF
OFF State of Undervoltage Seal-in
3304 U< 10.0..125.0 V 80.0 V Undervoltage Seal-in Pickup
3305 T-SEAL-IN 0.10..60.00 sec 4.00 sec Duration of Undervoltage Seal-in
3312 T END 0.00..60.00 sec;∞ 3.00 sec T END: Final Time Delay
3306 ZONE Z1 0.05..130.00 Ohm 2.91 Ohm Impedance Zone Z1
3307 T-Z1 0.00..60.00 sec;∞ 0.10 sec Impedance Zone Z1 Time Delay
3308 ZONE Z1B 0.05..65.00 Ohm 4.99 Ohm Impedance Zone Z1B
3309 T-Z1B 0.00..60.00 sec;∞ 0.10 sec Impedance Zone Z1B Time
Delay
3310 ZONE Z2 0.05..65.00 Ohm 4.16 Ohm Impedanz Zone Z2
3311 ZONE2 T2 0.00..60.00 sec;∞ 0.50 sec Impedance Zone Z2 Time Delay
3313 POWER SWING ON
OFF
OFF Power Swing Blocking
3314 P/SPOL-TPOL 0.10..30.00 Ohm 8.00 Ohm Distance betw. Power Swing
-Trip-Pol.
3315 dZ/dt 1.0..600.0 Ohm/s 300.0 Ohm/s Rate of Change of dZ/dt
3316A BLOCKING OF Zone Z1
Zone Z1 and Z2
Zone Z1 Power Swing Blocking locks out
3317A T-ACTION P/S 0.00..60.00 sec;∞ 3.00 sec Power Swing Action Time
F.No. Alarm Comments
03953 >Imp. BLOCK >BLOCK impedance protection
03956 >Extens. Z1B >Zone 1B extension for impedance prot.
03958 >Useal-in BLK >Imp. prot. : BLOCK undervoltage seal-in
03961 Imp. OFF Impedance protection is switched OFF
03962 Imp. BLOCKED Impedance protection is BLOCKED
03963 Imp. ACTIVE Impedance protection is ACTIVE
03966 Imp. picked up Impedance protection picked up 03970 Imp. I> & U< Imp.: O/C with undervoltage seal in
03967 Imp. Fault L1 Imp.: Fault detection , phase L1 03968 Imp. Fault L2 Imp.: Fault detection , phase L2 03969 Imp. Fault L3 Imp.: Fault detection , phase L3
03977 Imp.Z1< TRIP Imp.: Z1< TRIP
03978 Imp.Z1B< TRIP Imp.: Z1B< TRIP 03979 Imp. Z2< TRIP Imp.: Z2< TRIP
03980 Imp.T3> TRIP Imp.: T3> TRIP
03976 Power Swing Power swing detection
F.No. Alarm Comments
Out-of-Step Protection (ANSI 78)