I.L. 17562
Page 87
PR 0.3 Effective 8/99
Note that these are phasor (vector) operations with a phasor result. The positive sequence
phasors in phases B and C have the same magnitude as the phase A positive sequence phasor,
but lag the phase A component by exactly 120 and 240 degrees respectively. This balanced set
of phasors drives the motor's useful work.
To calculate the negative sequence component in phase A, rotate the phase B current phasor
120 degrees in the negative direction and the phase C phasor 240 degrees in the negative
direction. Refer to Figure 8.5. The formula for I
A2
is:
I
A2
= I
A
+ (I
B
∠
-120
°
) + (I
c
∠
- 240
°
)
3
The negative sequence phasors in phases B and C have the same magnitude as the phase A
positive sequence phasor, but lead the phase A component by exactly 120 and 240 degrees
respectively. This balanced set of phasors represents the net effect of magnitude or phase
unbalance and only heats the rotor.
Certain harmonics in the phase currents produce torques in the rotor, just like positive and
negative sequence currents. In particular, the 7
th
and 13
th
and certain higher harmonics act like
positive sequence. The 5
th
, 11
th
, and certain other higher harmonics act like negative sequence.
This can also influence motor performance and heating. The MP-3000 sequence calculations
also capture these harmonic currents and include their effect in the thermal modeling.
Prior to the use of a microprocessor in a multifunction motor protection relay, there was no
reasonable way of modeling the total heating effects of the positive and negative sequence
components on a continuous basis. Therefore, oversimplified assumptions were used with
available relays. This resulted both in nuisance tripping and in motor burnouts or life reduction.
The MP-3000 uses a unique, patented calculation for determining these values from current
samples and modeling their effects. The effective current squared, as used in the calculation for
rotor heating, is:
I
2
=
I
1
2
+
kI
2
2
Here
1
2
2
is weighted by k because of the disproportionate heating caused by the negative
sequence current component. The effects of both the positive and negative sequence currents
are accurately taken into account. Their
combined
effect is incorporated into a rotor protection
algorithm that effectively keeps track of the temperature of the rotor.
It is not necessary to pick an arbitrary phase unbalance setting to trip the motor, although such
an unbalance trip function is additionally included in the relay to speed up tripping without
heating for grossly unbalanced conditions. As long as the combined effect of the positive and
negative sequence currents does not approach the motor damage curve, the MP-3000 will allow
the motor to run.
8.2.3 Thermal Bucket -
The MP-3000 models heating as the filling of a thermal reservoir or
accumulating bucket whose size is determined by the thermal capacity of the motor. This
capacity is calculated in the relay from motor nameplate constants. The filling is proportional to
effective I
2
over time, where effective I
2
includes the disproportionate heating effect of negative-
sequence currents. Cooling is also modeled as draining of the bucket. The loss of equilibrium
between heating and cooling leads to an eventual thermal trip. The thermal bucket filling in
percent can be observed continuously on the MP-3000 display, via data communications, or via
the 4-20 mA transducer output.
Содержание MP-3000
Страница 18: ...Page 18 I L 17562 PR 0 3 Effective 8 99 Figure 4 1 MP 3000 Pushbuttons...
Страница 19: ...I L 17562 Page 19 PR 0 3 Effective 8 99 Figure 4 2 MP 3000 LED Indicators...
Страница 72: ...Page 72 I L 17562 PR 0 3 Effective 8 99 Figure 6 1 Panel Cutout Dimensions...
Страница 73: ...I L 17562 Page 73 PR 0 3 Effective 8 99 Figure 6 2 Faceplate Dimensions...
Страница 74: ...Page 74 I L 17562 PR 0 3 Effective 8 99 Figure 6 3 MP 3000 Case Depth Dimensions...
Страница 75: ...I L 17562 Page 75 PR 0 3 Effective 8 99 Figure 6 4 Universal RTD Module Mounting Dimensions...
Страница 76: ...Page 76 I L 17562 PR 0 3 Effective 8 99 Figure 6 5 Rear Panel Terminals...
Страница 77: ...I L 17562 Page 77 PR 0 3 Effective 8 99 Figure 6 6 Typical Ct Circuits and Motor Control Wiring...
Страница 78: ...Page 78 I L 17562 PR 0 3 Effective 8 99 Figure 6 7 Typical ac Supply and URTD Wiring...
Страница 79: ...I L 17562 Page 79 PR 0 3 Effective 8 99 Figure 6 8 Alternatives for Discrete Input Wiring...
Страница 80: ...Page 80 I L 17562 PR 0 3 Effective 8 99 Figure 6 9 RTD Wiring to URTD Module...
Страница 88: ...Page 88 I L 17562 PR 0 3 Effective 8 99 Figure 8 1 System Overview Figure 8 2 Torques from Sequence Currents...
Страница 100: ...Page 100 I L 17562 PR 0 3 Effective 8 99 Figure 9 1 Rotor Temperature Tracking...
Страница 101: ...I L 17562 Page 101 PR 0 3 Effective 8 99 Figure 9 2 Motor Protection Curve...
Страница 102: ...Page 102 I L 17562 PR 0 3 Effective 8 99 Figure 9 3 Underload Jam Protection Curve...
Страница 103: ...I L 17562 Page 103 PR 0 3 Effective 8 99 Figure 9 4 Motor Protection Curve Example without RTDs...
Страница 104: ...Page 104 I L 17562 PR 0 3 Effective 8 99 Figure 9 5 Motor Protection Curve Example with RTDs...
Страница 105: ...I L 17562 Page 105 PR 0 3 Effective 8 99 Figure 9 6 Motor Start and Run Cycles...
Страница 109: ...I L 17562 Page 109 PR 0 3 Effective 8 99 P5L8 40 Incomplete Sequence time 1 60s OFF 1 240s...