Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Polyphase
Reexamination Certificate
2001-03-28
2003-01-14
Sherry, Michael (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Polyphase
C361S036000
Reexamination Certificate
active
06507184
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to power systems, and more particularly, to methods and apparatus for differential current measurement in three-phase power systems.
BACKGROUND
Differential current measurement is a technique used in a wide variety of power system applications. For example, the technique is often used in the protection of power system equipment, such as transformers, generators, motors, and the like. Generally, differential current measurement techniques involve monitoring the current at both an input terminal and an output terminal of a device, normalizing the measured input and output currents to compensate for changes in phase and magnitude of the measured currents that may be introduced by the device during normal operation, and then comparing the normalized input and output currents. If the difference between the normalized input and output currents is zero, then the device presumably is working properly. On the contrary, a detected difference between the normalized input and output currents may indicate a fault within the device. In response to the detection of a fault, a relay or other circuit breaker may be triggered to shut off power to the device in order to prevent further damage.
FIG. 1
is an example of how differential current measurement may be employed in the protection of a three-phase power distribution transformer
10
. As shown, each phase a, b, and c of the power system is connected to a respective primary winding (high-voltage side) of the transformer
10
via a respective input of a first terminal
12
. Similarly, a second terminal
14
provides an output from the secondary winding for each phase (low-voltage side). Current transformers
16
a
,
16
b
, and
16
c
can be used to obtain a measure of the current flowing into each phase of the first terminal (I
1a
, I
1b
, and I
1c
). Similarly, a measure of the current flowing out of each phase of the second terminal (I
2a
, I
2b
, and I
2c
) can be obtained by respective current transformers
18
a
,
18
b
, and
18
c.
Transformers like that shown in
FIG. 1
are often employed either to step-up or step-down an input voltage or current. This naturally introduces a change in the magnitude of the voltage or currents entering and leaving the device. With a transformer, the magnitude of this change is dependent upon the ratio of the number of turns, N
1
, in the primary winding of the transformer to the number of turns, N
2
, in the secondary winding. Specifically, the input current (I
1
) to the primary winding will be equal to N
2
/N
1
times the current output from the secondary winding (I
2
). Thus, I
1
=N
2
/N
1
(I
2
). The turns ratio of the transformer must be taken into account when normalizing the input and output currents during the differential current measurement process.
The magnitude and phase of the input currents can also be affected during normal operation of a transformer by the manner in which the primary windings (high-voltage side) for each phase and the secondary windings (low-voltage side) for each phase are connected, or wired. Two common ways to wire the multi-phase windings on one side of a transformer are referred to in the art as the Wye (Y) configuration and the Delta (&Dgr;) configuration. Different combinations of these and other configurations can be used to wire the respective primary and secondary windings. In the example of
FIG. 1
, the primary windings of the transformer are connected in a Wye (Y) configuration, and the secondary windings are connected in a delta (&Dgr;) configuration. A Y-&Dgr; wiring configuration of this type will introduce a phase shift of 30 degrees, and a change in magnitude between the input and output currents by a factor of
1/{square root over (3)}. That is, ignoring the turns ratio, I
1
=1/{square root over (3)}∠+30° (I
2
), where I
1
is the input current to the primary winding and I
2
is the output current from the secondary winding. Other wiring configurations will result in other phase shift magnitudes. Generally, the known wiring configurations for distribution transformers in use in the power distribution industry induce phase shifts that are some multiple of 30°. Different phase shifts result in different changes in current magnitude. These changes in magnitude and phase must also be taken into account when normalizing the input and output currents during the differential current measurement process.
With respect to the phase and magnitude changes caused by the wiring configuration at the primary and secondary windings, one way to cancel out those changes to achieve normalization is to wire the current transformers
16
a-c
,
18
a-c
in such a way as to cancel out the affect of the transformer wiring configuration. Generally, however, power distribution system customers do not favor such a solution, particularly because the variety of different wiring configurations for the primary and secondary transformer windings requires a different current transformer wiring configuration to provide the appropriate normalization in each case. Consequently, this form of physical normalization, commonly referred to as current transformer phasing, makes transformer installation much more difficult and costly.
Because of the difficulties with current transformer phasing, the general approach to normalization with respect to magnitude and phase changes caused by the wiring configurations of transformers today is to digitize the measured currents obtained with the current transformers
16
a-c
,
18
a-c
, and to then perform the normalization functions digitally on a programmable processor or microcontroller. In this manner, different transformer wiring configurations can be accommodated by simply reprogramming the processor for each different case, while using the same physical connection for the current transformers. Transformer installation is thus simplified and less costly.
FIG. 1
illustrates this form of digital processing. As shown, the current measurements obtained by the respective sets of input and output current transformers
16
a-c
,
18
a-c
, are digitized and passed through respective normalization functions
20
,
22
to produce normalized input ({overscore (I
1a
)}, {overscore (I
1b
)}, and {overscore (I
1c
)}) and output ({overscore (I
2a
)}, {overscore (I
2b
)}, and {overscore (I
2c
)}) currents in which changes in magnitude and phase introduced by the transformer
10
during normal operation are factored out. A differential current function
24
then calculates a differential current for each phase from the normalized input and output currents (I
Opa
={overscore (I
1a
)}−{overscore (I
2a
)}; I
OPb
={overscore (I
1b
)}−{overscore (I
2b
)}; and I
OPc={overscore (
1c
)}−{overscore (I
2c
)}). During normal operation of the transformer
10
, each I
OP
current should equal zero. However, a fault within the transformer
10
should result in a non-zero reading. The non-zero reading can be used as an indication of a transformer fault. Upon detecting such a non-zero reading, a relay or circuit breaker can be triggered to interrupt power to the transformer to prevent further damage.
Another consideration to be taken into account in performing differential current measurements in a multi-phase power system, particularly in differential current measurements used for protection of transformers, is the presence of a zero-sequence current component. Power system connections that allow ground or zero sequence current to flow will add to the individual phase currents, and where it is present on only one side of the device will create a non-zero combination that will result in a false differential current reading that could be interpreted as a fault within the device (i.e., “internal fault”) when in actuality the fault has occurred in the line outside of the device (i.e., “external fault” or “through fault”). Because of this possibility, in addition to compensating for the phase shifts introduced by the device, it is also necessary for the normalization tech
ABB Automation Inc.
Sherry Michael
Tang Minh N.
Woodcock & Washburn LLP
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