Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-55 973.334.3100
the ground transmits only the negative half-cycle measurements. The messages used to transmit the
measured current are called CCD (Charge Comparison Data) messages.
When a current measurement is transmitted to the remote relay two additional pieces of information
are included in the message. The message includes a flag to indicate the status of the local fault
detectors. The message also includes information regarding the delay between when the current
measurement was taken and when the message was transmitted to the remote end.
13.5.1.2.4 Nesting
When a relay receives a CCD message from the remote relay it must compute how long ago the actual
current measurement was made. As stated above the message itself includes information indicating
how long the data was held in queue at the remote end prior to transmission. Two additional pieces of
timing information are also required to determine how old the data is.
The relay keeps track of the channel delay in the system (how long it takes between transmitting a
message and receiving it at the other end. The last piece of timing information required is the delay at
the local end between when a message is received and the data is actually processed.
These three delay times (remote queuing time, channel time, and local queuing time) are added to
determine how long ago the remote relay made the current measurement that is being processed. This
is used to determine which of the locally calculated half-cycle currents was measured at the same time.
This process is called nesting.
The fact that the comparisons are made based upon half-cycle measurements results in some notable
characteristics of the algorithm. First, the channel delay is canceled by the algorithm so it does not
compromise the integrity of the protection (although longer channel delays will result in delayed trip
times). The system can tolerate greater than 27ms of channel delay and still provide protection.
The other benefit is that exact delay compensation is not required. Modest errors in the calculated
delays can be tolerated while still allowing the received CCD message to nest with the correct local
half-cycle. Delay compensation errors on the order of ± 4ms will not cause a nesting error.
13.5.1.2.5 Operate Level Calculation
The differential current that will cause the relay to operate is the operate level. The operate level is
based upon several user settings and the dynamic conditions of the line.
13.5.1.2.6 Bias Setting
The user selects the bias level depending upon the system application. There is one setting for the
phases and one for ground.
13.5.1.2.7 Bias Boost
The user may chose to boost the bias level following a breaker closing. This reduces the relay’s
sensitivity and accommodates line charging currents. Phase and ground have independent settings for
both the boost level and the duration of the boost. The boost function may be disabled by setting the
boost level to zero.
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