Schweitzer Engineering Laboratories SEL-321 - Calculating Overcurrent Element Test Quantities

7-18 Maintenance and Testing Date Code 20011026
SEL-321/321-1 Instruction Manual
The steps necessary to perform an overcurrent element test are shown below.
Step 1.
Select element under test. Briefly review the specifications for the element under
test. If the element under test is supervised by other conditions, be sure that you
understand the inputs necessary to satisfy the supervisory conditions, and the
overcurrent condition.
Step 2. Verify the element setting by executing the SHOWSET command via a terminal
connected to a relay serial port, or by examining the relay settings via the front panel
LCD display using the Access Level 2 SET command.
Step 3.
Select the method used to indicate assertion of the element under test. Typically,
this is accomplished by monitoring an output contact programmed to indicate only
the condition under test.
Step 4.
Make test source connections. If an unsupervised, single-phase overcurrent element,
such as the 50H element, is being tested, a single current source is all that is required
to perform the test. If a directionally supervised negative-sequence overcurrent
element is to be tested, voltage-sources are also required to satisfy the directional
supervision conditions.
Make test source connections based upon the requirements of the test and the
availability of sources. Figure 7.3 through Figure 7.7 illustrate the various source
connections you may use.
Step 5.
Determine the signals required to operate the element. For unsupervised elements,
simply use the equations shown on the following page. Fulfilling the requirements
of the supervisory conditions may be more complicated. Refer to the test procedures
appropriate to the supervising element for details.
Step 6.
Apply the test signals.
Step 7.
Monitor the output contact assigned to indicate the state of the element under test to
ensure that the element asserts. Review the event reports to ensure that supervisory
conditions and the element under test asserted appropriately.
Calculating Overcurrent Element Test Quantities
The SEL-321 Relay is equipped with several different types of overcurrent elements. Each has a
specific purpose and a slightly different method of testing. The types of overcurrent elements,
the signals they operate from, and the elements of that type are listed below.
Element Type
Operates based upon the magnitude of: Relay Elements
Phase A-phase or B-phase or C-phase current.
Test using single-phase current.
50H, 50M, 50L1, 50L2,
50L3, 50L4
Three-Phase A-phase, B-phase, and C-phase current.
Test using three-phase current.
3P50R
Phase-to-Phase Vector difference between two phase
currents. Test using one or two phase
currents.
50PP1, 50PP2, 50PP3,
50PP4