Hands-On Relay Testing Session
Restricted Earth Fault (REF) Element Settings
Exercise Objectives
After completing this exercise, you should be able to do the following:
•
Identify REF settings and protection requirements.
•
Define REF settings recommendations.
•
Calculate effective settings for the hands-on activities.
•
Validate relay settings.
In this exercise, you will determine protection settings for the REF function for the example transformer
data provided. You will perform calculations and then enter protection settings into ACSELERATOR
QuickSet® SEL-5030 Software.
Finally, after sending your settings to the relay, you will validate element assertion and relay operation.
REF Protection
REF protection is a technique for sensitive detection of ground faults in a grounded wye-connected
transformer winding. Because it employs a neutral current transformer (CT) at one end of the winding and
a normal set of three CTs at the line end of the winding, REF protection can detect only ground faults
within that particular wye-connected winding. For REF to function, the line-end CTs must also be
connected in wye because the technique uses a comparison of zero-sequence currents. Delta-connected
CTs cancel out all zero-sequence components of the currents, eliminating one of the quantities the REF
element needs for comparison.
SEL-387A REF Implementation
The REF implementation in the SEL-387A Current Differential and Overcurrent Relay uses a directional
element (32I) that compares the direction of an operating current derived from the line-end CTs with the
polarizing current obtained from the neutral CT. A zero-sequence current threshold and positive-sequence
restraint supervise tripping. Because the SEL-387A has two REF elements, you can apply separate
elements to each of the wye windings of a wye-wye-connected transformer. The neutral CT connects to
one of the relay inputs (wye-delta with no grounding bank) or two relay inputs (wye-wye or wye-delta
with grounding bank). The three current inputs are labeled IN1, IN2, and IN3.
The directional element compares the polarizing current (neutral CT current) to the operating current
(IRW1 or IRW2) and indicates forward (internal) fault location or reverse (external) fault location. The
forward (internal) indication occurs if the fault is within the protected winding, between the line-end CTs
and the neutral CT.
Restricted Earth Fault (REF) Element Settings
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The element operate time is determined by an extremely inverse-time overcurrent curve (Curve U4) at the
time dial (TD) setting 0.5 and with 50GPn as the pickup setting (see Figure 1). Relay Word bit 32IF
(forward fault) torque-controls the timing curve, and IN1 operates the timing function. The curve resets in
one cycle if the current drops below the pickup or if 32IF deasserts. When the curve times out, Relay
Word bit REFP asserts.
Figure 1 REF Protection Output (Extremely Inverse-Time Overcurrent)
Data Requirements
You will need the following information to calculate the REF setting:
•
Transformer Winding 1 and Winding 2 CT ratios
•
Transformer neutral CT ratio
Hands-On Activity 1: Calculate REF Element Settings
In this hands-on activity, you will determine the settings for applying the REF element to transformer
Winding 2. The relay uses the residual currents from a single winding to calculate the polarizing quantity
(see Figure 2).
Figure 2 Two-Winding Transformer Application Single-Line Diagram
System Data
Transformer rated MVA is 50
Winding 1 phase CT is 200:5, wye connected
Winding 2 phase CT is 2000:5, wye connected
Neutral CT is 1200:5 A
Restricted Earth Fault (REF) Element Settings
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Step 1
In ACSELERATOR QuickSet, select Group 1 > Set 1 > REFn.
Step 2
Determine and enter the Polarizing Quantity REFn (n = 1 or 2) setting.
REFn = __________________
Step 3
In ACSELERATOR QuickSet, select the Group 1 > Set 1 > General Data settings.
Step 4
Determine and enter the Neutral (IWN2) CT Ratio.
CTRN2 = __________________
Step 5
Select the Restricted Earth Fault settings.
Step 6
Verify that the Restricted Earth Fault Torque Control setting is at the default setting of 1.
Step 7
Determine and enter the REFn Current Sensitivity Level.
50GP1 = __________________
50GP2 = _________________
Note: For this application, enable both Winding 1 and Winding 2 REF functions.
The neutral current sensitivity threshold can be set as low as 0.25 times nominal current (0.25 A for a 5 A
nominal CT current), the minimum neutral current sensitivity of the relay. However, the minimum
acceptable value of 50GP must meet the following two criteria:
•
50GPn must be greater than the highest unbalance expected by load conditions.
•
50GPn must be greater than a minimum value determined by the relationship of the CTRn values
used in the REF function.
⎡ CTRMAX ⎤
50GPn = ≥ 0.05 • Inom • ⎢
⎥
⎣ CTRNn ⎦
In this example, n = 2, representing transformer Winding 2.
Restricted Earth Fault (REF) Element Settings
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Step 8
Use the Sequential Events Recorder (SER) to capture the operation of the REF elements under test.
Select the Report > SER > SER Trigger Lists settings.
Step 9
Add the following Relay Word bits to SER 4:
32IE1 and 32IE2: REF logic enabled
32IR1 and 32IR2: external ground fault
32IF1 and 32IF2: internal ground fault
50GN1 and 50GN2: neutral current pickup
REFP1 and REFP2: REF inverse-time overcurrent
Step 10
Save your settings, and send them to the relay.
Hands-On Activity 2: Validate REF Element
In this hands-on activity, you will validate the REF element enable logic, fault direction logic, and
inverse-time overcurrent logic. You will use Relay Word bits to monitor the following conditions:
•
External ground fault
•
Internal ground fault
•
Inverse-time overcurrent
Set Up Monitoring
To monitor element Relay Word bits, you will set up user-defined target light-emitting diodes (LEDs) in
the ACSELERATOR QuickSet human-machine interface (HMI).
Step 1
Select Tools > HMI > HMI.
Step 2
In the Device Overview panel, from the HMI categories to the left, select Targets, and scroll down to
Target Rows 50 and 51. Monitor the REF Relay Word bits found in each target row.
49
LB9
LB10
LB11
LB12
LB13
LB14
50
50GC1
50GN1
32IE1
32IR1
32IF1
REFP1
51
50GC2
50GN2
32IE2
32IR2
32IF2
REFP2
52
*
*
*
*
*
*
Figure 3
Restricted Earth Fault (REF) Element Settings
REF Targets in SEL-387A HMI
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REF Enabled Logic
In this test, you will verify that the REF Relay Word bit asserts for the following current values:
|IN1| ≥ 50G • INOM1
Step 1
Reset the target LEDs.
The instructor will inject the current into the IN current input.
Step 2
Verify the following HMI targets:
•
The 32IE1 Relay Word bit asserts.
•
The 50GN1 Relay Word bit asserts.
Fault Direction Logic
In this test, you will verify the REF directional pickup as follows:
•
For an external fault, IN is at 0 degrees.
•
For an internal fault, IN is at 180 degrees.
Step 1
Reset all relay targets.
The instructor will inject an external fault, followed by an internal fault.
Step 2
Verify the following HMI target LEDs:
•
The 32IR1 target LED asserts for an external fault.
•
The 32IF1 target LED asserts for an internal fault.
Inverse-Time Test
In this test, you will verify the time characteristic of the inverse-time overcurrent element. From Figure 1,
you can see that the curve shape is U4 with a TD of 0.5.
Step 1
Calculate the expected operate time (tp) for IWn = 2 • 50GPn • (CTRNn/CTRn) at 0 degrees using the
following equation or the TOC_Calculator.xls spreadsheet:
5.67 ⎞
⎛
tp = TD • ⎜ 0.0352 + 2
⎟
M −1 ⎠
⎝
Restricted Earth Fault (REF) Element Settings
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Step 2
Clear the SER using the SER C serial port command or the ACSELERATOR QuickSet HMI menu.
The instructor will inject the current magnitude for the test.
Step 3
Issue the SER serial port command, or select SER from the ACSELERATOR QuickSet HMI menu.
Step 4
Verify the following:
•
50GNn is asserted.
•
32IEn is asserted.
•
32IFn is asserted.
•
REFPn is asserted.
•
The operate time is as expected (the difference between 50GNn and REFPn).
I-44 HANDS ON
Date: 05/31/12
161/14.4 KV SUBSTATION
FID=SEL-387A-R606-V0-Z004003-D20040628
#
DATE
TIME
ELEMENT
STATE
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
05/31/12
09:06:41.704
09:06:41.727
09:06:44.047
09:06:47.577
09:06:47.577
09:06:47.596
09:08:17.450
09:08:17.473
09:08:17.829
09:08:22.328
09:08:22.328
09:08:22.345
09:15:28.946
09:15:28.969
09:15:29.323
50GN1
INTERNAL
REFP1
INTERNAL
50GN2
REFP1
50GN2
INTERNAL
REFP2
INTERNAL
50GN2
REFP2
50GN2
INTERNAL
REFP2
Restricted Earth Fault (REF) Element Settings
FAULT
FAULT
FAULT
FAULT
FAULT
Time: 09:19:57.654
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Asserted
Asserted
Asserted
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