SEL Transformer Differential Element_EP_20120723.pdf

Hands-On Relay Testing Session  Differential (87) Element Element Settings Exercise Objectives After completing this exercise, you should be able to do the following: •

Identify differential settings and protection requirements.

•

Define settings recommendations for transformer differential protection.

•

Calculate effective settings for system examples.

•

Validate relay settings.

In this exercise, the instructor will introduce the SEL-387A Current Differential and Overcurrent Relay differential protection settings and demonstrate how to calculate these settings. You will then perform your own calculations and enter protection settings into ACSELERATOR QuickSet SEL-5030 Software. After sending your settings to the relay, you will validate the settings and verify the  proper assertion assertion of the elements.

®

Student Resources and References The following files are available with this exercise: •

SEL Presentation 1081, “SEL-387A Relay: Two-Winding Current Differential and Overcurrent Relay” ( 1081_SEL387A_EP_20120710.pdf )

•

Transformer nameplate (I-44_Nameplate.pdf )

•

Three-line ac diagram ( I-44_ThreeLine.pdf )

•

•

SEL Technical Paper 6261, “Considerations for Using Harmonic Blocking and Harmonic Restraint Techniques on Transformer Differential Relays” (6261_ConsiderUsingHarmonic_KB_20061019.pdf ) SEL Technical Paper 6451, “Percentage Restrained Differential, Percentage of What?” (6451_PercentageRestrained_MT_20110614.pdf )

You can also refer to the SEL-387A Instruction Manual (available at http://www.selinc.com) for further  information about protection and logic functions.

SEL-387A Phase Differential Protection The SEL-387A phase differential (87) element can protect your system with optional dual-slope  percentage  percentage current differential differential detection. detection. Percentage differential protection is more sensitive and secure than traditional differential protection; the dual-slope characteristic compensates for current transformer (CT) ratio mismatches, CT ratio errors, CT saturation, saturation, and errors due to tap changing.

Differential (87) Element Settings

Page 1 of 9

20120723 20120723

Protection Schemes The SEL-387A allows you to choose transformer differential protection with harmonic blocking, harmonic restraint, or both, providing stability during transformer inrush conditions. Even-numbered harmonics (second and fourth) provide security during energization, while fifth-harmonic  blocking provides security for overexcitation conditions.

Differential Element Characteristics Figure 1 shows the relay characteristic. You can set the characteristic as either a single-slope, percentage differential characteristic or as a dual-slope, variable-percentage differential characteristic.

IOP

Operating Region

Slope 2 (SLP2) Slope 1 (SLP1) 60%

25%

087P = 0.3

Restraining Region

IRS1 = 3

IRT

Figure 1 Percentage Restraint Differential Characteristic

The SEL-387A has three percentage differential elements: 87R1, 87R2, and 87R3. These elements employ operate (IOP) and restraint (IRT) quantities that the relay calculates from the winding input currents. Tripping occurs if the operate quantity is greater than the curve value for that particular restraint quantity. A minimum pickup level for the operate quantity must also be satisfied. For added security during transformer energization, Slope 2 is used in place of Slope 1 for 10 cycles. The four settings that define the characteristic are as follows: •

•

O87P is the minimum IOP level required for operation. SLP1 is the initial slope, beginning at the origin and intersecting O87P at IRT = O87P • (100/SLP1).

•

IRS1 is the limit of IRT for SLP1 operation, intersecting where SLP2 begins.

•

SLP2 is the second slope, which must be greater than or equal to SLP1.

With careful selection of these settings, you can closely duplicate the characteristics of existing differential relays that have been in use for many years. The SEL-387A also has three unrestrained differential elements: 87U1, 87U2, and 87U3. The unrestrained elements detect very high differential current that clearly indicates an internal fault. These elements compare the IOP quantity to a setting value (U87P)—typically about 10 times TAP—and trip if this level is exceeded.

Differential (87) Element Settings

Page 2 of 9

20120723

The unrestrained elements respond only to the fundamental frequency component of the differential current and are not affected by the percentage restraint, harmonic blocking, or harmonic restraint functions.

Transformer Differential For transformer current differential applications similar to the one illustrated in Figure 2, the SEL-387A offers differential protection that can correctly compensate for the ratio and phase shift of the power  transformer. X1

H1 X3 X0 H2

H3 200:5

 A B C

H1

X1

H2

X2

H3

X3 X0

X2 2000:5

50 MVA 151 kV 14.4 kV

2    0    6  

2    0    5  

2    0   4  

2    0    3  

2   2    0    0   2   1  

2   1   2  

2   1   1  

2   1    0  

2    0    9  

2    0    8  

2    0   7  

SEL-387A ICW1 IBW1 IAW1

ICW2 IBW2 IAW2

Figure 2 Transformer Percentage Differential Protection Scheme Connections

Figure 2 defines the required connections for a basic transformer differential protection scheme. The CTs located at the high side of the power transformer are connected to relay inputs IAW1, IBW1, and ICW1. The CTs located at the low-side terminals of the power transformer are connected to relay inputs IAW2, IBW2, and ICW2.

Hands-On Activity 1: Enter Transformer Differential Element Settings In this hands-on activity, you will calculate and enter transformer differential protection settings using the real-world data and recommendations given below each setting. Write your answers in the space  provided.

Transformer Data Rated 50 MVA Rated high side, 161 kV (DAB) Rated low side, 14.4 kV (wye) High-side bushing CT ratio (wye connected) Low-side bushing CT ratio (wye connected)  Neutral bushing CT ratio Differential (87) Element Settings

Page 3 of 9

20120723

Step 1 In the Transformer Differential Elements settings, enable differential protection. E87 = _________________  The relay automatically calculates the Tap 1 and Tap 2 settings based on the transformer MVA data obtained from the transformer nameplate. If the MVA setting is set to OFF, you must calculate the Tap 1 and Tap 2 settings. The following equations are used to calculate the Winding 1 and Winding 2 tap settings and the CT  primary currents: Ipri

=

MVA •1000 kV • 3

TAP

=

Ipri CTR 

For delta-connected CTs, TAP

=

Ipri • 3 CTR 

Step 2 Determine and enter the Restrained Element Operating Current PU in multiples of tap. O87P = __________________  Set high enough to avoid operation because of steady-state CT errors and transformer magnetizing current. The typical O87P range is 0.3 to 0.5. For this example, use the relay default setting O87P = 0.3. Set to approximately 50 percent of the O87P setting. Set longer than the maximum clearing time for external faults.

Step 3 Determine and enter the Restraint Slope 1 Percentage. SLP1 = __________________  To determine the maximum error for setting the percentage of the differential element, consider the following: •

•

Relay tap mismatch (this can be 0 with the proper CT ratio selection). Power transformer ratio (nominal tap to minimum tap). No-load taps can be up to 5 percent, and autotransformers are typically 10 percent.

•

Protective relay error (up to 5 percent).

•

CT ratio error (3 to 5 percent).

•

Magnetizing inrush (up to 5 percent).

Differential (87) Element Settings

Page 4 of 9

20120723

Step 4 Determine and enter the Restraint Slope 2 Percentage. SLP2 = __________________  Set Slope 2 in the range of 50 to 70 percent to avoid problems with CT saturation for high fault currents.

Step 5 Determine and enter the Restraint Current Slope 1 Limit. IRS1 = __________________  The suggested IRS1 setting, expressed as a percentage of winding current, is 3.0.

Step 6 Determine and enter the Unrestrained Element Current PU in multiples of tap. U87P = __________________  The instantaneous unrestrained current element is intended to react quickly to very heavy current levels that clearly indicate an internal fault. The typical setting range is from 8.0 to 10.0.

Step 7 Determine and enter the Second-Harmonic Blocking Percentage. PCT2 = __________________  Transformer simulations show that magnetizing inrush current usually yields more than 30 percent of  IF2/IF1 (compensated second-harmonic differential current/compensated fundamental differential current) in the first cycle of the inrush. The typical setting range is 12 to 15 percent. A setting of  15 percent usually provides a margin for security.

Step 8 Determine and enter the Fourth-Harmonic Blocking Percentage. PCT4 = __________________  Relays typically use even harmonics to detect inrush conditions and to prevent misoperations resulting from inrush. The largest even-harmonic current component is usually the second harmonic, followed by the fourth harmonic. Use fourth-harmonic blocking to provide additional security against inrush conditions; set PCT4 to less than PCT2.

Step 9 Determine and enter the Fifth-Harmonic Blocking Percentage. PCT5 = _________________  Fourier analysis of transformer currents during overexcitation indicates that a 35 percent fifth-harmonic setting is adequate to block the percentage differential element. To disable fifth-harmonic blocking, set PCT5 to OFF. Differential (87) Element Settings

Page 5 of 9

20120723

Step 10 Determine and enter the Fifth-Harmonic Alarm Threshold. TH5P = __________________  Use the presence of the fifth-harmonic differential current to assert an alarm output during startup. This alarm indicates that the rated transformer excitation current has been exceeded. At full load, a TH5P setting of 0.1 corresponds to 10 percent of the fundamental current.

Step 11 Determine and enter the Fifth-Harmonic Alarm Delay. TH5D = __________________  The TH5D delay setting prevents the relay from indicating the transient presence of fifth-harmonic currents. When TH5P = OFF, TH5D is hidden. Consider triggering an event report if the transformer excitation current exceeds the fifth-harmonic threshold.

Step 12 Determine the need for the dc ratio blocking setting. DCRB = ___________________  Some magnetizing inrush cases contain very little harmonic content but a dc offset. The SEL-387A can detect the dc offset and use it in the blocking (not restraint) logic. The dc ratio blocking feature applies to inrush cases with little harmonic content, but a high dc offset. The measurement principle is that of  waveshape recognition, distinguishing between the time constants for inrush current that typically are longer than the time constants for an internal fault. Elements DCBL1, DCBL2, and DCBL3 are combined to form Relay Word bit DCBL, which sets the threshold of the dc ratio blocking function. The factory default is DCRB = N.

Step 13 Determine whether your application requires harmonic restraint and/or harmonic blocking features. Enable Harmonic Restraint and/or Harmonic Blocking accordingly. HRSTR = __________________  Restrained elements (87R1, 87R2, and 87R3) determine whether the IOP quantity is greater than the restraint quantity. This characteristic is modified by increasing the restraint current as a function of the second- and fourth-harmonic content in the input currents for the harmonic restraint elements (87HR1, 87HR2, and 87HR3). Set HRSTR to Y to activate the harmonic restraint element 87HR. A setting of  Y defeats common harmonic blocking. IHBL = __________________  While the restrained differential elements are making decisions, a parallel blocking decision process occurs with specific harmonic magnitudes in the IOP quantities. When IHBL is set to N, any differential element will prevent tripping of  any restrained differential element.

Differential (87) Element Settings

Page 6 of 9

20120723

Step 14 Verify the trip logic. Relay Word bits 87R and 87U are assigned to the trip TR3 equation. The factory default setting OUT103 is assigned to TRIP3.

Step 15 When you are finished, your settings will be verified in a class discussion.

Hands-On Activity 2: Validate Transformer Differential Element In this hands-on activity, you will validate the Winding 1 compensation setting, W1CTC, by injecting currents into the relay that simulate full load. The example transformer low-voltage winding is connected in wye, and the high-voltage winding is connected in delta. This results in a phase shift in current and voltages for each of the windings.

Validation Test Functional Description In this transformer-connected example, the high-voltage-side winding currents and voltages lead the lowvoltage-side winding currents and voltages by 30 degrees. Figure 3 is the step-up transformer winding connection diagram.

Figure 3 Example Transformer Winding Connection Diagram

Figure 4 illustrates the location of the CT for a transformer phase differential protection scheme. The IAW1, IBW1, and ICW1 relay inputs measure the load current on the transformer high side. The IAW2, IBW2, and ICW2 relay inputs measure the load current on the transformer low side.

Figure 4 Transformer Phase Differential CT Location

Differential (87) Element Settings

Page 7 of 9

20120723

For 1.0 per unit (pu) of load flowing through, the primary currents measured by the relay inputs are as follows: IAW1 = 179.3 at 0.00

IAW2 = 2000 at 150.00

IBW1 = 179.3 at –120

IBW2 = 2000 at 30.00

ICW1 = 179.3 at 120

ICW2 = 2000 at –90.00

Validate Transformer Phase Differential Settings The instructor will use the SEL-AMS (Adaptive Multichannel Source) to inject secondary quantities into the relay terminals to simulate 1.0 pu of load flowing through the transformer. A single-state test will be  performed. The instructor will inject current and voltage as defined in Table 1. Table 1 Simulated Load for Single-State Test; 1.0 pu Load; Simulation State 1 is 1 minute IAW1

4.48 at 0.00

IBW1

4.48 at –120.00

ICW1

4.48 at 120.0

IAW2

5.01 at 150.00

IBW2

5.01 at 30.00

ICW2

5.01 at –90.00

Step 1 Reset the relay targets by pushing the {TARGET RESET} pushbutton on the relay front panel.

Step 2  Clear the Sequential Events Recorder (SER) entries by communicating directly with the relay as follows: •

Select Communications > Terminal. Establish relay communications, and then issue the SER C command. OR 

•

Select Tools > HMI > HMI to access the human-machine interface (HMI) menu in ACSELERATOR QuickSet.

Step 3 After the instructor uses the SEL-AMS to inject current into the relay, verify the following: •

The relay is correctly measuring the primary values of the currents and voltages.

•

The differential element is stable (IOP values are very small, and IRT values are close to 2.0).

•

There are no targets or SER entries related to the phase differential element.

Differential (87) Element Settings

Page 8 of 9

20120723

Hands-On Activity 3: Validate Percentage of Slope The instructor will use the SEL-AMS to inject secondary quantities into the relay terminals to simulate a fault internal to the transformer or within the zone of the transformer differential protection. Use the SEL-5401 Test System Software front-panel function. The instructor will inject current and voltage as defined in Table 2. Table 2 Simulated Load for a Single-State Test; IW1 = 1.0 pu Load; Slowly Increase IW2 Until 87R Asserts IAW1

4.48 at 0.00

IBW1

4.48 at –120.00

ICW1

4.48 at 120.00

IAW2

5.01 at 180.00

IBW2

5.01 at 60.00

ICW2

5.01 at –60.00

Step 1 Reset the relay targets by pushing the {TARGET RESET} pushbutton on the relay front panel.

Step 2 Clear the SER entries. You can perform this task by using either the SER C serial port command or the ACSELERATOR  QuickSet HMI.

Step 3 After the instructor uses the SEL-AMS to inject current into the relay, do the following: •

•

Issue the MET DIF serial command, or select Meter Differential from the ACSELERATOR  QuickSet HMI menu options. Verify that (IOP/IRT) • 100 = the SLP1 setting ± 5 percent.

SEL-387A

Operate

(pu)

IOP1 0.37

IOP2 0.37

IOP3 0.37

Restraint

(pu)

IRT1 2.37

IRT2 2.38

IRT3 2.37

2nd Harmonic (%)

IOP1F2 0.00

IOP2F2 0.26

IOP3F2 0.00

4th Harmonic (%)

IOP1F4 0.26

IOP2F4 0.26

IOP3F4 0.00

5th Harmonic (%)

IOP1F5 0.26

IOP2F5 0.00

IOP3F5 0.00

Differential (87) Element Settings

Page 9 of 9

20120723