Restricted earth-fault protection function block description
Restricted earth-fault protection function block description
Document ID: PRELIMINARY VERSION
Budapest, October 2009
Restricted earth-fault protection
User’s manual version information
Version Date Modification Compiled by
Preliminary 30.10.2009. Preliminary version, without technical information Petri
PRELIMINARY VERSION 2/16
Restricted earth-fault protection
CONTENTS
The zero-sequence current calculation (Zero-sequence current calculation ) ............7
The zero-sequence differential characteristics (Differential characteristics) ...............8
PRELIMINARY VERSION 3/16
Restricted earth-fault protection
1 Restricted earth-fault protection function
The restricted earth-fault protection function is basically a low-impedance differential protection function based on zero sequence current components. It can be applied to transformers with grounded neutral. The function compares the measured neutral current and the calculated zero sequence current component of the phase currents and generates a trip command if the difference of these currents is above the characteristics.
1.1 Structure of the restricted earth-fault protection algorithm
Fig.1-1 shows the structure of the restricted earth-fault protection (DIF87N) algorithm.
IprimL1
IprimL2
Preparation
Fourier
IL1
Fourier
IL2
Zero sequence current calculation
-
Directional decision
DIF87N
IprimL3
Fourier
IL3
Binary outputs
IN
Fourier
IN
Differential characteristics magnitude
Decision logic
Measured values
Parameters
Status signals
Figure 1 1 Structure of the restricted earth-fault protection algorithm
The inputs for the preparation are: the sampled values of three primary phase currents, the sampled value of the neutral current.
The outputs of the preparation are: the RMS values of the fundamental Fourier components of the phase currents and that of the neutral current.
The inputs for the DIF87N function are: the RMS values of the fundamental Fourier components of the phase currents and that of the neutral current, parameters, status signal.
The outputs of the DIF87N function are: the binary output status signal, the measured values for displaying.
PRELIMINARY VERSION 4/16
Restricted earth-fault protection
The software modules of the differential protection function:
Fourier calculations
These modules calculate the basic Fourier current components of the phase currents and that of the neutral current individually. These modules belong to the preparatory phase.
Zero sequence current calculation
This module calculates the zero sequence current components based on the Fourier components of the phase currents. These modules belong to the preparatory phase.
Directional decision
This module compares the direction of the neutral current and that of the calculated zero sequence current. In case of small zero sequence components of the high fault currents in the phases, this decision improves the stability of the function.
Differential characteristics
This module performs the necessary calculations for the evaluation of the “percentage d ifferential characteristics” and decides if the differential current is above the characteristic curve of the differential protection function. This curve is the function of the restraint current, which is the maximum of the phase currents and the current of the neutral point. The result of this calculation is needed for the decision logic.
Decision logic
The decision logic module combines the status signals, binary and enumerated parameters to generate the trip command of the function.
The following description explains the details of the individual components.
PRELIMINARY VERSION 5/16
Restricted earth-fault protection
1.2 The Fourier calculation (Fourier)
These modules calculate the basic Fourier current components of the phase currents and that of the neutral current individually. The magnitudes of these currents are matched to each other using the current transformer rated current values. These modules belong to the preparatory phase.
IprimL1
IprimL2
IprimL3
IL1Four
IL2Four
IL3Four
INFour
IN
Fourier
Figure 1 2 Principal scheme of the Fourier calculation
The inputs are the sampled values of: the three phase currents of the primary side (IprimL1, IprimL2, IprimL3) the current measured at the neutral point (IN)
The outputs are the basic Fourer components of the analyzed currents (IL1Four, IL2Four,
IL3Four, INFour).
The integer parameters
of the Fourier calculation are listed in Table 1-1.
Parameter name Title Unit Min Max Step Default
Parameters for the current magnitude compensation:
DIF87N_TRPri_IPar_ TR Primary comp. %
DIF87N_TRNeut_IPar_ TR neutral %
20
100
500
1000
1
1
100
500
Table 1 1 The integer parameters of the Fourier calculation
PRELIMINARY VERSION 6/16
Restricted earth-fault protection
1.3 The zero sequence current calculation
(Zero sequence current calculation )
This module calculates the zero sequence current component based on the Fourier components of the phase currents. This module belongs to the preparatory phase.
The inputs are the basic Fourier components of the phase currents (IL1Four, IL2Four,
IL3Four).
The outputs are the Fourier components of the calculated zero sequence current (3IoFour).
IL1Four
IL2Four
IL3Four
3IoFour=IL1Four+
IL2Four+
IL3Four
3IoFour
Figure 1 3 Principal scheme of the zero sequence current calculation
1.4 Directional decision (Directional decision)
This module compares the direction of the neutral current and that of the calculated zero sequence current. In case of small zero sequence component of the high fault currents in the phases, this decision improves the stability of the function.
For the directional decision, the positive directions are drawn in Figure 1 4 . In this system, if
the angle between the calculated zero sequence current 3Io and the measured neutral cu rrent IN is out of the range of ±90 degrees, then the restricted earth fault protection can be blocked, the status signal DIF87N_StDir_GrI_ (Dir.element Start) is set to TRUE value. The blocking is decided in the decision logic of the function, using the binary parameter
DIF87N_DirCheck_BPar_.
PRELIMINARY VERSION 7/16
Restricted earth-fault protection
IL1
IL2
IL3
IN
Figure 1 4 Positive directions for the directional decision
The inputs are the Fourier components of the calculated zero sequence current (3IoFour) and that of the neutral current (INFour).
The binary output status signal
of the directional decision is shown in Table 1 2 .
Binary output signals Signal title Explanation
DIF87N_StDir_GrI_ Dir.element Start
Directional blocking signal of the zero - sequence directional element
Table 1 2 The binary output status signals of the directional decision
3IoFour
INFour
BLOCK
INFour
DIF87N_StDir_GrI_
3IoFour
Figure 1 5 Principal scheme of the directional decision
1.5 The zero sequence differential characteristics
(Differential characteristics)
This module performs the necessary calculations for the evaluation of the “percentage differential characteristics”, and decides if the differential current is above the characteristic curve of the zero sequence differential protection function. This curve is the function of the restraint current, which is the maximum of the phase currents and the current of the neutral point. The result of this calculation is processed in the decision logic.
PRELIMINARY VERSION 8/16
Restricted earth-fault protection
The differential current is calculated using the following formula:
Diff Current IL1Four IL2Four IL3Four INFour
The restraint current is calculated using the following formula:
Bias Current MAX(IL1Fou r , IL2Four , IL3Four , INFour)
Based on these values, the zero sequence differential protection characteristics are shown in
Diff Current
Slope=
DIF87N_f2_IPar__
DIF87N_f1_IPar_
Bias Current
DIF87N_f2Brk_IPar_
Figure 1-6 The zero sequence differential protection characteristics
IL1Four
IL2Four
IL3Four
INFour
Diff
Current
Slope=
DIF87N_f2_IPar__
DIF87N_f1_IPar_
DIF87N_f2Brk_IPar_
Bias Current
DIF87N_GenTr_GrI_i _
Measured values
Figure 1 7 Principal scheme of the zero sequence differential protection
PRELIMINARY VERSION 9/16
Restricted earth-fault protection
The measured values
of the restricted earth-fault protection function are listed in Table 1 3 .
Measured value Title Dim.
Explanation
DIF87N_Id_OLM_
DIF87N_Bias_OLM_
Diff
Current
Bias
Current
In %
In %
The calculated differential current
The calculated restraint current
Table 1 3 The measured analogue values of the zero sequence differential protection characteristics
The binary output status signal of the differential protection characteristics is shown in
Binary output signals Signal title Explanation
DIF87N_GenTr_GrI_i General Trip_i
Internal general trip signal of the differential characteristics
Table 1 4 The binary output status signals of the zero sequence differential protection characteristics
The parameters of the differential protection characteristics are explained in the following tables.
The integer parameters
of the differential protection characteristics are listed in Table 1 5 .
Parameter name Title Unit Min Max Step Default
Base sensitivity:
10 50 1 30 DIF87N_f1_IPar_ Base sensitivity
Slope of the second section of the characteristics:
%
DIF87N_f2_IPar_ Second part
Break point of the characteristic line:
- 50 100 1 70
DIF87N_f2Brk_IPar_ Break point - 100 200 1 125
Table 1 5 The integer parameters of the zero sequence differential protection characteristics
The binary input signal serves to increase the base sensitivity by 4 times. The conditions of increasing are defined by the user, applying the graphic equation editor.
Binary input signal Explanation
DIF87N_UnSens_GrO_
Output status of a graphic equation defined by the user to increase the base sensitivity by 4 times
Table 1 6 The binary input signal of the zero sequence differential protection characteristics
PRELIMINARY VERSION 10/16
Restricted earth-fault protection
1.6 The decision logic (Decision logic)
The decision logic module combines the status signals, binary and enumerated parameters to generate the trip command of the function.
DIF87N_StDir_GrI_
DIF87N_DirCheck_BPar_
DIF87N_Blk_GrO_
DIF87N_GenTr_GrI_i
DIF87N_Oper_EPar
AND
OR NOT
AND
DIF87N_GenTr_GrI
_
Figure 1 8 The logic scheme of the restricted earth-fault protection function
Binary input signals Signal title Explanation
DIF87N_GenTr_GrI_i
DIF87N_StDir_GrI_
General Trip_i
Dir.element Start
Internal general trip signal of the differential characteristics
Directional blocking signal restricted earth-fault protection function
Table 1 7 The binary input status signals of the zero sequence differential protection characteristics
The restricted earth-fault protection function has a binary input signal, which serves the purpose of disabling the function . The conditions of disabling are defined by the user applying the graphic equation editor.
Binary input signal Explanation
DIF87N_Blk_GrO
Output status of a graphic equation defined by the user to disable the differential protection function.
Table 1 8 The binary input signal of the restricted earth-fault protection function
The binary output signals of the restricted earth-fault protection function are listed in Table
Binary output signals Signal title Explanation
DIF87N_GenTr_GrI_
DIF87N_StDir_GrI_
General Trip
Dir.element Start
General trip signal of the zero sequence differential protection characteristics
Directional blocking signal of the restricted earth-fault protection function
Table 1 9 The binary output status signals of the zero sequence differential protection characteristics
PRELIMINARY VERSION 11/16
Restricted earth-fault protection
The enumerated parameter
of the restricted earth-fault protection function is shown in Table
Default Parameter name Title Selection range
Parameter to enable the zero sequence differential protection function:
DIF87N_Oper_EPar_ Operation Off,On On
Table 1 10 The enumerated parameters of the restricted earth-fault protection function
The Boolean parameter
of the restricted earth-fault protection function is shown in Table
Parameter name Title
DIF87N_DirCheck_BPar_ Directional check
Default Explanation
True
Enabling the directional checking of the measured and calculated zero sequence currents
Table 1 11 The Boolean parameters of the restricted earth-fault protection function
The restricted earth-fault protection function generates a trip signal if the differential current as the function of the bias current is above the differential characteristic lines and the function is not blocked and the operation of the function is enabled by parameter setting.
Blocking can be caused by the directional decision if it is enabled by parameter setting and the angle of the currents is in the blocking area or the user has composed a blocking graphic equation, and the conditions result a TRUE value for the blocking.
PRELIMINARY VERSION 12/16
Restricted earth-fault protection
1.7 Technical summary
1.7.1 Technical data
Function
Operating characteristic
Reset ratio
Unrestrained differential setting
Base sensitivity
Operate time
Reset time
*To be defined by types tests
Effective range* Accuracy*
1.7.2 The measured values
The measured values
of the restricted earth-fault protection function are listed in Table 1 12 .
Measured value Title Dim.
Explanation
DIF87N_Id_OLM_
DIF87N_Bias_OLM_
Diff
Current
Bias
Current
In %
In %
The calculated differential current
The calculated restraint current
Table 1 12 The measured analogue values of the restricted earth-fault protection function
The parameters of the restricted earth-fault protection function are explained in the following tables.
PRELIMINARY VERSION 13/16
Restricted earth-fault protection
1.7.3 The parameters
The enumerated parameter
of the restricted earth-fault protection function is shown in Table
Parameter name Title Selection range
Parameter to enable the zero sequence differential protection function:
DIF87N_Oper_EPar_ Operation Off,On
Default
On
Table 1 13 The enumerated parameters of the restricted earth-fault protection function
The Boolean parameter
of the restricted earth-fault protection function is shown in Table
Parameter name Title
DIF87N_DirCheck_BPar_ Directional check
Default Explanation
True
Enabling the directional checking of the measured and calculated zero sequence currents
Table 1 14 The Boolean parameters of the restricted earth-fault protection function
The integer parameters
of the restricted earth-fault protection function are listed in Table
Min Max Step Default Parameter name Title Unit
Parameters for the current magnitude compensation:
DIF87N_TRPri_IPar_ TR Primary comp.
DIF87N_TRNeut_IPar_ TR neutral
Base sensitivity:
%
%
% DIF87N_f1_IPar_ Base sensitivity
Slope of the second section of the characteristics:
DIF87N_f2_IPar_ Second part
Break point of the characteristic line:
DIF87N_f2Brk_IPar_ Break point
-
-
20
100
10
50
100
500
1000
50
100
200
1
1
1
1
1
100
500
30
70
125
Table 1 15 The integer parameters of the restricted earth-fault protection function
Floating-point parameters
The restricted earth-fault protection function has no floating-point parameters.
Timer parameters
The restricted earth-fault protection function has no timers.
PRELIMINARY VERSION 14/16
Restricted earth-fault protection
1.7.4 The binary input status signals
The restricted earth-fault protection function has a binary input signal, which serves the purpose of disabling the function . The conditions of disabling are defined by the user, applying the graphic equation editor.
The other binary input signal serves to increase the base sensitivity by 4 times. The conditions of increasing the base sensitivity are defined by the user, applying the graphic equation editor.
Binary input signal
DIF87N_Blk_GrO
DIF87N_UnSens_GrO_
Explanation
Output status of a graphic equation defined by the user to disable the differential protection function.
Output status of a graphic equation defined by the user to increase the base sensitivity by 4 times
Table 1 16 The binary input signal of the restricted earth-fault protection function
1.7.5 Binary output status signals
The binary output status signals of the restricted earth-fault protection function are listed in
Signal title Explanation Binary output signals
DIF87N_GenTr_GrI_ General Trip
General trip signal of the restricted earth-fault protection function
Table 1 17 The binary output status signals of the restricted earth-fault protection function
1.7.6 The function block
The function block of the restricted earth fault protection function is shown in Figure 1 9 . This
block shows all binary input and output status signals that are applicable in the graphic equation editor.
Figure 1 9 The function block of the restricted earth-fault protection function
PRELIMINARY VERSION 15/16
Restricted earth-fault protection
1.8 Example for the calculation of settings of the zero sequence differential protection function
As an example, the transformer data:
Sn = 25 MVA
U1/U2 = 120/22 kV/kV
Y(d)y6
Current transformers:
CT1
CT2
600/1 A/A
800/1 A/A
CTN 250/1 A/A
The selected base current is the primary rated current of the transformer: 120 A. If a lower current value is selected, the zero sequence differential protection becomes more sensitive.
REF on the secondary side:
I1np = 120 A
I2np = 656 A
INp = 656 A
The setting parameters:
On the secondary side of the CT I2n = 0.82 A
On the secondary side of the CTIN = 2.6 A
Parameter name Title Setting value
DIF87N_TRPri_IPar_
DIF87N_TRNeut_IPar_
TR Primary comp.
TR neutral
82%
260%
The base sensitivity refers to the selecte d “base current” expressed in %, the slope is the ratio of the differential current and the bias current expressed in %.
MICROENER - 49 rue de l'Université 93160 Noisy le Grand France - +33 (0)1 48 15 09 09 - info@microener.com
PRELIMINARY VERSION 16/16
