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Description

This relay comprises of overcurrent protection elements such as 50P (instantaneous or definite-time phase overcurrent), 50N (instantaneous or definite time neutral overcurrent), 51P (inverse-time phase overcurrent) and 51N (inverse-time neutral overcurrent) with directional protection elements such as 67P (phase directional) and 67N (neutral directional). 

The directional overcurrent relay (50-51-67) trips if and only if the fault current magnitude is above the threshold (50-51) and it is in a specific direction (67). The direction can be forward or reverse depending on the type of fault and type of directional element which is activated.

This document focuses on the directional elements of the relay (67P and 67N). For information regarding the overcurrent model, refer to 50-51 relay documentation.


Table of Contents

The 67 model determines the direction of the fault current based on the phase difference between the phase currents and phase-to-phase voltages (67P) or the phase difference between zero-sequence current (derived from neutral current) and either zero-sequence voltage or ground current (67N). The direction enables the overcurrent relay to trip or acts as a blocking signal. The 50-51-67 or Directional Overcurrent Relay trips whenever the monitored current exceeds the predefined setting and is in the desired direction. This is true for both phase overcurrent and neutral overcurrent.

The figure below shows the concept of how the relay operates.

2019-10-10_9-36-05.jpg

The direction discrimination principle is based on phase angle comparison between a set of phasors, one of which is used as a reference. The reference value is called the polarizing quantity and the value which is being compared is the operating quantity. In the case of 67, the operating quantity is always the fault current. By shifting the quantities by a specified value called the Element Characteristic Angle (ECA) or Relay Characteristic Angle (RCA), the model can determine the operating regions of the relay in terms of fault current direction. The directional element can act as a tripping mechanism or blocking for the overcurrent model.

Directional Elements

The model contains two directional elements: Phase Directional (67P) and Neutral Directional (67N).

67P Phase Directional Overcurrent Element

The operating quantity in 67P element is the phase current and the polarizing quantity is the line-to-line voltage. During a fault, the phase voltage is low so to be able to determine the direction, the line-to-line voltage is considered as a reference . The voltage is shifted by an amount of the ECA (Electric Characteristic Angle) specified by the user to find the operating regions of the relay. The phase difference between the operating quantity and polarizing quantity is compared with 90 degrees and it is determined whether the fault current is in forward (trip) or reverse (block).

                                                          If angle difference < 90  => forward direction

                                                          If angle difference > 90  => reverse direction

If phase-to-phase voltage drops to lower than 0.1 pu, the voltage memorized from 3 cycles ago is used for direction discrimination. The user has a choice of sending a blocking signal when voltage memory expires by enabling or disabling a setting (Block after voltage memory expiration) in the relay. When the setting is enabled, the directional element will block the phase overcurrent operation under directional control when voltage memory expires. If it is disabled, the phase overcurrent functionality can trip under directional control when voltage memory expires.

If phase current is higher than the threshold of 0.05 pu and based on the directional discrimination, the output of the 67 element can be forward (1) or reverse (0).

Forward direction is used for tripping the phase overcurrent and reverse is used for blocking. So, the overcurrent relay trips if a phase fault current is above a certain threshold and the direction is in forward [1].

67N Neutral Directional Overcurrent Element

The 67N element has one operating quantity which is the zero-sequence current:

                                                          I0 = (1/3) * IN = (1/3) * (IA+ IB+ IC)

The positive sequence current acts as a restraint for better relay performance and is subtracted from the zero-sequence current with a factor of K. So, the operating current would be:

                                                          Iop = 3 * (I0 - I1 * K) , K = 1/16

K is recommended to be set to 1/16 [2] for neutral directional overcurrent settings but can be changed.

Based on the polarizing quantity, there are different polarization modes for 67N. If Voltage Polarization is chosen by the user, the polarizing quantity is the zero-sequence voltage:

                                                         Polarizing Quantity = Vpol = V0 = (1/3) * VN = (1/3) * (VA + VB + VC)

If Current Polarization is chosen, the polarizing quantity is the ground current which can be taken from the ground CT. (IG)

                                                          Polarizing Quantity = Ipol = IG

If Dual Polarization is chosen, both polarizing quantities are used for direction discrimination.

Direction discrimination is the same principle as it is for 67P. Based on the phase angle difference between the polarizing and operating quantities, direction of the fault current is determined.

Characteristic Angle (ECA) is used in voltage polarization. The user can specify the pickup current and limit angles which are used for directional discrimination in both forward and reverse directions. It should be noted that the pickup value is separate than the one used for 50N or 51N and is typically a very low value. The phase angle difference between the operating quantity and polarizing quantity is compared with Forward Limit Angle and Reverse Limit Angle which determines the operating regions of the 67N element.  

                                                          if angle difference < forward limit angle => forward direction (1)

                                                          if angle difference < reverse limit angle => reverse direction (1)

Because of this method, there are two outputs for 67N. One shows the direction in forward which can be 0 or 1 and the other shows the direction in reverse which also can be 0 or 1. There are non-operation zones which the output for both forward and reverse direction is zero. So, if the output for reverse direction is zero, it does not mean the output for forward direction is 1 and vice versa.

There are areas where the direction could be forward for current polarization but reverse for voltage polarization. In Dual polarization mode, the forward direction is considered as the main direction of the 67N element. So if both forward and reverse directions are true, the 67N output shows 1 for forward and 0 for reverse.

The output of the neutral directional element is always a tripping signal which is sent to the neutral overcurrent element. The user can choose whether the tripping signal is sent from forward direction or reverse. This gives more options to users.

The overcurrent relay trips if a neutral fault current is above the threshold (for 50-51N) and its direction is either forward or reverse [3].

Table below summarizes the different directional elements in 67.

Relay Type

Polarization mode

Operating Quantity

Polarizing Quantity

Output

67P

Voltage

IA

VBC ∠ ECA

One output:

1: FWD

0: REV

IBVCA  ECA
ICVAB ∠ ECA

67N

Voltage

I0 ∠ ECA-V0

Two outputs:

0 or 1: FWD

0 or 1: REV

Current

I0IG

Dual

I0 ∠ ECA or I0-V0 or IG

Mask and Parameters

General Parameters

NameUnitDescription
FrequencyHzThe frequency of the measured current signals. Both 50 and 60 Hz are supported within the model.
Base currentABase value for current
Base voltageVBase value for voltage
Sample TimesBlock sample time
Enable directional components-To enable the directional elements of the relay model (67P, 67N)


The parameters for 50P, 51P, and 51N are covered in the confluence page for 5051 relay model.

Phase directional overcurrent

NameUnitDescription
Enable phase directional 67P-Enable the use of 67P for control of phase overcurrent
ECA (67P)degECA angle for 67P
Blocking signal when voltage memory expires-Enable sending a blocking signal when voltage memory expires in 67P

Neutral directional overcurrent

NameUnitDescription
Enable neutral directional 67N-Enable the use of 67N for control of neutral overcurrent

Choose directional element for tripping




-

Select either forward or reverse current as tripping signal for neutral overcurrent control
Forward (1)Forward direction
Reverse (2)Reverse direction
Positive sequence restraint factor (K)-The factor used for positive sequence restraint in calculating the operating current for neutral directional overcurrent 67N
ECA (67N)degECA angle for 67N
Polarization mode-Polarization mode for 67N
Current (1)Current polarization
Voltage (3)Voltage polarization
Dual (2)Dual polarization
Enable ground calculation (current polarization)-Enable ground current and voltage calculations that are used for current polarization
Forward direction pickup currentpuPickup current for forward directional discrimination
Reverse direction pickup currentpuPickup current for reverse directional discrimination
Forward direction limit angledegLimit angle for forward directional discrimination
Reverse direction limit angledegLimit angle for reverse directional discrimination

Inputs and outputs

Inputs

NameDescription
IabcThree-phase currents
VabcThree-phase voltages
IGGround current taken from the ground CT
VGGround voltage taken from the ground VT
ResetThe reset signal for the relay. The relay will be reset when this signal becomes 1

Outputs

NameDescription
TripThe output of overcurrent relay. By default, it is 0, if an overcurrent scenario is detected and fault current is in a desired direction, it becomes 1.
mThe output is a vector of multiple signals, to monitor the response of each protection element and direction of each element.
trip5051The output of overcurrent relay separate from directional elements.
trip50PA three-dimension output of 50P, in the order of phase A, B, C
trip51PA three-dimension output of 51P, in the order of phase A, B, C
trip50NOutput of 50N
trip51NOutput of 51N
trip505167PThe output of phase directional overcurrent 505167P.
dir67PA three-dimension output of 67P direction, in order of phase A, B, C. 
trip505167NThe output of neutral directional overcurrent 505167N.
dir67NFA one dimension output of 67N forward direction.
dir67NRA one dimension output of 67N reverse direction. 

References

[1] GE Industrial Systems, “D60 Line Distance Relay: UR Series Instruction Manual”, Section 5.5.7.e: Phase Directional Overcurrent, (2009 GE Multilin)

[2] Bogdan Kasztenny, Dave Sharples, Bruce Campbell, Marzio Pozzuoli, “Fast Ground Directional Overcurrent Protection: Limitations and Solutions”, Page 16, (2000 GE Grid Solutions)

[3] GE Industrial Systems, “D60 Line Distance Relay: UR Series Instruction Manual”, Section 5.5.8.d: Neutral Directional Overcurrent, (2009 GE Multilin)

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