Negative Sequence Directional Overcurrent (67Q)
1. Introduction
It is important for the Protection Engineer to understand symmetrical components. Digital multi-function numerical relays use symmetrical component voltages and currents for a large number of protection functions, such as 67Q negative sequence directional overcurrent , 67G ground directional overcurrent , 67P three phase balance directional , 46,47, 50G. For example the Beckwith Digital Feeder Protection and Control System Model M-7651A below in Figure 1.1 (see www.beckwithelectric.com for more detail).
This paper discuss negative sequence directional overcurrent protection function (67Q) of digital protection relay and calculate voltage and current measured by protective relays during unbalanced system conditions such as a single phase line to ground fault , line-line to ground fault and line to line fault.
Symmetrical Components provides a practical methodology for understanding and analyzing power system operation during unbalance conditions and also to understanding 67Q negative sequence directional overcurrent protection function. “In a sense Symmetrical Components can be called the language of the relay engineer or technician”. In this paper discuss the setup and simulation of application of negative sequence overcurrent directional (67Q) base on Upgrading Distribution Protection and Control. The simulation was done in PT Jasa Rekayasa Mandiri Protection and Control System Workshop (www.jasarekayasamandiri.com) by using Simulator JRM-SM-M-7651A
2. Symmetrical Components for Negative Sequence Directional Overcurrent
In this section, we will discuss the symmetrical components for negative sequence directional application and unbalance faults analysis.
2.1. Symmetrical Components
Standard power system equations assume a balanced 3Φ system. What to do when it is not balanced? Need a technique to de-construct the unbalanced currents into a set of balanced currents to do the calculations. Symmetrical Components to the rescue. 101 years ago, on 6/28/1918, CL Fortescue of Westinghouse presented a paper on Symmetrical Components to AIEE. Each phase of voltage and current is made up of 3 separate components (or sequences) such that the sum of these components make up the total (see Figure 2.1.1):
Although the phases are unbalanced, the individual sequence networks are balanced. This allows us to go from 6 degrees of freedom to just 2. Any individual sequence component can never exist alone in 1 phase i.e. if any sequence is in 1 phase, then it must exist in all 3 phases. And each sequence component must be equal in magnitude in all 3 phases. Ia0, Ib0, and Ic0 also have equal phase angles. While Ia1, Ib1, Ic1 and Ia2, Ib2, Ic2 do not have equal phase angles, but they are always 120° apart as defined. Zero sequence phasors are of equal magnitude and in phase with each other. Zero sequence phasors do not rotate in sequence but they do rotate in time.
Shorthand method for representing 2 of the phases (typically B and C), using the reference phase (typically A phase) shifted by the appropriate phase angle (see Figure 2.1.2). The “a” rotates a vector by 120° and “a2” rotates a vector by 240°.
Combining “a” operators with sequence phases (see Figure 2.1.3):
Negative-sequence and zero-sequence quantities typically only exist during system unbalance or unbalanced faults. Therefore protection functions that operate on these quantities can be set to be very sensitive.
2.2. Network Equations for Current and Voltage
The network equations for current as follows. Re-writing the phase equations now with “an” operators included.
If no phase designation, assume it is in reference to a phase and solving equations for sequence values:
The network equations for voltage as follows:
If no phase designation, assume it is in reference to a phase and solving equations for sequence values:
2.3. Example Symmetrical Component Calculation
Given the following unbalanced currents in Amperes because single line to ground fault:
Calculate the symmetrical component values using the equations:
Given the following unbalanced voltages in kV because line to line fault
Calculate the symmetrical component values using the equations:
3. Negative Sequence Directional Overcurrent Function (67Q)
In this section, we will be discussed Negative-Sequence Directional element of Beckwith Digital Distribution and Protection and Control model M-7651A (www.beckwithelectric.com).
3.1. Negative Sequence Directional Overcurrent Element
Each Negative-Sequence Directional Overcurrent element can be configured as directional or non-directional. forward or reverse looking operation depends upon the setting for the Maximum Sensitivity Angle. Figure 3.1.1 “Negative-Sequence Directional Overcurrent – Directional Characteristic” illustrates setting an element to be forward looking for unbalanced faults on a distribution feeder. 60 to 70 degrees is a typical line angle for a distribution feeder. Figure 3.1.2. Shows the phase relationship between the polarizing voltage and negative sequence current for an unbalanced fault in the forward direction with respect to the relay on a purely reactive system.
Up to five independent Negative Sequence Directional Overcurrent elements can be enabled, 67Q #1 through 67Q #5. Each directional element has two Angles, a Maximum Sensitivity Angle, and an Angle Band. Figure 3.1.3. Illustrates the directional characteristic when the Angle Band is enabled. This is the umbrella characteristic and adds security to the directional decision.
The choice of polarizing voltage is negative-sequence voltage V2. The operate current is the negative sequence current I2. Ranges and increments are presented in Figure 3.1.5.
3.2. Minimum Polarization Voltage
The directional element can be selected to either trip or block trip when the polarizing voltage drops below a settable level. The minimum level is 5 Volts and the setting is 2.0 to 10.0 percent of the nominal voltage. The Nominal Voltage can be Line to Ground or Line-Ground to Line – Line.
Use this option to prevent unwanted operation for cases such as heavy load coupled with standing system unbalance. Choose settings for reliability (trip) if the relay is located in a weak area of the power system (i.e., low magnitude negative-sequence voltage during unbalanced faults).
3.3. Definite/Inverse Time Characteristic
Each element can be configured to operate on a Definite (Figure 3.1.5) or an Inverse Time Overcurrent characteristic (Figure 3.1.6.)
4. Setup for Negative Sequence Directional Overcurrent
In this section will discuss the setup and simulation of application of negative sequence overcurrent directional base on Upgrading Distribution Protection and Control in one of Power Plant (example).
4.1. Setup System Digital Relay
The setup system of in Figure 4.1.1, setup relay inputs in Figure 4.1.2, setup relay outputs in Figure 4.1.3, and setup of relay voltage configuration in Figure 4.1.4.
4. 2. Setup Setpoints of Negative Sequence Directional Overcurrent Protection
The setpoints of 67Q in Figure 4.2.1 and directionally view of 67Q in Figure 4.2.2
5. Fault Simulation for of Negative Sequence Directional Overcurrent
5.1. Injected Voltage and Current for Fault Simulation
The injected voltage for simulation as follows:
Calculate the symmetrical component values using the equations:
The injected current for simulation as follows
Calculate the symmetrical component values using the equations:
The voltage and current metering and vector diagram of Relay M-7651A after current and voltage injected base on the calculation above is shown in Figure 5.1.1
5.2. Fault Data Recorder and Oscillograph Analysis
Fault Recorder for Fault No. 68 (67Q#1 10/7/2020, 22.48:19.599) for Fault Simulation in section 5 is shown in Figure 5.2.1.
Figure 5.2.2 Sequences of Event Viewer for Fault No. 2477.
Figure 5.2.3 Sequence of Events Detailed Record No. 2477
Figure 5.2.4 Oscillograph of Original Wave Form of Voltage and Current Fault No. 2477
Figure 5.2.5 Oscillograph of RMS Wave Form of Voltage and Current Fault No. 2477
Base on the Data on Fault Recorder, Sequence of Event and Oscillograph:
Date of Fault: 07 October 2020
Element Operated: No. 67Q#1
Start Time: 22.48.14.419 PM
67Q#1 Pickup at 22.48:19. 500 PM (Current Ia = 10.54 A, Ib = Ic = 0 Ampere), Va = 10.21 V)
67Q#1 Time Out at 22.48: 19. 599 PM (Ia = 15.03 A, Ib = Ic = 0 Ampere, Va = 28.52 V). Time delay: 100.048 mseconds.
5.3. 67Q zone of protection test
Test 67Q for trip and block trip is performed with secondary negative sequence voltage reference V2 with zero degree and magnitude higher than minimum V2 block voltage. The magnitude of negative sequence current I2, higher than pickup setting with some angle degrees in 67Q trip and block zone.
6. Simulator JRM-SM-M7651A
For simulate the application and testing of Negative Sequence Directional Overcurrent use Simulator JRM-SM-M7651A. The power sensing is shown in Figure 6.1.1. The control diagram is shown in Figure 6.1.2., and the picture of simulator is shown in Figure 6.1.3.
7. General Specification of Beckwith M-7651A
Figure 7.1.1 , Figure 7.1.2, Figure 7.1.3 and Figure 7.1.4 are shown the general specification, standard and optional features , single line diagram , and external connection of Beckwith Digital Distribution Protection and Control System model M-7651A , that have negative sequence directional overcurrent protection function and other functions. The others protection function (see single line diagram) is needed for electrical distribution system, small power generation, and also renewable power generation such as wind turbine, small hydro, PV.
