power system background - uidaho.edu · 1 power system background originally, the model power...
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Power System Background
Originally, the model power system was constructed by Idaho Power, the main
Southern Idaho electrical utility, headquartered in Boise, Idaho. Idaho Power built the
system to test protection equipment such as relays and breakers. They also used the
model power system to model part of their own transmission system around the Midpoint
area in lower-central Idaho. In the mid-1990’s, Idaho Power donated the system to the
University of Idaho.
The University of Idaho has made several modifications to the system including
but not limited to (1) a fault matrix in which three faults can be placed on the system
either simultaneously or in an evolving manner (2) the ability to load impedance faults
onto the system and (3) incorporating SEL relays into the system.
Looking at each of the modifications in turn, the fault matrix is three sets of
thyristors controlled by three separate microcontrollers. Using a computer, a user can
program the microcontrollers to fire the thyristors for a specified period of time. While
the thyristors are being fired they act as a short circuit where the fault matrix is connected
to the system.
As a senior design project in 2001-2002, a group attempted to make it possible to
have impedance associated with a fault. At the writing of this guide this feature does not
yet function properly.
The SEL relays where donated to the University by Schweitzer Engineering Labs,
an industry leading relay design firm based out of Pullman, Washington. The relays are
microprocessor based and can be programmed for a variety of protection purposes. Some
protection purposes include, but are not limited to, overcurrent protection, under or over
voltage protection, and frequency protection. The relays also allow a user to piece
together logic blocks in which the relays will send a trip signal in the event of multiple
occurrences. Thanks to SEL, the University has been able to use the model power system
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to enlighten students on the operation of relays, give students hands on experience with
actual power system configurations, research system designs, test relay settings, and
perform many other academic studies.
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Main Parts of the Model Power System
The model power system consists of many different components. The following
components are listed here with an explanation because they are mentioned often in the
user guide.
(1) Transmission Lines
The power system consists of four separate transmission lines that can be connected
in any fashion the power system operator desires. Each transmission line is basically
a pi model consisting of a variable series impedance, a parallel capacitance to ground,
capacitance between the lines, and mutual inductances. The series impedance is
variable from .1+j*1 to 1+j*10 in .1+j*1 increments (Figure 1).
Figure 1: Variable Line Impedance
The parallel capacitance to ground and capacitance between lines is fixed at a
constant value of 470 µF. Series capacitors can also be switched in and out of the
system. For a more detailed view of the system look at the model power system AC
schematic taped to the wall in the power lab.
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(2) Four Breakers
There are four three phase breakers connect to the system (Figure 2). Each of these
breakers is connected to a relay through a communication module located at the very
top of the model power system.
Figure 2: Six Single Phase Breakers (Two Lines)
The breakers closest to L1 and L3 are able to reclose if the “reclose switch” is on and
if given a specific command from the relays. All breakers have manual open and
close piston handles (Figure 3) connected to them, enabling the power system
operator to energize and de-energize the lines whenever desired.
Figure 3: Breaker Piston Handle
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(3) SEL Relays
Each breaker on the system is connected to a specific relay. The breakers off of L1
and L3 are connected to SEL351S overcurrent relays and the breakers off of L2 and
L4 are connected to SEL321 distance relays. The overcurrent relays act like a fuse,
when the current running through them crosses a certain threshold, which is
programmed into the relay, the relay will send a trip command, after a specified
period of time, to the breaker and the breaker will open. The distance relays, rather
than using thresholds, use zones to determine the location of a fault. These zones are
specified in ohms. The relay will determine which zone the fault is located, by taking
voltage and current measurements, and trip according to elements set for that zone.
In the lab, the protection schemes are modified using Ewan Telnet, a program
installed on the computer closest to the South wall.
Figure 4: SEL 321 Distance Relay
(4) LEMs
The primary tool used to measure voltages and currents on the model power system
are voltage and current LEMs. The LEMs can be connected anywhere on the system
to measure voltages and currents. The LEMs sample the line to line voltages and
currents at discrete intervals and using the program GLGraphMain, installed on the
computer closest to the North wall, these readings can be acquired and seen on a
graph. In order to be able to view the values of the voltages and currents, the data
points stored on the computer must be converted into a readable graph in MatLAB.
The procedure to do this can be found on page 16 of the manual.
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(5) Fault Matrix
There are three sets of thyristors connected to the fault matrix. Thyristors act like
diodes that require a firing signal before they can turn on. The firing signal comes
from the microcontroller connected to the thyristors. Using the program
GLGraphMain, the power system operator can program the thyristors to fire for a
specified period of time, thereby creating a virtual short on the system. This virtual
short created on the system is known as a fault. Using the microcontrollers, the user
can place single line to ground faults, line to line faults, double line to ground faults,
and three phase faults on the system for any amount of time, at up to three faults at a
time. There is also the option of having evolving faults, such as a fault beginning as a
single line to ground fault and then becoming a three phase fault.
Figure 5: The Fault Matrix
(6) M/G Set
The M/G set is a 5kW, 120 Volt, synchronous machine. The M/G set is located in the
southwest corner of the lab however the controls for the M/G set are located on the
right side of the model power system. Using the controls on the model power system,
the power system operator can adjust the speed of the machine and the voltage at the
terminals of the machine. In order to run the machine in conjunction with the infinite
bus (Avista) there is a synchroscope to allow the power system operator to close the
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breaker connecting the two at the correct time (in a fashion similar to the light bulbs
in the power lab).
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Startup Procedures for the Model Power System
To ensure safety, because of the high voltage, two people must always be in the room
while running the power system.
1) Set Up
a) Ensure that the two mushroom head “Emergency Stop” buttons located near the
“System Power” and the “M/G Set” controls are pulled out.
Figure 6. Emergency Stop Button
b) Make sure that the shunt trip breaker on the south wall of the lab is reset (on).
c) Double check that the configuration patched together on the panel jacks is
electrically valid and safe. (i.e. Make sure the jumper cables to the M/G set are
disconnected if the M/G set is not to be used).
d) Ensure all four pistol grip breaker control handles show green (rotate the handles
to your left) indicating that they are open.
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2) System Start
a) Remove the padlock and turn on the main breaker located on the south wall.
b) Flip on the “Communication and DC Control” switch located on the left side of
the power system to supply power to the relay control modules and the breakers.
c) Press the “System Power On” button to activate the system. The source light
should come on indicating there is a voltage on the line.
Figure 7. System Power Buttons and Source Light
3) M/G Set Operation (If the M/G Set is to be Used)
a) Look over the M/G Set equipment in the southwest corner of the lab for readiness
and safety.
b) Ensure that the circuit breakers where the M/G Set is to be patched into the
system are open (green). Before the M/G Set can be electrically connected into
the system it must be started up and synchronized with Avista.
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c) Start the M/G Set and drive electronics by pressing the system on button. You
should hear the generator start up.
d) Before synchronizing, wait approximately 1 minute after starting for the
controller timer to engage the drive clutch. When the clutch engages, this will be
obvious from a sudden change in noise, the generator light on the right end of the
power system should illuminate.
Figure 8. M/G Set Panel Jacks and Light
e) Adjust the speed and voltage controls appropriately using the frequency,
synchroscope, and voltage meters. Adjust the voltage to 120 volts and adjust the
speed so that the frequency meter reads 60Hz and the rotation of the synchroscope
needle slows way down.
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Figure 9. Frequency and Syncroscope Meters
f) The needle on the synchroscope should be rotating very slowly if close to
synchronous speed. Close the generator into the rest of the power system when
the synchroscope needle is pointing directly upward.
Note: To enhance understanding of the synchroscope, if the needle is pointing
directly downward and is not rotating, this is the point at which the M/G set is
exactly 180 degrees out of phase with the system.
4) System Stop
a) Only use the mushroom head “Emergency Stop” switch for emergencies (they
will trip the shunt trip breaker).
b) Return all pistol grips to the “green” (open) position before turning the system
power off to ensure that the breakers are actually reset (tripped open).
c) Press the “System Power Off” button to turn off everything (including the M/G
Set) except for the shunt trip breaker.
d) Turn off the main breaker and lock it if done with the system.
e) Turn off the Communication and DC Contol switch.
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Initiating Faults onto the Model Power System
The model power system has the ability to have one, two or three faults
simultaneously occur on the system. These faults can be single line to ground, double
line to ground, line to line or three-phase. To initiate a fault on the system, follow the
instructions given below.
1) Connect the fault matrix to the system at the node where the fault is desired.
System X, Y, and Z are three different microcontrollers. This allows the user to
connect up to three faults at different locations on the system.
2) Go to the computer closest to the north wall in the lab and open a program called
“GL Graph Main”. There is a shortcut on the desktop for easy access.
Figure 10. Screen shot of the GL Graph Main program.
3) Click on the “Initialize” menu and select “Driver Linx”. It should be set on
Keithley KPCI-3100 Series, if it is click OK, if not then select it and click OK.
4) Click again on the “Initialize” menu and select “Subsystem”, then “Analog
Input”. This step is required to receive readings from the LEMs patched into the
system.
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5) Click on the “Options” menu and select “Channels/Gains”.
a. Click on the “Load” button on the top of the window
b. From the load window, click on “Root”, then select “SeniorDesign”, then
select the file “12channels.cgo”.
Figure 11. Screen shot of the “Channels/Gains” window.
6) This is an optional step. If the user wishes to alter the sampling rates of the
LEMs, click on the “Options” menu and select “Rates/#Samples”. To change the
values, simply click and retype the values according to the project.
Sampling Rate 18000 Hz
Sampling Period 5.55556e-00
Channel Sampling Rate 3000 Hz
Cycles 60
Samples (per Channel) 3000
Time 1 sec
Table 1. Default Values for the Rates/#Samples.
7) Click on the “Terminal” menu and select “Start”. This will bring up the Transient
Network Analysis window where the user can program a fault.
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Figure 12. Screen shot of Transient Network Analysis window.
8) There are three microcontrollers that can be used. To access the first one type
“uC21” then enter. To gain access to the second type “uC22” and type “uC23” to
access the third.
Note: Nothing appears on the screen when attempting to access microcontrollers.
9) Once accessed, the name of the microcontroller, uC21, uC22, or uC23 will appear
on the left side of the screen. Hit “V” then enter to go into Verbose Mode. This
will allow explanation of how to enter a fault to later be brought up on the screen
10) Type “F” and return to enter Fault Mode. The program will want five values
entered to program a fault. The first number is the section, for all intents and
purposes use a “0” for this. The second number is the duration in cycles of the
fault to be put on the system. This value can be between 0-200 cycles. The third
number is the type of fault:
0 is no fault
1 is a line A to line C fault
2 is a line A to line B fault
3 is a line A to line B to line C fault
4 is a line A to ground fault
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5 is a line A to line C to ground fault
6 is a line A to line B to ground fault
7 is a line A to line B to line C to ground fault
The fourth number is the initial fault degree, use a “90” for this value. And the
fifth number is the variable impedance degree, use a “0” for this value.
An example of a fault entry is:
0 60 7 90 0
This is a three-phase to ground fault for 60 cycles in segment zero with an initial
fault degree of 90. After the values are entered, press enter twice and the “uCxx”
will return on the left side.
11) After the “uCxx” returns, enter “X” then enter to exit the microcontroller. Close
the TNA window and press the “Acquire” button on the GL Graph Main window.
This will send the fault to the system.
12) After the fault has been sent, click on the display button and the waveforms from
the system will appear on the screen. These values however are unit-less and
need to be sent to a Matlab file to be converted.
13) The fault programmed will remain on the microcontroller until the user exits GL
Graph Main, the user re-enters the TNA Terminal window and programs a new
fault, or the user re-enters the TNA terminal accesses the microcontroller and
types “E” to erase the contents on that microcontroller.
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Transferring Plots to Matlab
Unfortunately the sketches in the GLgraphMain program do not have scales along
the axis to allow accurate measurements to be read from the graphs. To create voltage
and current sketches with axis scales, one option is to transfer the data to Matlab.
1. In the “File” pull down menu of the GlgraphMain program select “save as” and save
the data as a dt file. (i.e. C:\seniordesign\sample.dt)
2. Next open up the file in “Notepad”. You should see columns of data. Each column
represents different measurements taken over time. The first three columns are the
three phases of current taken with LEM1. The next three columns are the line to line
voltage readings taken with LEM1. While the next three columns are the current
readings taken with LEM2. Etc.
3. To get this data into the right format for Matlab, delete the top row that includes only
two numbers as shown below.
12 3000 0.000056 Delete These Two Numbers-1.059570 3.784180 -2.583008 0.000000 -0.131836 0.102539 -1.748047 3.813477 -1.855469 -0.029297 0.131836 -0.087891
-1.899414 3.745117 -1.689453 0.034180 -0.136719 0.068359 -2.529297 3.715820 -0.957031 -0.063477 0.136719 -0.053711
-2.656250 3.598633 -0.786133 0.063477 -0.146484 0.048828 -3.183594 3.447266 -0.029297 -0.092773 0.141602 -0.034180
-3.291016 3.276367 0.166016 0.087891 -0.126953 0.019531 -3.598633 2.954102 0.859375 -0.107422 0.126953 -0.014648
-3.676758 2.783203 1.000977 0.102539 -0.117188 -0.004883 -3.676758 2.114258 1.748047 -0.112305 0.097656 0.043945
-3.720703 1.977539 1.835938 0.102539 -0.087891 -0.039063 -3.579102 1.191406 2.558594 -0.122070 0.068359 0.068359
-3.583984 1.059570 2.602539 0.107422 -0.058594 -0.068359 -3.369141 0.258789 3.232422 -0.112305 0.048828 0.068359
-3.300781 0.073242 3.276367 0.122070 -0.029297 -0.097656 -2.973633 -0.664063 3.666992 -0.117188 -0.004883 0.117188
-2.861328 -0.810547 3.662109 0.112305 -0.009766 -0.112305 -2.246094 -1.567383 3.793945 -0.087891 -0.029297 0.122070
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4. Next scroll down the file all the way to the bottom of the file until you see some data
similar to this shown below. Delete this block of data shown below.
Vxab 1 1 5.000000 0.000000 0.000000 1.000000 -1.000000
Vxbc 1 1 5.000000 0.000000 1.000000 0.000000 -1.000000
Vxca 1 1 5.000000 0.000000 1.000000 1.000000 -1.000000
Ixa 1 1 5.000000 1.000000 0.000000 0.000000 -1.000000
Ixb 1 1 5.000000 1.000000 0.000000 1.000000 -1.000000
Ixc 1 1 5.000000 1.000000 1.000000 0.000000 -1.000000
0 0 0.000000 1.000000 1.000000 1.000000 -1.000000
0 0 0.000000 1.000000 0.500000 0.000000 -1.000000
Vyab 1 1 -5.000000 0.443137 0.443137 0.776471 -1.000000
Vybc 1 1 -5.000000 0.443137 0.776471 0.443137 -1.000000
Vyca 1 1 -5.000000 0.219608 0.556863 0.556863 -1.000000
Iya 1 1 -5.000000 0.776471 0.443137 0.443137 -1.000000
Iyb 1 1 -5.000000 0.556863 0.219608 0.556863 -1.000000
Iyc 1 1 -5.000000 0.556863 0.556863 0.219608 -1.000000
c1 0 0 -2.000000 1.000000 1.000000 1.000000 -1.000000
c2 0 0 -2.000000 1.000000 0.854902 1.000000 -1.000000
Vzab 0 0 -5.000000 1.000000 0.141176 0.749020 -1.000000
Vzbc 0 0 -5.000000 0.749020 0.427451 0.749020 -1.000000
Vzca 0 0 -5.000000 0.498039 0.713726 0.749020 -1.000000
Iza 0 0 -5.000000 0.000000 0.713726 0.247059 -1.000000
Izb 0 0 -5.000000 1.000000 0.568627 0.247059 -1.000000
Izc 0 0 -5.000000 0.000000 0.854902 0.000000 -1.000000
0 0 0.000000 1.000000 1.000000 1.000000 -1.000000
5. Save the resulting notepad file as a .mat file. (i.e. C:\seniordesign\sample.mat)
6. Open up Matlab and create a file that includes the script on the next page. This file is
also already saved on the computer in the senior design directory under the name
plot_command.
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plot_commmand script
clear all;
addpath c:/seniordesign;
load sample.mat -ascii;
time=(0:2999)*.00066666666; %Note: TNA is set to take 3000 samples
y1=sample(:,7); %Note: This takes three different columns (7,8,and 9) of data from % the output file (ex. sample.mat) and definesy2=sample(:,8); % the three different columns of data as three variables
y3=sample(:,9);
cf=10; %The current factor on the lems is 10
vf=81.4/sqrt(3); %The voltage factor on the lems is 81.4 line to line. The voltage is %divided by the square %root of three to make it a line to neutral voltage
figure(1);plot(time,y1*vf,time,y2*vf,time,y3*vf);grid;xlabel('Time');ylabel('Voltage');title('Longline Zone 1 Fault Voltage');
% end of program
You will most likely need to modify the matlab script so that the program will
plot the data you specify. Follow the steps below.
7. First after the “addpath” command, specify the directory that the file is in.
8. Next after the “load” command enter the .mat file name followed by “–ascii”.
9. Define the columns of data you want plotted into variables. In the example script
above three variables were defined. For example here is how a variable is defined
( i.e. variable name = filename(:,column number))
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10. Next after the “plot” command, enter in the variables you want plotted. For each
variable to be plotted you will need to specify the x-axis, which is time.
11. For each variable in the plot command multiply by vf if it’s a voltage or cf if it’s a
current. These values are the voltage and current transform factors currently
configured for the LEMs.
12. Run the program. The program should give the currents and/or the line to neutral
peak voltages specified. (Note: For unblanced faults the voltage factor must be
changed. Dividing by the square root of 3 only works for balanced systems.)
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Programming the SEL Relays
Before the user begins, the following settings must be known:
(1) Which relay do I want to read/change settings?
(2) In which group in this relay should I be operating?
If unsure about these questions first, please be sure not to permanently change any
setting in the relay (the program always asks to save settings, say NO) and second, find
out the answers by talking to Dr. Brain Johnson.
There is one relay for each of four breakers on the model power system. SEL 351S
relays are connected to the breakers closest to L1 and L3 and SEL 321 relays are
connected to the breakers closest to L2 and L4.
!IMPORTANT!
Current measurements into the SEL relays are scaled down by a factor of two. This is
important when creating current protection settings.
The four relays are also connected to ports on the SEL 2030 communication relay.
Using the computer closest to the South wall, the user can telnet into the SEL 2030
communication relay. Once the user has accessed the 2030, the user can use this relay
communicate with each of the other four relays to read/change settings.
This guide is not going to go into extreme depth to explain all the settings in the
two different types of relays due to the complexity associated with them, but it will go
over some basic changes that can be made. Everything explained, except for how to
navigate around Ewan Telnet, can also be found in the instruction manuals for these
relays.
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To access a particular relay and read/change setting, follow the steps provided below:
1) Go to the computer closest to the South wall and open Ewan Telnet.
2) Once in Ewan Telnet, the first thing to do is gain Level 1 access to the SEL 2030
communication relay. To do this enter “ACC” and then enter the password
“OTTER”.
3) The next step is to gain Level 2 access. To do this enter “2AC” and then enter the
password “TAIL”.
4) To see a list of possible commands at this point type “WHAT”.
5) In order to begin changing settings in the relays the user must know which relays
are connected to which ports. Here is a list showing the relay connected to each
port. This can also be found by typing “WHO”
PORT 1: SEL 321 on L2
PORT 2: SEL 321 on L4
PORT 3: SEL 351S on L1
PORT 4: SEL 351S on L3
6) First, look at programming the SEL 321 on L2. Type in “PORT 1”.
7) Follow steps (2) and (3) once again to gain Level 2 access to the relay.
8) Now enter the group to read/change settings in by typing “GRO #” where # is the
number of the group desired. Each relay has six separate groups that are
unrelated, allowing for six completely different protection schemes for each relay.
9) Now, to view the settings in this group type in “SHO”. A list of acronyms will
stream down the window. Each one of these acronyms has a different meaning
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which can be found in the Instruction Manual of the relay starting on page 5-25.
A few of the more important settings are listed below:
DIR1: This is the direction in which the relay is looking for its Zone 1
mode of protection. This is used in conjunction with many of the other
settings to determine whether or not the relay should trip. If the relay is
looking forward (F) and the fault is behind it, it will not "Zone 1 trip".
DIR2: This is the direction in which the relay is looking for is Zone 2
mode of protection.
Z1P: This is entered as in Ohms*sec. This is the impedance the relay
looks down the line for its Zone 1 mode of protection.
Z2P: This is the same as Z1P except Z2P is how far down the line the
relay looks for Zone 2. So, if the fault is within the distance specified in
Ohms*sec and is not in Zone 1, then the Zone 2 elements will pick up.
50PP1: This is the overcurrent entered in Amps*sec. If the current goes
above this value in Zone 1 (about 15 ohms down the line, depending on
the Z1P setting) then the relay will send a trip signal to the breaker it is
connected to.
50PP2: This is the same as 50PP1 except this overcurrent value is entered
for Zone 2 (30 ohms down the line). There will be more impedance
between the breaker and fault in this case so the value entered here is
generally smaller than the value entered for 50PP1.
10) Although many settings will stream down, these are not all of the settings
available to the relay. Setting at the end of the stream may or may not show up
based on settings at the beginning of the stream. For example, if there is no
Residual Time-Overcurrent element settings wanted then the user will enter “N”
under the E51P setting. When NO is entered under this setting, 51PC, 51 PP,
51PTD and many other settings will not show up because these extra settings are
only needed when E51P is enabled.
11) To be able to change/read all the settings in turn one at a time type “SET” and
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then press return.
12) The user can now go through the settings one at a time by hitting enter. If the
user desires to change any of the settings simply enter the new value and proceed.
13) Once the user has went through all the settings a listing of all the settings will
once again be displayed with all the changes. At this point a save settings prompt
will appear and the user must decide whether to save these settings or not.
14) In order to go directly to one setting simply type “SET A” where A is the
acronym of the setting. This will take the user directly to that setting. If that is
the only change desired then type "END". At this point all the setting will once
again stream down the screen and the user will be prompted about whether to
save.
15) Once the user has used the relay multiple times he/she may desire a faster process.
In this case, type "SET A TERSE". By typing TERSE the program will skip the
sep of listing out all the settings and go directly to the save prompt. This saves
quite a bit of tie when experimenting with different settings.
16) Now take a look at the SEL 351S. This relay is very similar to the 321 except the
351S is meant to be more of an overcurrent protection relay rather than a
directional relay. For this reason most of the acronyms are different. A few of
the more important acronyms are listed below:
E50P: This is where the user must specify how many levels of protection
are desired.
E32: Specifies whether or not directional control is desired.
E79: Specifies whether the relay will send one or more reclosing signals to
the breaker (Maximum of 4).
50P1P: This is the Level 1 phase instantaneous/definite time overcurrent
protection element on the relay. If the current ever exceeds this value the
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breaker will trip instantaneously.
50P2P: This is the same as 50P1P except this is the Level 2 current
threshold.
67P2D: This is the time the relay waits to trip in the if currents exceed the
level 2 threshold. When this is set to zero, the relay will trip
instantaneously regardless of Level.
79OI1: Here a time is entered specifying how long the relay waits before it
sending a reclosing signal.
79OI2: This is how long the relay will wait before it tries reclosing a
second time.
17) Rather than work with zones like the 321, the 351 works with levels. In the most
basic sense this relay acts as a fuse. The relay will send trip signals based on the
threshold the current crosses , if the current goes above the Level 1 threshold it
will trip instantaneously whereas if the current goes above the Level 2 threshold it
rip based on the time delay specified (6P2D).
18) Navigating through Ewan telnet for the 351 is the same as the 321. See steps (8)-
(15) above.
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Appendix
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Appendix Table of Contents
Case Study 1 Long-Line 27
Case Study 2 Parallel-Line 34
Case Study 3 Time-Overcurrent 37
Case Study 4 Split-Line 41
Case Study 5 Reclosing 43
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Case 1. Long-Line Study
The SEL 321 relay is a Phase and Ground Distance, Directional Overcurrent
Relay. This relay can be set to have different zones of protection. Zone 1, for example,
could be set to be 20 Ω down the line or any other value impedance value desired.
Generally, zone 1 is set to be about eighty percent of the line between breakers. Any
fault that the relay detects in zone 1 will trigger an instantaneous trip. Zone 2 could then
be set to be a hundred percent or more down the line. For any fault that occurs in zone 2
the relay would delay before tripping, waiting for another relay to respond or waiting for
some sort of communication telling it what to do. For this study use the group 5 settings.
The zone settings are shown in the table below.
Zone Impedance/Distance Time Delay Direction
1 13.91 ohms - F
2 30 ohms 20 cycles F
3 10 ohms 0 cycles R
Table A.1 Zone Settings for SEL-321 Relay (L2)
This study will explore the operation of the SEL 321 relay located at L2. Set up
the system so that the four lines on the system are in series like shown in the figure on the
next page. Set all four lines so they are at maximum impedance. (Note: For this study to
prevent the SEL-351s relay at L3 from tripping you may have to increase the Phase
Inst./Def.-Time Overcurrent Element Threshold 50P1P).
28
Figure A1. Long-Line Study Schematic
For the first part of this study place a three phase to ground fault outside of the
zone of protection of the relay at L2. Zone 2 protection for this relay reaches only part of
the way down L3. So place a fault between L3 and L1 and see what happens. The
setup of this fault is shown on the in Figure A2.
Figure A2. 3-Phase Fault out of SEL 321 Zone of Protection
29
The next two figures show the three-phase voltage and current at the fault point.
As expected no trip occurred because the fault was out the SEL 321s zones of protection.
So, while the fault occurred, there was a lag in voltage and spike in current.
Figure A3. Long-Line Fault Beyond Zone 2 Voltage
Figure A4. Long-Line Fault Beyond Zone 2 Current
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-150
-100
-50
0
50
100
150
Time
Vol
tage
Longline with Fault Beyond Zone 2
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-6
-4
-2
0
2
4
6
Time
Cur
rent
Longline with Fault Beyond Zone 2
30
Next for the second part of the study, place a fault between L4 and L3. This fault
is in the zone 2 region of protection for the relay at L2. If the test operates correctly then
the relay will delay for a set number of cycles to wait for another relay closer to the fault
to possibly respond, then when no other relays respond, the SEL 321 at L2 will trip its
breaker. Adjusting Z2PD sets this delay for zone 2. The set up for this part is shown
below.
Figure A5. 3-Phase Fault in SEL 321’s Zone 2 Protection
31
The next two figures show the three-phase voltage and current at the fault point.
The plots both indicate there was a delay before the relay tripped the breaker.
Figure A6. Long-Line Fault Zone 2 Voltage
Figure A7. Long-Line Fault Zone 2 Current
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
-150
-100
-50
0
50
100
150
Time
Vol
tage
Longline with Fault in Zone 2
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
-10
-5
0
5
10
Time
Cur
rent
Longline with Fault in Zone 2
32
Finally, for the last part of this study, place the fault between L2 and L4. This
fault will be within the relay’s zone 1 range of protection. For this fault the relay should
trip instantaneously. The setup for this part is shown below.
Figure A8. 3-Phase Fault in SEL 321’s Zone 1 Protection
The next two figures show the three-phase voltage and current at the fault point.
As you can see, the relay took about 3 cycles to respond to the fault. This is considered
an instantaneous trip.
Figure A9. Long-Line Fault Zone 1 Voltage
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
-150
-100
-50
0
50
100
150
Time
Vol
tage
Longline with Fault in Zone 1
33
Figure A10. Long-Line Fault Zone 1 Current
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
-10
-5
0
5
10
15
Time
Cur
rent
Longline with Fault in Zone 1
34
Case 2. Parallel Line Study
This setup will demonstrate the communication aided tripping between the SEL-
321 relays. The relays are set in group 5 to work under Permissive Overreaching
Transfer Trip Logic (POTT). This communication scheme allows the relays at L2 and
L4 to isolate a fault between the relays to allow current to continue to flow through the
other line and serve the load. Again set all four lines so that they are at their maximum
impedance. The setup for this study is shown below.
Figure A.11 Parallel Line
The fault between the relays at L2 and L4 is in both of their zones of protection.
For a normal setup, the relay at L2 would trip instantaneously, but then the current would
flow to the fault from the other direction through L4 and then the relay at L4 would trip
after a small delay. Rather than let this delay happen and inconvenience the load, a
communication scheme can be set up enabling the relays to trip at the same time in the
event of a fault between them. So, when a fault is place between L2 and L4, the SEL 321
at L2 will trip instantaneously while sending a signal to the SEL 321 at L4, telling it to
also trip instantaneously. The fault is then isolated from the rest of the system. The three
phase voltages and currents on the bottom branch at the breaker on L2 are shown in the
next two figures.
35
Figure A.12 Voltage at L2 Breaker
Figure A.13 Current at L2 Breaker
The next two figures show the voltages and currents of the other line, at the
breaker on L1, which was left online so that the load could be served, uninterrupted. The
0.05 0.1 0.15 0.2 0.25-20
-15
-10
-5
0
5
10
15
20
Time
Cur
rent
Parallel Line with Isolated Fault
0.05 0.1 0.15 0.2 0.25
-150
-100
-50
0
50
100
150
Time
Vol
tage
Parallel Line with Isolated Fault
36
voltage remained constant because measurements were taken at the infinite bus and as
expected, when the other line went down, all the current originally running through it was
forced through the top line. This causes more losses on the system, but more importantly,
the load is still served.
Figure A.14 Voltage at L1 Breaker
Figure A.15 Current at L1 Breaker
0.05 0.1 0.15 0.2
-150
-100
-50
0
50
100
150
Time
Vol
tage
Parallel Line with Isolated Fault
0.05 0.1 0.15 0.2 0.25
-6
-4
-2
0
2
4
6
8
Time
Cur
rent
Parallel Line with Isolated Fault
37
Case 3. Time-Overcurrent Study
This setting will demonstrate the levels of operation for the SEL-351s relay. The
Level 1 threshold current (50P1P) is set at 5 Amps and the Level 2 threshold current
(50P2P) is set at 2 Amps. There is no time delay associated with the Level 1 phase
instantaneous overcurrent element but for the Level 2 phase instantaneous overcurrent
element a 30 cycle time delay is set (67P2D).
If the configuration seen on Figure A.13 is placed on the system, the current
measured by the relay will exceed 5 Amps (don’t forget about the CT factor of 2 between
system and relay) and the relay will send and instantaneous trip signal to the breaker.
Figure A.16 Time-Overcurrent Study Setup For Level 1
38
The following two plots show the instantaneous trip for the level 1 threshold.
Figure A.17 Level 1 Instantaneous Trip Voltage
Figure A.18 Level 1 Instantaneous Trip Current
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
-10
-5
0
5
10
15
Time
Cur
rent
Level 1 Instantaneous Trip
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
-150
-100
-50
0
50
100
150
Time
Vol
tage
Level 1 Instantaneous Trip
39
If the configuration of Figure A.15 is used, the current will not exceed 5 Amps
however it will exceed the 2 Amps Level 2 threshold. In this case, the relay will still
send a trip command but it will wait the time specified by the 67P2D setting, which is 30
cycles in this case.
Figure A.19 Time Overcurrent Study Setup for Level 2
Again, the following two figures show the voltage and current readings at L1 for
this kind of trip.
Figure A.20 Level 2 30 Cycles Delayed Trip Voltage
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-150
-100
-50
0
50
100
150
Time
Vol
tage
Level 2 30 Cycle Delay Trip
40
Figure A.21 Level 2 30 Cycles Delayed Trip Current
The last two plots show the relay acted properly by delaying 30 cycles before the
trip. As mentioned earlier, the threshold for level 2 can be adjusted by changing 50P2P.
The time delay for level 2 can be adjusted by varying 67P2D.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-10
-5
0
5
10
Time
Cur
rent
Level 2 30 Cycle Delay Trip
41
Case 4. Split-line Study
This case represents a simple secondary protection scheme. A load is being
served off of L3 but there is a fault on L4. Ideally, the SEL 321 should trip
instantaneously allowing the load to still be served by Avista.
Figure A.22 Split-Line Study Schematic
A problem that might present itself in a system like this is incorrect settings for
the relays. If for example Level 1 of the relay is set to be too sensitive, both the SEL 351
on L1 and the SEL 321 on L4 will trip instantaneously. Now, the load can not be served
by Avista, this is not our ideal case. Also, if 67P2D, the phase instantaneous overcurrent
level 2 delay, is set to zero, the same problem presents itself. So, one way to fix this
problem would be to set the phase instantaneous overcurrent thresholds of the SEL 351
on L1 extremely high. In this case, however, the user gives up secondary protection (and
you have a relay doing nothing but taking measurements). If the SEL 321 on L4 does not
operate correctly, or is set incorrectly, then a fault will remain on the system indefinitely
and the load will suffer. So, the best way to set up the power system protection for this
case is the following:
42
SEL 351 on L1:
50P1P: 5.8 Amps
50P2P: 2 Amps
67P2D: 20 cycles
SEL 321 on L4:
50PP1: 2 Amps
For the settings above, the SEL 321 on L4 will trip instantaneously but the SEL 351 will
see a level 2 fault. If for some reason the SEL 321 does not trip, the SEL 351 will trip
after a 20 cycle delay, giving the first relay plenty of time to do its job before the
secondary protection kicks in.
43
Case 5. Reclosing Study
This study explores the reclosing capability of the SEL-351s relay. The group 5
settings have reclosure settings enabled for both the 351 relays. Each of the relays is set
to have 2 reclosure attempts otherwise known as shots. For each shot, the user can
specify the delay in cycles before the breaker is reclosed. These time delays are specified
under the settings 790I1 and 790I2 for the first and second reclosures respectively.
Other settings associated with reclosure are the 79RSD and 79RSLD settings.
79RSD is the reset time from the reclose cycle state. For example if the relay was able to
successfully reclose the breaker into the fault on the first shot than the relay would wait
the specified 79RSD time before resetting.
79RSLD is the reset time from the lockout state. When the relay is in its lockout
state reclosure is no longer possible. The relay will reset from lockout after the time
interval 79RSLD has passed.
The figure below from the SEL-351s manual shows the different states for
reclosure logic.
Figure A.23 Figure of Reclosing Sequence
44
To begin working with a reclosing relay use the configuration in Figure A.17
below.
Figure A.24 Schematic For Reclosing Study
The following two figures show the voltage and current for a double reclosure.
After the first reclose, the fault was still present on the system, so the breaker reopened.
Next, after another specified time delay, the breaker reclosed into the system again. The
fault by that time was gone so no trip after the second reclose was necessary.
Figure A.24 Double Reclose Voltage
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-300
-200
-100
0
100
200
Time
Vol
tage
Double Reclose Voltage
45
Figure A.25 Double Reclose Current
To further understand reclosing and the different sequences of reclosures try
different lengths of faults and different reclosure time settings. Also watch the LED
lights on the front panel of the SEL-351s. You can watch the relay cycle through
different reclose sequence states (i.e. lockout to reset, or cycle to reset, etc.).
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
-20
-15
-10
-5
0
5
10
15
20
Time
Cur
rent
Double Reclose Current