Distance Protection Tutorial

Contents Introduction ......................................................................................................2 Distance Relay Modeling .................................................................................2 Setting the Distance Relay...............................................................................8 Creating and Editing a Path ...........................................................................11 Adding More Relays.......................................................................................15 Creating a New Path......................................................................................16 Creating a Time-Distance Plot .......................................................................16

Distance Protection Tutorial Introduction This tutorial demonstrates the modeling and editing of protective devices typically found in transmission networks. The network that is used can be found, as an application example, in the 1987 edition of Protective Relays Application Guide (PRAG) published by GEC Measurements, paragraph 11.32. Some differences from the original example in the text have been introduced to demonstrate specific PowerFactory applications, as well as to model a more realistic example. As it is assumed that the user is familiar with basic editing of data, the network has been prepared for use, only requiring the editing of protection devices. Instructions to perform load flows and the observations of the results are thus left to the students discretion. It is also assumed that the student has completed the overcurrent protection tutorial so that the basics of relay modeling are familiar.

Distance Relay Modeling The textbook example uses a ‘Quadramho’ relay. In this tutorial however, a ‘Micromho’ relay will be used, which is very similar. The Micromho type characteristics are available in the tutorial project library. The steps we follow to model the relay are as follows: ► Right click on the cubicle feeding Line G from Station P. Select New Devices / Relay Model…. as is shown below.

► A relay element data input window opens, where the new relay is named “Relay G”. ► We select the relay type using the select button and look for the relay type in the project library. The project library should open with the relay type filter activated.

► There is only one distance relay type saved in the library, this has been placed there for use in this tutorial and of course this relay type is the Micromho that we want to use in this example. ► We select this Micromho relay by double clicking on the relay type icon. The relay element data input window is now updated as shown below.

► Select Create CT to model a CT input to the relay. The CT data input window as shown below, opens.

► The CT element can be given a special name such as “CT G”, but this is not absolutely necessary. ► Again we need to select a CT type from the project library. This is done by pressing selecting a type from the project library once again. Then select the “600/1 CT Type” from the project library. ► The CT element data window is updated to show a 600/1 ratio. Of course, had the CT type been a multi-ratio CT, we would also need to select the CT ratio. ► The Location is not specified and therefore the CT is automatically modeled in the same cubicle as the relay. A specific location, other than the local cubicle, would only be used if current measurement was required from a different feeder that that in which the relay is located. ► Press OK and the CT element is correctly modeled and visible in the relay element model. ► Now a VT element must be created. To do this, the Create VT button is pressed. A element data input window, as shown below, opens.

► The VT is given a name “VT G”. ► Note that the VT is defined in terms of a Type and Secondary Type. In other words the VT model consists of a separate primary and secondary. Firstly the primary type is defined by selecting the relevant type using the selection button. A drop down menu appears and we select the VT that is available in the project library. ► Now the Secondary Type is selected from the project library using hte normal type selection procedure, this time for the Type in the Secondary section. Use the “Voltage Transformer Secondary” that is available in the project library. ► Press OK and we are back to the relay element input window, but this time with a VT modeled in the relay. ► Just as with real systems, we need to be sure that the relay model type is correct for the application. Double click on the Measurement element in the relay, and we notice that the relay has been rated with a nominal current of 1 A and a nominal voltage 110 V. These values correctly match the CT and VT input values. Pressing OK closes the measurement element. The Relay G has been modeled in place, but has not yet been set. This is our next step.

Setting the Distance Relay The relay elements are set individually using the same settings proposed in the textbook, as follows: ► Double click on the polarizing element. The window shown below opens.

► Note that the line k0 (described as kn, residual compensation factor adjustment in the ref. book) value is automatically calculated and displayed. As should be expected, the value of 0.49 at an angle of 7.8 degrees matches the textbook exactly. By pressing Assume k0, the k0 setting is changed to 0.48, which is the closest available setting to 0.4893 for this relay. Press OK. ► Double clicking the starting element opens the next setting window:

► The starting element consists of earth fault and over-current elements. It is important that these elements are set sensitively enough to pick up for all faults at the end of the setting zones. To determine this sensitivity we can use PowerFactory to calculate the 3-phase and earth fault currents at the end of zone 3 for relay on Line G. Using a fault impedance of, say, 10 Ohms, we give us a conservative value for setting the starting elements. For this tutorial the busbar at ‘Substation R/B1’ at the end of Line J is faulted, using the complete calculation method. Respective resultant fault currents of 600 A and 410 A for 3phase- and earth fault through Line G are calculated. ► Set the Current, 3*I0 to 0.6 sec.A and Current I>> to 1 sec.A. Press OK. ► Double click on the earth fault measuring element for phase 1 called “PGZ1”. The window shown next opens.

► The secondary ohm impedance values of the first line are automatically calculated and shown. Assuming we want to set this element to 80% of the impedance of Line G, we calculate a value of 8.78 sec.Ohms (10.981 x 80%). Set the Replica Impedance to 8.78 and the Relay Angle to 65 deg. The branch angle reach is automatically calculated as 79.93% of the line impedance, confirming that the setting is correct. Press OK. ► The Zone 2 reach must be set to cover the protected line plus 50% of the shortest adjacent line or 120% of the protected line whichever is the greater. For the application under consideration Zone 2 is set to cover the protected line plus 50% of the shortest adjacent line. Using the same procedure as for setting PGZ1, we set PGZ2 Replica Impedance to 15.37 sec ohm and the Relay Angle to 65 deg. ► Again we set PGZ3 using the same procedure as for PGZ1 and PGZ2. This time we set the PGZ3 Replica Impedance to 65.89 sec ohm and Relay Angle to 65 deg. The Character Angle is kept at 90 deg (to maintain a circular tripping characteristic) and the Offset Impedance is set to 2.2 sec ohm. ► The phase elements of PPZ1, PPZ2 and PPZ3 are all respectively set to be exactly the same as the earth fault elements of PGZ1, PGZ2 and PGZ3. ► Double clicking on the Z2GD element (earth fault timer), opens the following window:

► Select time Z2GD Time Setting to 0.3 s. Press OK. ► Repeat this procedure for Z3GD, setting the Time Setting to 0.6 s. ► The same procedure is used to set Z2PD and Z3PD timers to 0.3 s and 0.6 s respectively. ► The last element to be set is the logic element. In most cases, such as this one, it needs no setting. However, should we wish to trip a different breaker to the one in the same cubicle as the relay, we would need to define this here. For this exercise, we will not set the logic unit.

Creating and Editing a Path When there are several relays in a system and one would like to check the settings of some of these distance relays, in series, it is beneficial to define a path. We define a path as follows: ► Multi-select the busbars and lines from Station P Busbar B3 (132 kV) to Station R Busbar B1 by clicking on each of the elements along this path, while holding down the Control key. ► Right click anywhere on this multi-selection. A drop down menu appears. Select Path… / New….as shown.

The following input window appears:

► The path to be created can be given a unique name for identification. Press OK. ► The path selected should appear in red on the single line diagram. ► Right click anywhere on the path and select Path… / Create R-X Plot on the drop down menu as shown next.

An RX Plot appears showing the settings of Relay G, as well as some line impedances. Note that the earth fault and phase fault impedance elements are on top of each other for each zone. This can be seen by double clicking on, say, the outer zone setting (Zone 3). The following window appears:

► Relay elements can be set directly from the RX plot by double clicking on the displayed characteristic. In case of there being more than one plot being on top of another, as we have here, a window will open in which we must then select the relevant relay setting to be edited. ► After selecting the element to be set or changed, press the Edit Object tool on the toolbar, and the setting sheet of the selected element appears. Alternatively, double click on the element icon to arrive at the setting sheet. ► Double click anywhere on the diagram (but not on a plot) and the relay plot editor appears. Select Options and the window shown below appears.

► Select “Zone 2” in the Branches, Z options. Press OK and OK again to return to the graphic. ► A new line has appeared. Double click on the new vertical line and we see that this is represents the impedance of transformer T6.

Adding More Relays The aim of any protection engineer is to ensure that coordination between different distance and overcurrent relays is correct. This coordination can be checked using RX plots, Time-Distance plots and time Overcurrent plots. Defining paths for the relays to be coordinated is a tool that may be used in order to make maximum use of these different plots. Before this can be demonstrated, more relays must be added to our project. In the next few steps, we add an overcurrent relay to the source side (Station Q side) of Line K, and a distance relay at the source side of Line J, as follows: ► Right click on the Station Q cubicle connected to Line K. Select New Devices…/ Relay Model. ► Name the relay “Line K OC”. ► Select the “Standard OC Relay” type relay from the library. ► Select Create CT. From the library select the type to be a 400/200/1 CT and press OK. ► Note that the CT defaults to the lowest available ratio of 200/1. We want to use the 400/1 ratio and must select it in the Primary Tap drop down menu. Press OK. ► Set the three-phase over-current element to 5 p.u. and the time multiplier to 0.2 (double click on the Toc3Ph element field to access these setting fields). Press OK. ► Right click on the Station Q cubicle connected to Line J. Select New Devices…/ Relay Model. ► Name the relay “Relay J”. ► Select the “Micromho” type relay from the library. ► Select Create CT. On the window that appears, select the Type arrow down. From the library select the 600/1 CT and press OK. ► Select Create VT. Define both primary and secondary VT type as before for “Relay G”. ► Set the relay as follows: PGZ1 = PPZ1 = 18 sec.Ohm; PGZ3 = PPZ3 = 60 sec.Ohm; Z3 Offset Impedance = 0;

PGZ2 = PPZ2 = 30 sec.Ohm; Relay Angle = 65 degrees; Characteristic Angle = 90 degrees

The new distance relay “Relay J” is already in the defined path. The relay can either be added to the existing RX plot, or a new RX plot could be generated containing all relays in the path. The second option is chosen: ► Right click on the red path in the grid and select Path… / Create R-X Plot. ► A new RX plot appears showing both “Relay G” and “Relay J” impedance plots.

Creating a New Path Say we need to check the tripping coordination between “Relay G” and “Line K OC” relays. One way to do this would be to use a time-distance plot. First a new path needs to be defined: ► Multi-select the new path shown below holding down the control key. Make sure the Station Q 132 kV bussection cubicle/ breaker is also selected in the path, or you will receive a warning “Path not complete”. To do this, you may need to enlarge the area around the bussection. ► Select Path… / New….

► A dialogue for the new path appears with the path colour as green (this can of course be changed). Select OK. The new path will appear in green.

Creating a Time-Distance Plot ► Right click on the newly created green path and select Path… / Create Time-Distance Diagram. Make sure that you do not right click on a combined path, but select a part of the path that is unique to the green path in order to create the correct diagram.

► Press Execute on the window that opens. ► Two plots are shown, but these need to be further defined. Double click anywhere on the plots. The screen shown will appear.

► Only a forward plot is required. In the drop down menu next to “Diagrams”, select Forward. ► For the “Reference Relay, Forward” select “Relay G”. ► Press OK. ► The curves may not appear immediately as the scale could be incorrect. Press the “Scale X-Axis Automatically” and “Scale Y-Axis Automatically” buttons on the second toolbar, and the curves should appear as shown next.

From the diagram it is noticed that the distance relay (Relay G) will operate faster than the overcurrent relay. This would cause incorrect tripping. To set this right, take the following steps: ► Double click on the green curve of the over-current relay. Change the “Current Setting” to 1.5 p.u. and “Time Dial” to 0.1. ► Press OK. ► Double click on the Zone 2 part of the red distance relay (Relay G). The PPZ2 window opens. Select Timer. The Z2PD window opens. Set the “Time Setting” to 0.5 seconds. Press OK and OK. ► Double left click on the Zone 3 part of the red distance relay. The PPZ3 window opens. Select Timer. The Z3PD window opens. Set the “Time Setting” to 1.0 seconds. Press OK and OK. ► Press the “Rebuild” button on the second toolbar. ► After the recalculation has been completed, rescaling the Y-Axis may be required. This is done by pressing the “Scale Y-Axis Automatically” button on the second toolbar. ► The Time-Distance diagram now appears as shown below.

It is now clear that for three-phase faults without any fault impedance along the green path, tripping coordination will be correct.