Var Control. Adding a transformer and transformer voltage regulation. engineers loadflow program. The control system engineers loadflow.

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1 November 2012 Adding a transformer and transformer voltage regulation to the control system engineers loadflow program The control system engineers loadflow program The loadflow program used by this website is principle tailored for 1. developing and testing voltage regulation routines in subsystems. 2. applying settings to reactive plant control routines. 3. understanding voltage regulation events in subsystem. The load flow program provides a steady state voltage solution for a particular subsystem loading condition. The load flow program is an excel VBA program that uses the Gauss-Seidel iteration method to find a solution. The excel workbook is separated into three sheets, the load flow worksheet,the subsystem worksheet and miscellaneous worksheet called potpourri. The subsystem worksheet starts with a diagram of the subsystem under investigation. The loadflow program s results will take up about the first 28 rows of the subsystem worksheet and the subsystem diagram can be placed below the loadflow results. The power and reactive power values can be transferred down to the diagram by referencing the loadflow result s row and column as a formula. The load flow sheet is the entry point for the subsystem parameters. There are two tables the bus connection table and the busbar table. The bus connection table is where the transmission line or transformer π equivalent circuits are added adjacent the assigned source busbar and destination busbar numbers. (see transformer tapchanger control article for details on the transformer π equivalent circuit) The busbar table is the entry point for peak busbar loads and generation and the percentage of the peak load required for a particular solution. Also the busbar name and the assumed busbar volts can be changed. The potpourri worksheet is for miscellaneous tables used to store data. The subsystem diagram below show the subsystem impedance parameters in blue. These are subsystem values that are entered on the loadflow worksheet.

2 Page 2 The bus connection table below shows an entry for the main system transformer connected between source bus one and destination bus two. The reactance and transformer tap are the only fields entered. The transformer tap is reference from the subsystem worksheet as show in the formula bar. For an initial run place a value of say 1.02 in the subsystem worksheet cell referenced by the transformer tap cell. The connection status is discussed later and the field is left blank. To add a line to the bus connection table select the source bus and destination bus numbers and add the resistance reactance and line charging fields in per cent on 100MVA base. The diagram below shows the entries for lines between buses 2 and 3 and between buses 2 to 5. The value in cell F27 is the line conductance in pu and the value in cell G27 the line series susceptance in pu. Cells H27 and I27 are shunt susceptance in pu split at each end of the line. These values are used by the loadflow program.

3 Page 3 The busbar table is shown below. This is where the busbar peak load and any generation is added in MW and MVArs. The load value can be adjusted by using the percentage of peak load column. Busbar 1 is the swing bus at which the voltage is specified and the power and reactance power are the variables. The busbar names can be added for clarity. busbar volts loadflow results are displayed in the calculated bus volts column in pu quantities. The calculated bus volts column can be transferred to the subsystem diagram and multiplied by the base volts. The main system 132 busbar volts on the subsystem worksheet diagram cell is selected and the formula bar shows the formula that is transferred from the loadflow worksheetcalculated bus volts column. Reactive plant can be added to busbars such as shown on busbar 6 where 12MVAr is shown. The

4 Page 4 All of the loadflow commands are on the custumize quick access tool bar at the very top of the excel window. The circled i will run the loadflow program. However before running the loadflow program the transformer off-nominal tap ratio has to be added to the transformers. As shown in the adjacent diagram below the transformer symbol place tap= in the left cell and a tap number in the adjacent cell. The transformer tapchanger ratio icon is then selected on the quick access toolbar. tapchanger_ratio bus_volts_and_loadflow

5 Page 5 The transformer tapchanger ratio userform is displayed with two options a main system transformer or a subsystem transformer. The main system transformer is selected in this case and the tap position cell below the transformer symbol is added to the userform. The off-nominal tap ratio formula will be added to the right of the tap position. The off-nominal tap ratio has to be transferred back to the loadflow worksheet as shown in the formula bar below. The off-nominal tap operation needs to be repeated for all subsys- tem transformers. The transformer voltage regulation now needs to be added to the transformers. The transformervr table is selected on the quick access toolbar and userform completed as shown below.

6 Page 6 The transformer voltage regulation table is added as shown in the diagram adjacent below the transformer tap position. The tapchanger command has been highlighted in red and the per cent error shows that about five tapchange operations are required. The setpointv in kv is the voltage on the loadline. The error is the difference between the setpointv and the busbar volts expressed as a per cent of nominal volts. The tap position needs to be raised to tap 7 before the tapchanger raise command is removed. The error need to be below 0.9 percent to be in the deadband. The three diagrams show the tap position adjusted to show the difference tapchanger commands. The load drop compensation on the main system transformer only boosts the voltage to 105 percent of nominal. This is because generation in the subsystem can be kept at a voltage below 110 percent.

7 Page 7 The subsystem worksheet diagram with all of the transformer tapchangers and voltage regulation added is shown below. The subsystem substations loading is peak load with the subsystem transformer boosting the distribution system voltage to 105 per cent. A 12MVAr capacitor is in service at Kaputar 66kV busbar. The subsystem substation loading is adjusted to light loading conditions at about 30 per cent of peak load with the subsystem transformer regulating the distribution system voltage to 100 per cent. The Kaputar 12MVAr capacitor has been left in service. This has the affect at light loading of cause the main system transformer to be near the tapchanger limit (tap 2). The main system voltage can be above 105 per cent and this would cause the tapchanger to reach its limit. The control system engineer is responsible for keeping the tapchangers within limits and the capacitor would need to be remove from service for light loads. Capacitor are entered into the loadflow program in the busbar table on the loadflow worksheet. The capacitor MVArs at busbar column required the nominal rating of the reactive plant. The nominal rating is the same value as the susceptance in per cent for 100MVA base. Multiplier top and bottom by V b = B * V 2 b / V b * I b *100 = MVAr rating / 100 MVA * 100 = MVAr rating %B susceptance = B/B base *100 = B / I b /V b *100 = B*V b /I b * 100