System for voltage regulation, control protection and monitoring of state of changers under power transformer load, voltage regulators, capacitor banks and similar

Abstract

System for voltage regulation, control, protection and monitoring of state of changers under power transformer load, voltage regulators, capacitor banks and similar”, is preferably applied to power transformers ( 1 ) and voltage regulators and similar, through a microprocessor ( 2 ) it provides innovating functions that make up for deficiencies of currently existing voltage relays, highlighting pre-programming, by means of man-machine interface such as, for example, keyboard and display ( 3 ), or serial communication ( 4 ) of several settings and different voltage regulation parameters, each adequate setting for a certain prevailing load condition, performed by means of dry contact inputs ( 5 ), contact outputs ( 6 ) destined for self-diagnosis signaling and alarms ( 7 ); voltage regulation allows parameter selection to be made by means of daily and hourly programming through an internal clock ( 8 ); said system (S) also has the non-volatile memory function ( 9 ); CDC blocking function ( 10 ) that directly acts on the motorized drive ( 11 ) of the CDC; and maintenance assistant functions of the changer under load ( 10 ).

1 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, preferably applies to power transformers ( 1 ) and voltage regulators and similar, used in electric power generation, transmission and distribution systems, usually changing the load output voltage value ( 13 ) in relation to input voltage ( 14 ), with windings equipped with taps ( 15 ), connected to an Onload Tap Changer (CDC) that makes tap change with energized transformer (T) and connected load, to that end the CDC has a motorized drive mechanism (M) that obeys input signals to raise and lop the tap ( 16 ); to perform the voltage regulator function, output voltage measurement is made ( 17 ) by means of a power transformer (PT) and load current (′ 17 ) by means of a current transformer (CT), characterized by proposed regulation system (S) consisting of a microprocessor ( 2 ) that with said voltage and current measurements ( 17  and  17 ′) provides new functions and features, allowing to be programmed by means of a man-machine interface such as, for example, key board and display ( 3 ), or serial communication ( 4 ) several settings and different voltage regulation parameters, each setting suitable for a given prevailing load condition, performed by means of dry contact inputs ( 5 ), output contacts being ( 6 ) destined for self-diagnosis and alarm signaling ( 7 ); voltage regulation allows parameter selection to be made by means of daily and hourly programming through an internal clock ( 8 ); said system (S) also has the non-volatile memory function ( 9 ); CDC blocking function ( 10 ) that directly acts on CDC motorized drive ( 12 ); tap maintenance assistant function, CDC maintenance assistant function; smart programming and correct synchronism and programming signaling function. 
     
     
         2 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by the regulation system (S) proposed herein having six or more regulation parameter sets adjustable in independent manner, each set having parameters described below:
 Desired rated voltage in load;   Voltage variance percentage allowed over and below rated voltage;   Timing adjustment for CDC actuation in order to correct voltage;   Choice of linear or reverse timing type;   Parameters referring to voltage drops in the line.   
     
     
         3 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 2  characterized by selecting the voltage regulation parameter set used in system operation to be made by means of dry contact inputs ( 5 ). 
     
     
         4 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 3  characterized by combination of open and closed contacts in these inputs in order to indicate to the voltage regulation system which parameter set is to be used. 
     
     
         5 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 4  characterized by the combination of open and closed contacts in these inputs being able to be encoded in decimal manner, an input contact directly corresponding to each parameter set. 
     
     
         6 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 5  characterized by the combination of open and closed contacts in these inputs being able to be encoded in binary manner, with the combination of input contact state forming a number that corresponds to the respective parameter set. 
     
     
         7 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 6  characterized by the voltage regulation system allowing regulation parameter set selection used in system operation to be made by means of a daily and hourly programming, consisting of weekdays and time ranges for activating each parameter set; for each parameters set weekdays are selected (Sunday-Saturday), in addition to the time range defined by range start hour, minute, and second and end hour, minute and second; within selected days when the set will be used for voltage regulation; based on its real-time internal clock ( 8 ) that keeps day, week, month, year, weekday, hour, minute and second information. 
     
     
         8 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 7  characterized by based on daily and hourly programming made by voltage regulation system making the choice of regulation set to be used at present time. 
     
     
         9 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by proposed voltage regulation system allowing independent CDC operation timing values to be adjusted for conditions in which voltage is over or below allowed tolerance margins. 
     
     
         10 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by proposed system allowing, in addition to tolerance margins for onload voltage, programming several CDC voltage deviation ranges (for each deviation arrange one time setting is programmed for increasing voltage and one setting for reducing voltage). 
     
     
         11 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 10  characterized by allowing a customized timing mode, in which operation timer decreases precisely as desired as voltage deviation increases. 
     
     
         12 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by regulation system proposed herein having a mass memory function, in which measurement values made by the system are recorded in non-volatile memory ( 9 ) obeying the programmed interval by the operator. 
     
     
         13 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 12  characterized by the function having features:
 variables to be recorded may be freely chosen by the operator among all measured or calculated variables by the system, such as measured voltage in transformer, onload voltage, load current, powers, frequency, power factor, CDC tap position;   for each selected variable for recording by user, recording mode may be chosen among the options: instantaneous value (measured value at recording time), arithmetic mean of measurements made since the last recording in memory, quadratic mean of measurements made since the last recording in memory, maximum amount of measurements since the last recording in memory or minimum value of measurements made since the last recording in memory;   the same measurement can be selected for recording in memory in several different modes, for example, the “onload voltage” variable may be selected for recording with   instantaneous value, mean value, maximum value and also minimum value.   
     
     
         14 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by preventing certain failures in CDCs proposed voltage regulation system having the “CDC blocking” function. 
     
     
         15 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 14  characterized by the tripped CDC blocking function, based on measuring CDC tap position, having the following features:
 It considers normal CDC operation the condition in which the voltage regulation system issues a signal to the CDC to raise or lower tap position and after that the CDC tap position undergoes a change in the same direction of sent signal (raise or lower the tap);   It considers that the CDC is tripped if a change in measured CDC tap occurs without the voltage regulation system having previously sent a signal requesting this change, or further if the voltage regulation system issues a signal for tap change and the CDC makes a change in the opposite direction to issued signal;   In order to prevent false tripped CDC alarms, due, for example, to tap changes manually requested by the operator directly on the motorized drive panel, it may be programmed in the voltage regulation system to only consider that the CDC is tripped if several successive tap changes occur, being the number of tap changes for considering that the CDC is tripped programmed by the operator;   When a tripped CDC situation is detected, as described above, an output contact is activated in the voltage regulation system; this contact can be connected by the operator in order to switch off the CDC drive motor supply, forcing him in this manner to stop tap changes and avoid the consequences such as forced transformer switch off or damage to loads connected to it.   
     
     
         16 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by in order to allow quick detection of inoperative changer, before voltage reaches too high or too low values, the voltage regulation system proposed herein has the “inoperative CDC detection” function”. 
     
     
         17 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 16  characterized by characterized by the following features:
 It is considered that CDC is found in normal condition if the voltage regulation system issues a signal to the CDC to raise or lower tap position and after that the CDC tap position undergoes a change in the same direction as sent signal (raise or lower the tap, detected by measuring CDC position) within the time period programmed by the operator;   It is considered that the CDC is inoperative if the voltage regulation system sends a signal to the CDC to raise or lower tap position, and after that CDC tap position does not undergo a change within the time period programmed by the operator (the change is detected by measuring CDC position);   In order to prevent false inoperative CDC alarms, such as, for example, during maintenances in which the CDC is kept inoperative, the operator may enable or disable this function in the voltage regulation system;   When inoperative CDC situation is detected, as described above, the condition is signaled by an alarm indication in the voltage regulation system that, can also activate an output contact for remote alarm indication;   The alarm indication will remain active in the voltage regulation system and the alarm contact will remain activated until the operator makes the manual alarm reset informing the voltage regulation system that he is already aware of the problem;   the operator may also enable the voltage regulation system to make the inoperative CDC alarm automatic reset, the automatic reset being made as soon as a new CDC tap change attempt is successfully made.   
     
     
         18 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by proposed voltage regulation system having the CDC Maintenance Assistant function. 
     
     
         19 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 18  characterized by the CDC Maintenance Assistant function having the following features:
 Total CDC operation number count, since it is going into operation, by measuring the CDC position. Every time the measured tap position changes the CDC operation counter increases the number of required operations for making referred tap change (one operation in most cases, there may also be several operations in case of CDCs with intermediate positions);   Partial CDC operation number count since the last inspection or maintenance made by the operator, by measuring the CDC position, in similar manner to above-described total operation number count;   Initial and partial operation counters stored in the voltage regulation system non-volatile memory; initial operation numbers of both counters (total and partial) adjustable by the operator, allowing utilization of the CDC, also in old equipment, in addition to allowing operation count correction in the event of a failure in the voltage regulation system.   A warning indicating that the operation number for CDC maintenance when partial operation count (since the last maintenance) has exceeded the number of operations for maintenance adjusted by the operator; with the possibility of programming an output relay for remote signaling;   CDC average daily operation count, made by dividing the sum of total operation number in the last X days by the number of X days, X being a time window programmable by the operator between one and three hundred and sixty five days;   Approximate Y remaining time calculation for reaching the number of operations for maintenance, according to the ratio below:   
       
         
           
             
               Y 
               = 
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               No 
                               . 
                               
                                   
                               
                                
                               of 
                             
                              
                             
                                 
                             
                              
                             operations 
                              
                             
                                 
                             
                              
                             for 
                              
                             
                                 
                             
                              
                             maintenance 
                           
                           ) 
                         
                         - 
                       
                     
                   
                   
                     
                       
                         ( 
                         
                           partial 
                            
                           
                               
                           
                            
                           operation 
                            
                           
                               
                           
                            
                           counter 
                         
                         ) 
                       
                     
                   
                 
                 
                   Average 
                    
                   
                       
                   
                    
                   daily 
                    
                   
                       
                   
                    
                   operations 
                 
               
             
           
         
         Warning indication that the number of operations for CDC maintenance will be reached within Y days, when remaining time calculation for Y maintenance is lower than the number of advance days for maintenance warning adjusted by the operator between zero and three hundred and sixty five days, with the possibility of programming an output relay ( 6 ) for remote signaling; 
         The maintenance warning indication will remain active in the voltage regulation system and the alarm contact(s) will remain activated until the operator makes a manual warning reset, informing the voltage regulation system that maintenance has already been performed. Once the reset is made, partial operation count since the last maintenance is zeroed and warnings are switched off for maintenance. 
       
     
     
         20 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claims 18  characterized by in addition to the above-listed, the “CDC maintenance assistant”, further having the following features:
 Every time a CDC tap change is made, detected by tap measuring, the voltage regulation system takes a load current measurement at the time of tap change and raises it to the power of 2; obtained value is added to the already existing one in the total changed current sum recorder since the beginning of the CDC operation, which is kept in a non-volatile memory in the voltage regulation system;   As described above, every time a CDC tap change is made, detected by means of tap measurement, the voltage regulation system takes the load current measurement at the time of the change and raises it to the power of 2; obtained value is also added to the already existing value in another recorder, this one dedicated to the sum of partial change current since the last CDC maintenance, which is kept in the voltage regulation system non-volatile memory;   Initial values of both changed current sum records (total and partial) adjustable by the operator, allowing using the CDC maintenance assistant also in old equipment, in addition to allowing sum corrections in the event of a failure in the voltage regulation system;   A warning indication that the changed current sum for CDC maintenance has been reached when the partial sum record (since the last maintenance) exceeds the sum value for maintenance adjusted by the user, with the possibility of output relay programming for remote signaling;   Daily increment average calculation in the current sum changed by the CDC, made by dividing the increment in the current sum in the last W days by VV number of days, W being a time window programmable by the operator between one and three hundred and sixty five days;   Approximate remaining time Z calculation for reaching sum value for maintenance, according to the formula:   
       
         
           
             
               Z 
               = 
               
                 
                   
                     ( 
                     
                       Sum 
                        
                       
                           
                       
                        
                       value 
                        
                       
                           
                       
                        
                       for 
                        
                       
                           
                       
                        
                       maintenance 
                     
                     ) 
                   
                   - 
                   
                     ( 
                     
                       partial 
                        
                       
                           
                       
                        
                       sum 
                        
                       
                           
                       
                        
                       recorder 
                     
                     ) 
                   
                 
                 
                   Daily 
                    
                   
                       
                   
                    
                   increment 
                    
                   
                       
                   
                    
                   average 
                    
                   
                       
                   
                    
                   in 
                    
                   
                       
                   
                    
                   sum 
                 
               
             
           
         
         A warning indication that the number of operations for CDC maintenance will be reached within Z days, when remaining time for maintenance Z calculation is lower than the number of advance days for maintenance warning adjusted by user between zero and three hundred and sixty five days, having the possibility of programming the output relay for remote signaling; 
         The maintenance warning indication remains active in the voltage regulation system and alarm contact(s) will remain activated until user makes manual warning reset, informing the voltage regulation system that maintenance has already been performed, Once this reset is made, the partial sum recorder of changed currents since the last maintenance is zeroed and maintenance warnings are switched off. 
       
     
     
         21 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by proposed system performing the “smart master-commanded-individual” function. 
     
     
         22 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 21  characterized by smart master-commanded-individual programming having the following features:
 Inhibiting the possibility of selecting any transformer as master if there is already any other transformer selected as master. In this event in which there is already a selected master, all other transformers can only be selected as commanded or individual;   Inhibiting the possibility of selecting any transformer as commanded if there is not yet a transformer selected as master. In this event, in which there is not yet a selected master, all other transformers can only be selected as master or individual;   Inhibiting the possibility of changing the programming of the transformer selected as master to commanded or individual if there is one or more transformers selected as commanded, in this way avoiding that there will be transformers in commanded without there being a master.   
     
     
         23 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 22  characterized by in addition to what has already been commented above, the “smart master-commanded-individual programming” function having as features:
 Rejecting the selection change of any transformer from individual mode to commanded mode if its measured tap position is not equal to a master transformer one, avoiding thereby unnecessary alarm issuing due to tap discrepancy;   If it is insisted in the attempt to program a transformer as commanded without its measured tap position being equal to a master transformer one, in addition to rejecting this programming the voltage regulation system will issue an “invalid parallelism programming” alarm, warning the operator about the situation.   
     
     
         24 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by proposed system performing the “smart bank-individual phase programming” function. 
     
     
         25 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 24  characterized by smart “bank-individual phase” programming having the following features:
 Inhibiting the possibility of selecting any phase of a single-phase transformer bank for operation in the “individual phase” mode if the bank is selected for operation in parallel with other banks, that is, if the bank is selected in master or commanded mode.   Inhibiting the possibility of selecting any phase of a single-phase transformer bank for operation in the “individual phase” mode if the bank is selected for operation in automatic mode, that is, with CDCs being controlled according to load voltage and   current measurements.   Only allowing selecting one of the phases of a single-phase transformer bank for operation in “individual phase” mode if the bank is selected for operation in individual and manual mode.   Automatically changing any phase of a single-phase transformer bank that is selected for operation in “individual phase” mode to “bank” mode if the bank is selected for operation in master, commanded or automatic mode.   
     
     
         26 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 25  characterized by in addition to as commented above, the “smart bank-individual phase programming” having the following features:
 Rejecting the selection change of any phase of a transformer from “individual phase” mode to “bank” mode if its tap position is not equal to phase A one, adopted as a reference, avoiding thereby unnecessary alarm issuing due to tap discrepancy.   If it is insisted in the attempt to program a phase in the “bank” mode without its tap position being equal to that of phase A, in addition to rejecting this programming the voltage regulation system will issue an “invalid parallelism programming” alarm, warning the operator about the situation.   
     
     
         27 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 1  characterized by proposed system further performing the “correct synchronism and programming signaling” function. 
     
     
         28 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claim 27  characterized by the “correct synchronism and programming signaling” function giving permission for closing the circuit breaker that connects the winding of each transformer in parallel with the others, allowing automating the synchronism checking function before connection in parallel. To that end, correct synchronism and programming signaling can be made through a serial communication network or further by activating an output signal for each transformer. 
     
     
         29 . SYSTEM FOR VOLTAGE REGULATION, CONTROL, PROTECTION AND MONITORING OF STATE OF CHANGERS UNDER POWER TRANSFORMER LOAD, VOLTAGE REGULATORS, CAPACITOR BANKS AND SIMILAR, in accordance with  claims 27  characterized by the “correct synchronism and programming signaling” function having the following features:
 In selected transformers in individual mode no correct synchronism and programming signaling is issued, preventing transformers in this condition from being electrically connected in parallel to others (because there is no, in this case, a commitment to keep equal tap position to master transformer position);   In each transformer selected in commanded mode it is signaled that synchronism and programming are correct only if transformer tap position is equal to master tap position and if current individual master-commanded situation is valid (there is only one master transformer);   In selected transformer is master mode it is always signaled that synchronism and programming are correct, because tap position of this transformer as taken as a reference for selected transformers as commanded and is considered correct by default.