Abstract:
A portable auxiliary cooling system for cooling electrical power transformers is provided with an intelligent controller that is programmable for functioning in a plurality of different operational modes. A plurality of sensors monitor cooling system equipment operation, transformer top oil temperature and transformer current. The cooling system controller includes a user-interface panel, a modem and/or other digital communications circuitry, data storage memory and a computer or microprocessor that performs real-time heat removal rate computations and automatically controls heat pump and fan operation in response to the computed results. System performance data, sensor readings and alarm condition indications are indicated on a user-interface panel and may also be provided to a remote station or operator using conventional digital communication facilities.

Description:
This invention relates generally to cooling systems for electrical power transformers and, more particularly, to an intelligent controller for a portable auxiliary cooling system used for cooling electrical power transformers. 
   BACKGROUND OF THE INVENTION 
   Power transformers utilized throughout the electrical power generation, transmission and distribution industry are equipped with a variety of cooling systems designed to remove excess heat generated during transformer operations. Although these installed cooling systems typically have adequate cooling capacity to maintain transformer winding temperatures within acceptable limits, auxiliary cooling may become necessary during periods of extreme environmental conditions and/or abnormally high power loading conditions. These auxiliary cooling systems may be employed to prevent heat-related damage to transformer windings as well as to increase the working life span of a transformer by reducing the time the transformer is operated at elevated temperatures. 
   Conventionally, such auxiliary cooling systems consist of a basic heat exchanger that is either turned on/off manually or automatically by a conventional thermostat mechanism. Even in those instances where the auxiliary transformer cooling system is simply left running continuously, the operator often does not know if the auxiliary cooling system is performing adequately. Consequently, it would be highly desirable to have an auxiliary transformer cooling system that would provide a reliable means of removing heat from the transformer, and alert the operator if the auxiliary cooling system fails to perform properly or the transformer heat load exceeds the heat removal capacity of the auxiliary cooling system to provide adequate cooling to the transformer windings. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention provides an auxiliary “add-on” cooling system for electrical power transformers. Basically, the auxiliary cooling system of the present invention comprises a programmable “intelligent” controller and a portable compact heat exchanger unit that operate together as a stand-alone heat removal system which may be used, for example, as an auxiliary “add-on” cooling system for electrical power transformers. The intelligent controller of the present invention includes a programmable computer or micro-controller that turns on the auxiliary cooling system at an operator-selected top oil temperature and/or a set of anticipatory conditions (such as a combination of top oil temperature, ambient temperature, time-of-day, and/or transformer phase current). During auxiliary cooler operation, the micro-controller performs real-time heat removal rate computations, while the system provides local and remote indication of cooler heat removal rates and alerts the operator if the auxiliary cooler is failing to achieve the predicted heat removal rate for the ambient conditions. This feature enables detection of cooling malfunctions—such as fouling of heat transfer surfaces—which are not detectable by other means. A plurality of sensors are used for monitoring various operational conditions of the cooling system and the transformer (e.g., top oil temperature and phase current) and the intelligent controller provides warning and alarm condition indications in response to operator programmable set-points for the monitored conditions. The micro-controller also alerts the operator of sensor degradation and failures. Onboard memory is provided for storing operating history, cooling system performance, a historical plot of transformer operating temperatures, and all auxiliary cooling system events and alarm conditions. Digital communications circuitry is included for using conventional landline or wireless links to provide a remote configuration capability, and to permit remote measuring and tracking of cooling system performance and sensor readings as well as providing alarm condition notifications. Additionally, the auxiliary cooling system is equipped with an independent temperature-sensitive switch that will initiate fail safe operation of the auxiliary cooling system by bypassing the automatic features of the system in the event the transformer oil temperature exceeds a factory-set temperature. 
   Automated control of the cooling processes using a programmable “intelligent” controller ensures that a power transformer may be operated during peak load conditions with a high degree of confidence and assurance. The programmability of the controller permits tailoring the cooling process to specific or unique transformer cooling application requirements. A further beneficial aspect of the intelligent controller is that it permits the cooling system to be easily integrated with existing electrical power substation control systems and other data acquisition systems. In addition, the intelligent controller of the present invention enables real-time remote monitoring of performance data and system operational status and provides immediate dispatch of warning and alarm notifications of predetermined system conditions. A still further beneficial aspect of the present invention is that the cooling system may be programmed to automatically perform periodic cycling of fan and pump mechanisms during periods of extended idleness to prevent the motors/pump from seizing. These and other features disclosed herein enhance the overall utility and versatility of the cooling system and enable it to be easily integrated into existing electrical power substation control systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is schematic diagram of the auxiliary cooling system; 
       FIG. 2  is a flow diagram of an example control program executed by the intelligent controller of the cooling system; and 
       FIG. 3  is a graph illustrating example heat load removal capacity of the transformer cooling system of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The cooling system of the present invention may be used as an auxiliary or “add-on” cooling system in conjunction with existing transformer cooling systems. A general schematic diagram illustrating an example arrangement of the cooling system and controller arrangement of the present invention for providing intelligent cooling for an electrical power transformer is shown in FIG.  1 . Basically, the cooling system ( 100 ) of the present invention comprises an “intelligent” controller  101 , a compact heat exchanger unit  102  and a transformer coolant (typically oil) pump  103  that are packaged together in a portable compact configuration so as to function as a stand-alone heat removal system. In a conventional manner, the pump  103 , pumps coolant out of the power transformer  115  via piping  104 , through an orifice plate  105 , through heat exchanger  102  and back to the power transformer  115 . The orifice plate  105  is used to determine coolant flow in the closed system. 
   The “intelligent” controller  101  comprises at least a computer or micro-controller (CPU  110 ), a modem and/or conventional digital communications circuitry  111 , data storage memory  112  and a control/user-interface  113 . Transformer oil pump  103  is provided with a current sensor (CT 1 ), orifice plate  105  is provided with a differential pressure sensor (DP 1 ), heat exchanger  102  is provided with fan  114  and a fan current sensor (CT 2 ). A transformer phase current sensor (CT 3 ) and a transformer coolant temperature sensor (TS 1 ) are provided for the transformer being cooled ( 115 ) and at least two other temperature sensors (TS 2  and TS 3 ) are used to measure inlet and outlet coolant temperatures of heat exchanger  102 . An ambient air temperature sensor (TS 4 ) is also provided. Signals from all of the sensors are provided to CPU input ports  113  of controller  101 . 
   In an exemplary embodiment, auxiliary cooling system  100  operates in either one of two primary modes of operation: “Manual” mode and “Automatic” mode. (These “modes” of operation refer only to the manner in which the pump and fan operations are initiated—since all of the “intelligent” computation, indication and alarm features are always operational and available.) In either mode of operation, alarm and system performance data may be displayed both locally and remotely. To accommodate the operating requirements of a particular transformer  115  with which cooling system  100  is used to cool, an operator may program system control, warning and alarm condition set-points either locally via user interface  113  or from a remote location using a configuration utility or other conventional communications software for communicating with controller  101  over a digital communications link via onboard modem  111 . Onboard communications circuitry  111  is provided for connecting to and utilizing telephone lines and/or other available digital communications medium (such as, for example, other landline links, WI-FI wireless, RS-232, RS-485 or Ethernet lines). Sensor readings, alarm indications and performance data may also be provided to a remote recipient. Historical event, alarm and warning condition data are stored in controller onboard memory  112  and may be used by CPU  110  for conducting system performance analysis. Controller  101  may also be connected into a SCADA monitoring system via, for example, RS-485, Ethernet, or electrical relay connection (not shown) for triggering an alarm mechanism arranged to selectably indicate different types of cooling system faults such as: pump failed, fan failed, low flow, no flow, loss of power, flow sensor failed, or a particular thermal performance alarm condition. 
   Auxiliary cooling system  100  may also be remotely controlled using known communications protocols (e.g., Modbus or command line protocols) via modem, RS-232, rs-485, Ethernet, or cellular communication path to enable remote starting, stopping, programming, and monitoring. Differing levels of access protection for remote programming/control may be easily implemented using, for example, a conventional password based authorization scheme. 
   A program flow diagram for an example intelligent control software process executed by the intelligent controller of the cooling system (when the system is in “Automatic” mode) is shown in FIG.  2 . Using control panel/user interface  113 , an operator inputs values for the transformer coolant (top oil) temperature, T on , at which the cooling unit is to begin cooling, and a top oil temperature, T off , at which the cooling unit returns to standby. Other operational control conditions, set-points and parameter values such as, for example, a minimum operating current for pump  103  or a value for a maximum current for fan  114 , may be programmed and input by the operator via the control panel/user interface front as well. 
   As indicated at  201  of  FIG. 2 , controller  101  continually monitors the transformer coolant temperature from sensor TS 1  to determine whether it is greater than the operator selected set-point condition temperature, T on , for initiating cooling. At the same time, controller  101  ( FIG. 1 ) also continually monitors the transformer temperature and transformer phase current sensors (TS 1  and CT 3 ) and determines whether certain preprogrammed “anticipatory conditions” are satisfied (e.g., specific predetermined combinations of ambient temperature, transformer load, time of day, etc.). If the transformer coolant fluid temperature measured by TS 1  is greater than T on , or one or more other preprogrammed anticipatory conditions exist (block  202 ), the pump and heat exchanger fan are started, as indicated at block  203 . 
   Next, as indicated at  204 , controller  110  checks a pump current reading from current sensor CT 1  against a predetermined operator-programmed set-point for a minimum current value and a fan current reading from current sensor CT 2  is also checked against a predetermined operator-programmed set-point for a maximum current value. If the sensed fan and/or pump currents are beyond the preprogrammed minimum and maximum values, the fan and/or pump operation is terminated (block  205 ) and a fault alarm condition indication is generated (block  206 ). Otherwise, controller  110  next proceeds to check the coolant flow rate measured by coolant flow rate sensor DP 1 . If after expiration of a preprogrammed delay period (for example, 30 sec), the coolant flow rate as measured by DP 1  is not within a range between preprogrammed minimum and maximum values (block  207 ), then the cooling operations are terminated (block  205 ) and a fault alarm condition indication is generated (block  206 ). If, after expiration of the delay period, the coolant flow rate as measured by DP 1  is within the predetermined range, then controller  110  computes a value for a Measured Heat Removal rate (MHR). The value of MHR is computed using conventional techniques from heat exchanger coolant inlet and outlet temperature data obtained from sensors TS 2  and TS 3  and the coolant flow rate obtained from sensor DP 1 . 
   After computing a value for MHR, controller  110  determines the heat removal rate of the heat exchanger for the existing ambient conditions by computing a Calculated Heat Removal rate (CHR). The value for CHR is computed by conventional techniques using data obtained from heat exchanger temperature sensor TS 2 , ambient air temperature sensor TS 4  and the coolant flow rate from sensor DP 1 , as indicated at block  209 . As indicated at block  210 , the controller next determines whether the value of the ratio of MHR to CHR lies within a predetermined/preprogrammed “acceptable” range (for example, between 0.8 and 1.2). If the ratio is not within the preprogrammed range of acceptable values, a alarm condition indication is generated (block  211 ). 
   If the computed ratio is within the preprogrammed range, then as indicated at block  212 , the transformer coolant (top oil) temperature sensor reading from TS 1  is compared against the operator selected set-point condition temperature, T off . If the reading from TS 1  is greater than T off , then cooling operation continues and the sensor monitoring and heat removal computation functions (blocks  204  to  211 ) are performed continually until the reading from TS 1  falls below the preset T off  value. If the reading from TS 1  is lower than T off , and the system is not being controlled by an anticipatory condition ( 202 ), then the cooling is stopped and the fan and pump operations are shut down, as indicated at block  213 . Controller  110  then continues to monitor the transformer coolant temperature from TS 1  and compares it against the preset T on  value to determine if cooling operations should be reinitiated, as indicated back at block  201 . 
   Although not explicitly illustrated in  FIG. 2 , routines for controlling onboard digital communications circuitry (or modem)  111  for sending sensor readings, alarm indications and performance data to a remote station may also be integrated into the control software processes of FIG.  2 . For example, blocks  204  through  211  include an additional step or steps for sending sensor data, alarm indications/warnings, computed system performance data and other information to a remote facility/recipient via onboard communications device  111 . Similarly, routines may be included for programming the intelligent controller to receive data and commands from a remote facility for setting cooling system operating parameter data, selecting operating mode and set-point values for triggering various cooling system operations. Historical operating and performance data for the cooling system and the transformer may also be stored in the controller memory. Such data my be used, for example, to: i) compare an actual cooling performance of the cooling system versus an expected performance over time; ii) assess the effectiveness of the cooling system; iii) record times, duration and temperature profile of the transformer whenever it is detected as operating at predetermined elevated temperatures; and iv) determine when specific monitored events occurred. In addition, although not explicitly illustrated in  FIG. 2 , controller  101  of auxiliary cooling system  100  may also be programmed with a routine for initiating periodic operation of heat exchanger fan  114  and/or coolant pump  103  during extended idle periods to prevent seizing of moving machinery components. A programmer of ordinary skill will appreciate that software routines for performing such tasks may be easily implemented without undue experimentation using conventional programming techniques. 
   A graph illustrating the potential heat load removal capacity for an example transformer cooling system as presented above is presented in FIG.  3 . Heat load removal rates are illustrated for five different coolant pumping rates of 10 GPM, 20 GPM, 30 GPM, 40 GPM and 50 GPM. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.