采用可編程邏輯控制器直接負荷控制.docx

中原工學院畢業(yè)設(shè)計（譯文）Direct load control using a programmable logic controllerAbstractThis paper presents a fully automated programmable logic controller (PLC) based direct load control system. Unlike other existing systems, it gives the consumer the privilege to share in the load shedding policy. After receiving the warning alarm from the PC at the utility control center, the consumer has the chance to switch off any desired load. If he does not act within a predefined period, the PLC will take the required action based on the scenario decided by the utility. Having the consumer decide his own priorities release the utility company from blind decisions. This is one of the most important contributions of the proposed system. Also the load restoration process is scheduled to avoid the creation of a new peak during reenergization. This is done using 48 built-in timers with an accuracy of less than one cycle (10 ms). The hardware at the customer side may be integrated with a digital energy meter to form a complete energy management system. 1999 Elsevier Science S.A. All rights reserved.Keywords: Programmable logic controller; Load control; Energy management1.IntroductionToday, electric utilities are facing a rapid increase in demand. Increasing the generation capacity and expanding the transmission and distribution networks are not sufficient on long-term basis. Many utilities attempt to use the existing plants and facilities more effectively rather than try to build new power plants. As the cost of producing electricity increases, utilities are extending their activities into the customer domain. In order to increase efficiency, utilities are controlling how electric energy is used. Thus they shift from a supply only viewpoint to demand side activities. These activities and measures are known as demand side management (DSM). The DSM measures are classified into direct load control (DLC) or indirect load control. A great deal of research and development has occurred in the field of DSM and DLC over the last decades. The concepts of DSM and its implementation are well covered in several technical publications 1 5 Several utilities have gained valuable experience by adopting load control methods. The IEEE working group summarizes the results from several ongoing projects at major power systems 6. The survey concludes that significant reductions in demand are possible through control of water heaters and air conditioners. The experiences vary in control strategy and communication methods. Willett et al. analyzed load savings, perceived comfort, and temperature in- creases in an experimental DLC program 7,8. Arntzen et al. presented a low-cost microcomputer-based DLC scheme for a small electric utility 9. The scheme is particularly suitable for small- to medium-sized public utilities. The system is designed around an 8-bit 68HC11 microcontroller and uses an addressing scheme to control up to four loads at each of 4096 sites. The integrated operation of the system has been tested with satisfactory results. This system controls two loads by switching them on and off and uses external relays to read the input signals and control the consumer appliances. However, there is no feedback to check the application of the DLC system and the customer does not play a role in the scheme. Robert Roman and Robert Wilson introduced a radio controlled load management system 10. They used a programmable logic controller (PLC) to control customer loads directly. The system arranges for customer loads to be either on or off. Controllers have been installed for a refrigeration storage customer at three separate locations. Theunits have been in service since 1993. However, the scheme does not enable the customer to participate in the DLC process. It is a one way communication system that does not include internal system feedback. This paper presents a fully automated PLC-based direct load control system. Unlike most existing systems, it enables the consumer to share in the load shedding policy. Load restoration will also have to be scheduled to avoid the creation of new peaks during re energization. This is done using 48 built-in timers with an accuracy of less than one cycle (10 ms). The proposed system hardware and software are described in Section 2 and Section 3, respectively. Section 4 provides a description of the system operation. Implementation and test results are discussed in Section 5. Conclusions are given in Section 6.2 The hardware structureThe system hardware consists of two major components. One is a personal computer at the utility site and the other is a PLC at the consumer location.2.1 The personal computerThe PC is used as a control center that initiates the commands to the PLCs. The commands consist of a set of instructions that deliver the alarm signals and the scenario that is to be executed. The PC serves as a control center for many consumers using the common network protocols. The RS 232 serial interface is used to carry the operating commands to the PLCs at the consumer locations.2.2 The PLCThe programmable logic controller is used to energize or de-energize the loads at the consumer side. It has an input module that receives the commands from the control center as a train of pulses. The PLC software is designed to convert the number of pulses into meaningful information such as the alarm duration and the scenario to be executed. Consumer loads are energized via the PLC output module. The output module of the PLC has eight outputs. One of the outputs is used to sound an alarm and the remaining seven are used to control the consumer loads as shown in Fig. 1. Having larger output modules can increase the number of loads per consumer. The output modules are available in eight, 16 and 32 ports. The PLC used in the experiment had eight ports end was enough to demonstrate the concept of this research. Fig. 2 shows the system setup for N number of consumers.3 System software architectureThe proposed system has two software algorithms. The first software algorithm is written in C+ to enable the utility operator at the control center to select the warning period and the operation scenario. The other software algorithm is written in the native language of the PLC, which is an assembly-like language. It can be converted into a ladder diagram to make it easy to read. The function of the PLC algorithm is to energize and de-energize the loads at the consumer side.The concept of the drum counter has been used in the PLC environment. The drum counter is a very powerful instruction that controls up to 16 loads. Each of the 16 outputs (loads) will be disconnected automatically if the pulse frequency is within a specific range. The frequency will be supplied by the control center based on the desired scenario. Table 1 shows en example of such operations keeping in mind that the input pulses are loaded automatically to the drum counter register via the RS232 link.It should be noticed that more than one load can be disconnected at a time if two lower end upper limits are equal or are overlapping.4 System operationIn order to interact with the hardware control rou- tines, the operator must be provided with an environ- ment where it is easier to view and interact with different parameters in the control loop. The user in- terface uses the visual environment provided by the Visual Basic. The algorithm for the user interface is summarized as follows: The program starts by an inquiry whether to start or to quit. If the intention is to start the routine, then the program asks about the nature of control, i.e. feeder or customer. The associated ID is received from the operator. Next, the control scenario is selected. Based on each scenario, a different control strategy is selected. The appropriate signals are sent to the hardware interface routine. At any stage, the program can be aborted.The operator selects the scenario according the re- quirements of the utility. Once a scenario is selected, a set of predefined signals is transmitted from the master station center to the PLC at the customer side.The PLC takes the appropriate decision according to the incoming signals. It activates the warning system to give the customer the opportunity to participate in the control process. If the customer responds within the warning period by switching off the number of loads the utility has determined, then the alarm will be shut off. If the customer takes no action, then the PLC will take control. The scenarios are designed as follows:one load off cycling scenario; two load off cycling scenario; turn off a certain number of loads scenario and the entire shedding scenario; restoration processor; reset.4.1 One load cycling scenarioThis scenario implies the cycling of the load alternately. As per the schedule of control pulses, the protocol generates signals a certain number of pulses. During the warning period, the customer can participate and switch off any one of the loads. If the customer turns off at least one load, the alarm is turned (off). If the customer does not respond, the PLC will start turning (off) one load at a time for the specified period. This scenario uses a 30 min cycling period; the alarm is then turned off. When the cycling ends, the alarm flashes for 30 s. It should be pointed out that the control center operator could change the 30 min period to any desired value.4.2 Two load cycling scenarioThis scenario requires two loads being off at a time. As per the schedule look-up table, which is stored in the PLC memory, the protocol generates the control signals for two load cycling. If the customer complies by turning off at least two chosen loads, the alarm is turned off. However, if there is no response, the PLC will randomly start turning off two loads for a specific period. When the cycling ends the alarm will flash for 30 s.Serial numberTimeW/o DLCWith DLCLoad118.2258.225All on21:018.2258.225All on31:028.2258.225All on41:038.2258.225All on51:048.2258.225Start DLC61:058.2258.1375L171:068.2258.1375L181:078.2258.1375L191:088.2258.1375L1101:098.2258.1375L2111:108.2258.1375L2121:118.2258.1375L2131:128.2258.1375L2141:138.2258.1375L2151:148.2258.1375L3161:158.2258.1375L3171:168.2258.1375L3181:178.2258.1375L3191:188.2258.1375L3201:198.2254.2875L4 & m1211:208.2254.2875L4 & m1221:218.2254.2875L4 & m1231:228.2254.2875L4 & m1241:238.2254.2875L4 & m1251:248.22518.2875L5 & m2261:258.2254.2875L5 & m2271:268.2254.2875L5 & m2281:278.2254.2875L5 & m2291:288.2254.2875L5 & m2301:298.22522.1375L6311:038.2258.1375L6321:318.2258.1375L6331:328.2258.1375L6341:338.2258.1375L6351:348.2258.225End DLC4.3 Control any number of loads and the entire shedding scenarioSometimes the utility requires the turning off of some or all the loads. It sends a specific signal for this scenario, which is to energize the alarm for ten minutes. The alarm is different from the one and two cycling processes. It is a continuous audio signal. It starts to turn off the loads in sequence one at a time or as required. In the case of an emergency, the utility can apply the entire shedding scenario by switching off all the loads.4.4 Restoration processThe restoration process is programmed and arranged to avoid the creation of a second peak. The loads are thus turned (on) in a timely sequential manner.4.5 ResetTo reset the system during a cycling mode, the utility sends a 1 min continuous pulse.5 System implementation and resultsA prototype of the proposed DLC system has been implemented in the laboratory. Fig. 3 shows the block diagram of the DLC system. It is used to control different loads. Four scenarios are applied to control these loads. The experimental setup arranges the restoration process to avoid the creation of a new peak 11.5.1 Single load cycling scenario implementationTwo 1/4 hp motors and six 100 W lamps are used. The loads are connected to the output of the PLC. After executing the DLC system software, the demand is recorded at each interval. The control period of the loads will not be more than 3 h at a time. However it can be programmed based on the utility policy. Table 2 shows the demand following the application of the single load cycling scenario. Fig. 4 shows the demand variations over the time. The effect of the starting current is seen at the restoration process. Taking the moving average of actual load curve after applying the DLC will show the demand variations over the time period. At point a, motor 1 and load 4 are off. The rest of the loads are (on). At the next cycle, motor 1 and load 4 are (on). This is shown at point b; the rest of the loads are (on). At point c, the cycling process is finished. All the loads are (on). The effect of the starting current is shown at points b and c. Motor 1 and motor 2 create these two spikes at b and c, respectively. The two loads cycling scenario was also applied with similar results.5.2 The entire shedding scenarioThe entire shedding scenario was tested using the prototype DLC. Fig. 5 shows the graphical display of the data. Table 3 shows the data when applying the entire shedding scenario.The DLC is applied at point a in Fig. 5. All the loads are off at that instant. Then, after a period of time determined by the utility, the restoration process starts. This is happening at point b, c, d, to point f. The two motors are (on) at point d and f, respectively.SerialTimeW/o DLCW DLCLoad118.2258.225 All on21:018.2258.225 All on31:028.2258.225 All on41:038.2258.225 All on51:048.2258.225 Start DLC61:058.2250.000 All off71:068.2250.000 All off81:078.2250.0875 All off91:088.2250.0875 Restore101:098.2250.0875 All off except L1111:108.2250.0875 All off except L1121:118.2250.0875 All off except L1131:128.2250.175 All off except L1 & L2141:138.2250.175 All off except L1 & L2151:148.2250.175 All off except L1 & L2161:158.2250.175 All off except L1 & L2171:168.2250.175 All off except L1 & L2181:178.2250.175 All off except L1 & L2191:188.2250.2625 All off except Ll & L2 & L3201:198.2250.2625 All off except L1 & L2 & L3211:208.2250.2625 All off except L1 & L2 & L3221:218.2250.2625 All off except L1 & L2 & L3231:228.2250.2625 All off except L1 & L2 & L3241:238.22518.200 L5 & M2 & 16251:248.2254.200 L5 & M2 & 16261:258.2254.200 L5 & M2 & 16271:268.2254.200 L5 & M2 & 16281:278.2254.200 L5 & M2 & 16291:288.22522.1375 L6301:298.22522.1375 L6311:038.2258.1375 L6321:318.2258.1375 L6331:328.2258.1375 L6341:338.2258.225 End DLC6 ConclusionsA DLC system has been designed, implemented and evaluated. The proposed DLC system meets the objective that it was designed for.The proposed DLC system is a flexible tool that can be easily implemented and adapted to future applications. The system offers an improved solution due to its reliability, flexibility and efficiency. It allows consumers to control loads based on their priorities. The warning period is adjustable; load restoration does not generate a new peak. The hardware at the customer side may be integrated with a digital energy meter to form a complete energy management system.It is worth mentioning that the scaling of the proposed system depends on the cost, customer acceptance and discomfort. Future work will address these