Electric Power System.doc

Unit One Electric Power System Text A Components of Power Systems Modem power systems are usually largescale，geographically distributed，and with hundreds to thousands of generators operating in parallel and synchronouslyThey may vary in size and structure from one to another, but they all have the same basic characteristics： (1)Are comprised of threephase AC systems operating essentially at constant voltageGeneration and transmission facilities use threephase equipmentIndustrial loads are invariably three-phase；single-phase residential and commercial loads are distributed equally among the phases SO as to effectively form a balanced threephase system (2)Use synchronous machines for generation of electricityPrime movers convert the primary energy(fossil，nuclear, and hydraulic)to mechanical energy that is，in turn，converted to electrical energy by synchronous generators (3)Transmit power over significant distances to consumers spread over a wide areaThisrequires a transmission system comprising subsystems operating at different voltage levels The basic elements of a modem power system in USAare shown in Fig61Electric power is produced at generating stations(GS)and transmitted to consumers through a complex network of individual components，including transmission lines，transformers，and switching devicesIt is common practice to classify the transmission network into the following subsystems：Transmission system；Subtransmission system；Distribution system Fig1 Basic elements of a power system The transmission system interconnects all major generating stations and main load centers inthe systemIt forms the backbone of the integrated power system and operates at the highestvoltage levels(typically,230 kV and above in USA)。The generator voltages are usually in therange of 11 to 35 kVThese are stepped up to the transmission voltage level，and power istransmitted to transmission substations where the voltages are stepped down to the subtransmission level(typically,69 to 1 38 kV)The generation and transmission subsystems are often referred to as the bulk power system The subtransmisson system transmits power in small quantifies from the transmission substations to the distribution substationsLarge industrial customers are commonly supplied directly from the subtransmission systemIn some systems，there iS no clear demarcation between subtransmission and transmission circuitsAs the system expands and higher voltage levels become necessary for transmission，the older transmission lines are often relegated to subtransmission function The distribution system represents the final stage in the transfer of power to the individual customersThe primary distribution voltage is typically between 40 kV and 345 kVSmallindustrial customers are supplied by primary feeders at this voltage levelThe secondary distribution feeders supply residential and commercial customers at 120240 VSmall generating plants located near the load are also connected to the subtransmission or distribution system directlyInterconnections to neighboring power systems are usually formed at the transmission system levelThe overall system thus consists of multiple generating sources and several layers of transmission networksThis provides a hiigh degree of structural redundancy that enables the system to withstand unusual contingencies without service disruption to the customersNew Words and Expressionsin parallel 并联地synchronously ad同步地facility n容易，方便灵活，熟练(便利)条件 (常用复数)设备，装置，工具，器材primary energy 一次能源fossil n化石，石块hydraulic a水力学的，水压的，液压的，液力的mechanical energy 机械能subsystem n子系统，分系统，辅助系统switching device 开关装置，开关设备subtransmission n二次输电subtransmission system 分支输电系统，二次输电系统distribution system 配电系统substation n变电所，变电站，分站，分所feeder n馈线，馈电电路redundancy n多余，冗余contingency n意外事故，偶然事故Notes1. Modem power systems are usually largescale，geographically distributed，and with hundreds to thousands of generators operating in parallel and synchronously现代电力系统通常是规模大，地域分布广，具有成百上千并列同步运行的发电机组。2.3.ExercisesITranslate the following words or expressions into English 1. 2 3. 4 5. 6. 7. 8. 9. 10.II Fill in the blanks according to the passage.IIn power plants convert the primary energy to mechanical energy, and convert mechanical energy to electric energy2The generator voltages are usually stepped up to the transmission voltage level by 3Residential and commercial customers are supplied by the secondary distribution feeders at 4In USA，the voltages of the transmission system are typically 5. 6.7.8.9.10.IIIAnswer the following questions according to the passage.1What components is a modern power system comprised Of?2What is the function of the distribution system?3What is the function of the transmission system?4What is the function of the subtransmission system?5What is the characteristics of a modern power system in network structure?. Retell the elements of a power system based on Fig. 1.Text BPower System Security电力系统安全Up until now we have been mainly concerned with minimizing the cost of operating a power system. An overriding factor in the operation of a power system is the desire to maintain system security. System security involves practices designed to keep the system operating when components fail. For example, a generating unit may have to be taken off-line because of auxiliary equipment failure. By maintaining proper amounts of spinning reserve, the remaining units on the system can make up the deficit without too low a frequency drop or need to shed any load. Similarly, a transmission line may be damaged by a storm and taken out by automatic relaying. If in committing and dispatching generation proper regard for transmission flows is maintained, the remaining transmission lines can take the increased loading and still remain within limit.到目前为止，我们一直主要关心最大限度地降低电力系统的运行成本。电力系统运行中最重要的就是维护系统的安全。系统安全实际上涉及设计：当系统中部分设备发生故障时，系统还能正常运行。例如，一套发电机组因为附属设备出现故障而不得不停止工作。通过维持适量备用发电能力，系统内仍在工作的发电机组可以弥补发电量的不足而不会使频率下降太多，也不需要拉闸断电。同样，一条输电线路可能遭受暴风雨而受损，自动中续保护器则会使其与电网断开，如在发电调度时，保持对输送电流的适度关注，其余的输电线路就会承担相应的负荷，而电力系统仍处于安全。Because the specific times at which initiating events that cause components to fail are unpredictable, the system must be operated at all times in such a way that the system will not be left in dangerous condition should any credible initiating event occur. Since power system equipment is designed to be operated within certain limits, most pieces of equipment are protected by automatic devices that can cause equipment to be switched out of the system if these limits are violated. If any event occurs on a system that leaves it operating with limits violated, the event may be followed by a series of further actions that switch other equipment out of service. If this process continues, the entire system or large parts of it may completely collapse. This is usually referred to as a system blackout._。由于电力系统的设备设计成在一定的限度内工作，大多数设备在超出限度时，自动保护装置会使设备与系统断开而保护设备。假设系统内任何事件使系统超限运行，随后会产生一系列的反应，使其他设备停止工作。如果这一过程继续，整个电力系统或系统大部分可能完全瘫痪。这通常被称为电力系统断电。An example of the type of event sequence that can cause a blackout might start with a single line being opened due to an insulation failure; the remaining transmission circuits in the system will take up the flow that was flowing on the now opened line. _. This type of process is often termed a cascading outage. Most power systems are operated such that any single initial failure event will not leave other components overloaded, specifically to avoid cascading failures. 产生这类断电事件的原因可能始发于单一线路由于绝缘失效引起开路，系统内其余的传输线路将承担开路线路所输送的电流。如果其中一条线路负荷太大，由于中续保护行为，它将断开，因而使剩余的线路负荷太大。这一过程常被称为连锁跳闸停电。大多数电力系统都不会由于单一始发故障而使其他设备过载，特别要避免连锁跳闸停电Most large power systems install equipment to allow operations personnel to monitor and operate the system in a reliable manner. We will lump these under the commonly used title system security. 大多数大型电力系统都会安装设施使操作人员能够可靠地监控和运行电力系统。我们将把这些纳入“电力系统安全”标题之下。System security can be broken down into three major functions that are carried out in an operations control center. 在运行控制中心执行任务时，系统安全可以分成三个主要功能。System MonitoringSystem monitoring provides the operators of the power system with pertinent up-to-date information on the conditions on the power system. Generally speaking it is the most important function of the three. From the time that utilities went beyond systems of one unit supplying a group of loads，effective operation of the system required that critical quantities be measured and the values of the measurements be transmitted to a central location. Such systems of measurement and data transmission，called telemetry systems, have evolved to schemes that can monitor voltages, currents, power flows, and the status of circuit breakers and switches in every substation in a power system transmission network. In addition, other critical information such as frequency, generator unit outputs and transformer tap positions can also be telemetered. With so much information telemetered simultaneously, no human operator could hope to check all of it in a reasonable time frame. For this reason, digital computers are usually installed in operations control centers to gather the telemetered data, process them, and place them in a data base from which operators can display information on large display monitors. More importantly, the computer can check incoming information against prestored limits and alarm the operators in the event of an overload or out-of-limit voltage. 系统监控 系统控制功能为电力系统操作人员提供与电力系统相关的最新状态信息。一般而言，它是三个功能中最重要的功能。从电力公司超越一台机组系统供应一组负荷之时起，电力系统有效的运行要求测量重要的数据，并把测到的数据传送到控制中心。这样的测量和数据传送的遥测系统已经发展成为可以监控电压、电流、电能流和电力系统输电网络内每一变电站的线路断路器和开关状态的装置。此外，其他重要信息，如频率、发电机组输出和变压器抽头位置也可以遥测。这么多数据同时遥测，操作人员不可能在合理的时间内全部核实。_。更重要的是，计算机可以把输入数据与以前存储的限制数据进行对比，并在过载或超限电压时向操作人员发出警示。State estimation is often used in such systems to combine telemetered system data with system models to produce the best estimate (in a statistical sense) of the current power system conditions or “state”.这样的系统常应用状态评估把遥测系统数据与系统模型相结合，作出现在电力系统状态最佳的评估(统计学意义上的)。Such systems are usually combined with supervisory control systems that allow operators to control circuit breakers and disconnect switches and transformer taps remotely. Together, these systems are often referred to as SCADA systems, standing for supervisory control and data acquisition system. The SCADA system allows a few operators to monitor the generation and high voltage transmission systems and to take action to correct overloads or out-of-limit voltages.这样的系统通常与监控系统相结合，使操作人员可以遥控线路断路器、遥控断开断路器和变压器抽头。这些系统一起被称为SCADA系统，即监控和数据采集系统。这个系统只需少数操作人员监控发电、高压输电系统，并采取措施纠正过负荷或过电压。Contingency Analysis The second major security function is contingency analysis. The results of this type of analysis allow systems to be operated defensively. Many of the problems that occur on a power system can cause serious trouble within such a quick time period that the operator could not take action fast enough. This is often the case with cascading failures. Because of this aspect of system operation, modern operations computers are equipped with contingency analysis programs that model possible system troubles before they arise. These programs are based on a model of the power system and are used to study outage events and alarm the operators to any potential overloads or out-of-limit voltages. For example, the simplest form of contingency analysis can be put together with a standard load-flow program such as described above, together with procedures to set up the load-flow data for each outage to be studied by the load-flow program. This permits the system operators to establish defensive operating states where no single contingency event (i.e., a single failure) will cause overloads and/or out-of-limit voltages. (This analysis effectively develops operating constraints that may be used in the economic dispatch and unit commitment program). Several variations on this type of contingency analysis scheme involve fast solution methods, automatic contingency event selection, and automatic initializing of the contingency load flows using actual system data and state estimation procedures.偶发事故分析 偶发事故分析是第二个主要的安全功能。它的分析结果使电力系统可以受保护地运行。电力系统发生的许多问题由于操作人员采取措施不够迅速，在短时间内可以引起很严重的麻烦，如连续跳闸断电。因为系统运行的这种情况，现代化的运行计算机都装有偶发事故分析软件。这些软件以电力系统的模型为基础，在事故出现前模拟可能出现的系统故障，并用来研究停机事件、警示操作人员任何潜在的过载或超限电压。比如，最简单的偶发事故分析程序与负荷流分析软件研究的每一次停机负荷流数据建立的程序结合使用。这使系统操作人员可以确立防卫式运行状态，任何单一故障、单一偶发事件将不会产生过载和(或)超限电压。这种分析有效地确立了可以用与经济调度和机组调度软件的运行限制条件。偶