统频率的变化是由于发电机的负荷功率与原动机输入功率之间失去均衡所致．因而调频与有功功率调理是不可分开的。电力系统负荷是不时变化的，而原动机输人功率的改动则较迟缓，因而系统中频率的动摇是难免的。负荷的变动状况能够分红几种不同的重量：种是频率较高的随机重量，其变化周期普通小于10s，是靠单元机组调速系统来自动调速完成的，即一次调频，一次调频的特性是由汽轮发电机组自身的调理系统直接调理，因此其响应速度快；第二种为脉动重量，变化幅度较大，变化周期在10 s～3 min之间；第三种为变化很迟缓的持续重量。后两种要经过改动汽轮发电机组的同步器来完成，即经过平移高速系统的调理静态特性，从而改动汽轮发电机的出力来到达调频的目的，称为二次调频。当二次调频由电厂运转人员就地设定时，称为就地手动控制。
The change of the system frequency is caused by the unbalance between the load power of the generator and the input power of the motor. Therefore, frequency modulation and active power regulation are inseparable. The load of power system varies from time to time, while the change of the power of the prime mover is slow, so the frequency fluctuation in the system is inevitable. Load changes can be divided into several different weights: the first is the random weight with higher frequency, whose change period is usually less than 10 seconds, which is accomplished by automatic speed regulation of the unit unit speed control system, namely primary frequency regulation, whose characteristics are directly regulated by the regulation system of the turbogenerator unit itself, so its response speed is fast; the second is the fluctuating weight. The change period is between 10 s and 3 min, and the third is the continuous weight which changes slowly. The latter two are accomplished by altering the synchronizer of the turbogenerator unit, that is, by adjusting the static characteristics of the translation high-speed system, the output of the turbogenerator is changed to achieve the purpose of frequency modulation, which is called secondary frequency modulation. When secondary frequency modulation is set locally by power plant operators, it is called in-situ manual control.
The change of power system frequency has a direct impact on consumption rate and load distribution among power plants. For example, when the frequency changes, the generator set and auxiliary equipment deviate from the rated operating conditions, so their efficiency decreases. Power plants operate under uneconomic conditions, but also affect the economic operation of the entire power grid. When the frequency is too low, it will endanger the safe operation of the whole system.
Therefore, one of the main tasks of power system operation is to stop monitoring and controlling the frequency. When the input power and load power of the system unit lose balance and the frequency deviates from the rated value, the control system must adjust the output of the unit to ensure that the frequency deviation of the power system is within the allowable range (the normal allowable deviation should not exceed (+0.2Hz).
The adjustment of active power and frequency as well as the magnitude and speed of adjustment are accomplished by adjusting the intake of steam turbine.
When one or more units are used to adjust the frequency, the load distribution among the units will be changed, which will affect the economic operation of the power system. Therefore, frequency regulation is closely related to the economic distribution of power system load. When adjusting the system frequency, it is requested that the system frequency be maintained within the regular range. In addition, the system load should be allocated economically among generators.
完成电力系统频率和有功功率自动控制的的系统称为自动发电控制( Automatic Gen-eration Control，AGC)。自动发电控制系统主要由电网调度中心的能量管理系统(EMS)、电厂端的远方终端(RTU)和分散控制系统的谐和控制系统通道三局部组成。完成自动发电控制系统闭环自动控制必需满足以下根本请求：
Automatic Gen-eration Control (AGC) is a system that accomplishes automatic control of power system frequency and active power. The automatic generation control system is mainly composed of three parts: energy management system (EMS) of power grid dispatching center, remote terminal (RTU) of power plant and harmonic control system channel of decentralized control system. To complete the closed-loop automatic control of the automatic generation control system, the following basic requirements must be met:
(1) The thermodynamic automatic control system of power plant units must operate in an automatic mode, and the harmonic control system must operate in a "harmonic control" mode.
(2) The energy management system, microwave channel and RTU of power dispatching center must be in normal working state, and the dispatching load instructions of motor, Furnace Harmonic and control system can be changed directly from the terminal CRT of power dispatching center's energy management system. Machine and furnace harmonic control system can directly receive the "request" and "release" signals of automatic generation control and the analog signals of "dispatch load instructions" (the standard interface is 4-20 mA DC) from the energy management system. Energy management system can accept the response signals, harmonic control mode signals and AGC input signals of the unit harmonic control system.
(3) The absolute bias between the "dispatching load instruction" signal issued by the energy management system and the practical output of the power plant units must be controlled within the allowable range.
(4) Units operate in a harmonic control mode, and the load is set by the operator as in-situ control, while the load is directly controlled by the power grid dispatching center as remote control under dispatching instructions. The switching between local control and remote control is bidirectional and undisturbed. In the local control, the dispatching load instruction automatically tracks the real power of the unit; in the remote control, the output load instruction of the manual load setter of the harmonic control system automatically tracks the dispatching load instruction.
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