【作者】 魏强；

【导师】 韩学山；

【作者基本信息】 山东大学 ， 电力系统及其自动化， 2015， 博士

【摘要】 当今,节能减排、清洁化利用能源势在必行,对电力系统的运行和控制提出了更高的要求,在以资源充分优化与配置为目标的前提下,如何改变传统方法所带来的弊端,如何实现分布自治、全局统筹是亟待解决的问题,分散与集中已成为当今电力系统运行与控制面临的主要矛盾。本文就如何抑制低频振荡、提高暂稳性能,实现新的负荷跟踪机制,以及自动潮流实现等问题,提出发电机转子角相对不变的控制的新思路,并以此为核心,开展对这些问题的深入研究与探索,符合当今电力系统发展形势,具有重要的理论意义和工程价值。目前电力系统稳定器(PSS)主要通过比较发电机与相邻发电机的相对位置和速度来提供阻尼,这种机制决定了相邻的成员可以比较容易的维持相对位置,但是相距更远的发电机就只能起间接的作用。由此,随着电网规模的扩大,假使不同区域间的发电机间发生振荡时,就很难通过这种模式来平息。如果设法将每个发电机组的转子角都在全球时钟定位系统(GPS)确定的旋转坐标系中固定下来,就可保证区域间发电机组的相对位置不变,振荡发生后自然容易平息。这是本研究当初欲达到的目的。随着研究的深入,发现转子角控制对提高暂稳裕度也有显著效果,这是因为在现有的运行方式下,每个发电机的绝对转子角都在不断变化,大扰动后就不可能根据角度量测进行控制,发电机只能靠电网重新拉入同步。当系统中全面实施转子角控制时,所有发电机都会精确的保持额定转速并运行在各自的角度目标附近。发生扰动后,发电机即可根据本地信息,根据控制规则,通过汽门操作或电气制动加速或减速,向事故前目标值回归。从而在不依赖远程量测的前提下使系统回归有序状态,恢复稳定。为了验证转子角控制的可行性及相应自动负荷跟踪的机制,在山东大学动模试验室进行了相关的动模试验。试验表明：(1)现有的测量精度和控制技术完全可以实现定转子角控制,并且可以同时指定多个发电机的目标转子角,也不存在能控性/能观性/稳定性问题。(2)发电机并入运行于定转子角模式的电网后,转子角亦随之固定下来,此外,如果电网在大扰动后暂时变为变频变角度模式,也可以在频率偏差较小时,很方便的重新启动定角度模式。(3)电网中多种模式(定功率/仅有一次调频/定角度)的发电机可以并列运行,在负荷快速增长或者大扰动发生时,电网中的发电机都会参与调节,但调整结束后负荷增量仅由定角度机组分担。定角度机组越多,则调整过程中发电机角度滞后越少。此外,如果运行于定转子角模式的发电机到达出力上限,该机就会失去维持角度的能力,但相邻的发电机可在负荷增长时提供支援,因此系统频率仍可维持不变。由于励磁会导致发电机内电势在负荷增加后显著改变,所以不能使用直流潮流来计算负荷增加后的发电机出力。可以在扩展对应发电机内电势的节点后,由牛顿-拉弗逊法或利用本文自PQ分解法派生的新算法来进行潮流计算。后者和传统PQ分解法的主要区别为,在数据准备阶段生成B阵时需要考虑到d轴电抗。在迭代阶段,还需要在求得各母线电压修正量后,额外修正B的部分元素才能求解得到各节点角度修正量,并进一步计算内电势大小修正量。计算实践表明,新的潮流算法计算速度快,收敛性好,可以作为新运行模式下优化和调度运行的潮流算法。值得注意的是,本文选择通过汽轮机来实现转子角控制。这是因为角度(及频率)波动的背后是有功的波动,所以通过汽轮机输出变化来达到功率平衡是最直截了当的做法。但是,通过汽轮机调整也有动作迟缓、影响调门寿命等问题。不过应该看到,使用汽轮机实现转子角控制并不是唯一的选择。在具体实现时,也完全可以通过调整励磁(或协调励磁与汽轮机)来达到控制转子角的目的。只是那时本文所述的益处和影响将以完全不同的形式来展现而已。从这个角度讲,本文所采取的具体做法和形式并不重要,重要的是背后的想法和这种想法所能带来的本质性变化,这一变化会实现分散、就地、自动的平衡与控制。当然,也会给传统的调度与控制带来方法和机制上的改变。综上,本文具有一定新意的工作主要体现如下：(1) 发电机转子角控制的新思想。基于这一思想,给出了使用比例、积分、微分(PID)控制器实现转子角控制的具体方法。这种方法的主要目的是力图固定每个发电机在GPS确定的旋转坐标系中的位置,从而让整个电网中发电机组成的队形保持不变,并使电网频率时刻渐进维持在额定频率上。当每一发电机组都实现了这一控制时,在一定条件下,伴随扰动预期可实现发电与用电的自动平衡。(2) 转子角控制抑制低频振荡的作用机理。通过该控制器的引入,分析了转子角控制器产生阻尼转矩的特性和转子角控制器的稳定性。并进一步分析了转子角控制器抑制区域间振荡的机理：由于实施转子角控制后频率和角度不再随机波动,因而就有可能通过本地量测获取区域间振荡信息,并据此施加合适的阻尼转矩,最终得到提高区域间振荡阻尼比的效果,仿真验证了这一想法的有效性。(3) 转子角控制提高暂稳性能的作用机理。进一步,分析了汽轮机调门操作和电气制动的等效性,给出了转子角控制器在单机无穷大系统中提高极限切除时间的算例,并论证了转子角控制器通过适当改变机械功率增加减速面积的具体机理。分析了暂稳过程中转子角控制器补偿模块的作用和局限性,并给出了多机系统中通过调门操作提高极限切除时间的算例,表明其有效性。(4) 转子角控制模式下的负荷跟踪机理和动模验证。分析了新控制模式下的负荷跟踪机制,并将新的机制与传统一次调频/自动发电控制(AGC)配合机制做了对比。此外,还分析了负荷增长后的增量分配原则。通过动模试验验证了转子角控制思想的可行性,并验证了新模式下负荷自动跟踪的机制。(5) 定转子角控制下的准稳态潮流计算。在给定转子角度前提下,分析了实施转子角控制后潮流计算与现有潮流计算不同之处,并以PQ分解法为基础,派生了新的不需要扩展发电机内电势节点的潮流计算算法,实现潮流的自动计算。更多还原

【Abstract】 The power system operation and control face more problems with the development of smart grid and clean energy (like wind power and photovoltaic). The problems include how to solve traditional problems (like low frequency oscillations) better and how to implement distributed autonomous control (with central coordination). A new principle is proposed in this paper, namely, the principle of generator rotor angle control. This principle tries to fix generator rotor positions in GPS coordinate system so that their relative position can be fixed. This method can suppress low frequency oscillation, increase transient stability, implement automatic load following at the same time. The researches on these aspects are of great theoretic and possible practical value.Currently PSS provide damping torque by measuring speed and relative position between local generators. This mechanism cannot take effect between generators far apart. Consequently, remote speed (and position) informations are needed to suppress inter-area low frequency oscillations. This problem can be solved if the angle position of every generator rotor is fixed in the spinning coordinate system determined by GPS. This way, the oscillations can be suppressed more easily, and the power system will change from chaos to order.The rotor angle controller can also improve transient stability. For the time being, the absolute rotor angle of every generator is changing from time to time, so it’s less likely to improve transient stability by using the rotor angle measurement. Things are no longer so after absolute rotor angle controllers are deployed across the power system. The frequency of the power system will remain constant, and the absolute rotor angle of every generator will be kept at the aim value given by the dispatching center most of the time. After a fault happens, the angle controller can then give order to turbine valve or power electronic braking devices to accelerate or decelerate the generator rotor so that it comes back to its original position. Consequently, stability and order will be restored. This is the reason why rotor angle control can improve transient stability.Some experiments are carried out in the laboratory of Shandong University to verify the practicability of the new principle and the new load following mechanism. Physical experiment shows that:(1) It is possible to implement absolute rotor angle control based on measurement and control technology nowadays. All generator rotor angles can be fixed on their aim values in the GPS coordination system. There’s no stability/controllability/observability problem. (2) The rotor angle of a generator stops changing after it is connected to a power system that runs on fix angle mode. Besides, if a power system changes to variable angle mode after large disturbance, it can come back to fix angle mode easily when the frequency deviation is small enough. (3) The physical experiments also show that the generators operating on different modes (constant power/primary frequency control/rotor angle control) can run side by side. The primary frequency control and AGC currently used in power system can be fulfilled in a new manner under the new control strategy. The generators can follow loads quickly and precisely without intervention of a dispatching center. The increase of any load will be shared between all generators running on fix angle mode. Besides, if a generator running on fix angle mode reaches its output up-limit, it will lose the ability to maintain angle. But nearby generators will provide assistance if the loads keep on growing, so system frequency can be kept at the rated value.Because the magnitude of the generator internal potential source changes dramatically for different levels of load demands, the DC power flow calculation method should not be used for power systems running in the angle control mode. The Newton-Raphson method or a new power flow calculation method derived from the PQ decoupled method should be used instead. The differences between the new method and the traditional PQ decoupled method are as follows: When calculating B matrix before the iteration, the diagonal elements should be modified by adding the corresponding d-axis admittance. During the iteration, this mtrix should be modified again after voltage magnitude corrections are got. The angle corrections of all buses can be got by using the modified matrix. The new magnitude of generator internal source can then be calculated. The new method also converges quickly. It may be the recommended power flow calculation method since the algorithm and codes currently used can be reused after slight modification.Please note that, since the position (and the speed) variations reflect active power imbalance, changing turbine mechanical power is selected as the means to fulfil rotor angle control in this paper. However, this is not the only choice. The principle of rotor angle control can also be fulfilled by exciter control since exciters act more quickly and precisely when compared with turbines. But the benefits will take other forms at that time. From this point of view, what’s important is not the forms (and possible difficulties) of methods used in this paper, but the idea (and the benefits) behind the appearance.The main achievements of our research are as follows:1) The principle of generator rotor angle control and its implementation by means of PID control. The main aim of the principle is to fix generator rotor position is the coordinate system determined by GPS, so that the form composed by generators remain unchanged, and system frequency be fixed at the rated value.The automatic balancing between generation and loads can also be fulfilled after this principle is implemented across the system.2) The method to suppress inter-area low frequency oscillations more effectively. After the angle controllers are deployed, the rotor angle and speed remain constant under normal conditions, so it is possible to get information of all kinds of oscillations (including inter-area low frequency oscillation) through local measurements. When used together with PSS, the angle controller can suppress inter-area oscillations effectively without help of any remote measurement. Simulation also proves its effectiveness.3) The method to improve transient stability by means of rotor angle control. After fault happens, rotor angle controller can give orders to turbine or braking devices to accelerate or decelerate the generator rotor so that it moves back towards its original position. The decelerating area can be increased by these means. Simulation results in both single machine infinite bus system and multi machine system are given. The critical clearing time can be increased dramatically in both cases.4) The new load following mechanism and its experimental verification. Analysis shows that the primary frequency control and automatic generation control currently used can be fulfilled in a new manner under the new control strategy. The generators can follow loads quickly and precisely. The load balancing can be reached without intervention from the dispatching centre. Moreover, the new mechanism can distribute load variance mainly to nearby generators. This can decrease network losses and tie line power flow fluctuations.5) The new power flow calculation method used under rotor angle control mode. A new method is derived from PQ decoupled method, and a comparison is made between the new method and the traditional power flow calculation method.更多还原