Content
- Introduction
- 1. Researches
- 1.1 The concept of asynchronous mode and the electrical center of swings
- 1.2 Theoretical overview of existing methods ECC definitions
- Summary
- Source list
Introduction
The violation of the stability of the power system is a constant and serious threat. Therefore, it seems useful to identify in advance signs of changes in the operating conditions of the electric power system in the direction of deterioration of sustainability.
One of the most dangerous consequences of violation of stability is the emergence of an asynchronous mode, to prevent its occurrence and prevent automatic elimination of the asynchronous mode (ALAR) is used. For correct and effective operation of automation, it is necessary to install it on dangerous sections constructed on electric swing centers (ЕCoO).
The purpose of this work is to assess the effectiveness of methods for determining ETSK, identify the most practical of them and compile an accessible and understandable methodology for determining ЕCoO.
Achieving these goals is achieved by solving the following tasks:
- Review of literature on the topics of AR and ЕCoO;
- Review of existing methods for determining ECO in electrical network in the event of asynchronous progress in power system;
- Evaluation of methods in terms of accuracy and ease of definition electric center of swing;
- Identify the most appropriate method for practical
targets, for example, for calculating ALAR installation locations and location
ETSK, and theoretical goals, for example, training in educational
institutions and places of advanced training, goals.
1. Researches
1.1 The concept of asynchronous mode and the electrical center of swings
Emergency emergencies, including asynchronous modes, occurring in the event of a violation of stability, represent a serious danger to the power system, since they can lead to the development of an accident and deenergize the responsible consumers. Consider a more asynchronous mode.
Asynchronous mode (AP) – is a consequence of a violation of a static or dynamic stability [1], which can be caused by:
- overload of transit connections with capacities in excess of the maximum acceptable values for stability;
- failure of switches or protection in case of short circuit in the power supply network;
- failure or lack of effectiveness of PA;
- asynchronous switching of links or generators;
- loss of excitation of powerful generators;
- operation of the power system with unacceptably low voltage on generators and in the main network;
- emergency shutdown of high power;
- disabling one or more loaded network elements of sections of the main network;
- work with unacceptably low frequency;
- a combination of several factors [2].
Thus, there are two types of AR, with a loss of arousal on the generator and without a loss of arousal, the second case is also called asynchronous operation. Let us consider in more detail the operation mode during asynchronous progress.
The physical essence of the AR is that the movement of the rotors of synchronous machines of one group (separate generators, all generators of power stations, power systems, synchronous engines) occurs with an angular speed different from the angular speed of movement of the rotors of synchronous machines of the other group (power system, unified power system).
AR characterized by:
- Stable deep fluctuations in voltage, current and power;
- A change in the mutual angle of the EMF of the generators at least with one of the power plants relative to the EMF of the generators of any other power plant of the power system by an angle exceeding 360°;
- The emergence of the frequency difference between the parts of the synchronous zone, out of synchronism, while maintaining the electrical connection between them;
- The emergence of additional currents in the closed circuits of the rotor, causing overheating of the rotor; – The emergence of equalizing currents comparable to short–circuit currents;
- Voltage drops in the network;
- The emergence of the ЕCoO point, which can move around the system and lead to the self-disconnection of groups of energy-receiving consumer installations, which are near the ETSK, and the disabling of the responsible mechanisms of the own needs of power plants;
- Changing the direction of reactive power flow [2].
Thus, the resulting processes in asynchronous mode are dangerous for the entire power system, and for its individual parts. Therefore, in order to prevent the development of accidents and de-energize the responsible 16 consumers, nowadays, automation of the prevention of violation of stability, dividing automation and other means of emergency control automation are widely used.
For the correct operation of the dividing automatics, it is necessary to install it on dangerous sections formed when an asynchronous mode occurs. These sections are formed by a set of ЕCoOlocated on parallel elements of the system.
This paper is devoted to the development of the optimal method for determining the electric center of oscillation (ЕCoO), when an asynchronous mode (AR) occurs. Let us consider in more detail the processes occurring in the ES when an AR occurs, using the example of a single–machine ES.
We will consider the left-hand group of synchronous machines as a power station, the right, as an integrated power grid. We will consider the connection between them in electrical distances, for example, in the form of resistances, which may include overhead lines, resistances of transformers, other elements and, of course, synchronous machines themselves. Let us select a point at some electrical distance from the power plant (L1), where we will be interested in voltage. Distance to the electrical system will be L2.
In normal mode, the movement of the rotors of all synchronous machines occurs at the same speed, which is called the synchronous speed. In this case, a small deviation of the speed of individual synchronous machines or their groups is allowed, but the increase does not go into an asynchronous process and with the subsequent attenuation of this process. Such phenomena are called synchronous oscillations [1].
Figure – Model of the considered single–machine power system
(animation: 10 frames, 5 repeat cycles, 52.6 kilobytes)
Supposed power plant transmits a certain power to the power grid. This power is determineted by:
where P – is the power transmitted by the line, MW;
E1 – module of the EMF values of the station, kV;
E2 – module of the EMF value of the system, kV
Х – is the resultant, equivalent resistance between power plant and power system;
δ – the mutual angle of the EMF station and system. Equivalent resistance between the power station and grid, divided into two sections:
Where:
XL1 – is the reactance of the region L1, Ohm;
XL2 – reactance of the L2 section L2, Ohm;
The vector diagram of the considered scheme is shown in Figure 2.
Figure 2 – Vector diagram of a single machine system in normal mode
Since the vector of the EMF of the power system is directed along the real axis, the vector of the EMF of the power plant is conveniently represented using the exponential function:
With some error it is possible to determine the current circulating between the power station and the power system:
Knowing the amount of current, according to [1], the voltage in the place of interest can be defined as:
Moving the point of measurement of voltage, you can find a point where the voltage is minimal. This point is called the point of minimum voltage (TMN). If we talk about electrical distances, TMN is just in the middle: XL1= XL2.
Next, we move from the normal mode to the AR, in the new conditions the angle will constantly change. Considering the change in voltage at the point ТМН for different values of different values for the angle, the vector diagram of the voltage shown in Figure 3 was obtained.
Figure 3 – Vector diagram of voltages at various angle values
For each angle value, the magnitude of the voltage at the center of the swings is determined. We note that as the angle δ increases, the voltage at the center of the swings decreases; at an angle of 180°, it becomes zero. We show the change in voltage for the ЕCoOpoint and the current in the network at AR.
Figure 4 – Graph of changes in voltage and current in the network when AR
From the above graph it can be seen that the voltage in the swing center reaches zero at an angle of 180°, with the same angle the current of the asynchronous stroke reaches a maximum, it is comparable in magnitude with the current of a short circuit in the same place (swing center). Thus asynchronous the mode is unacceptable, we will consider in more detail ways of fight against AR.
To combat the AR, you can perform the following activities:
- Reducing the active power of the generators that are out of synchronicity and operate in asynchronous mode;
- . Forcing the excitation current of generators, which leads to an increase in their EMF and, consequently, to a more rigid binding to the power system, as well as a decrease in voltage fluctuations [3];
- By means of emergency control automation, which can turn off the generators or the load, and, if necessary, produce a division of the power system into synchronously operating parts [2].
Successful combat asynchronous mode can lead to the restoration of normal synchronous mode. This phenomenon is called resynchronization.
Considering changes in the angle of motion of the rotor of the generator when AR, it is clear that the angle periodically passes through zero (or through an angle of 360°). It is at this moment that resynchronization becomes possible. However, for successful resynchronization, it is necessary that the acceleration pad be smaller than the deceleration pad. Under these conditions, the acceleration site is calculated from the angle (more precisely, from n • 360 °), which complicates the resynchronization process. Therefore, to facilitate resynchronization, the turbine power is reduced and the generator excitation current is forced.
1.2 Theoretical overview of the existing methods for determining the ECC
As noted earlier, to assess the state of the current mode and carry out quick and effective measures to eliminate emergency conditions, as well as solving design and modernization problems of emergency control automation, it is necessary to know about weak network sections (weak links) that make up dangerous sections.
One of the first step in solving these problems is to identify these weak areas and points of the ЕCoO, which are their part. This paper describes 5 ways to determine the location of the ETSK, i.e. electric and geographical remoteness of the ЕCoOfrom one of the considered tires.
Considered methods for the definition of ЕCoO:
- The TMN method;
- The method for determining the ЕCoOby operating parameters (current and voltage) in the AR;
In this technique, a sign of the presence of ЕCoOon the line in the AR is the presence of the point of minimum voltage (TCH) on this line in the normal mode.
An important difference in identifying weak lines based on the TMN analysis is the ability to identify weak links and potentially dangerous sections long before the onset of buckling, which allows using the sign of the presence of the ТМН to monitor and control the power system in real time.
It is shown that for the line segment from the node with the voltage U0 up to the node with the voltage U=U×e j×α voltage distribution of the considered:
where x – reactance of the considered area, Ohm;
U0 – full voltage at the beginning of the phase, kV;
U – total voltage at the end of the phase, kV.
The scheme of the considered area is shown in Fig.5.
Figure 5 – Homogeneous line section
In this case, the magnitude of the square of the voltage module will have a minimum at the point:
Where: Xmin – electrical remoteness of the point of TMN, Ohm;
Where: Xmin is the electrical remoteness of the TMN point, Ohm;
and the magnitude of the voltage in TMN:
Where νx2 – the value of the modulus of the square voltage at the point ТМН. Condition hit TMN on the site in question: xmin ∈ [0;1].
Considering TMN with zero voltage value, i.e. passing to ETSK and taking the angle conditions equal to 180° for an arbitrary value of, the position of the TMS will be determined by the simplified formula:
Thus, when cranking vectors U0, U at asynchronous operation at the moment when the angle between the voltages of the section reaches 180°, the MST is combined with the point of the electric center of oscillations. With a smaller angle of reversal of the stress vectors, the presence of TMN can be considered as a pointer to the line with the ЕCoO. The voltage distribution diagram along the power transmission lines is presented in Figure 6
Figure 6 – The diagram of voltage distribution along power lines
It can be seen from the diagram that the magnitude of the voltage square in the Z1 and Z3 sections varies almost linearly, and in the Z2 section, starting with a certain value of the EMF angle, it has a characteristic feature – the difference in the minimum of the square of the voltage module.
The position of TMN is determined by the structure and parameters of the scheme. TMN is detected already at relatively small turning angles and long before the voltage reaches the absolute minimum voltage at the ETSK point [5]. The geographical remoteness of TMN is determined by the following formula:
The geographical distance of the ЕCoO, in turn, is determined by the following formula:
There is a question about the legitimacy of the application of this method on the lines with intermediate load, adjacent sections of homogeneous and heterogeneous lines.
So with adjacent homogeneous lines, the ЕCoOlocation can be determined both for each section of the adjacent line and for the entire section of the homogeneous line as a whole, which is more expedient and confirmed by the calculations made in the work.
If adjacent nonuniform lines are considered, an example of the line is shown in Figure 7, then according to [6], the ЕCoO location should be determined separately for each of the adjacent sections, i.e. consistently consider sections Z1 and Z2.
Figure 7 – Adjacent section of a non–uniform line
The presence of intermediate load does not directly affect the method of determining the ЕCoO on adjacent sections of the line, but leads to a shift in the location of the ЕCoO.
With the occurrence of AR in the ES, stable, there are deep fluctuations in voltage, current and power. This method is aimed at determining the ETSK in AR based on currents and voltages. Electrical distance, i.e. line resistance at an angle, ETSK from one of the tires according to [7] is according to the formula:
Where ХЭЦК – – electric remoteness ETSK, Ohm;
U – phase value of line bus voltage with ECC, kV;
Having determined the electrical distance of the ETSK from the bus, it is necessary to determine its geographical distance, i.e. determine which kilometer of the line is the ETSK. To do this, it is necessary to determine the resistance of the power lines in the normal mode, and make a ratio. Line resistance in normal mode is determined by the formula:
The geographical distance of the ЕCoO is determined by the formula:
Where lЭЦК – geographical distance of the ECC location from the bus, km.
Source list
- Профилактические испытания изоляции оборудования высокого напряжения, Бажанов С.А., М., ЭНЕРГИЯ, Москва 1968 г.,72 с.
- Обслуживание электрических подстанций/ О.В. Белецкий, С.И. Лезнов, А.А Филатов. – М.: Энергоатомиздат, 1985, – 416 с.
- Обслуживание генераторов, Чернев К.К. – М.: Энергоатомиздат, 1989 г., 592 с.
- Электрические системы и сети, Идельчик В.И. Учебник. – М.: Высшая школа, 2003. – 463 с.
- Электрические машины, Кацман М.М. Учебник. – М.: Высшая школа, 2003. – 463 с.
- Рожкова Л.Д., Козулин В.С., Электрооборудование станций и подстанций 3–е изд., перераб. и доп. Учебник для техникумов. М.: Энергоатомиздат, 1987. – 648 с.
- Неклепаев Б.Н. Электрическая часть электростанций и подстанций. Учебник для вузов – 2–е изд. М. Энергоатомиздат, 1986. – 640 с.
- Сайт компании
DIMRUS
Измерение частичных разрядов в изоляции статоров высоковольтных электрических машин [Электронный ресурс] – Режим доступа: Компания DIMRUS – (дата обращения: 15.10.18). - Сайт компании
DGM KZ
Краткая информация о частичных разрядах (ЧР) и их измерении [Электронный ресурс] – Режим доступа: Компания DGM KZ – (дата обращения: 17.10.18). - Выдержка из книги Бажанов С.А. Профилактические испытания изоляции оборудования высокого напряжения [Электронный ресурс] – Режим доступа: Большая энциклопедия нефти и газа – (дата обращения: 14.11.18).
- Выдержка из книги Бажанов С.А. Профилактические испытания изоляции оборудования высокого напряжения [Электронный ресурс] – Режим доступа: Большая энциклопедия нефти и газа – (дата обращения: 10.12.18).
- Сушка изоляции генераторов [Электронный ресурс] – Режим доступа: Электронный ресурс KazEDU – (дата обращения: 16.12.18).