Synchronous machines are called devices with the rotor speed, in which it is always equal to or a multiple of the same indicator of the magnetic field inside the air gap, which is created due to the current passing through the armature winding. The operation of this type of machine is based on the principle of electromagnetic induction.
Excitation of synchronous machines
Synchronous machines can be excited by electromagnetic action or by a permanent magnet. In the case of electromagnetic excitation, a special DC generator is used, which feeds the winding, in connection with its main function this device called the causative agent. It is worth noting that the excitation system is also divided into two types according to the method of exposure - direct and indirect. The direct excitation method means that the shaft of the synchronous machine is mechanically connected directly to the exciter rotor. The indirect method assumes that in order to make the rotor rotate, another motor is used, for example, an asynchronous electric machine.
It is the direct excitation method that has received the greatest distribution today. However, in cases where the excitation system is expected to work with powerful synchronous electric machines, independent excitation generators are used, the winding of which is supplied with current from another DC source, called a subexciter. Despite all the bulkiness, this system allows you to achieve greater stability in operation, as well as finer tuning of characteristics.
Synchronous machine device
A synchronous electrical machine has two main components: an inductor (rotor) and an armature (stator). The most optimal and therefore common today is the scheme when the armature is placed on the stator, while the inductor is located on the rotor. A prerequisite for the functioning of the mechanism is the presence of an air gap between these two parts. The armature in this case is a fixed part of the device (stator). It can consist of either one or several windings, depending on the required power of the magnetic field that it must create. The stator core, as a rule, is recruited from separate thin sheets of electrical steel.
The inductor in synchronous electrical machines is an electromagnet, while the ends of its winding are brought directly to the slip rings on the shaft. During operation, the inductor is excited by direct current, due to which the rotor creates an electromagnetic field that interacts with the magnetic field of the armature. Thus, due to the direct current that excites the inductor, a constant frequency of rotation of the magnetic field inside the synchronous machine is achieved.
The principle of operation of synchronous machines
The principle of operation of a synchronous machine is based on the interaction of two types of magnetic fields. One of these fields is formed by an armature, while the other arises around an electromagnet excited by a direct current - an inductor. Immediately after reaching the operating power, the magnetic field created by the stator and rotating inside the air gap, couples with the magnetic fields at the poles of the inductor. Thus, in order for the synchronous machine to reach its operating speed, it takes a certain time to accelerate it. After the machine accelerates to the required frequency, the inductor is powered by a DC source.
The excitation of the synchronous motor is provided from a separate thyristor exciter.
Static devices are used to excite synchronous machines. The designed motor is equipped with a semiconductor static thyristor exciter. The advantages of thyristor exciters lie in their small dimensions and low weight, practical inertialessness, wide control possibilities and the low power required for this, measured in watts, high efficiency.
Non-standard values of rated excitation voltages of SM caused non-standard TVU voltages and required the development of special transformers, which made it possible to reduce the installed power of transformers, increase the efficiency and power factor of TVU.
The following basic requirements are imposed on excitation systems, which it must satisfy:
1) reliable DC power supply to the excitation winding of the motor rotor in any operating mode;
2) stable regulation of the excitation current when the motor load changes from zero to nominal at a given voltage level;
3) sufficient speed;
4) forcing excitation;
5) fast damping of the magnetic field in the motor.
On fig. 3 shows the excitation circuit from a thyristor exciter.
Rice. 3 - Excitation circuit from a thyristor exciter
The excitation system consists of the following main units: external network U c, auxiliary network SN, excitation transformer TV, thyristor converter TP, start-up resistance R pz, thyristor key TK, automatic excitation regulator ARV, voltage transformer TN and current transformer TT.
The excitation current is controlled by changing the firing angle of the thyristors. When starting a synchronous motor at a subsynchronous speed, the thyristors open with an angle corresponding to the overhead excitation voltage. The duration of forcing is usually about 1 s. The maximum steady-state voltage of the exciter (ceiling) U pot during forcing must be at least 1.4 of the rated excitation voltage of the synchronous motor U v.n. For large engines, almost always U opt \u003d (1.7-2.0) U v.n (especially with thyristor excitation). The excitation system must be designed for a forcing duration of 50 seconds.
The thyristor exciter controls the start and stop of the engine and therefore there is no need for a control station. At start-up, when an alternating emf is induced in the rotor winding, the winding must be connected to a resistor in order to create a closed circuit for the negative half-wave of current. The resistor can be linear and non-linear, switched on only during the start-up or permanently connected. In the latter case, it also protects thyristors from overvoltage during transients in the engine. When starting a synchronous motor, the thyristor converter is locked, the excitation winding is connected to a discharge resistor through a thyristor key, which is two anti-parallel thyristors. By the end of the start, when the voltage on the rotor winding drops, the thyristor converter is turned on, and the thyristors of the key are locked.
Start
The process of starting the engine is very complicated, since it is impossible to accelerate the rotor from a stationary state to synchronous speed due to the synchronous torque due to the inertia of the rotor. If you try to start the engine by simultaneously applying voltage to the stator winding and to the rotor winding from the exciter, then the synchronous torque resulting from the interaction of the rotor and stator fields will be alternating with a frequency of 50 Hz. As a result, a special starting winding or damper winding is provided for starting, which helps to dampen the oscillation of the rotor as a result of transients. In a clearly pole synchronous machine, the starting winding is a short-circuited squirrel-cage winding. Its rods are located in the grooves of the pole piece. Segments of neighboring poles are also connected and form a common short circuit ring.
The starting characteristics of VDS 375 vertical synchronous motors are designed for starting pumps in difficult conditions and are designed for reactor starting from low voltage.
The start is carried out in two stages: at the first stage, due to the interaction of the stator field with the starting winding, an asynchronous torque occurs, the engine starts up to a subsynchronous speed; on the second, voltage is applied to the excitation winding and, under the influence of an electromagnetic moment, the machine is pulled into synchronism.
The process of starting the SM is accompanied by large starting currents and intense heating of the windings, especially starting, so restarting for many powerful machines is not allowed without cooling. With frequent starts, the damper winding must have a reinforced structure, as well as the fastening of the stator winding.
The characteristics of the excitation system are determined by a combination of the properties of the field winding power supply and automatic control devices. Excitation systems must provide:
1) reliable power supply of the rotor winding of a synchronous machine in all modes, including in case of accidents;
2) stable regulation of the excitation current when the load changes within the nominal;
3) sufficient performance;
4) forcing excitation.
Excitation systems are classified depending on the power source-excitation winding into dependent (self-excited) and independent. Z dependent - powered by the main or additional winding of the armature of the excited generator. Independent powered by other sources (from the plant's auxiliary buses, from an exciter or an auxiliary generator).
Among independent excitation systems, there are:
a) direct excitation systems, in which the exciter or auxiliary generator rotor is on the same shaft as the rotor
synchronous machine or interfaced with it by a speed reducer;
b) indirect excitation systems, in which the rotor of the exciter or auxiliary generator is driven by a synchronous or asynchronous motor specially installed for that purpose.
Until the 1960s, direct electrical excitation systems, in which the excitation winding of the synchronous machine is powered by a collector DC generator - the exciter (Fig. 24.26, a).
In accordance with GOST 533-76, GOST 5616-81 and GOST 609-75, turbo and hydro generators and synchronous compensators can only have the most reliable direct excitation system or self-excitation system. But electric machine excitation systems, according to switching conditions, cannot be used in turbogenerators with a capacity of 200 MW and higher, in which the excitation power exceeds 800-1000 kW.
V. is now becoming more common valve excitation systems. They are used for synchronous motors and generators of small power, as well as for large turbogenerators, hydro generators and synchronous compensators, including for power limiting installations.
There are three main types of valve excitation systems.
1. Independent valve excitation system(Fig. 24.26, b) in which the excitation winding is powered by an auxiliary synchronous generator, the rotor of which is mounted on the main generator shaft. In the rectifier circuits, in this case, semiconductor valves (silicon diodes or thyristors) are used, assembled according to a three-phase bridge circuit. When regulating the excitation of the generator, both the control capabilities of the rectifiers and the possibility of changing the voltage of the auxiliary generator are used.
2. Brushless excitation system, which differs from an independent valve system (Fig. 24.26, b) by the fact that it has an inverted auxiliary synchronous generator, in which the alternating current winding 3 placed on the rotor. Rectifier 5, powered by this winding, is located on the main generator shaft. The advantage of this system is the absence of sliding contacts, which in powerful turbogenerators must be designed for thousands of amperes.
3 . Self-excitation system(Fig. 24.26, in), in which the field winding is powered from the main or additional armature winding. Rectification of alternating current is carried out using thyristors. Energy extraction is carried out with the help of transformers 9 and 7, connected respectively in parallel and in series with the stator winding. Transformer 7 allows for forcing excitation in case of close short circuits, when the voltage on the armature winding is significantly reduced. The self-excitation system has a higher reliability and lower cost compared to other systems due to the absence of an exciter or an auxiliary generator in it.
Important parameters of excitation systems are the nominal rate of rise of the excitation voltage, the nominal excitation voltage, the excitation forcing ratio.
Rated excitation voltage- voltage at the terminals of the excitation winding when it is supplied with the rated excitation current and the resistance of the winding, reduced to the calculated operating temperature.
Excitation forcing ratio- the ratio of the highest steady value of the excitation voltage to the rated excitation voltage.
A special device is provided in the excitation circuit, with the help of which it is possible to quickly reduce the excitation current to zero in an emergency ( extinguish the magnetic field). For example, in case of internal short circuits in the stator winding, the field is quenched using a field quenching machine, which closes the excitation winding to a special quenching resistor.
To keep the synchronous machine in synchronism with a decrease in the mains voltage during remote short circuits, they resort to forcing its excitation current. Forcing is carried out automatically by the relay protection of the machine. The efficiency of forcing is characterized by the multiplicity of excitation forcing.
When considering the principle of operation of a synchronous generator, it was found that a source of MMF (inductor) is located on the rotor of a synchronous generator, which creates a magnetic field in the generator. With the help of a drive motor (PD), the generator rotor is set into rotation with a synchronous frequency n 1 . In this case, the magnetic field of the rotor also rotates and, mating with the stator winding, induces an EMF in it.
Synchronous motors are structurally almost the same as synchronous generators. They also consist of a stator with a winding and a rotor. Therefore, regardless of the operating mode, any synchronous machine needs the process of excitation - induction of a magnetic field in it.
The main method of excitation of synchronous machines is electromagnetic excitation, the essence of which is that the excitation winding is located on the poles of the rotor. When a direct current passes through this winding, an MMF of excitation occurs, which induces a magnetic field in the magnetic system of the machine.
Until recently, special independent excitation DC generators, called exciters B, were used to power the excitation winding (Fig. 1.1, a) , the excitation winding of which (OV) received DC power from another generator (parallel excitation), called a subexciter (PV). The rotor of the synchronous machine and the exciter and subexciter armatures are located on a common shaft and rotate simultaneously. In this case, the current enters the excitation winding of the synchronous machine through slip rings and brushes. To control the excitation current, adjusting rheostats are used, which are included in the excitation circuit of the exciter (r 1) and subexciter (r 2).
In synchronous generators of medium and high power, the process of regulating the excitation current is automated.
In high-power synchronous generators - turbogenerators - sometimes inductor-type alternators are used as an exciter (see § 23.6). At the output of such a generator, a semiconductor rectifier is turned on.
Rice. 1.1.
The adjustment of the excitation current of the synchronous generator in this case is carried out by changing the excitation of the inductor generator.
A non-contact electromagnetic excitation system has been used in synchronous generators, in which the synchronous generator does not have slip rings on the rotor.
In this case, too, an alternating current generator is used as an exciter (Fig. 1.1, 5), in which winding 2, in which EMF is induced (armature winding), is located on the rotor, and excitation winding 1 is located on the stator. As a result, the armature winding of the exciter and the excitation winding of the synchronous machine turn out to be rotating, and their electrical connection is carried out directly, without contact rings and brushes. But since the exciter is an alternating current generator, and the excitation winding must be supplied with direct current, then at the output of the exciter armature winding, a semiconductor converter 3 is switched on, mounted on the shaft of the synchronous machine and rotating together with the excitation winding of the synchronous machine and the exciter armature winding. The DC power supply of the excitation winding 1 of the exciter is carried out from the sub-exciter (PV) - a DC generator.
The absence of sliding contacts in the excitation circuit of a synchronous machine makes it possible to increase its operational reliability and increase efficiency.
In synchronous generators, including hydrogenerators (see § 1.2), the self-excitation principle has become widespread (Fig. 1.2, a), when the AC energy necessary for excitation is taken from the stator winding of the synchronous generator and through a step-down transformer and a rectifier semiconductor converter (PP) is converted into DC energy. The principle of self-excitation is based on the fact that the initial excitation of the generator occurs due to the residual magnetism of the magnetic circuit of the machine.
Rice. 1.2.
On fig. 1.2, b shows a block diagram of an automatic self-excitation system of a synchronous generator (SG) with a rectifier transformer (VT) and a thyristor converter (TC), through which AC power from the SG stator circuit, after being converted to direct current, is fed into the excitation winding. The thyristor converter is controlled by means of an automatic excitation regulator ARV, to the input of which voltage signals are received at the output of the SG (through the voltage transformer VT) and the load current of the SG (from the current transformer TT). The circuit contains a protection unit BZ, which provides protection of the excitation winding and the thyristor converter of the TP from overvoltage and current overload.
In modern synchronous motors, thyristor exciter devices are used for excitation, which are connected to an alternating current network and automatically control the excitation current in various engine operating modes, including transient ones. This excitation method is the most reliable and economical, since the efficiency of thyristor exciters is higher than that of DC generators. The industry produces thyristor exciter devices for various excitation voltages with a permissible DC value of 320 A.
Excitatory thyristor devices of types TE8-320/48 (excitation voltage 48 V) and TE8-320/75 (excitation voltage 75 V) are most widely used in modern series of synchronous motors. The excitation power is typically between 0.2% and 5% of the useful power of the machine (lower value applies to large machines).
In synchronous machines of low power, the principle of excitation by permanent magnets is used, when permanent magnets are located on the rotor of the machine. This method of excitation makes it possible to save the machine from the excitation winding. As a result, the design of the machine becomes simpler, more economical and more reliable. However, due to the scarcity of materials for the manufacture of permanent magnets with a large supply of magnetic energy and the complexity of their processing, the use of excitation by permanent magnets is limited only to machines with a power of no more than a few kilowatts.
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EXCITATION SYSTEMS FOR SYNCHRONOUS MACHINES
Most synchronous machines have electromagnetic excitation. DC sources for excitation windings are special excitation systems, which are subject to a number of important requirements:
1) reliable and stable regulation of the excitation current in any operating mode of the machine;
2) sufficient speed, for which excitation forcing is used, i.e., a rapid increase in the excitation voltage to a limit value called the ceiling. Forcing excitation is used to maintain stable operation of the machine during accidents and in the process of eliminating their consequences. The ceiling excitation voltage is chosen at least 1.8-2 of the rated excitation voltage. The rate of voltage rise during excitation forcing must be at least 1.5-2 rated voltages on the slip rings of the rotor per second;
3) rapid damping of the magnetic field, i.e., a decrease in the excitation current of the machine to zero without a significant increase in the voltage on its windings. The need to extinguish the field arises when the generator is turned off or damaged in it.
Several systems are used to excite synchronous machines. The simplest of them is an electric machine excitation system with a direct current exciter (Fig. 15). In this system, a special DC generator is used as a source. G.E., called the pathogen; it is driven from the shaft of a synchronous generator, and its power is 1-3% of the power of a synchronous generator. Synchronous machine excitation current I c is relatively large and amounts to several hundreds and even thousands of amperes. Therefore, it is regulated by rheostats installed in the excitation circuit of the exciter. The excitation of the exciter is carried out according to the self-excitation scheme (Fig. 15) or independent excitation from a special DC generator GEA, called a sub-exciter (Fig. 16). The subexciter works with self-excitation, and the resistance of the resistor R w2 does not change during generator operation.
To quench the magnetic field, an automatic field quenching machine (AGP) is used, which consists of contactors To 1 and To 2 and quenching resistor R p . Field blanking is carried out in the following order. When the contactor is on To 1 contactor turns on To 2, closing the excitation winding to a quenching resistor having resistance r p ≈5 r at where r c is the resistance of the excitation winding. Then the contactor opens To 1 and the current in the excitation winding circuit of the generator decreases (Fig. 17).
The excitation current could be reduced to zero by opening only one contactor To 1 without including a quenching resistor. The excitation current in this case would disappear almost instantly. But an instant break in the excitation circuit is unacceptable, because due to the high inductance of the excitation winding L in it a large EMF of self-induction would be induced e= - L in ∙ diin/ dt, exceeding the rated voltage by several times, as a result of which a breakdown of the insulation of this winding is possible. In addition, the contactor To 1 when it was turned off, significant energy would be released stored in the magnetic field of the field winding, which could cause the destruction of the contactor.
Forcing excitation when using the circuits in fig. 15 and 16 is carried out by shunting a resistor R sh1 included in the excitation circuit of the exciter.
Recently, instead of electric machine systems, valve excitation systems with diodes and thyristors are increasingly used. These excitation systems can be built for high power and are more reliable than electric machines.
There are three types of valve excitation systems: a self-excited system, an independent excitation system and a brushless excitation system.
In a valve system with self-excitation (Fig. 18), the energy taken from the armature winding of the main generator is used to excite the synchronous machine G, which is then converted by the static converter PU into direct current power. This energy enters the excitation winding. The initial excitation of the generator occurs due to the residual magnetization of its poles.
In a valve independent excitation system (Fig. 19), the energy for excitation is obtained from a special exciter GN, made in the form of a three-phase synchronous generator. Its rotor is mounted on the shaft of the main generator. The alternating voltage of the exciter is rectified and supplied to the excitation winding.
A kind of valve independent excitation system is a brushless excitation system. In this case, an AC exciter armature with a three-phase winding is placed on the shaft of the main synchronous machine. The alternating voltage of this winding, with the help of a rectifier bridge mounted on the machine shaft, is converted into a constant voltage and directly (without rings) is fed to the excitation winding of the main generator. The excitation winding of the exciter is located on the stator and receives power from an independent source.