The mode of operation of a synchronous machine in parallel with the network at a synchronous speed is called synchronous.

Consider an implicit-pole machine switched on for parallel operation, neglecting the active resistance of the phases of the armature winding ().

The armature winding current will be equal to

Reactive power change. Synchronous compensator mode.

If all the conditions for switching on the generator for parallel operation are met, the armature current is zero, the machine is idling. If the excitation current of the generator after synchronization is increased, then, and there is a current lagging behind 90 el. deg. (Fig. 3.23, a). The machine will supply inductive current and reactive power to the network. If the excitation current of the generator is reduced, then a leading relative current occurs (Fig. 3.23, b). The machine will supply capacitive current to the network and consume reactive power from the network.

A synchronous machine that does not carry an active load and is loaded with reactive current is called synchronous compensator.

Active power change. Generator and engine mode.

In order for the machine connected to parallel operation to generate active power, to work in generator mode, it is necessary to increase the mechanical torque on the shaft (Fig. 3.23, c). In this case, a current lagging behind arises. The value of the active power of the generator is

If, on the contrary, we slow down the rotor of the machine, creating a mechanical load on its shaft, then the EMF will lag behind by an angle, current by an angle (Fig. 3.23, d). In this case, the active power will be equal, the machine will operate in engine mode, consuming active power from the network.

When data is exchanged at the physical layer, the unit of information is a bit, so the physical layer means always maintain bit-by-bit synchronization between the receiver and the transmitter.

The link layer operates on data frames and provides synchronization between the receiver and transmitter at the frame level. It is the responsibility of the receiver to recognize the start of the first byte of the frame, recognize the boundaries of the frame fields, and recognize the end of the frame flag.

It is usually sufficient to ensure synchronization at these two levels - bit and frame - so that the transmitter and receiver can ensure a stable exchange of information. However, if the quality of the communication line is poor (this usually applies to switched telephone channels), additional means of synchronization at the byte level are introduced to reduce the cost of equipment and increase the reliability of data transmission.

This mode of operation is called asynchronous or start-stop. Another reason for using this mode of operation is the presence of devices that generate data bytes at random times. This is how the keyboard of a display or other terminal device works, from which a person enters data for processing by a computer.

In asynchronous mode, each byte of data is accompanied by special start and stop signals. The purpose of these signals is, firstly, to notify the receiver of the arrival of data and, secondly, to give the receiver enough time to perform some timing-related functions before the next byte arrives. The start signal has a duration of one clock interval, and the stop signal can last one, one and a half, or two clocks, so one, one and a half, or two bits are said to be used as a stop signal, although these signals do not represent user bits.

The described mode is called asynchronous because each byte can be slightly shifted in time relative to the bit clocks of the previous byte. Such asynchronous transmission of bytes does not affect the correctness of the received data, since at the beginning of each byte, the receiver is additionally synchronized with the source due to the "start" bits. More "free" time tolerances determine the low cost of the equipment of the asynchronous system.

In synchronous transfer mode, there are no start-stop bits between each pair of bytes. User data is collected in a frame that is preceded by sync bytes. The sync byte is a byte containing a pre-known code, such as 0111110, which notifies the receiver that a data frame has arrived. Upon receiving it, the receiver must enter into byte synchronization with the transmitter, that is, correctly understand the beginning of the next byte of the frame. Sometimes several sync bytes are used to provide more reliable synchronization between the receiver and transmitter. Since the receiver may have problems with bit synchronization when transmitting a long frame, self-synchronizing codes are used in this case.

In normal operation, two moments act on the generator shaft (we assume that we can neglect the moment of resistance due to friction in the bearings and the resistance of the cooling medium): turbine moment Mt, which rotates the generator rotor and tends to accelerate its rotation, and synchronous electromagnetic torque Mem, opposing the rotation of the rotor. In the event of an imbalance between the turbine torque and the electromagnetic (braking) torque of the generator, depending on the severity of the disturbance, synchronous oscillations or asynchronous generator operation may occur.

Asynchronous mode (asynchronousregime) – transient mode in the power system, characterized by non-synchronous rotation of part of the generators of the power system.

Asynchronous modes can result from:

Violations of static stability due to an increase in the transmitted power over power lines of an excess value;

Violations of dynamic stability due to emergency disturbances (short circuits, shutdown of generating equipment or electrical installations of the consumer);

Non-synchronous switching on of power lines and generators;

Generator excitation losses.

It should be noted that the asynchronous modes of operation of an unexcited and excited synchronous machine differ significantly from each other.

1. Asynchronous mode of an excited synchronous machine

As an example, consider the transition of a generator to an asynchronous mode of operation due to a violation of dynamic stability (see Fig. 1) in the event of a short circuit with a power line disconnection.

A characteristic feature of this dependence is the presence of a clearly defined maximum and minimum. The difference between the asynchronous mode and synchronous swings in terms of current change lies only in the magnitude of the maximum current value in the swing cycle and in the duration of these swings. Since the angle during synchronous swing can theoretically reach its critical value, it is impossible to distinguish asynchronous mode from synchronous swing only by the value of the current. Therefore, ALAR devices based on the detection of an asynchronous mode by current fluctuations are configured to work on the second, third, etc. asynchronous mode loop. In other words, a selectively asynchronous mode can be detected only by long-term current fluctuations with an amplitude not less than a given one and a period not exceeding the calculated one.

Dependence of voltage change and mutual angle between two voltage vectors in asynchronous mode

The expression for determining the voltage at intermediate points is determined in accordance with the second Kirchhoff law according to the following formula:

Relative remoteness of the controlled point with voltage from the point with voltage .

In asynchronous mode, the EMF vector of a synchronous machine that has fallen out of synchronism begins to rotate relative to the EMF vector of machines operating synchronously. It should be noted that in the general case, the rotation of the vector can occur both clockwise and counterclockwise:

counterclock-wise are accelerating

If the power system vector #2 rotates clockwise, then this indicates that the generators of power system No. 2 slow down regarding the generators of the power system No. 1.

As an example, consider the rotation of the vector of system No. 2 in the presented design scheme "clockwise".

Analysis of the obtained expressions shows that at the moment of divergence of the voltage of system No. 1 and system No. 2 at an angle of 180 degrees (asynchronous rotation), the active power changes its sign, and the value of reactive power reaches its maximum value. This feature of power change at the moment of asynchronous cranking is used by various manufacturers in ALAR devices, regardless of the element base (electromechanical or microprocessor devices).

In the general case, the hodograph of the total power vector (S= P+ j Q) at the measuring point (installation of the power relay) is an ellipse (dependence of P on Q) when the angle changes. Features of changing the power hodograph in the asynchronous cycle make it possible to identify the moment of the onset of the asynchronous mode, if it is possible to fix the transition of the indicated hodograph from the range of angles ~0<δ<180° в диапазон ~180 0 <δ<360 0 при выполнении дополнительного условия, характеризующего зону δ≈180°.

Dependence of resistance change in asynchronous mode

The resistance at the terminals of the resistance relay is determined as the quotient of dividing the voltage at the controlled point by the current

Taking into account the ratio between the voltage modules at the ends of the power line the resulting expression can be transformed into the following form:

Analysis of the obtained expression shows that the resistance hodograph is a circle (ellipse) displaced relative to the origin. Depending on the ratio of the voltage modules at the ends of the power line, the resistance change characteristic has a different form.

Dmitry Ivanov, 10 December 2013

In this article, we will get acquainted with the synchronous mode of operation of the WoodmanUSB module. It is in it that you can get the maximum data transfer rates. What is the fundamental difference between this mode and the asynchronous mode that we considered earlier? In synchronous mode, in addition to the read / write line, a separate clock line must also be used ( CLK), and the control signals for reading and writing must be quite accurately tied in time to the clock signal. Thanks to this synchronization, WoodmanUSB allows you to receive data transfer rates up to 220 Mbps.

Let's start with the basics. There are several options for synchronous mode. First of all, it is necessary to select a mode with internal and external clocking. With external clocking, the clock signal is fed to the CLK line of the module (works as an input) from an external device. With internal clocking, the module itself generates a clock signal and outputs it to the CLK line (works as an output). The external device is clocked by this signal. The module can generate two clock frequencies: 30 and 48 MHz.

Now let's look at what needs to be done at the program level in order to work with the module's PORTB port in synchronous mode. Here everything is very simple. It is only necessary to pass the desired constant to the function WUSB_SetupPortB()- and you can use the read / write functions as before without any changes. The WUSBdrv.dll library defines three constants for synchronous mode: SYNC_MODE_EXTERNAL_CLK - the clock signal will be external relative to the module (supplied by an external device to the module's CLK line), SYNC_MODE_INTERNAL_CLK_30MHZ - an internal clock signal with a frequency of 30 MHz (issued outside through the CLK line) and SYNC_MODE_INTERNAL_CLK_48MHZ - also most, only the frequency of 48 MHz.

//SYNC_MODE_EXTERNAL_CLK 0x0C //SYNC_MODE_INTERNAL_CLK_30MHZ 0x14 //SYNC_MODE_INTERNAL_CLK_48MHZ 0x1C WUSB_SetupPortB(SYNC_MODE_INTERNAL_CLK_30MHZ);

I repeat once again that working with read / write functions in synchronous mode does not differ in any way from that considered earlier in asynchronous mode.

Now let's look at a timing diagram illustrating the "relationship" between the clock signal and the read/write control signals.

1. Synchronous mode. Reading data from the module by an external device

Table 1.1

Table 1.2

2. Synchronous mode. Writing data to the module by an external device

Table 2.1 Synchronous mode parameters for internal clocking

Parameter	Description	Min	Max
	Clock period

Table 2.2 Synchronous mode parameters for external clocking

Parameter	Description	Min	Max
	Clock period
	Read signal preset time
	Read hold time
	Data preset time on PORTB lines
	Data hold time on PORTB lines

Now let's do a little test illustrating the potential transfer rates in synchronous mode. We will leave the ideology as in the last article - we do not really process the data itself, we generate only read / write signals and in this mode also a clock signal. Also, let's decide in which of the subtypes of the synchronous mode we will work. I suggest using it with internal clocking at 48 MHz, since everything is more difficult with an external clock, rather strict requirements for timing characteristics must be observed. The test device schematic is shown below. As can be seen from the figure, the read/write control signals coincide with the clock signal, which, in the internal clocking mode, is output "outside" the module via the CLK line.

We use the program from the last article. The only change that needs to be made is to call the function WUSB_SetupPortB() with the SYNC_MODE_INTERNAL_CLK_48MHZ parameter. Screenshots of test results are shown below.

I think you will agree that the results are not bad at all. In total, we can say that the synchronous mode is noticeably more difficult in terms of hardware implementation than the asynchronous one, but its use allows you to get the maximum data transfer rates. The complexity of the hardware implementation is due to the fact that in reality for data transfer it is necessary to analyze the states of the buffers in order to avoid data loss when they overflow this time, then it is necessary to make a correction for the fact that the external device must be fast enough to provide the specified temporal characteristics of the generation of control signals and their clock synchronization.

Here, the transmitter and receiver act independently and exchange a sync bit pattern at the beginning of each message chip (frame). There is no fixed relationship between one message frame and the next. This is analogous to communication devices such as a computer keyboard, where input may occur with long, random pauses between keystrokes.

Rice. 2.13. Asynchronous data transfer

The initially selected baud rate determines the polling rate (with the exception of "Autobaud" systems). The channel polling rate at the receiver is high, typically 16 times the bit rate, to accurately determine the center of the sync pattern (start bit) and its duration.

Rice. 2.14. Sync Signal Extraction

The data bits are then determined by the receiver by polling the channel at times corresponding to the middle of each transmitted bit. They are defined by adding for; each subsequent cycle of the value of the duration of the bit, starting from the middle of the start bit. For an eight-bit serial transmission, this polling is performed for each of the eight data bits, and the final sample is taken during the ninth time slot. The last sample is used to determine the stop bit and confirm that synchronization is preserved until the end of the message frame. Rice. 2.15 illustrates the process of receiving data asynchronously.

Rice. 2.15. Asynchronous data reception

2.4.4. Synchronous transmission

Here, the transmitter and receiver establish initial synchronization, then continuously transmit data, maintaining it throughout the entire transmission session. This is achieved through special data coding schemes, such as Manchester Encoding, which ensure that the transmitter's clock signals are continuously written to the transmitted data stream. In this way it is possible to keep the receiver synchronized down to the last bit of the message, which can be up to 4500 bytes (36000 bits) long. This makes it possible to efficiently transmit large data frames at high speeds. A synchronous system packs a lot of characters together and sends them in a continuous stream called a block. Each block has a header containing a start delimiter for initial synchronization and information about the block, and a trailing part for error checking, etc. An example of a synchronous transmission block is shown in Figure 2.16.