Synchroscope

The synchroscope is an electrotechnical meter , which in substations and power plants is used, the interconnection of synchronous AC voltage sources such as electric generators or the merger between power grids ensure.

In the course of the connection, three parameters - voltage, frequency and phase position  - of the different network areas are compared with one another. The successful switching process takes place at the synchronization point , that point in time at which the three variables are within permissible small (ideally none) deviations from one another. If alternating voltage sources or alternating voltage networks that are asynchronous to one another are not interconnected to form the synchronization point, voltage jumps and malfunction of other electrical devices (e.g. protective devices) in the network can occur. In the worst case of asynchronous interconnection, the incorrect interconnection results in an electrical short circuit equal.

construction

Synchronisiersäule with synchronoscope, double voltage and double tongue-rate monitor

The synchronization device (synchronization column) contains all the measuring devices required for synchronization and consists of

• Double tension meter
• Double frequency meter
• Switch indicator synchronoscope

In operations services, manual synchronization, semi-automatic synchronization and fully automatic synchronization, which is almost exclusively used in larger interconnected networks, are still common in special cases for smaller systems . The fully automatic synchronization offers the advantage that switching errors can be excluded with a high degree of probability.

synchronization

The manual synchronization is carried out by connecting the respective branches or parts of the power network in parallel by switching on the associated circuit breaker , taking into account the intrinsic operating time of the switch, in the synchronization point. A so-called synchronization socket wrench is used here, with which the circuit breaker of the outgoing circuit is switched on, taking into account the synchronization conditions.

Electrotechnical systems are essentially designed according to the one-key method, whereby a synchronizing socket wrench is used for the entire system. With the two-key method, the synchronization switch is actuated in the feeder to be switched in parallel and the feeder used for parallel connection.

The semi-automatic synchronization takes place by connecting the respective feeders and power supply units in parallel by switching on the associated circuit breaker in the synchronization point with the parallel switching device. The parallel switching device issues the switch-on impulse for the circuit breaker after selecting the relevant branch or outlet, taking into account the switching default time and the switching slip. The voltage and frequency in the network are not influenced by the device. The switching preset time is the time that must be preset for the switch-on pulse of the circuit breaker, taking into account the breaker operating time as well as the control and relay times, in order to enable parallel switching at the synchronous point. Switching slip is the maximum permissible frequency difference that depends on the network constants and the power to be synchronized or on the generator design, in order to avoid larger equalizing currents with oscillations when connected in parallel.

Fully automatic synchronization takes place by connecting the respective power supply units in parallel by switching on the associated circuit breaker in the synchronization point with a parallel switching device that works in conjunction with a voltage and frequency balancing device. The control system allows synchronizing today have more functions due to many different system configurations. In order to cover all necessary system configurations, modern synchronization devices have several types of synchronization. These can be for example:

• Generator with mains or line
• Synchronous or asynchronous networks or lines
• Synchronization check for manual connection
• Busbar quick changeover
• Measurement of the circuit breaker operating time

Establishing the connection to the network

The synchronous machine should be mechanically coupled to a working machine (e.g. turbine ). The connection of the anchor strands to the rigid net has not yet been established. The circuit breaker as the connection point is open. The working machine drives the synchronous machine at any speed . The field winding of the synchronous machine is fed with direct current . Since the armature does not carry any current, a magnetic field is only built up from the pole system in the present open circuit with open armature terminals , which is then equal to the resulting field. ${\displaystyle n}$

The armature winding of the synchronous machine is designed in star connection . The magnetic flux that is decisive for the voltage induction in the strand under consideration here has the angular frequency and induces the voltage of the same frequency. This is observed as the terminal voltage ${\displaystyle a}$ ${\displaystyle {\underline {\Phi }}_{h}={\underline {\Phi }}_{p}}$ ${\displaystyle \omega =2\pi pn}$ ${\displaystyle {\underline {e}}_{hp}=-j\omega (w\xi _{1}){\underline {\Phi }}_{p}}$

${\displaystyle {\underline {u}}={\underline {u}}_{p}=j\omega (w\xi _{1}){\underline {\Phi }}_{p}}$

a voltage, the amount of which is proportional because of the speed and because of the excitation current and the frequency of which is given by the current speed of the synchronous machine. Your instantaneous value can be represented as ${\displaystyle \omega =2\pi pn}$ ${\displaystyle n}$ ${\displaystyle {\hat {\Phi }}_{p}\sim I_{fd}}$

${\displaystyle u_{p}=\omega (w\xi _{1}){\hat {\Phi }}_{p}\cos(\omega t+\phi _{up})={\hat {u}}_{p}\cos(\omega t+\phi _{up})}$.

The voltage of the network to which the synchronous machine is to be connected shows the magnitude and the circular frequency . This voltage can be represented as follows for the strand under consideration ${\displaystyle {\hat {u}}_{n}}$ ${\displaystyle \omega _{n}}$ ${\displaystyle a}$

${\displaystyle {\underline {u}}_{N}={\hat {u}}_{N}\cos(\omega _{N}t+\phi _{uN})}$.

The circuit breaker as a connection to the rigid network can be switched on without a current starting to flow if the voltage is present at every instant before switching on

${\displaystyle u_{s}=u_{N}-u=0}$

is. From this it follows as the synchronization conditions that the voltages of the synchronous machine and of the network must correspond to the three determinants of a sine value according to the effective value, frequency and phase position. So it must apply:

1. ${\displaystyle U=U_{N}}$,
2. ${\displaystyle \omega =\omega _{n}}$or or and ${\displaystyle 2\pi pn=\omega _{N}}$ ${\displaystyle n=n_{0}={\frac {f_{N}}{p}}}$
3. ${\displaystyle \omega _{u}=\omega _{uN}}$

The simplest way of observing compliance with the synchronizing conditions is with the so-called synchronizing column. The two voltmeters (voltage synchronous machine or voltage of the network) are combined in a double voltmeter, with the pointers of both measuring mechanisms being assigned to a scale. The two frequency meters are also designed as double frequency meters, in which the rows of vibrating tongues lie next to one another in a housing. So-called phase lamps are used to observe the phase angle or the phase shift of the voltages and . ${\displaystyle u}$ ${\displaystyle u_{N}}$

Circuit variants

The synchronization conditions are met when the two voltages (synchronous machine and mains) have already been brought into agreement with regard to their frequencies and amounts and then also have the same phase position.

When switching the phase lamps, a distinction is made between dark switching and light switching . With dark switching, if the synchronization condition is fulfilled, there is no voltage across the switch contacts, so that the lamps connected in parallel to the contacts do not light up. With light switching, the phase lamps are not connected in parallel to the circuit breaker contacts, but between the conductors. The phase lamps then light up brightly in the synchronization point.

The voltages for the phase lamps are provided by the voltage converters of the two systems. Nowadays the phase lamps are only used in trial operation. The synchronoscope is used instead of the lamps . This is a device in which a pointer shows the deviation of the speed of the machine from the required synchronous speed through the direction and speed of its movement and, through its position in relation to a fixed mark, shows the phase shift between the two voltages. Its mode of operation is based on the comparison of two rotating fields, one of which is built up from the network and the other from the synchronous machine. By adjusting the speed member of the prime mover, the pointer moves faster or slower. The parallel switching takes place when the pointer slowly reaches the mark of the synchronization point in clockwise direction.

In order to carry out a proper synchronization by hand, the synchronization device is provided with a parallel switching lock. This prevents switching on if the voltage difference is too great or the speed of the drive machine is incorrect.

The measures that are required to meet the synchronization conditions are referred to as synchronization. The rotary field equality of the networks is implemented before the systems are commissioned for the first time. Then one begins to bring the frequencies of the two voltages into agreement. To do this, the speed of the synchronous machine must be changed accordingly. This is done by intervening in the speed control element of the drive machine. Then the amounts of the two voltages are adjusted by changing the excitation current of the synchronous machine. This sequence is necessary because in the other case the machine voltage, if its amount were first adjusted to that of the mains voltage by changing the excitation current, would also change during the subsequent frequency adjustment by changing the speed. ${\displaystyle f=pn}$

After the two voltages match in terms of frequency and magnitude, they can only differ in terms of their phase position. This phase shift is eliminated by again briefly intervening in the speed control elements of the drive machine. This changes the frequency of the machine voltage during this time. This short-term frequency change must end in the case of phase equality when there is no longer any voltage across the circuit breaker contacts. If the synchronization conditions are met, the circuit breaker can be switched on without equalizing currents starting to flow. In this case one speaks of a fine synchronization . The machine is connected to the network with a dead armature. By again intervening in the speed control element of the drive machine, an active power conversion is now forced. ${\displaystyle {\underline {u}}_{s}=0}$

literature

• G. Müller: Electrical machines . VEB Verlag Technik, Berlin.
• A. Varduhn, W. Nell: Handbook of electrical engineering . Fachbuchverlag GmbH, Leipzig.
• L. Schauer, A. Reissmann: Operation of electrical systems . VEB German publishing house for basic industry, Leipzig.