Electrical transmission or interconnection systems – Plural supply circuits or sources – Connecting or disconnecting
Patent
1987-02-03
1989-12-12
Shoop, Jr., William M.
Electrical transmission or interconnection systems
Plural supply circuits or sources
Connecting or disconnecting
363 72, 368 46, H02J 100, G04C 1100
Patent
active
048869813
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE PRESENT INVENTION
The present invention relates to a circuit for synchronizing a plurality of power units with one another, and more, particularly to the type used in power stations for telecommunication systems.
Usually the latter are constituted by a plurality of power units (such as inverters, etc.) connected in parallel with one another. The parallel connection is used when the power station has to generate an amount of power that a single power unit alone could not supply and therefore, the required power is supplied by a plurality of power units connected in parallel one with another. Besides the number of units needed to provide the operating power, at least one additional unit is usually required in order to have a hot, active reserve in case one of the operative units fails. In almost all applications there is the problem that the units must be synchronized with one another and/or with the electrical power distribution line (i.e: the mains).
In particular, synchronizing operations are necessary when the units are DC-DC converters because, in case of a phase difference among the single units, low frequency interferences which lap oer the useful signal, may be registered.
On the other hand, when the power units are inverters, the synchronizing means are absolutely necessary, either to avoid damages described below, or to overcome the technical troubles which may arise from the switching power station-power distribution line.
It is useful to point out that the inverters are constituted by units adapted to receive a direct current on the input side and to deliver an output of alternating current having the same characteristics of the power distribution line.
The above mentioned switching must be carried out however with phase continuity, otherwise the single units will be pulled in to the mains at differentiated speeds and cause a phase irregularity from the single units which compose the power station.
The inverters of the latest generation are characterized by the fact that they have an output impedance that is sensibly low in order to approximate the characteristics of an ideal voltage generator. This characteristic implies a perfect synchronism among the inverters of a same station; because if one of the units is not in phase with the remaining units, the power delivered by the remaining units is transferred into the unit which is out of phase causing possible damage to it.
In accordance with well known solutions, the synchronization troubles mentioned above may be solved by sending to a common branch point the clock signals generated by the single inverters. A reference clock signal is thus created at the branch point that has a frequency equal to the arithmetical average of the inverters' frequencies. The devices are synchronized with the reference clock signal.
This system however presents the inconvenience that the pulling in field for the different oscillators must be limited in order to maintain the average frequency within acceptable values. Furthermore, the connection or disconnection of an inverter causes a transient frequency on the inverters actually operating.
Other solutions provide for a generation of a master clock signal through a central oscillator on which the single inverters are synchronized. This solution does not present the inconvenience of the connection transient described above, (because the inverter is connected once the synchronization has been adjusted), but on the other hand the following troubles may arise:
it maybe necessary to have a slave oscillator, operating in phase with the master oscillator in order to carry out the switching with phase continuity;
it introduces limits to the system's reliability due to the circuits which provide for the master-slave switching;
the cost of the structure described above (master oscillator, slave oscillator, switching circuits) constitutes a large part of the cost for a station when the station is only composed of two inverters.
The invention, as claimed, is intended to solve the problem of realizing a circui
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Lentini Salvatore
Patti Giuseppe
Italtel Societa Italiana
Logan Sharon D.
Shoop Jr. William M.
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