Parallel connection

Parallel connection for redundant operation

Two SITOP power supplies of the same type can be connected in parallel through diodes for a redundant configuration. 100% redundancy only exists for two power supplies when the total load current is no higher than that which one power supply can supply alone and when the supply for the primary side is also implemented redundantly (i.e. a short-circuit on the primary side will not trigger a shared fuse which would disconnect both power supplies from the mains).

Parallel connection with decoupling diodes for redundant operation is permitted for all SITOP power supplies. The V1 and V2 diodes are used for decoupling. They must have a blocking voltage of at least 40 V (when decoupling from 24 V power supplies) and it must be possible to load them with a current equal to or greater than the maximum output current of the respective SITOP power supply. For diode dimensioning, see the following note "General information on selection of diodes".

The ready-to-use add-on SITOP PSE202U redundancy modules are available as a simple alternative to diode dimensioning (Article No.: 6EP1962-2BA00, 6EP1964-2BA00, 6EP1961-3BA21) and SITOP RED1200 (Article No.: 6EP4346-7RB00-0AX0, 6EP4347-7RB00-0AX0, 6EP4348-7RB00-0AX0) for redundant interconnection of two power supplies.

Parallel connection of two SITOP power supplies for redundant operation

General information on selection of diodes:

The diodes must be dimensioned for the maximum dynamic current. This can be the dynamic current during startup in the short-circuit case, or the dynamic current during a short-circuit in operation (the larger of the two values should be taken from the relevant technical specifications).

To dissipate the significant power loss of the decoupling diodes (steady short-circuit current rms value x diode conductive-state voltage), the diodes must be equipped with suitably dimensioned heat sinks.

An additional safety margin is recommended, because the output capacitor integral to the power supply generates an additional peak current in the short-circuit case. This additional current flows only for a few milliseconds so it is within the period in which diodes are permitted to be loaded with a multiple of the rated current (8.3 ms, known as the permissible surge current for diodes).

Example

Two 1-phase SITOP PSU8200 power supplies with 10 A rated output current (Article No.: 6EP1334-3BA10) are connected in parallel. The dynamic current in the event of a short-circuit during operation is approx. 30 A for 25 ms.

The diodes should therefore have a loading capability of 40 A to be safe, the common heat sink for both diodes must be dimensioned for the maximum possible current of approximately 24 A (steady short-circuit current rms value) x diode conductive-state voltage.

Parallel connection for performance enhancement

To enhance performance, identical types of most SITOP power supplies can be connected in parallel galvanically (the same principle as parallel connection for redundant operation, but without decoupling diodes):

The types permitted for direct galvanic parallel connection are listed in the relevant technical specifications under "Output, parallel connection for performance enhancement".

Requirement:

• The output cables connected to terminals "+" and "-" of every power supply should be installed with an identical length and cross-section (or the same impedance) to the common external linking point.
• The power supplies connected in parallel must be switched simultaneously using a common switch in the mains supply line (e.g. using the main switch available in control cabinets).
• The output voltages of the power supplies must be measured under no-load operation before they are connected in parallel and are permitted to differ by up to 50 mV. This usually corresponds to the factory setting. If the output voltage is changed in case of variable power supplies, the "-" terminals should first be connected and then the voltage difference between the "+" output terminals measured under no-load conditions before they are connected. The voltage difference must not exceed 50 mV.

Note

With a direct galvanic connection in parallel of more than two SITOP power supplies, further circuit measures may be necessary for short-circuit and overload protection!