Determining the dynamic overload capability

Function overview

The rated DC current specified on the unit rating plate (maximum permissible continuous DC current) may be exceeded in operation. The extent to which this value is exceeded and how long this lasts are subject to certain limits, which are explained in more detail in the following.

The absolute upper limit for the value of the overload currents is 1.8x the rated DC current. The maximum overload duration depends on the time characteristic of the overload current as well as on the load history of the unit and also depends on the specific unit.

Each overload must be preceded by an underload (load phase with load current < rated DC current). Once the maximum permissible overload duration has elapsed, the load current must return to at least an absolute value ≤ the rated DC current.

The dynamic overload duration is made possible by thermally monitoring the power section (I²t monitoring). I²t monitoring uses the time characteristic of the actual load current to calculate the time characteristic of a substitute value for the increase of the depletion layer temperature of the thyristors above the ambient temperature. In this case, unit-specific properties (e.g. thermal resistances and time constants) are incorporated in the calculation. When the converter unit is switched on, the calculation process starts with the initial values that were determined before the shutdown/line supply failure. The environmental conditions (ambient temperature and installation altitude) must be taken into account when setting a parameter.

I²t monitoring responds when the calculated substitute depletion layer temperature rise exceeds the permissible value. Two alternatives can be parameterized as response:

• Alarm with a reduction of the armature current setpoint to the rated DC current or
• Fault with unit shutdown

I²t monitoring can be disabled. In this case, the armature current is limited to the rated DC current.

Configuring for the dynamic overload capability

The configuring sheets contain the following information:

• The maximum overload duration t_an when starting with a cold power section and specified, constant overload,
• The maximum zero current interval t_ab (maximum cooling down time) until the "cold" thermal state of the power section is reached, and
• Fields of limiting characteristics for determining the overload capability during thermally stabilized, intermittent operation with overload (periodic duty cycles)

Technical support personnel from the local Siemens office can provide assistance with the selection of units for duty cycles involving multiple duty stages and cycle times in excess of 300 s.

Remark: The power section is considered to be "cold" if the calculated substitute depletion layer temperature rise is less than 5 % of its maximum permissible value. This state can be queried using a binary assignable output.

Structure of the fields of limiting characteristics for intermittent operation with overload

The fields of limiting characteristics refer to a duty cycle of the intermittent overload operation with a total duration (time period) of 300 s. Such a duty cycle comprises two time sections - the base-load duration (armature current actual value ≤ rated DC current) and the overload duration (armature current actual value ≥ rated DC current).

Each limiting characteristic represents a unit-specific maximum base-load current for a specific overload factor (limiting base-load current, specified as a % of the rated DC current) over the minimum base-load duration (limiting base-load duration). For the remaining duration of the duty cycle, the maximum permissible overload current is determined by the overload factor. If no limiting characteristic has been specified for the required overload factor, then it will be subject to the limiting characteristic for the next highest overload factor.

The fields of limiting characteristics are valid for a duty cycle of 300 s. Using basic calculation algorithms, duty cycles can be configured with duty cycle durations of longer than or shorter than 300 s. This will now be shown using two basic tasks.

Characteristic example for basic tasks 1 and 2

Basic task 1

• Given:
  Unit, cycle duration, overload factor, overload duration
• To be found:
  (Min.) base-load duration and max. base-load current
• Solution:

Example for basic task 1

• Given:
  
  • Unit with 30 A
  • Cycle duration 113.2 s
  • Overload factor 1.45
  • Overload duration 20 s
• To be found:
  
  • (Min.) base-load duration
  • Max. base-load current
• Solution:
  
  • Limiting characteristic for a unit with 30 A
  • Overload factor 1.5
  • Overload duration₃₀₀ = 300 s/113.2 s Г— 20 s = 53 s в†’
  • Max. base-load current = 44 % I_rated = 13.2 A

Basic task 2

• Given:
  Unit, cycle duration, overload factor, base-load current
• To be found:
  Maximum overload duration, minimum base-load duration
• Solution:

Example for basic task 2

• Given:
  
  • Unit with 30 A
  • Cycle duration 140 s
  • Overload factor 1.15
  • Base load current = 0.6 Г— I_rated = 18 A
• To be found:
  
  • Maximum overload duration
  • Minimum base-load duration
• Solution:
  
  • Limiting characteristic for a unit with 30 A
  • Overload factor 1.2
  • Base-load current = 60 % I_rated в†’
  • Overload duration₃₀₀ = 127 s
  • Max. overload duration = 140 s/300 s Г— 127 s = 59 s
  • Min. base-load duration = 140 s - 59 s = 81 s

Explanation of terms:

Base-load duration₃₀₀ = min. base-load duration for 300 s cycle duration (300 s - overload duration)

Overload duration₃₀₀ = max. overload duration for 300 s cycle duration

6RA8013-6DV62-0AA0 15 A/four-quadrant operation 400 V, 6RA8013-6FV62-0AA0 15 A/four-quadrant operation 480 V

6RA8018-6DV62-0AA0 30 A/four-quadrant operation 400 V, 6RA8018-6FV62-0AA0 30 A/four-quadrant operation 480 V

6RA8025-6DS22-0AA0 60 A/two-quadrant operation 400 V, 6RA8025-6FS22-0AA0 60 A/two-quadrant operation 480 V, 6RA8025-6GS22-0AA0 60 A/two-quadrant operation 575 V

6RA8025-6DV62-0AA0 60 A/four-quadrant operation 400 V, 6RA8025-6FV62-0AA0 60 A/four-quadrant operation 480 V, 6RA8025-6GV62-0AA0 60 A/four-quadrant operation 575 V

6RA8028-6DS22-0AA0 90 A/two-quadrant operation 400 V, 6RA8028-6FS22-0AA0 90 A/two-quadrant operation 480 V

6RA8028-6DV62-0AA0 90 A/four-quadrant operation 400 V, 6RA8028-6FV62-0AA0 90 A/four-quadrant operation 480 V

6RA8031-6DS22-0AA0 125 A/two-quadrant operation 400 V, 6RA8031-6FS22-0AA0 125 A/two-quadrant operation 480 V, 6RA8031-6GS22-0AA0 125 A/two-quadrant operation 575 V

6RA8031-6DV62-0AA0 125 A/four-quadrant operation 400 V, 6RA8031-6FV62-0AA0 125 A/four-quadrant operation 480 V, 6RA8031-6GV62-0AA0 125 A/four-quadrant operation 575 V

6RA8075-6DS22-0AA0 210 A/two-quadrant operation 400 V, 6RA8075-6DV62-0AA0 210 A/four-quadrant operation 400 V, 6RA8075-6FS22-0AA0 210 A/two-quadrant operation 480 V, 6RA8075-6FV62-0AA0 210 A/four-quadrant operation 480 V, 6RA8075-6GS22-0AA0 210 A/two-quadrant operation 575 V, 6RA8075-6GV62-0AA0 210 A/four-quadrant operation 575 V

6RA8078-6DS22-0AA0 280 A/two-quadrant operation 400 V, 6RA8078-6DV62-0AA0 280 A/four-quadrant operation 400 V, 6RA8078-6FS22-0AA0 280 A/two-quadrant operation 480 V, 6RA8078-6FV62-0AA0 280 A/four-quadrant operation 480 V

6RA8081-6DS22-0AA0 400 A/two-quadrant operation 400 V, 6RA8081-6GS22-0AA0 400 A/two-quadrant operation 575 V

6RA8081-6DV62-0AA0 400 A/four-quadrant operation 400 V, 6RA8081-6GV62-0AA0 400 A/four-quadrant operation 575 V

6RA8082-6FS22-0AA0 450 A/two-quadrant operation 480 V, 6RA8082-6FV62-0AA0 450 A/four-quadrant operation 480 V

6RA8085-6DS22-0AA0 600 A/two-quadrant operation 400 V, 6RA8085-6FS22-0AA0 600 A/two-quadrant operation 480 V, 6RA8085-6GS22-0AA0 600 A/two-quadrant operation 575 V

6RA8085-6DV62-0AA0 600 A/four-quadrant operation 400 V, 6RA8085-6FV62-0AA0 600 A/four-quadrant operation 480 V, 6RA8085-6GV62-0AA0 600 A/four-quadrant operation 575 V

6RA8086-6KS22-0AA0 720 A/two-quadrant operation 690 V

6RA8086-6KV62-0AA0 760 A/four-quadrant operation 690 V

6RA8087-6DS22-0AA0 850 A/two-quadrant operation 400 V, 6RA8087-6FS22-0AA0 850 A/two-quadrant operation 480 V

6RA8087-6DV62-0AA0 850 A/four-quadrant operation 400 V, 6RA8087-6FV62-0AA0 850 A/four-quadrant operation 480 V, 6RA8087-6GV62-0AA0 850 A/four-quadrant operation 575 V

6RA8087-6GS22-0AA0 800 A/two-quadrant operation 575 V

6RA8088-6LS22-0AA0 950 A/two-quadrant operation 830 V, 6RA8088-6LV62-0AA0 950 A/four-quadrant operation 830 V

6RA8090-6GS22-0AA0 1100 A/two-quadrant operation 575 V, 6RA8090-6GV62-0AA0 1100 A/four-quadrant operation 575 V

6RA8090-6KS22-0AA0 1000 A/two-quadrant operation 690 V, 6RA8090-6KV62-0AA0 1000 A/four-quadrant operation 690 V

6RA8091-6DS22-0AA0 1200 A/two-quadrant operation 400 V, 6RA8091-6FS22-0AA0 1200 A/two-quadrant operation 480 V, 6RA8091-6FV62-0AA0 1200 A/four-quadrant operation 480 V, 6RA8091-6DV62-0AA0 1200 A/four-quadrant operation 400 V

6RA8093-4DS22-0AA0 1600 A/two-quadrant operation 400 V, 6RA8093-4DV62-0AA0 1600 A/four-quadrant operation 400 V, 6RA8093-4GS22-0AA0 1600 A/two-quadrant operation 575 V, 6RA8093-4GV62-0AA0 1600 A/four-quadrant operation 575 V

6RA8093-4KS22-0AA0 1500 A/two-quadrant operation 690 V, 6RA8093-4KV62-0AA0 1500 A/four-quadrant operation 690 V, 6RA8093-4LS22-0AA0 1500 A/two-quadrant operation 830 V, 6RA8093-4LV62-0AA0 1500 A/four-quadrant operation 830 V

6RA8095-4DS22-0AA0 2000 A/two-quadrant operation 400 V, 6RA8095-4DV62-0AA0 2000 A/four-quadrant operation 400 V

6RA8095-4GS22-0AA0 2000 A/two-quadrant operation 575 V, 6RA8095-4GV62-0AA0 2000 A/four-quadrant operation 575 V

6RA8095-4KS22-0AA0 2000 A/two-quadrant operation 690 V, 6RA8095-4KV62-0AA0 2000 A/four-quadrant operation 690 V

6RA8095-4LS22-0AA0 1900 A/two-quadrant operation 830 V, 6RA8095-4LV62-0AA0 1900 A/four-quadrant operation 830 V

6RA8096-4GS22-0AA0 2200 A/two-quadrant operation 575 V, 6RA8096-4GV62-0AA0 2200 A/four-quadrant operation 575 V

6RA8096-4MS22-0AA0 2200 A/two-quadrant operation 950 V, 6RA8096-4MV62-0AA0 2200 A/four-quadrant operation 950 V

6RA8097-4GS22-0AA0 2800 A/two-quadrant operation 575 V, 6RA8097-4GV62-0AA0 2800 A/four-quadrant operation 575 V

6RA8097-4KS22-0AA0 2600 A/two-quadrant operation 690 V, 6RA8097-4KV62-0AA0 2600 A/four-quadrant operation 690 V

6RA8098-4DS22-0AA0 3000 A/two-quadrant operation 400 V, 6RA8098-4DV62-0AA0 3000 A/four-quadrant operation 400 V

Load classes

In order to be able to adapt the SINAMICS DC MASTER as simply as possible to the load profile of the driven machine, in addition to the individual dimensioning using the limiting characteristics of the dynamic overload capability, these can also be dimensioned using pre-selected load cycles that are simple to parameterize.

Note:
SINAMICS DC MASTER does not monitor whether the load class - set using parameters - is maintained. If the power section permits it, the unit can operate for overload durations in excess of those defined by the load class. This means that the driven machine of the mechanical system is not protected against overload!

The overload duration that is actually permitted for the power section in question is always longer than the duration defined by the load class. SINAMICS DC MASTER monitors whether the overload duration that is actually permitted for the power section is being maintained.

Load class (parameter)	Load for the converter	Load cycle
DC I	IDC I continuous (IdN)
DC II	IDC II for 15 min and 1.5 Г— IDC II for 60 s
DC III	IDC III for 15 min and 1.5 Г— IDC III for 120 s
DC IV	IDC IV for 15 min and 2 Г— IDC IV for 10 s
US rating	IUS for 15 min and 1.5 Г— IUS for 60 s Note: With this setting, for all unit types, an ambient and/or coolant temperature of 45 °C is permissible.

Duty cycles for two-quadrant operation

Duty cycles for four-quadrant operation