Coordination of MCCB and high-voltage side OCR

When there is an OCR on the high-voltage side, the low voltage side MCCB must establish a coordinated relation with that OCR. The configuration shown in Fig. 4. 35 will be reviewed here. The power receiving OCR’s CT ratio, tap value and dial settings are determined by the coordination with the power company substation’s feed OCR. At the same time, the following conditions are considered.

If an instantaneous tripping element is provided, the setting value must be 10-times or more than the transformer’s rated current so the breaker does not malfunction with the transformer’s exciting rush current.

To protect the transformer from short-circuits, the breaker must function in less than 2s when a current 25-times the rated current flow. Fig. 4. 36 is a figure to review the coordination relation shown in Fig. 4. 35. The figure shows the values converted to the low-voltage side.

 (1) Setting the OCR

The rated primary current is 87.5A, so the CT ratio is 150/5. Due to the relation with the substation’s feed OCR, the timedelay dial is normally 0.2s or less at the restricted section. If an instantaneous element is included, it is set to 1s or less. The dial No. 2 operating characteristics for the Mitsubishi general-purpose relay MOC-E are shown here. When considering coordination with the downstream MCCB, the inertia must also be considered and is shown with a dotted line. The instantaneous tripping element is q shown above, and is set to 30A here.

(2) Setting the electronic MCCB

Consider the 800A and 600A settings for MCCB. For the reasons explained in the next section, the MCCB type short-time delay tripping characteristics are set with the dial to 5-times the rated current.

(3) Coordination of OCR and electronic MCCB

The short-time tripping current value of the MCCB is 2 to 10-times the rated current. When set to 10-times, the 600A setting is 6000A, and the 800A setting is 8000A. In other words, when set to 10-times, the MCCB short-time delay tripping pickup value is larger than the OCR’s pickup value 4710A (secondary conversion). Thus, a favorable relation can be established by setting both to 5-fold so the resulting value is 4710A or less. The OCR has an instantaneous tripping element and the setting value is 30A (secondary conversion 28.3kA), so the OCR and MCCB selective tripping range is restricted to this current value.

(4) Coordination of electronic MCCB and downstream MCCB

Assume that a 250A frame is provided as the downstream MCCB. The model is MCCB (175A), and the maximum and minimum operating characteristics are as shown in Fig. 4. 36. It can be seen that a favorable selective tripping relation is established and that the maximum operating characteristics curve does not cross the MCCB operating characteristics curve.

Fig: 4.36