All Motors contribute to the main busbar short-circuit, reality or dream?

Credit to Author: AVELINO DE OLIVEIRA| Date: Mon, 16 Jul 2018 12:32:37 +0000

This post is part of the motor management series discussing various aspects of the motor integration in the electrical network and industrial process. This blog post presents a new chapter of this discussion with a case study of Motor contribution to the main busbar short circuit.

Correct short-circuit busbar sizing is an important design characteristic for every power distribution system and critical for ensuring the safety of the personnel and the equipment. In oil & gas platforms, where footprint and weight are cost savings factors, the optimum busbar sizing can bring substantial economies, especially on the main busbar. A lever for such optimization can be the control of the motor fault contribution or/and the short-circuit voltage of transformer (Ucc) or/and the impedance of the generator (X’’d). In the range below 17.5kV, a common fault-current limitation on the main busbar is 50kA sustained and 130kA peak current. When the values are higher this impacts the entire switchboard and circuit breaker ratings. The evaluation of the motor contribution and basic guidelines are the subject of this post.

In electro-intensive or heavy industries, the motors are the major energy consumers reaching up to 95% of the total consumption in Mining (90%), Oil and Gas (90%) and Water supply (95%). More information can be found in the “United States Industrial Electric Motor Systems Market Opportunities Assessment”, from “the US department of energy”. Consequently, their contribution to fault is also significant, even if the great majority of the current will be provided by the grid or the local generation.

For this case study, the example system is an oil and gas platform with 69% of motor loads on different busbar and voltage level. There are 3 main generators with total capacity of 153MVA at 13.8kV. On the main busbar are also connected 2 Medium Voltage Motors (8.74MW – 18.8MW). The rest of the rotating loads are connected at 6.6kV and 440V.

A preliminary short-circuit estimation shows on the main busbar the fault current from main generators will reach Isc ≈46kA and Ipeak≈127kA. This value is acceptable for the power distribution and switchboards design at 50kA – Ipeak=130kA (Common parameter for all suppliers).

The next step is to add the contribution of the large motors at 13.8kV. Their individual contribution is Isc ≈4.4kA – Ipeak≈11kA for the 18.8MW and Isc ≈1.9kA – Ipeak≈4.7kA for the 8.74MW.

Simply with these 2 motors and the generators the short-circuit current will exceed the objective switchboard rating, Isc ≈52.3kA and Ipeak≈142.7kA. Despite connecting cables and step-down transformers, the motors at 6.6kV and 440V will add some Isc≈5kA and Ipeak≈6.7kA to this already high value.

Getting back on objective values can be achieved by adding VSD (Variable Speed Drives) on the 2 large motors at 13.8kV or by modifying the main generators sub-transient impedance. The last is usually difficult as generators are first to order for their very long delivery time. In some cases, using low inrush current motors can be a very economical alternative.

For the case of interest, the short-circuit current will be reduced to values within the 50kA/130kA range with VSD on each of the large motors and motors (6.6kV and 440V) with low inrush current.

The table below gives typical values for motor starting and short-circuit current at 13.8kV:

The most efficient fault-current limitation is achieved with Variable Speed Drive (VSD) without AFE (Active Front End).

In conclusion, the fault-current contribution of motors is significant and a key for the optimization of the electrical equipment. In all the case, the motors need to be integrated of the short circuit calculation.

Learn more in the standards “IEC 60909”, a technical paper “CT158”  (Calculation of short-circuit currents), “United States Industrial Electric Motor Systems Market Opportunities Assessment”  from the US Department of energy  or see “our High Power Motor Management Panorama”.

Glossary

AFE: Active Front End.

IEC: International Electrotechnical Commission

Isc: Short circuit current.

Ipeak: Peak short-circuit current.

LV: Low Voltage.

MV: Medium Voltage.

RVSS: Reduced Voltage Soft Start

US: United States.

VSD: Variable Speed Drives.

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