Load forecasting

Load Forecasting, Chilled Warehouse

An electrical contractor asked for advice on a logistics distribution site where an additional warehouse had been built. The new facility was to provide chilled storage. The site had a number of warehouses and was supplied at 11kV to 3 sub-stations through 4 x 11kV/415V transformers. The authorised supply capacity(ASC) to the site was 2MVA; the previous peak demand to the site had been 950kVA.

It was planned that eight condensers linked with eight air handling units would be installed and the refrigeration contractors stated that each unit would have a demand of 250 Amps, approximately 200kVA. Therefore, the estimated increased demand was 1600kVA, theoretically the future peak demand would increase to almost 2600kVA and an application for increased supply availability would be required. The project scheduled four units to be commissioned initially and the remainder at a later date.

The site survey identified that the transformer serving the new warehouse was rated at 1500kVA (2000A) and was set to nominal tapping 3; offload the output terminal Voltage was 433V. The main breaker on the LV panel was rated at 2500 Amps and commissioned to operate at 1000 Amps; the existing shed lighting and power demand exceeded 100kVA  Clearly, the incoming circuit breaker rating, after adjustment, could support the increased load but, the transformer would not. However, the new equipment specification sheet highlighted the equipment power factor to be 0.82.

It was recommended that an application to increase the ASC to 3MVA should be made to the local distribution company and 800kVAr of power factor correction be installed at the LV incoming supply to the warehouse, this would effectively reduce the apparent load to within the rating of the transformer and protect against possible excess reactive charges. Also, considering transformer tap down was suggested to reduce the secondary Voltage (chiller equipment commissioning notes warned against Voltages in excess of 400V).  Additionally, this could reduce the kWh consumption and energy costs. 

Following the commissioning of the first four chiller units the site demands were monitored and it was evident that the loads the refrigeration engineers stated were not apparent and the increased demand was only about 400kVA. Installation and commissioning data from the internet highlighted that the figures quoted by the refrigeration engineers were based on ambient temperatures of 43degC and evaporator temperatures of 2.5degC. In practice the design was for A30/E10.

The conclusion was that, although the increased ASC had been agreed, it would not be necessary and therefore the monthly charges would not increase. Transformer tap down and PFC were still a recommendation but left with the site management for future consideration.

During the project a request was made by the site management to look at an energy efficiency review report from an energy consultancy, relating to a recommendation to install additional power factor correction capacitors in one of the main LV distribution boards. The budget cost for this work was £6000.00; inspection of this equipment identified that the circuit breaker had tripped. It was switched on and the staged capacitance improved the power factor to 0.99pf. This saved the cost of additional equipment.

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