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Topic Title: Maximum Demand
Topic Summary: Can the 30 minutes be extended
Created On: 02 June 2010 09:32 AM
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 02 June 2010 09:32 AM
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sfchew

Posts: 589
Joined: 10 December 2002

The operation of a particular plant is such that they will incur substantial MD for a particular 30 minute interval. In order to appear to be fair they are asking whether the period for the MD can be extended to 60 minutes instead of 30 minutes.

May be someone can throw some light to why 30 minutes is chosen in the first place?

Regards
Chris Chew
 03 June 2010 06:32 AM
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Al

Posts: 60
Joined: 02 February 2009

Chris,

I'm sure you are aware plant is rated for MD with most equipment such as transformers and cables suitable for short 30 minute overloads without monitoring.
I dont know why the operator would ask you this unless you are the service provider?

Hope I have interpreted your post correctly

Al
 09 June 2010 03:34 PM
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jafarpour

Posts: 198
Joined: 14 August 2005

The cyclical loads generally vary with the ambient temperature due to air conditioning and heating. However, summer and winter profiles are very different. Summer peak loads occur when peak temperatures occur.
The reverse occurs in winter. In fall and spring, not only is the ambient temperature mild, but there is little or no air conditioning or heating load. The thermal stress on a transformer with a cyclical normal load during fall and spring is negligible. It is also usually negligible during winter peak loading.
The thermal rating of a power transformer differs from the thermal rating of other current-carrying elements in a substation. Examples of other elements are conductor, buswork, connectors, disconnectors, circuit breakers, etc. The insulation system for these elements is air and solid support insulators. The cooling system is passive (ambient air). The thermal limits depend on the properties of the conductor itself. These elements are maximum-rated devices. In a power transformer, the cooling system is active.
The thermal limits depend on the dielectric and mechanical properties of the cellulose and oil insulation system. As a maximum-rated device, the transformer capability may be 200% of the maximum nameplate rating (IEEE, 1995).
If ambient conditions differ from the power transformer nameplate criteria, then the user must adjust the transformer capability accordingly. IEEE C57.91-1995 provides tables and equations for making these adjustments. A good approximation is an adjustment of 1% of the maximum nameplate rating for every degree C above or below the nameplate rating (IEEE, 1995). If the transformer operates in 40˚C average ambient, then the user must de-rate the nameplate kVA by 10% in order to meet the nameplate thermal-rating criteria. Conversely, operating in a 0˚C average ambient environment allows the user to up-rate the transformer by approximately 30%.



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 09 June 2010 08:34 PM
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alancapon

Posts: 5759
Joined: 27 December 2005

Chris,

A lot of commercial tarriffs in the UK are based on three charges. One is a capacity charge, based on ASC (Authorised Supply Capacity), this is the amount of electricity that the consumer has agreed they need to have from their supplier and that the supplier has agreed that their network will be able to provide continuously. The second part, is a pence per kilowatt hour charge for the electricity used. The third charge (the MD charge), is effectively a penalty charge for exceeding their ASC averaged over a half-hour period. This time period is used fairly extensively within the UK, and electricity trading is based on 48 half-hour periods for each day.

In terms of what can be done to reduce the MD penalties, there are generally two options. First would be the reduction of electricity usage to ensure that the ASC agreed with the supplier is not exceeded, the second would be to increase the agreed ASC, assuming that there is the necessary spare capacity in the local distribution network.

Regards,

Alan.
 10 June 2010 01:22 PM
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OMS

Posts: 19549
Joined: 23 March 2004

First would be the reduction of electricity usage to ensure that the ASC agreed with the supplier is not exceeded, the second would be to increase the agreed ASC, assuming that there is the necessary spare capacity in the local distribution network.


Other options might be displacement of load to avoid concurrent peaks (buffering of heating or chilled water perhaps, tanking of process flow maybe) or introduction of peak lopping from generator or turbine power sets - essentially reducing import to stay within declared ASC levels.

Regards

OMS

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Failure is always an option
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