Future Power Systems 1 - the Balance principle, Frequency and the Grid

Each power system is always in balance; generation = demand. On an AC system, this rule is maintanied in real time by the frequency, whose deviation from nominal (50 or 60 Hz) represents the difference between 'required demand' and 'delivered demand'.

The frequency has to be kept within strict limits to avoid system degradation; usually +/- 1% for normal operation. If the frequency deviates beyond 2%, automatic disconnection and measures are necessary to arrest the slide and avoid collapse.

When frequency changes, Synchronous Generators will instantaneously release or absorb inertial energy and some (resistive) demand will reduce. Extra geneneration (and increasingly demand) is set to provide additional response and backup for same so that any event, inclusing the loss of the largest infeed, can be compensated without an excessive frequency deviation.

We can try to show the deviation limits in terms of maximum excursion from 'generation requirement = required demand'.  This assumes that demand is 40% frequency sensitive. On a large power system only resistive load will react. Motors and other inductive loads are not frquency sensitive. This shows that the mismatch of generation delivered to that required has to be tightly controlled. 

So, we need to be able to predict both demand and generation, and ensure that the match is kept within tolerance, for all timescales from immediate out to planning. Also, the transport system must be secure - under both steady state conditions and for any credible fault, both transmission and distribution must not be overloaded, have unacceptable voltage excursions or be unstable.

To do this, it is necessary to predict and model the network loadings and voltage/stability conditions in detail.