**8.4 Average service availability index (ASAI)**

ASAI is the customer-weighted availability of the system and provides the same information as SAIDI. Higher ASAI values reflect higher levels of system reliability. This measure can be defined as

$$\text{ASAI} = \frac{\text{Customer hours service availability}}{\text{Customer hours service demand}} \text{ (pu)} \tag{27}$$

*FCT* ¼ ∑ *t* ∑ *k*

and miscellaneous expenses) and can be evaluated as

*VCT* ¼ ∑ *t* ∑ *k*

of that by an appropriate outage cost rate (*OCR*), as follows:

continuous flow as well as the least cost of its production.

*OCT* ¼ ∑ *t*

where

**A.2 Variable cost**

*Reliability Evaluation of Power Systems DOI: http://dx.doi.org/10.5772/intechopen.85571*

cost of unit of type *k* (SR/kWh).

**A.3 Outage cost**

outage cost rate in SR/kWh.

following equation:

**165**

being expressed in the following equation:

ð Þ *CAPk* � *CCk* � *NUk <sup>t</sup>* (A.1)

ð Þ *<sup>ϵ</sup>ESk* � *ESCk* � *NUk <sup>t</sup>* (A.2)

*SCT* ¼ *FCT* þ *VCT* (A.3)

ð Þ *<sup>ϵ</sup>ENS* � *OCR <sup>t</sup>* (A.4)

*OSCT* ¼ *SCT* þ *OCT* (A.5)

; .

; -

;

The variable cost (*VC*) represents the cost of energy supplied by the system. It is affected by the load variation, the type and size of generating units, and the number of hours of operation. Also, these costs are related to the cost of operation and maintenance (fuel, scheduled maintenance, interim spare parts, staffing, wages,

where : expected energy supplied by unit of type *k*; : energy supplied

The total system costs (*SCT*) for the entire expansion plan can be estimated by summing all the above individual costs at every stage of the planning period as

The outage costs, i.e., the cost of the expected energy not supplied ( ), were previously presented and discussed in Section 5. One method of evaluating is described in [8]. Therefore, estimating the outage cost (*OC*) is to multiply the value

where *ϵENS* is the expected energy not supplied (kWh lost) and *OCR* is the

The overall cost of supplying the electric energy to the consumers is the sum of system cost that will generally increase as consumers are provided with higher reliability and customer outage cost that will, however, decrease as system reliability increases or vice versa. This overall system cost (*OSC*) can be expressed as in the

The prominent role of outage cost estimation, as revealed in the above equation, is to assess the worth of power system reliability by comparing this cost (*OC*) with the size of system investment (*SC*) in order to arrive at the least overall system cost that will establish the most appropriate system reliability level that ensures energy
