Analysis of risk assessment algorithm from power supply interruption due to transmission line fault

The guaranteed reliable and uninterrupted supply of electrical energy to consumers is an important task. It can be noted that currently there is no well-structured system for ensuring reliability or security and preventing risk occurrence. In this report, risk assessment algorithms in case of accidents on transmission line are developed and tested by means of the analytical example.


Introduction
Reliability of consumer or consumer groups electricity supply is related to the technical state and operating conditions of external distribution networks, through which they are fed.
Decreasing of reliability from transmission line faults leads to a shortage of electricity to consumers, which is called "damage from electricity sacrifice" in cash-equivalent term, thus the study of risks from transmission line faults is relevant. The problem of reliability and risk is discussed by Billington R., Allan R., Beehler M, Guk Yu.B., [2,3,6]. In addition to this, modern authors such as Syrri A.L.A., Mancarella P., Junlakarn S., Ciapessoni E., consider this problem in their articles [9,8,5].
Nowadays, requirements that regulate the risk assessment and reliability calculation are either absent or obsolete. Consequently, the utility practice requires the theory and methods for reliability calculation taking into account the risks from faults.

What is Reliability?
Reliability refers to reliability connected with electrical mode (henceforth referred to as reliability) or service security. [7,8] Generally, reliability is the ability of a power system to hold over a new state due to disturbances without violation of power system stability. It characterizes the emergency procedures by the criterion (n-i), where n − is the total number of components and i − is the number of suddenly disconnected system components [6].

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Reliability of consumer energy supply can be defined as the difference between electrical supply with full consumer demand ( C W ) and electricity sacrifice due to accidents and other events ( W  ) to C W [4]: Service security is determined by the level of physical reliability of individual components and component connection layout.
The main indicators of service security are: -failure flow parameter [ω, failure per year] called SAIFI in [3] is the ratio of component number that failed per a time unit to the total number of the same tested components; recovery time [Tr, hour] − the ratio of total recovery time of technical devices for the selected fixed period to the number of its failures for the same fixed period.
The main reliability indicators of systems with various connections of recoverable components are calculated according to the serial and parallel connection of components [2].

Calculation of the damage from electricity sacrifice
Estimation of the damage from electricity sacrifice costs can be calculated as W is the expected load at the delivery point, kWh; 0 d , is the specific cost for energy which is not supplied, $/kWh [1].

Risk Assessments from power supply interruption
Qualitative analysis corresponds to the stage of risk identification, quantitative analysis allows determining the size of individual risks numerically and risks in general. [9]

Quantitative method
The calculation of this indicator is carried out by the assumption that production costs can be divided into conditionally variable Ccon−var (depending on the volume of production Bp) and conditionally constant Ccon−fix (independent of the volume of production). 100 con fix cr р con var where Vcr − the critical volume of transferred electricity, %.
To calculate the break-even point, the planning interval, for example, the year is chosen and full production capacity is achieved. The project is considered sustainable if Vcr <75 -80% from normal level. Advantages of calculation of the break-even point are the simplicity and clarity. [5]

Qualitative method
For a qualitative risk assessment of electricity supply, it is necessary to determine the main components of the risk and the severity of fault consequences (Table 1). The failure, which rapidly and with high probability can cause significant damage to the object and / or the environment, death or serious injuries of people, failure of the task III-critical failure The failure, which rapidly and with high probability can cause significant damage to the facility itself and/or the environment, disrupts the task, but creates a negligible threat to life and health of people II-not critical failure The failure that leads to delaying in the fulfillment of the task, decreasing the readiness and efficiency of facilities, but does not pose a threat to the environment, facilities themselves and human health I-with small consequences The failure, which leads to decreasing the quality of the function of facilities, does not pose a threat to the environment and human health To perform this type of analysis, each type of failure must be assigned. The level of occurrence probability is illustrated in Table 2.  According to the schedule in Figure 1, areas of equal priorities are singled out: 1 − high. A component whose failure requires special attention in the design. 2 − medium. A component whose failures require attention during development. 3 − low. A component that requires careful analysis of species, consequences and criticality of failures to confirm priority values.
The advantage of qualitative analysis is that it is performed in early stages of development, when the design of products is not fully defined and there is no quantitative data on reliability of all components; as a result, it is simple and evident.

Calculation of practical objectives
The problem situation: The plant is fed from two power sources: CHPP (Combined Heat and Power Plant) and System (Figure 2). Each transmission line can transfer total power required by the plant. However, CHPP can not produce more than 15.5 MW. Parameters are conventionally accepted and given in Table 3. The task is to define parameters of the failure flow in this system, the probability of failure, damage from the electricity sacrifice in a year, and then identify the risks.
Calculations: Probability of failure for circuits I and II: Based on these calculations, the breakers can be assigned for group E, transmission lines for group D, therefore, no analysis is required for the components. Comparing the obtained data according to the graph, shown in Figure 1, it can be concluded that all considered components of the system belong to the second priority of the component classification and it is not necessary to enter control systems and compensate the failure.
In the steady-state mode the risk assessment is defined by quantitative method (breakeven point) from the electricity sacrifice.
The scheme is reliable since Vcr <75 -80% from the normal level.
When an accident on transmission line occurs the risk assessment is calculated by quantitative method.
30 15 100 100 50 30 Taking into account the average recovery time of the line, the estimated damage from electricity sacrifice costs: Based on the calculation results, it can be noticed that the critical sale volume when one line is out of service has slightly increased, the project is remaining reliable.

The total algorithm of risk assessment
Based on the analytical problem solution, the algorithm is developed taking into account risks from accidents on transmission lines: 1. Visually determine the sections of the circuit with weak circuit reliability. 2. Modeling electric modes that detect "narrow" places by regime reliability, capturing areas with weak circuit reliability. 3. Define reliability indicators. 4. Determine the probability of failure. 5. Determine the damage from under-supply of electrical energy.