Introduction on Distribution System Losses

Introduction on Distribution System Losses

August 7, 2019

This loss is composition of 11kV loss, Transformer loss and LT loss. 11 kV loss contains technical and non-technical portion of loss. Transformer loss has two types of loss viz. iron loss which is fixed loss and copper loss which is variable loss. LT loss is composition of technical and non-technical loss.

There are mainly three areas where losses occur in a feeder. Those are 11 kV line loss, Transformer loss and LT line loss. 11 kV and LT line losses can be divided into technical and non-technical losses. Transformer loss consists of iron loss and copper loss.

A. Technical Losses

Technical losses are caused by the physical properties of the components of the power system such as power dissipated in transmission lines and transformers due to internal electrical resistance. Technical losses are caused by action internal to the power system and consist mainly of power dissipation in electrical system component such as transmission lines, transformers, measurement system, etc. Technical losses are possible to compute and control, provided the power system consists of known quantities of loads. Technical losses occur during transmission and distribution and involve substation, transformer, and line related losses. In a distribution feeder these include resistive losses of 11 kV feeder, the resistive losses in windings and the core losses of distribution transformer, resistive losses in LT lone and resistive losses in service cable. These types of losses are inherent to the distribution of electricity and cannot be eliminated. Technical losses are due to current flowing in the electrical network and generate the following types of losses:

Copper losses (I2R losses) that are because of the finite resistance of conductors.

Dielectric losses that are losses that result from the heating effect on the dielectric material between conductors

Induction and radiation losses that are produced by the electromagnetic fields surrounding conductors. Technical losses are possible to compute and control, provided the power system in question consists of known quantities of loads.

Following are the causes of technical losses:

(i) Harmonics distortion

(ii) Improper earthing at consumer end

(iii) Long single-phase lines

(iv) Unbalanced loading

(v) Losses due to overloading and low voltage

(vi) Losses due to poor standard of equipment.

B. Non-Technical Losses:

Non-Technical losses are caused by actions external to the power system or are caused by loads and condition that the technical losses computation failed to consider. They can be thought of as electricity that is consumed but not billed. It is important to differentiate this from electricity that is billed but where the bills are not paid. In the case of non-technical losses, the end user is unknown, or the amount of energy being consumed is uncertain. Non- Technical losses are more difficult to measure because these losses are often unaccounted for by the system operators and thus have no recorded information.

The three main types of non-technical lossesare:

  • Energy Theft
  • Errors in Unmetered Supplies
  • Conveyance error

Energy Theft

This is not energy that has been accurately billed but not paid; it is energy that has been illegally taken from the network through tampering with meters or other network assets. This is taken without the knowledge of an energy company and leads to differences between estimated and actual electricity consumption. Energy Theft increases everybody’s energy bills and creates serious electrical hazards for both those stealing the power and those working on the network. Energy theft is also done by tapping (hooking) on LT lines. Un-metered supply is an example of energy theft. Stealing by bypassing the meter or otherwise making illegal connections also creates non-technical loss.

Conveyance

These are losses that arise when electricity is consumed but not correctly recorded. Situations arise where energy is legally consumed but is not properly recorded in the national electricity settlement system. This can occur due to inaccuracies in meter readings, unregistered meter points, errors in registration or faulty meters. These errors result in a discrepancy between actual and measured consumption, meaning energy is lost in the system.

Unmetered Supplies

Unmetered supplies (UMS) are commonly used for the communal areas in council owned buildings, street lamps, bus stops and advertising boards. Unmetered supply customers provide inventories of their connected electrical equipment and estimated consumption. Although we audit these inventories and request accurate updates they are not always provided and may change frequently. The difference between UMS estimates and actual consumption creates a non-technical loss.

Technical losses will be simply calculated using load flow method of power system. This will be done because non-technical losses are more difficult to measure. As NTL cannot be computed and measured easily, but it can be estimated from preliminary results, i.e. the result of technical losses is first computed and subtracted from the total losses to obtain the balance as NTL. The technical losses are computed using appropriate load-flow studies and simulation.

Relationship between the Load and Loss Factors

In general, the loss factor cannot be determined from the load factor. However, the limiting values of the relationship can be found. Assume that the primary feeder shown in Figure 1 is connected to a variable load. Figure 2 shows an arbitrary and idealized load curve. However, it does not represent a daily load curve.

Assume that the off-peak loss is PLS,1 at some off-peak load P1 and that the peak loss is PLS,2 at the peak

load P2. The load factor is

FLS=𝑃𝑙𝑠,2βˆ—π‘‘+𝑃𝑙𝑠,1βˆ—(π‘‡βˆ’π‘‘)/𝑃𝑙𝑠,2βˆ—π‘‡ ……………….(6)

Where

Pls,1 is the off-peak loss at off-peak load

t is the peak-load duration

T-t is the off-peak-load duration

The copper losses are the function of the associated loads. Therefore, the off-peak and peak loads

can be expressed, respectively, as

PLS,1=kP12 …………………………….….(7)

PLS,2=kP22 ……………………………..…(8)

Where k is a constant. Thus, substituting Equations 7 and 8 into 6, the loss factor can be expressed as

FLS= {(kβˆ—P2βˆ—P2)βˆ—t+(Kβˆ—P1βˆ—P1)(Tβˆ’t)}/(kβˆ—P2βˆ—P2)βˆ—T………………….(9)

FLS=𝑑/T + 𝑃1βˆ—π‘ƒ1/(𝑃2βˆ—π‘ƒ2) * (π‘‡βˆ’π‘‘)/T……………….….(10)

By using Equations 3 and 10, the load factor can be related to loss factor for three different cases.

Case 1: Off-peak load is zero. Here,

PLS,1 = 0

Since P1= 0. Therefore, from Equations 3 through 10,

FLD=FLS= t/T ………………….(11)

That is, the load factor is equal to the loss factor, and they are equal to the t/T constant.

Case 2: Very short-lasting peak. Here,

t β†’ 0

hence in Equations 3 and 10,

(T-t)/T→1

therefore,

FLS=(FLD)2………………………..(12)

That is, the value of the loss factor approaches the value of the load factor squared.

Case 3: Load is steady. Here,

t β†’ T

That is, the difference between the peak load and the off-peak load is negligible. For example, if the customer’s load is a petrochemical plant, this would be the case. Thus, from Equations 3 through 10,

FLS β†’ FLD…………………………(13)

That is, the value of the loss factor approaches the value of the load factor.

Therefore, in general, the value of the loss factor is

FLD < FLS < FLD2……………………(14)

Therefore, the loss factor cannot be determined directly from the load factor. The reason is that the loss factor is determined from losses as a function of time, which, in turn, are proportional to the time function of the square load.

However, Buller and Woodrow developed an approximate formula to relate the loss factor to the load factor as

FLS = 0.3 FLD+0.7 FLD2……………(15)

FLS = k1 FLD+k2 FLD2………………(16)

Where

FLS is the loss factor, pu 

FLD is the load factor, pu