The Nature of Loads

8/10/2019 The Nature of Loads

1/42

Distribution of Electrical Energy

The Nature of Loads Ch.2

Dr. Fouad Zaro Assistant Professor

Electrical Engineering DepartmentPalestine Polytechnic University


2/42

Individual Customer Load

Distribution of Electrical Energy 2

In order to define the load,the demand curve is brokeninto equal time intervals.

The average value of the loadin an interval is defined as the15-minute kW demand .


3/42


the 15-minutemaximum kWdemand occurs at13:15 and has a valueof 6.18 kW.

The energy in kWh usedduring each 15-minutetime interval is

The total energy consumed during the day is the summation of all of the15-minute interval consumptions.The 15-minute average kW demand is computed by:


4/42

Load Factor

the ratio of the average demand to the maximum demand. From the utilitys standpoint, the optimal load factor would be 1.00.



5/42

Cust. # 2Cust. # 1

Cust. # 4Cust. # 3

Distribution Transformer Loading



6/42

Diversified Demand


the diversified demand is the sum of the all 15-minute kW demands foreach time interval for the group in that time interval.

The 15-minute maximum diversified kW demand of the transformer. Itoccurs at 17:30 and has a value of 16.16 kW

Transformer diversified demand curve


7/42

Load Duration Curve

Sorting in descending order, the kW demand of the transformer developsthe load duration curve.

The load duration curve plots the 15-minute kW demand versus thepercent of time the transformer operates at or above the specific kWdemand.

For example, the load duration curve shows the transformer operates witha 15-minute kW demand of 12 kW or greater 22% of the time.

This curve can be used to determine whether a transformer needs to be

replaced due to an overloading condition.Distribution of Electrical Energy 7


8/42

Maximum noncoincident kW demand

The 15-minute maximum noncoincident kW demand for the day is thesum of the individual customer 15-minute maximum kW demands.


Diversity Factor

diversity factor is the ratio of the maximum noncoincident demand of agroup of customers to the maximum diversified demand of the group.

The idea behind the diversity factor is that when the maximum demandsof the customers are known, then the maximum diversified demand of agroup of customers can be computed.


9/42

as viewed from the substation, the maximum diversified demand of a

feeder can be predicted by computing the total noncoincident maximumdemand of all of the customers served by the feeder and dividing by 3.2.



10/42

Demand Factor

The total connected load will be the sum of the ratings of all of theelectrical devices at the customers location.

The demand factor gives an indication of the percentage of electricaldevices that are on when the maximum demand occurs.

The demand factor can be computed for an individual customer but notfor a distribution transformer or the total feeder.



11/42

The utilization factor

The utilization factor gives an indication of how well the capacity of anelectrical device is being utilized.


Load diversity

Load diversity is defined as the difference between the noncoincidentmaximum demand and the maximum diversified demand.


12/42

Feeder Load


Feeder Demand Curve


13/42


14/42

I. Application of Diversity Factors


This maximum diversified demand becomes the allocated load for thetransformer.


15/42

Building Diversity Factors Table

Knowing the maximum demand for each customer. perform a load survey where the maximum diversified demand of groups

of customers is metered. selecting a series of locations where demand meters can be placed that

will record the maximum demand for groups of customers ranging from atleast 2 to 70. At each meter location the maximum demand of all downstream

customers must also be known.



16/42

II. Load Survey

Perform a load survey of similar customers in order to determine therelationship between the energy consumption in kWh and the maximumkW demand.

At the end of the survey period, the maximum demand vs kWh for eachcustomer can be plotted on a common graph.

Linear regression is used to determine the equation of a straight line thatgives the kW demand as a function of kWh.



17/42

linear regression algorithm



18/42

Example



19/42

Table 2.2 from textbook



20/42

Solution


1. Determine for each transformer the 15-minute noncoincident maximum kWdemand and, using the Table of Diversity Factors in Table 2.2, determine the15-minute maximum diversified kW demand.


21/42

Assumed PF of 0.9, the 15-minute maximum kVA diversified demand oneach transformer would be


The kVA ratings selected for the three transformers would be25 kVA, 37.5 kV, and 50 kVA, respectively.

With those selections, only transformer T1 would experience a significantmaximum kVA demand greater than its rating (135%).


22/42

2. Determine the 15-minute noncoincident maximum kW demand and 15-minute maximum diversified kW demand for each of the line segments.

Segment N1 to N2:

The maximum noncoincident kW demand is the sum of the maximumdemands of all 18 customers.

Noncoin. max. demand = 66.7 + 81.6 + 117.5 = 265.5 kWThe maximum diversified kW demand is the computed by usingthe diversity factor for 18 customers.Max. div. demand = 265.5/2.86= 92.8 kW



23/42

Segment N2 to N3:

This line segment sees 13 customers. The noncoincident maximum demandis the sum of customers number 6 through 18. The diversity factor for 13(2.74) is used to compute the maximum diversified kW demand.


Segment N3 to N4:This line segment sees the same noncoincident demand and

diversified demand as that of transformer T3.


24/42


25/42

III. Transformer Load ManagementA transformer load management program is used by utilities to determinethe loading on distribution transformers based upon a knowledge of thekWh supplied by the transformer during a peak loading month.

The program is primarily used to determine when a distribution transformer

needs to be changed out due to a projected overloading condition.

The results of the program can also be used to allocate loads to transformersfor feeder analysis purposes.

This method has an advantage because the utility will have in the billingdatabase the kWh consumed by each customer every month. As long as theutility knows which customers are connected to each transformer by usingthe developed equation, the maximum diversified demand (allocated load) oneach transformer on a feeder can be determined for each billing period.



26/42

IV. Metered Feeder Maximum Demand The major disadvantage of allocating load using the diversity factors is

that most utilities will not have a table of diversity factors. The process of developing such a table is generally not cost effective. The major disadvantage of the transformer load management method is

that a database is required that specifies which transformers serve whichcustomers. Again, this database is not always available.

An allocation factor (AF) can be determined based upon the meteredthree-phase kW or kVA demand and the total connected distributiontransformer kVA.

where Metered demand can be either kW or kVA, and kVA total = sum ofthe kVA ratings of all distribution transformers.

The allocated load per transformer is then determined by:

Transformer demand = AF kVA transformer



27/42

If there is no metered information on the reactive power or powerfactor of the feeder, a power factor will have to be assumed for eachtransformer load.

Modern substations will have microprocessor-based metering that willprovide kW, kvar, kVA, power factor, and current per phase.With this data, the reactive power can also be allocated. Since themetered data at the substation will include losses, an iterative processwill have to be followed so that the allocated load plus losses willequal the metered readings.



28/42

Example 2.2

Assume that the metered maximum diversified kW demand for the systemof Example 2.1 is 92.9 kW. Allocate this load according to the kVA ratingsof the three transformers. AF Metered demand


The allocated kW for each transformer becomes:


29/42

What Method to Use ?Four different methods have been presented for allocating load to distributiontransformers: Application of diversity factors Load survey Transformer load management Metered feeder maximum demand Which method to use depends upon the purpose of the analysis. If the purposeis to determine as closely as possible the maximum demand on a distributiontransformer, then either the diversity factor or the transformer loadmanagement method can be used. Neither of these methods should beemployed when the analysis of the total feeder is to be performed. Theproblem is that using those methods will result in a much larger maximumdiversified demand at the substation than actually exists.When the total feeder is to be analyzed, the only method that gives good results is

that of allocating load based upon the kVA ratings of the transformers.



30/42


31/42

Application of Diversity Factors


Assume that the voltage @ N1 is 2400V & the power factor of the loads is 0.9lagging. The impedance of the lines are: z = 0.3 + j0.6 /mile

The ratings of the transformers are


32/42



33/42



34/42


35/42



36/42



37/42



38/42



39/42

Load Allocation Based upon TransformerRatings

When only the ratings of the distribution transformers are known, thefeeder can be allocated based upon the metered demand and thetransformer kVA ratings



40/42

Example2.4

assume the voltage at N1 is 2400 volts andcompute the secondary voltages on the three transformers, allocating theloads based upon the transformer ratings. Assume that the metered kWdemand at N1 is 92.9 kW.

The impedances of the line segments and transformers are the same as inExample 2.3. Assume the load power factor is 0.9 lagging; compute the kVAdemand at N1 from the metered demand:



41/42



42/42

The Nature of Loads

Documents

Transcript of The Nature of Loads