EE 741
Spring 2014
Distribution System Load
Characteristics
Load Characteristics
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Some definitions
Relation between load and loss factors
Load management
Rate (tariff) structures
Metering
Conservation Voltage Regulation
Trend from linear to non-linear loads
Electrical Demand
Daily system demand Variation Curve
Yearly system demand variation Curve
Electrical Demand
2-day demand variation curve at a substation
Maximum demand = 2.4 kW
Demand Interval (i.e., period at
which the load is averaged: 15 min
24-h demand variation at 1 home
Average current, phase
voltage and power factor
variation on a local feeder
over a 6-hour period during
summer.
Frequency, real and reactive
power variation on a local
feeder over a 6-hour period
in summer.
Electrical Demand
Demand Factor (DF) = maximum demand/total connected demand.
Utilization Factor (Fu) = maximum demand/rated capacity of the system.
Coincidence Factor (i.e., inverse of Diversity Factor): Coincident
maximum demand/sum of individual maximum demands
Load Diversity (LD) = Σ of the peaks of individual loads – combined peak.
Load Duration Curve (LDC): see graph below
Relation between load and loss factors
Load Factor (FLD): average load/peak load.
Loss Factor (FLS) = average power loss/power loss at peak load.
Relation between the above two factors: refer to graph below
Under steady load:
loss factor → load factor
For short lasting peak:
loss factor → (load factor)2
The middle curve is an empirical
approximate formula for urban
areas.
For rural areas, replace 0.7 with
0.84 and 0.3 with 0.16.
Diversity Demand of various residential
loads
The curves of different loads in Fig. 2.13 in the book are derived
from field observations. These are useful when estimating loading
on transformers and feeders.
Load Management (i.e., DSM)
• DSM involves remotely controlling individual customer
loads. It is beneficial in many ways:
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Maximize the utilization of existing resources
Minimize more costly generation or power purchases
Defer capital expenditures
Reduce cold load pick-up during re-energization of circuits
Demand Response
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Demand Response (DR) is defined as changes in
electric usage by end-use customers from their
normal consumption patterns in response to
– changes in the price of electricity over time,
– or to incentive payments designed to induce
lower electricity use at times of high wholesale
market prices or when system reliability is
jeopardized.
It is expected that DR programs will be designed to
decrease electricity consumption or shift it from onpeak to off-peak periods depending on consumers’
preferences and lifestyles.
DR activities are defined as “actions voluntarily
taken by a consumer to adjust the amount or timing
of his energy consumption”.
Many electric companies around the world are
experimenting with DR.
A clothes dryer using a load
control switch to reduce peak
Demand.
Flat Rate Structure
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The utility’s revenue requirement is
determined by the PUC
The residential rate structure is rather
simple (see bill below)
– Basic Service Charge covers some
non-energy costs associated with
providing basic services such as bill
preparation, meters, meter reading,
and customer accounting.
– monies going to the State fund for
energy assistance and conservation
– Green and Renewable Energy fees
are to cover the rebates for such
resources
Commercial Customers are charged for (peak) demand. The local cost is $4.35/kW
Is some regions commercial customers are also charged for poor power factor. For
example, in Northern Nevada, the charge is $0.0014/kVARh for power factor
below 90%.
Time-Of-Use and Peak Day Rates
Rates based on amount of energy usage
Metering
• kWh meters (to measure energy)
• Demand meters (to measure peak power)
• kVARh or kVAh meters (used for power
factor measurement)
• Types of meters
– Induction-type meters
– Solid-state (electronic) meters
– Smart digital meters
Induction type kWh meter
• Disc speed proportional to real
power consumption.
• Registers (dials) record total
energy consumed to date.
kVARh and kVAh meters
• Power Factor Calculation from (P,S) or (P,Q)
P
PF  
S
P
P Q
2
2
• Need for operation at high power factor
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Lower current
Lower losses
Lower voltage drop
Release in capacity
• Utilities typically charge additional costs to large
customers who have a power factor below some
limit, which is typically 90%.
Demand Meter
• The power is averaged every
fifteen- or thirty- minute
interval.
• Each 15 (or 30) minute period
is known as a demand
interval.
• Demand (kW), unlike kWh’s,
however is not cumulative, as
demand is billed based upon
the highest 15 (or 30) minute
demand interval for the billing
period.
Electronic electric utility meter
• Electronic meters display the energy
used on an LCD display, and can
also transmit readings to remote
places.
• In addition to measuring energy used,
electronic meters can also record
other parameters of the load and
supply such as maximum demand,
power factor and reactive power
used.
• They can also support time-of-day
billing, for example, recording the
amount of energy used during onpeak and off-peak hours.
Smart meter
• Records kW, kVAR, KVA, PF, kWhr, peak
demand, power quality disturbances, etc…
• Communicates with utility Energy Management
System
• Remote power disconnect/reconnect
• Communicates with major home appliances
How does demand change with voltage?
(Conservation Voltage Regulation or CVR)
Let the demand by Po when the nominal voltage is Vo.
When changing the voltage be a small amount ΔV, assume
a change in demand ΔP takes place. Herein, the conservation
voltage regulation factor Kp is defined by
CVR Factor of individual appliances.
Does lower voltage operation results in energy conservation?
Load Linearity or Nonlinearity
The classical power system design and analysis is based under the
assumption that the load is linear.
Meanwhile, today’s load is more and more embedded with electronics
(i.e., becoming more and more nonlinear).
How does the current waveform on the feeder look like (given a more
widely diversified load)?
Current waveform of incandescent and CF light bulbs
Current waveform of desktop computer
Homework Assignment (from Chap. 2)
Solve Problems 4-7, 13, 18, and 23.