Load Characterization

Here we will discuss the different load types that are present in an aggregated load. Typical types of loads are described and various compositions are mentioned.

3.2.1 Load Types

According to the description presented in the previous section, the load composition of a particular area is characterized by the load class data, the composition of each one of the classes, and the characteristics of each single load component. The load class data is often grouped in industrial, residential, commercial and agricultural load data. The industrial load is mainly related to industrial processes, and most of the load corresponds to industrial motors, up to 95%. Heavy industries may include electric heating processes such as soldering. The residential load includes most of the devices related to housing habits, but also a big percent of electric heating and air conditioner units during winter and summer respectively. The commercial load corresponds to air conditioner units and a large percent of discharge lighting, and agricultural load to induction motors for driving pumps.

In general the different load components that constitute the different load classes can be included in one of the next four groups;

• Loads with ‘fast dynamic’ electrical and mechanical characteristics such as induction motors,

• Loads that under voltage excursions present significant discontinuities such as discharge lighting and motor protections,

• Loads whose response to voltage faults does not present significant discontinuities or delays such as incandescent lighting, and

• Loads with ‘slow’ characteristics such as electric heating.

A brief description of some important load components [Taylor, 1994], [Agneholm, 1999], [Kundur, 1994], [IEEELoad, 1993], induction motors, street lighting, electric heating and LTCs, on load tap changers, follows in this section.

Induction Motors

A large amount of power consumption goes to induction motors at residential, commercial and industrial areas. A common use of motors at residential and commercial areas is for the compressor loads of air conditioning and refrigeration. These loads require nearly constant torque at all speeds, and are the most demanding from a stability viewpoint. On the other hand, pumps, fans and compressors account for more than half of the industrial motor use.

Typically motors consume 60 to 70 % of the total power system energy and their dynamics are important for voltage stability and long-term stability studies.

Lighting

Mercury vapor, sodium vapor and fluorescent loads constitute the main kind of lighting used in industry and street lighting, and correspond to a large percent of the load composition in commercial areas. They are very sensitive to voltage variations, since they extinguish at about 80% voltage.

Mercury devices are based on the operation of an electric discharge, i.e.

when a mercury lamp is switched on it is characterized by a weak blue illumination, that will change into lighter white as long as the pressure and temperature increase. This process takes between 2 and 5 minutes to stabilize, and during that time the consumption corresponds to 40 to 50 % of the stationary value. After a lamp has been switched off, it needs some time,

glowing time, to cool down before the discharge can be re-ignited and then restarted. Sodium lamps work in the same way as mercury lamps. Since they work at higher pressure and temperature, the quality of the illumination is better, and the glowing time to be restarted is shorter. Fluorescent lamps are the most common type of light used in offices, supermarkets, and in general in commercial areas because their low production cost, and high efficiency to produce light. Just some seconds after the lamp is on, the power consumption reaches more than 90 % of its steady state value.

Thermal Loads

A large percent of loads in residential areas, water heaters, ovens, electric heating, and in industrial areas, soldering and molding machines, boilers, behave similar to a constant resistance in the short-term. Right after a voltage drop the possible variations from power in the input of the device hardly affect the temperature and therefore the resistance characteristic.

After some seconds, and since the heat production has decreased, the ‘on cycle’ of the thermostats in thermal loads will prolong in order to recover the temperature. Under low voltage conditions the temperature will then increase slower than in normal conditions during the ‘on cycle’ of the thermostat. On the other hand, those thermostats that are in the ‘off cycle’

will not respond to the voltage drop until they enter in the ‘on’ period, and therefore the temperature will drop to the same rate during the ‘off cycle’.

When the voltage is low the thermostats are mainly working on the ‘on cycle’ all the time, and therefore the load consumption is similar to the one under normal conditions. This type of load behaves as a constant power load in the long-term. In case of very extreme weather conditions, as in a cold winter day, the full restoration of the load may be impossible since the thermostats are already working 100 % in the ‘on cycle’.

Load Tap Changers

Load tap changer transformers do not correspond to a load component, but seen from the transmission system they may be considered as part of the load. After a disturbance, they restore the sub-transmission and distribution voltages to their pre-disturbance values, but they also affect the status of the voltage sensitive loads. The restoration of the voltage, and consequently the

increase of these loads may lead the system to voltage instability and collapse. The restoration process takes several minutes.

3.2.2 Load Composition

The composition of the load is strongly dependent on the time of day, month and season, but also on weather. In most cold countries the winters are characterized by high load consumption mainly related to electric heating, while during the summer the consumption is low and hardly affected by the small percent of air conditioner units. In warmer countries the situation is the opposite, and it is during the summer when the load consumption reaches the highest values due to connection of air conditioning loads. Both air conditioning and electric heating loads vary seasonally, but they are also strongly dependent on the weather conditions.

If there is cold and windy weather the electric heating demand will increase, while if there is humid weather the use of air conditioner units will increase.

Weekdays are mainly dominated by industrial and commercial loads. The industrial processes may also correspond to evening hours and weekend days. The commercial load consumption varies mainly when comparing weekdays and weekend, and the larger demand corresponds to the working hours.