Load power characteristics and operating curves

Load power characteristics and operating curves
  1. Characteristics of load power

In a steady state, the dependence of the active power and reactive power absorbed by the power system load on the power system frequency and supply voltage is called the static characteristic of the power system load, which is called the frequency characteristic of the load and the voltage characteristic of the load, respectively. Usually, the load that contains various forms of electrical equipment such as industrial and household is called comprehensive load. Unless otherwise specified, the load refers to the comprehensive load.

1.1 Voltage characteristics of load power

When the frequency of the power system is constant at the rated frequency, in the vicinity of the rated voltage, the dependency between the load power and the supply voltage is shown in Figure 1.
It can be seen from Figure 1 that the power absorbed by the load under the rated voltage (UN) is the rated power (PN, QN). Since the current always increases when the voltage rises, the rotation speed of the motor also increases, so the active power and reactive power absorbed by the load both fluctuate together with the voltage.

Figure 1 - Static Voltage Characteristics of Loads
Figure 1 – Static Voltage Characteristics of Loads

In order to facilitate the reference of the load characteristics in the calculation of the system, it is necessary to express it with a formula. According to the actual situation of the power system, the approximate expression of the load voltage characteristics is (2):

In the formula, a, b, and c are fitting coefficients. Proper selection of a, b, and c can make the relationship between the load power and the voltage change expressed according to formula (2) close to the actual change relationship.

1.2. Frequency characteristics of load power

Figure 3 shows the relationship between the active power and reactive power absorbed by the load with the frequency of the power system when the supply voltage is constant at the rated voltage.
It can be seen from Figure 3 that the active power absorbed by the load increases with the increase of frequency, which is because the speed of rotation of the motor increases the absorbed power of the machinery (such as blower, lathe, etc.) that it drives. The reactive power absorbed by the load decreases as the frequency increases, because the inductive reactance in the circuit increases with the increase of the frequency.

Figure 3 - Frequency characteristics of loads
Figure 3 – Frequency characteristics of loads

In a small area near the rated frequency, the frequency characteristic of the load can be linearized, that is, expressed by a straight line equation (4):

In the formula, Δf=f-fN is the frequency increment (Hz); Kp and Kq are the frequency coefficients of the active and reactive power of the load respectively, also known as the unit (frequency) regulation power (kW/Hz).

  1. Load operating curve

The load operation curve is referred to as the load curve, which represents the relationship between the power consumed by the load and the time, and is divided into the daily load curve and the annual maximum load curve.

2.1. Daily load curve

The daily load curve shows the change of the 24h load in a day in units of 1h, as shown in Figure 5.

Figure 5 - Typical daily load curve of power system with power
Figure 5 – Typical daily load curve of power system with power

The daily load curve can be expressed by some characteristic values: ① average load Pav; ② maximum load Pmax; ③ minimum load Pmin.
The area under the daily load curve is the electricity consumption throughout the day, denoted as Wd, and its expression is (6):

The average load is defined as (7):

The daily maximum load Pmax is also called the peak load, and the daily minimum load Pmin is also called the valley load. The ratio of the two is recorded as amin, which is called the minimum load factor, and its expression is (8):

The ratio of Pav to Pmax is recorded as kav, which is called the average load factor, and its expression is (9):

amin and kav reflect the intensity of load fluctuations in one day from two perspectives: the smaller the amin and kav values, the more intense the load fluctuations. The daily load curve is the basis for the operation and scheduling of the power plant. When the values ​​of amin and kav are smaller, the output adjustment of the power plant is required to be larger. Large and frequent output adjustment will make the operation of the power plant difficult and reduce the economics of operation. Therefore, some technical and economic measures should be taken to flatten the load curve and make amin and kav approach 1. For example, the factory implements a shift system, calculates electricity prices by time, and builds pumped-storage hydropower stations.

2.2. Annual maximum load curve

The annual maximum load curve takes months as the time unit, reflecting the change of the monthly maximum load in one year, as shown in Figure 10.

Figure 10 - Typical annual maximum load curve of power system with power
Figure 10 – Typical annual maximum load curve of power system with power

The annual maximum load curve is used to plan the balance of electricity (power) and electricity (energy) of the power system within a year, to determine the start-up of other power plants on the condition that hydropower does not abandon water, and to arrange equipment maintenance plans. Obviously, the trough of the annual maximum load is the best time to arrange unit maintenance.

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