The Difference Between Linear Load And Nonlinear Load

Blink: The difference between linear loads and nonlinear loads is: “When a sinusoidal voltage is applied to both, the current of the linear load is positive, and the current of the nonlinear negative is non-positive.” But in reality, Cummins In the daily use of diesel generator sets, users often do not know the characteristics of the load very well. The main reason is to mix the concept of power factor in it, thinking that only pure resistive loads are linear loads, while non-pure resistive loads are all nonlinear. In this paper, the definition, characteristics and application of linear and nonlinear loads are expounded and analyzed, and some basic concepts are clarified.

According to the characteristics of the load, it can be divided into linear load and nonlinear load.
1. Linear load Linearity means that when the voltage increases, the current also increases. The voltage divided by the current means that the resistance value remains unchanged, resulting in a load with a linear relationship between voltage and current. Common linear loads include heaters, incandescent lighting, transformers, direct-start or traditional peak-voltage start-up motors, etc. The phase relationship between linear negative root voltage and current can be divided into electric (such as electric lighting, electric furnace, incandescent lamp, etc.), inductive parts (such as induction motor, transformer, etc.), capacitive (such as capacitors, etc.) load.

2. Non-linear load
Non-linear means that when the voltage becomes larger, the current does not increase proportionally, resulting in a load with a nonlinear relationship between voltage and current. For example, capacitance and inductance are nonlinear loads. During the load operation process, the relationship between voltage and current often changes, and their waveforms are not the sine AC waves we often say, but contain rough distortion components. For example, in order to ensure that equipment in key departments such as network data centers, data exchange centers, financial settlement centers, and important system scheduling can receive uninterrupted power supply, these departments usually use “mains power supply + oil generator set backup + UPS power supply “The power supply system composed of. As the power supply equipment in the middle section from the mains power outage to the backup power generation, the UPS can invert the electric energy of the battery to supply power to important loads without delay, so as to ensure that the load is not powered on before the diesel generator set starts to supply power, and when the diesel generator set is put into operation When in use, the UPS is used as the load of the unit to charge the battery. The rectifier filter is one of the main components of the UPS. This nonlinear load will reflect a large number of high-order harmonics to the genset, of which the 5th and 7th The harm of sub-harmonic is the most serious, especially when the nonlinear load is large and the capacity of the generating set is small.

Definition and Characteristics of Linear Loads
In the national standard GB/17260-3 of UPS in my country, there is a clear definition of linear load: “3.2.6 Linear load is the kind of load whose negative resistance parameter (Z) is constant when a variable positive voltage is applied.
In an AC circuit, there are three types of load elements: resistance R, inductance L and capacitance C, and the results they cause in the circuit are different.
In a pure resistance circuit, the sinusoidal voltage U is applied to a resistor R, and the current] is also sinusoidal, and the phase of the current I and the voltage U is the same.
If the voltage u=Umsinwt, then i=lmsinwt; the effective value of the current l=U/R
The current passes through the resistance to generate heat, and the electric energy is converted into heat energy, that is, P=UI=I2R
1. Inductive reactance
In a purely inductive circuit, a sinusoidal voltage is applied to an inductance coil L. Because the current is alternating, an induced potential is generated in the line, so that although the current is still descending, the phase is full and then 90° ( electricity
The angle is T/2). If the voltage u=Umsinwt, then i=lmsin (wt-T/2) the effective value of the current I=U/ (2rf L) =U/X; XL=2f L, which is called inductive reactance. Current flows in the circuit, bringing electrical energy from the power supply to the coil, converting it to magnetic energy, which in turn converts the magnetism to electrical energy and returns it to the power supply. So there is no power dissipation in the circuit and the average power is zero.
Reactive power Q=UI=l2X

2. Capacitive reactance
In a purely capacitive circuit, the positive current is applied to a capacitor with a capacitance of C, because the current carries the charge and accumulates on the plate of the capacitor to generate capacitive current, so that the current is still negative. However, the phase leads the voltage by 90° (the electrical angle is T/2).
Such as voltage u=Umsinwt, then i=lmsin (wt+T/2); current effective value l=2rtfCU=U/Xc; Xc=1/ (2rfC), called capacitive reactance.

3. Reactance
Current flows in the circuit, bringing the electrical energy from the power supply to the capacitor, where it is converted into electric field energy, and then the electric field energy is converted back into electrical energy and returned to the power supply. So there is no power consumption in the circuit, and the average power is zero. Reactive power Q=UI=l2XC
Inductive reactance and capacitive reactance are generally referred to as reactance.

4. Impedance
On a linear load that generally has resistance R, inductance L, and capacitance, if a sinusoidal voltage is applied, the current is still sinusoidal, but the phase relationship between the current and the voltage is neither in phase nor 90° out of phase, but
There is a difference of one D angle. Such as voltage u=Umsinut, then i=lmsin (wtt). Effective current = UZZ is impedance, and its relationship with resistance and reactance is: Z2R2+X2, and reactance is the comprehensive value of reactance XL and capacitive reactance XC. The zero angle of phase difference is determined by the R, L, and C parameters in the load. 0 is positive when it is inductive, and 0 is negative when it is capacitive. tgp=X/R. Resistance Z, reactance X and resistance R form an impedance right-angled triangle. Apparent power on load
S=Ul, active power P=Ulcos, reactive power Q=Ulsinp, S2=P2+Q2, the three form a power triangle. It is not only the size of the load impedance that determines the load resistance, but also the size of the power factor. Comprehensively, in the linear load, there are pure power factor (1) and inductive (power factor less than 1), capacitive (power The number of days is less than 1), and purely inductive and purely capacitive (both power factors are 0). The above-mentioned loads are all linear loads. It cannot be considered that only pure positive loads with a power factor of 1 are linear, and other negative loads with a power factor of not 1 are not linear.
Diesel generator linear load circuit simulation is shown in Figure 4, [ and form an LC oscillator circuit, R and series will shift the phase output of the sinusoidal signal generated by the oscillator. Its linear load current waveform is shown in Figure 2,

Definition and characteristics of nonlinear load
In the national standard GB/17260-3 of UPS in my country, there is also a clear definition of nonlinear load: “3.27 nonlinear load is a negative impedance Z) that is not always a constant, and that varies with other parameters such as voltage or time.
kind of load. There are many types of nonlinear loads, most of which are rectified and filtered in the negative of UPS power supply, and the input of UPS is also rectified wave type. Therefore, a reference non-linear load (Reference non? The appendix of the standard is included in the standard, and this benchmark non-linear load is used to test the capability of UPS with non-linear load. In the UPS national standard GB/17260-3, this benchmark nonlinearity is also given in Appendix E
Load circuit, as shown in Figure 3. The reason why this circuit is a nonlinear load is that when a positive voltage U is applied to the input terminal, when the instantaneous value of the voltage is greater than the DC voltage on the capacitor, the power supply supplies power to the load R1 and charges the capacitor. When the instantaneous value of the voltage is less than the DC voltage on the capacitor, due to the blocking effect of the diode, the power supply no longer supplies power, and the discharge of the capacitor keeps the load current continuity. Therefore, the impedance presented by the load to the power supply changes with the instantaneous value of the voltage.
An important characteristic of nonlinear loads is that when a sinusoidal voltage is applied to the load, the current is not positive. The alternating current of the load circuit in Fig. 1 is discontinuous and peak. And Figure 4 is the voltage of this non-linear load
And the waveform diagram of the current, it can be seen that the current is a spike shape. To analyze and calculate the current and power in the nonlinear circuit, the method used is to use the Fourier function analysis method to replace the non-sinusoidal quantity with the equivalent sine quantity. Among many loads, nonlinear loads are very complex, and there are many types of current waveforms, such as peaks, double peaks, etc., and it is not enough to explain them only by their current magnitude. In order to illustrate the degree of difference between nonlinear and linear current, it is expressed by a parameter, which is the crest factor. In the GB/T7260-3 standard, it is said: “3.3.29 The crest factor refers to the ratio of the peak square root mean value of the periodic quantity. The root mean square value is the effective value usually called. Generally, the load of the maximum crest factor It is a personal computer with a crest factor of about 2.7. The current crest factor of a computer system is about 2.3. The crest factor of a sinusoidal current is 1.4. Therefore, the general UPS sets the crest factor of the non-linear load as 3, which is fully capable Meet the needs of the load. Especially the crest factor of the large PS is 3, there is no problem.

Summarize:
The voltage and current of the diesel generator load are not in a linear relationship, it is a nonlinear load, inductive load and capacitive load are both nonlinear loads, and a linear load is a resistive load. Pure inductance, such as an inductor connected in series with an electric anode, is an inductive load. The same capacitive negative means that the load as a whole exhibits the characteristics of capacitance, and it does not refer to pure capacitance. Linear load and nonlinear load are two basic loads in the circuit. These two loads are often encountered in UPS equipment and circuits, especially nonlinear loads. Therefore, there should be a clear and unambiguous understanding of the characteristics and differences of these two types of loads.