When buying a relay protection tester, salespeople often mention "linear power amplifier" or "switch type power amplifier". Most engineers are confused and do not know how much impact this parameter has on actual testing. HZJB-1200 insists on using high fidelity linear amplifiers, and the instructions specifically explain the reasons. Here, it is explained from a practical perspective.

The problem with switch type power amplifiers

The principle of a switch mode (PWM) power amplifier is to control the output through the conduction and cutoff of high-frequency switching tubes, similar to a switching power supply. The advantages are high efficiency, low heat generation, and low cost; The disadvantage is that the output signal is essentially a high-frequency modulated wave, which requires low-pass filtering to obtain an approximate sine output. When the filtering is incomplete, residual high-frequency components will enter the testing circuit, forming electromagnetic interference on other devices on site. At the relay protection testing site, the protection device itself is a precision measuring equipment, and this high-frequency interference will directly affect the accuracy of the test data, and even cause the protection device to malfunction or refuse to operate.

The working mode of linear power amplifier

The output tube of a linear amplifier (Class A or AB) always operates in the linear amplification region, and the output signal maintains a strict linear proportional relationship with the input signal, without high-frequency noise generated by switching actions. The cost is relatively low efficiency and high heat generation, requiring better heat dissipation design.

Actual impact on the experiment

The HZJB-1200 manual clearly states that the linear power amplifier ensures smooth waveform and excellent accuracy throughout the entire process from high current to low current, without causing high-frequency interference to the test site.

This is particularly important in the following situations:

Harmonic superposition test: It is necessary to accurately superimpose 2-20 harmonics onto the fundamental wave. Switching power amplifiers themselves contain a large number of harmonics and cannot precisely control the amplitude and phase of harmonic components.

Differential protection harmonic braking test: The braking coefficient is often only 15% to 25%, and the magnitude of harmonics is very small. Any background noise can affect the test results.

Low current test: When the protection startup value is set to a very small value, the linear amplifier can ensure linear output from zero to rated throughout the entire process without dead zones. The linearity of switch mode amplifiers is usually poor in the low current range.

For professional relay protection tests that pursue data reliability, linear amplifiers are a more rigorous choice.