Details of UPS Testing
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The primary function of an Uninterruptible Power Supply (UPS) is to continuously provide power to protected equipment during a general power outage, allowing the equipment to keep running. This ensures that IT systems, household appliances, and other devices can continue to operate or gain extra buffer time to complete data storage processes before shutting down. UPS systems are categorized into ON-Line and OFF-Line designs. The ON-Line UPS, which includes a complex filter and rectifier structure, offers the strongest power protection. It not only features an extremely short transfer time (approaching zero) when restoring power, but also provides superior output current quality.
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The OFF-Line UPS features the simplest design architecture. Its operation consists of rapidly transferring the power source to battery supply upon mains failure, thereby delivering backup power to the protected load. Unlike other UPS topologies, it provides only battery backup without active voltage regulation or power conditioning, resulting in limited output power quality and more basic functionality.
UPS power supply types can be divided into pure sine wave and simulated sine wave. The so-called sine wave refers to the AC power sent by the general mains. From the graph, it is a smooth sine wave; the other is the electric wave simulated by the square step wave. It is jagged like a staircase on the graph. Most of the models below 1000 VA on the market belong to this simulated sine wave.
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Power supplies for general information equipment typically use alternating current (AC). However, the batteries inside a UPS provide direct current (DC), so an inverter is required to convert the DC from the battery into a sine wave that the equipment can accept. For a UPS to convert DC into a pure sine wave similar to utility power, a high-performance inverter is needed. This increases the complexity and cost of the UPS, resulting in a higher selling price.
The impact of these two waveform types on information equipment is generally insignificant. For typical x86 servers, both simulated sine wave and pure sine wave outputs can operate the system without causing significant harm to the connected devices.
Sine wave: Sine wave is the waveform of normal AC power transmission. It has a smooth waveform and can be used for both inductive loads (such as motors, fans, hair dryers, etc.) and resistive loads (such as light bulbs, computers, etc.).
Simulated sine wave: (Simulated sine wave/step wave): A square wave that is very similar to a sine wave is obtained after continuous correction through the circuit. The degree of simulated sine wave simulation varies from UPS to UPS, and there is currently no standard.
Square wave: Square waves can be easily generated through electronic signal processing. The wave pattern is a regular high and low wave. Energy loss will occur in the process of high to low. Generally, it is acceptable for small household devices, but large devices or inductive loads with motors cannot use square waves.
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Simulating sine wave and square wave to design UPS equipment
Generally, the operation modes of AC electronic load design include constant current, constant resistance and constant power. Testing pure sine wave UPS is sufficient. As for UPS designed with simulated sine wave and square wave, many users will use general AC electronic load for testing.
The test results are as follows Load 1A, CC mode, CR mode or CP mode.
The test data all meet the specifications as follows.
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GW Instek's AEL-5000 series AC/DC electronic loads not only have common AC/DC electronic load operation modes, constant current, constant resistance and constant power, but also have exclusive linear constant current/constant power and constant voltage modes.
What is the difference between constant current (CC) and linear constant current/constant power (LIN CC/CP)?
Constant Current (CC): In constant current mode, a fixed current is drawn based on the set value. This current is independent of the voltage amplitude, value, and waveform—meaning that even if the voltage fluctuates or is non-sinusoidal, the current remains unaffected.
Linear constant current (LIN CC/CP): Linear constant current (LIN CC/CP) draws a fixed current or power according to the set value
1. LIN CC is not related to the voltage value but to the amplitude and waveform. That is, even if the voltage changes, especially if it is non-sinusoidal, it will not affect the current value and the current waveform will be the same as the voltage.
2. LIN CP is related to the voltage value as well as the amplitude and waveform, that is, even if the voltage changes or is non-sinusoidal, it will not affect the power value and the current waveform will be the same as the voltage.
For general electronic load testing with non-sinusoidal waveforms - such as simulated sine waves (stepped waves), triangular waves, or even distorted waveforms - constant resistance (CR) mode is typically used. In this mode, the load current varies with voltage fluctuations, since P = VI = (I*R) * I = I2R, when the voltage increases, the current increases accordingly; when the voltage decreases, the current decreases. This behavior does not meet the testing requirements for UPS systems.
In UPS reliability testing, the CP mode (constant power mode) is usually used for the following reasons:
1. Real load simulation
• In reality, most devices (such as servers, IT equipment, and medical equipment) usually consume power at a constant power rather than a constant current or constant resistance.
• The CP mode can more accurately simulate the working state of a real load, ensuring that the test environment is closer to actual applications.
2. Stable power output measurement
• In constant power mode, the power consumed by the load remains constant, even if the input voltage fluctuates, the power demand remains unchanged.
• This helps to evaluate the UPS's response capabilities under different input conditions and ensures consistency of test conditions.
3. Better reflects battery discharge characteristics
• The battery discharge characteristics of a UPS under constant power load are different from those under constant current load.
• Using the Constant Power (CP) mode allows for more accurate assessment of battery life, efficiency, and runtime, ensuring the reliability of the UPS under real-world load conditions.
Summary:
In UPS reliability testing, using Constant Power (CP) mode better simulates real-world load conditions and enables accurate evaluation of battery discharge characteristics, UPS operational efficiency, and inverter performance.
The details are here. The linear constant current (LIN CC/CP) mode load current waveform of the GW Instek AEL AC/DC electronic load is particularly suitable for testing non-sinusoidal UPS devices. The test waveforms are as follows.
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