The Role of UPS Systems in Data Centers and the Importance of Commissioning Tests
Overview
Because data centers aggregate critical IT operations for enterprise core systems, cloud services, healthcare, finance, and more, they must operate continuously 24/7/365 without interruption.
To deliver stable services, preparations must cover not only incidents such as fires or equipment failures, but also large-scale disasters like earthquakes, floods, and typhoons.
If operations were to stop, it could interrupt service providers' business, cause data loss, and halt services for end users; therefore, power systems in particular must maintain operation with high reliability even in the event of incidents.
This article introduces the role of UPSs (uninterrupted power supplies), the necessity of commissioning tests, and how to test power quality for data centers.
Roles of UPSs Beyond Outage Protection
UPSs are is composed of the following components.
- Rectifier: Converts AC voltage into DC voltage and smooths the output.
- Battery: Stabilizes UPS output by charging and discharging the DC power.
- Inverter: Converts DC voltage to AC voltage through high-speed switching of semiconductors.
- Static switch: Enables rapid transfer between inverter output and bypass output.
Data center UPS systems generally use the double-conversion UPS and serve several functions in addition to power outage protection. In a double-conversion UPS, the input AC power is first converted to DC and then inverted back to AC, allowing the output voltage and frequency to remain independent of the input.
Fig. 1. Basic double-conversion UPS configuration: rectifier, inverter, battery, switch
Protection Against Power Outages
This is the most fundamental role of a UPS. Even when input power is lost, the UPS continues supplying power to critical IT equipment. For data center owners, server shutdowns or service interruptions caused by even a few seconds of power loss are the least acceptable events; therefore, a UPS is required to maintain output under any circumstances.
Fig. 2. Shutdown of input voltage does not affect output voltage
Maintain Voltage and Frequency level
Even if the voltage or frequency fluctuates at the input, the UPS output must remain stable (VI*1, VFI*2). This is because servers and network equipment are sensitive to fluctuations in power quality, and even slight drops in voltage or frequency can cause malfunctions. A UPS must be capable of continuously supplying power of a constant voltage and frequency regardless of the condition of the external power source.
Fig. 3. Dip of input voltage does not affect output voltage
- *1:VI (voltage independent): A UPS system in which the output voltage is independent of the input. Even if the input voltage fluctuates, the output voltage remains largely unaffected.
- *2:VFI (voltage and frequency independent): A UPS system in which the output voltage and frequency are independent of the input. Even if the input voltage or frequency fluctuates, the output voltage and frequency remain largely unaffected.
Resilience to Sudden Changes in Load
If the load increases suddenly, a voltage drop may occur if the power supply cannot handle the output. However, voltage drops are not tolerated in data centers. In recent years, the massive parallel processing required for AI training has led to a sharp increase in power consumption, placing greater stability demands than ever on UPS systems to maintain stability during transients (brief changes in voltage or current during events such as load variations or switching). Furthermore, even in the presence of nonlinear loads—such as distorted current waveforms—it is essential to keep voltage distortion and abnormal fluctuations within acceptable limits.
Fig. 4. The output voltage remains stable even under sudden load fluctuations
Site Acceptance Testing During Commissioning
A Site Acceptance Test (SAT) is a test conducted after installation is complete to verify that the system functions as designed in the actual operating environment. Site Acceptance Tests for data center UPS systems are not conducted according to a standardized, one-size-fits-all checklist, but rather based on a test plan agreed upon by the manufacturer and the customer. The UPS standard IEC 62040-3 recommends that SATs focus on verifying only critical items not previously confirmed during factory testing, in order to ensure cost-effectiveness and avoid placing unnecessary stress on the equipment.
Test Example 1: Power Outage and Restoration Test
In data centers, it is essential that power supply to servers and network equipment remains uninterrupted even in the event of an instantaneous loss of commercial power. In the power outage and restoration test, the input power supply is disconnected while the UPS is operating in normal mode, and the output voltage fluctuations are evaluated when the system switches to stored energy mode (battery operation). Additionally, the UPS transient response is verified when the input power is restored after a specified period and the system returns to normal mode.
During the test, load equipment is operated, and the power input line is opened while current is flowing through the UPS to simulate a power failure. After the failure occurs, the UPS must operate in stored energy mode and maintain proper output voltage and frequency within specified limits even during transients.
Fig. 5. Switch is opened to simulate an AC input power failure
Test Example 2: Step Load Test (Output Voltage Stability)
In data centers, power consumption can fluctuate significantly in an instant due to factors such as server startup and shutdown, as well as load variations caused by AI training. The step load test verifies whether the UPS can maintain a stable output even under such load fluctuations. The test is conducted with the UPS in stored energy mode (battery mode). It simulates sudden load changes, such as a transition from no load to 100% load and a transition from 100% load to 20% load, to verify that there are no abnormalities in the output voltage during transients. The presence or absence of abnormalities is determined by plotting the deviation and duration on a graph of standard limit values to verify whether they fall within the specified range. At the actual measurement site, multiple unique test cases agreed upon by the manufacturer and the client are carried out according to a predetermined schedule.
Fig. 6. Measured deviation and duration to determine operational limits
Test Example 3: Switching Test Between Normal and Bypass Mode
A UPS contains a circuit that bypasses the main power path. In the unlikely event of a failure in the UPS unit itself, power flows through the bypass, allowing servers and communication equipment to continue operating. This state is called bypass mode and is specified in standards as a temporary operating mode triggered by a UPS malfunction or a command from an administrator.
If switching causes abnormalities in the output voltage, frequency, or phase, server operation will become unstable. In this test, transient phenomena are observed under actual wiring and load conditions. The test verifies that the UPS properly switches, then returns to normal mode.
Fig. 7. UPS bypass configuration for maintaining power during switching
How Power Quality Analyzers Can Help with SAT
Fig. 8. PQ3198 for measuring three-phase power
IEC61000-4-30 Class A Compliance
IEC 61000-4-30 strictly defines measurement algorithms and accuracy requirements for power quality parameters such as voltage, frequency, voltage fluctuations, and harmonics. Class A is defined within this standard as the highest-grade measuring instrument. This class offers high reproducibility of measurement results, enabling comparative verification across different manufacturers and models. This class is ideal for assessing compliance with standards, verifying contractual requirements, and serving as evidence in the event of issues or disputes.
Hioki’s PQ3198 power quality analyzer complies with IEC 61000-4-30 Ed. 3 Class A and is third-party certified. Its standardized measurement methods ensure reproducible results, supporting clear and reliable reporting of test data.
Simultaneous Measurement of Three-Phase Data and Events
When testing UPS systems, both long-term operating trends (time-series RMS values) and transient switching waveforms must be evaluated. The PQ3198 can record trend data based on RMS values calculated every half-cycle, along with event waveforms during transient events. This allows users to monitor long-term behavior across all three phases while also examining the exact moment an anomaly occurs.
Fig. 9. PQ ONE software application showing RMS trends (top) and event waveforms (bottom)
Simultaneous Measurement of Primary and Secondary Sides
When evaluating UPS switching performance, simultaneous measurement at multiple points is often required. The PQ3198 provides four measurement channels, each capable of simultaneously measuring voltage and current, with channels 1–3 grouped together and channel 4 isolated. The ability to measure the input and output sides simultaneously or trigger measurements on different circuits enhances measurement flexibility.
Fig. 10. Two different lines measured simultaneously
Measurement of Power Events and Harmonics
UPS testing involves more than just verifying power outages and voltage dips. In addition to harmonics trends, the PQ3198 can record a wide range of power events, including dips, swells, interruptions, and transient events. Because it enables verification of transient response during switching as well as waveform distortion and noise, it is useful not only for pass/fail evaluation but also for diagnosing the root causes of power quality issues.
Fig. 11. Transient waveform captured by the PQ3198. Voltage remains stable despite sudden load increases
By incorporating the PQ3198 into the commissioning process, objective and reproducible measurement data can be obtained, enabling reliable verification of UPS performance under actual operating conditions.
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