Method for judging the steady state of real-time electrical parameters of high-frequency online UPS
Core idea: steady state = the fluctuation range of each electrical parameter within the specified time does not exceed the technical index of national standard/model, and there is no continuous drift, large periodic oscillation and no over-limit alarm. High frequency online (double conversion online) focuses on four groups of real-time electrical parameters: input, rectifier bus, inverter output and battery.
I. Core real-time electrical parameters to be monitored
1. AC input (rectifier side)
Input voltage, input current, input frequency, input active/reactive power, power factor, THDi (total harmonic distortion of input current)
Basis for judging steady state (refer to GB/T 7260 for conventional power frequency commercial power)
Voltage: rated with short-time fluctuation ≤ 5% within the allowable window of rated input voltage;
Frequency: 50Hz, steady-state fluctuation ≤±0.2Hz；;
THDi: High-frequency UPS rectifier, with normal full load generally less than 5% (subject to the model index) and no continuous jump.
2. DC bus (rectified output, key of high frequency UPS)
The DC bus voltage of high frequency machine is the key point of steady state, which is more sensitive than that of power frequency machine.
Bus DC voltage, bus ripple voltage
Steady state: the bus voltage is stable at the design value without slow climb/drop; Small ripple, no large periodic jump.
Abnormal: the bus keeps drifting, the ripple is large and small, and the rectifier plate/capacitor is often aging.
3. Inverter output (the most important thing is to supply power to the load)
Output voltage, output current, output frequency, output active power, output voltage THDu (total harmonic distortion of voltage).
Steady-state criterion (GB/T7260.3-1)
Steady-state deviation of output voltage: ≤ 1% un (online inversion is normal steady state, transient state is another matter).
Output frequency: follow the mains supply when synchronizing the mains supply; When the internal crystal oscillator runs freely, it is generally ≤±0.1Hz.
Voltage THDu: ≤3% under linear load and ≤5% under nonlinear load.
Three-phase model: the three-phase voltage imbalance is less than 2%, and the three-phase current imbalance does not exceed the limit.
4. Battery branch
Battery voltage, battery charging and discharging current, cell voltage steady state: the current is very small and stable when floating; The floating charge voltage is stable, and there is no continuous rise/fall, and there is no frequent jump back and forth in charge and discharge;
Frequent switching between charging and discharging is a dynamic disturbance, not a steady state.
Second, the steady-state judgment operation steps (field operation)
1. Collect real-time data
Two ways: 1)UPS local LCD screen/monitoring card (SNMP, Modbus) continuous sampling record; 2) High-precision power quality analyzer (recommended, see waveform, harmonic and ripple), hung at the output end of UPS.
It is impossible to judge the steady state only by looking at the instantaneous value of a single point, and it must be observed continuously for a period of time.
2. Time window selection
Light load/normal load: continuous observation for 3 ~ 10 minutes.
Load change scenario: ensure that the load is basically constant before steady-state judgment; The condition that the load is constantly switched on and off does not belong to steady state itself.
Steady-state premise: stable load, no large load switching, no power disturbance. If the load is changing all the time, the UPS parameters will inevitably fluctuate, which is not a steady state.
3. Interpreting that three conditions are met at the same time is considered as steady state.
Condition 1: the average values of all electrical parameters are basically unchanged, and the average values of voltage/current/bus/power without unidirectional drift do not continuously increase or decrease slowly; Only a small amount of random noise exists.
Condition 2: The fluctuation amplitude is within the limit value within the manufacturer's index+national standard allowable steady-state error, and there is no periodic large oscillation (back and forth large fluctuation).
Distinguish: normal small noise (steady state) vs continuous oscillation (unstable control loop, unstable)
Condition 3: There is no alarm and the control mode is stable. UPS works stably in the double conversion online mode, and does not switch back and forth frequently in [mains inverter battery inverter]; No over-voltage, under-voltage, bus abnormality and harmonic overrun alarm.
Third, high-frequency UPS is easy to be confused: steady state vs transient state.
Steady state: the load and commercial power conditions remain unchanged, and the electrical parameters fluctuate slightly in a narrow range; The waveform is stable.
Transient state: the voltage will drop/peak for a short time due to the drop of mains supply, sudden load loading and unloading and switching events, which belongs to a dynamic process and cannot be used to judge the steady-state performance.
Pay special attention to the high-frequency on-line machine: the high-frequency PWM switch will bring its own tiny ripple, which is normal. Don't treat the switching ripple as unstable oscillation. Distinguishing method: ripple is a tiny pulsation with a fixed frequency; Instability oscillation is a low frequency, large amplitude, envelope swing back and forth.
Fourth, quick and simple judgment (there is no electric energy analyzer on site)
UPS monitors the background and calls up the trend curve (voltage, bus, output power)
Keep the load constant, and watch the trend for 5 minutes:
The curve is a narrow-band straight belt, with small burr → steady-state qualification.
The curve slowly drifts up/down, or periodically swings up and down sharply → steady-state abnormality (loop, capacitor, rectifier and inverter board problems).
Check the working mode: continuous on-line, not frequent battery jumping mode.
Five, common steady-state unqualified phenomenon
The DC bus voltage drifts slowly; Bus ripple continues to increase.
The output voltage oscillates periodically at low frequency (inverter control loop oscillation, a typical hidden danger of high frequency machine)
Input THDi continuously jumps sharply.
Three-phase output voltage imbalance continues to exceed the standard.
The floating charge current fluctuates greatly back and forth, and charging and discharging are frequently switched.
VI. Reference Standards
GB/T 7260.3-1 Uninterruptible Power Supply Equipment (UPS) Part 3-1: Methods for determining performance and test requirements; Product manufacturer's technical manual (the steady-state index given by UPS manufacturer is preferred as the judgment threshold, and the national standard is the minimum requirement)
