Special requirements of impact load on high frequency on-line UPS

Typical impact loads: motor, compressor, frequency converter, servo, laser equipment, capacitive surge loads, which are characterized by starting instantaneous current 3 ~ 7 times the rated value, short-term peak, inductive reactive power and frequency conversion feedback energy; High-frequency on-line UPS has no built-in power frequency isolation transformer, and the inverter IGBT has weak impact resistance. Compared with the power frequency machine, it has special hard adaptation requirements, which are explained in six dimensions:

I. Short-term overload capability (the core hard index)

The overload margin of ordinary civil high-frequency UPS is small, so the overload timing specification must be strengthened for impact load:

110% load: it can run continuously for a long time and cope with small load fluctuation.

125% overload: ≥10min, to deal with a small continuous overload.

150% overload: ≥ 30 ~ 60s, suitable for the conventional starting shock of the motor.

200% ~ 250% instantaneous overload: ≥ 100 ~ 200 ms, to undertake millisecond current peak.

Ordinary low-power high-frequency machines usually only support 150% overload for 10s, which is easy to start and trip, and cannot directly impact the load with high power.

Second, the peak coefficient (crest factor CF) matching requirements

The peak coefficient of conventional IT high frequency UPS is 3: 1; The peak value of impact load (motor, frequency converter, laser) can reach 5 ~ 8 times of steady-state current, and the peak value coefficient of UPS is required to be ≥5:1.

Matching formula:

SUPS≥S load ×CFUPSCF load

Consequences of insufficient peak value: instantaneous current overrun, inverter current limit, voltage drop, accidental overload, bypass or even shutdown protection.

Three, capacity selection and power factor adaptation requirements

Capacity must amplify redundancy.

The steady-state total load is recommended to be ≤ 70% ~ 80% of the rated capacity of ≤UPS; It is suggested that the capacity of UPS should be enlarged by 1.5 ~ 3 times to offset the starting impact.

Output power factor matching

The mainstream output pf of high-frequency UPS is 0.9; Motor inductive load PF = 0.6 ~ 0.8. If the load PF is lower than the UPS output PF, the active load capacity is compressed, so it must be expanded for accounting:

SUPS=PF load × pfuspsp load

It is strictly forbidden to match according to the steady-state power. It seems that the capacity is enough, and the overload protection will be started.

Four, dynamic response and output voltage regulation characteristics requirements

Fast dynamic response: when the load is impacted by step, the voltage drop/rise amplitude is ≤ 5%, and the recovery time is ≤20ms, so as to avoid motor stall and controller reset caused by sudden voltage drop.

Output waveform quality: pure chord output, linear load THDV < 3%, inductive shock load THDV < 5%; Prevent harmonic superposition from aggravating inverter heating and misoperation protection

Double transformation online mode zero switching (0ms), can not frequently cut bypass; Bypass and inverter must be locked in phase synchronously, and there is no circulating current impact when switching.

Five, topology, isolation and anti-reverse electromotive force design (high frequency machine short board key reinforcement)

There is no power frequency output isolation transformer in the high-frequency on-line UPS, which has two major risks: reverse electromotive force and braking feedback energy, and requires additional configuration:

Optional external output isolation transformer/series output reactor.

Restrain starting surge and absorb inductive back electromotive force

Isolate DC component, weaken harmonic reflection and prevent IGBT overvoltage breakdown.

The variable frequency/servo load must be equipped with an energy release loop.

The regenerative energy generated by motor deceleration braking flows back to the DC bus, and the common high-frequency machine has no energy dissipation resistor, which is easy to shut down due to DC overvoltage; The bleeder resistor/active feedback unit needs to be selected.

Industrial-grade enhanced high-frequency UPS (high-current IGBT, thickened bus, bus buffer absorption circuIT) is preferred for high-power impact scenarios, and it-type high-frequency machines in ordinary computer rooms cannot be used to directly impact heavy loads.

Six, protection logic optimization (to prevent false protection)

Overload protection with time-delay anti-shake: distinguish instantaneous impact from real short circuit, and avoid instantaneous tripping triggered by millisecond peak.

Sectional inverse time-limit overload curve: light overload, long delay, severe short circuit and quick trip, giving consideration to impact resistance and fault safety.

It has the function of current-limiting soft start: it supports the power-on sequence of back-end loads in batches, and prohibits multiple impact loads from starting at the same time.

Three-phase models require strong tolerance of three-phase imbalance, and single-phase impact will not lead to three-phase voltage imbalance and misoperation of phase-lack protection.

VII. Supporting Additional Requirements for Input Side

The front end is equipped with an input filter and a surge absorber, so as to prevent the harmonic backflow of the impact load from polluting the power grid, and at the same time avoid the power grid distortion from interfering with UPS rectification.

The rectifier has wide voltage adaptability, so it is not easy to cut the battery when the impact causes voltage drop, reducing the frequent battery switching loss.

Summary of typical rectification scheme of high frequency UPS with impact load

Selection: industrial reinforced high-frequency UPS, meeting peak coefficient ≥5:1 and strengthening overload timing.

Capacity: the steady-state load rate is ≤75%, and the impact load capacity is enlarged by 1.5 ~ 3 times.

Hardware: add output reactor/isolation transformer and braking energy release unit.

Usage: the load is started in batches in time sharing, and the protection delay parameters are optimized to avoid the impact of simultaneous start and stop