3.2.2 Residual Current Device
The main purpose of installing leakage current protection devices in power supply systems is to protect the safety of personnel and equipment. This is because when the insulation performance of electrical equipment in the system deteriorates, not only do the electrical equipment pose a hidden danger, but the safety of workers is also threatened.
Leakage current
1) Steady-state or static leakage current refers to the leakage current specified for the safe operation of a filter under 250V AC voltage conditions. It consists of two parts: capacitance current Lc and insulation resistance leakage current IL.
Ic=2πfUCy (1)
In the formula: f is the power supply frequency;
U is the voltage applied across the capacitor;
Cy is the common-mode capacitance.
As can be seen from equation (1), the capacitor current is proportional to the power supply frequency, the voltage applied to the capacitor, and the common-mode capacitance.
IL=U/R1 (2)
In the formula: RL represents the leakage resistance of the UPS uninterruptible power supply, which includes the volume resistance inside the capacitor and the insulation leakage resistance outside the capacitor.
- Unsteady-state or dynamic leakage current is the current generated during the transition process when the output relay operates and the inverter circuit is turned on by a sinusoidal power supply in a series circuit of equivalent resistance r, inductance L, and capacitance C. This transition process may generate a large, oscillating, and decaying leakage current. Where L = L1 and C = Cy.
In the formula: Um is the peak value of the power supply voltage;
ω is the angular frequency of the power supply voltage;
Ψ is the initial phase angle of the power supply voltage when the circuit is turned on.
In UPS uninterruptible power supply maintenance, ignoring the influence of resistance, substituting circuit parameters L=0.7mH and C=4.7nF into the above formula, the circuit oscillates because its free oscillation angular frequency is much higher than the power supply angular frequency of 314 rad/s. When Ψ=φ, the effective value of the overcurrent generated in the circuit is as high as 570mA or more. This may cause a timed leakage current protection device (RCD) with a current greater than 0.1s to trip. In reality, the initial phase angle Ψ of the sinusoidal power supply voltage when the circuit is turned on is a random quantity and is mostly not equal to the phase angle φ of the circuit’s AC impedance. Therefore, the resulting overcurrent may not necessarily exceed 30mA. In addition, the presence of resistance in the circuit reduces and attenuates the peak value of the overcurrent, so the resulting overcurrent may not necessarily exceed 30mA, and the time of overcurrent generation in the circuit may not necessarily exceed 0.1s.
In reality, among the eight power supply systems we developed, only one system experienced the phenomenon of the mains circuit breaker’s leakage protection tripping and cutting off the mains power during UPS no-load startup multiple times. However, the leakage protection tripping of the mains circuit breaker did not occur every time the UPS started under no-load conditions.
UPS maintenance when the above circuit is connected to a sinusoidal power supply. During the tests, we used a Hall current sensor to measure the leakage current, and the conversion ratio between the test voltage and the leakage current was 100mA/lV.