1) Fault Phenomenon: The equipment cannot be started, only the DC ammeter has indication during startup, while both DC voltage and medium frequency voltage have no indication.
Analysis:
a. Missing pulse in inverter trigger pulses;
b. Inverter thyristor breakdown;
c. Capacitor breakdown;
d. Short circuit or grounding on the load;
e. Open circuit or short circuit in the medium frequency signal sampling loop.
2) Fault Phenomenon: Difficult to start. After startup, the medium frequency voltage is more than twice the DC voltage, and the DC current is excessively high.
Analysis:
a. One thyristor in the inverter loop is damaged;
b. One inverter thyristor fails to conduct, that is, "three-leg" operation;
c. Open circuit or wrong polarity in the medium frequency signal sampling loop;
d. Fault in the inverter leading angle phase shifting circuit;
3) Fault Phenomenon: Difficult to start. After startup, the DC voltage can hardly reach full load or get close to full load, and the reactor has large vibration with dull sound.
Analysis:
e. Rectifier thyristor has open circuit, breakdown, soft breakdown or degraded electrical parameter performance
f. One group of rectifier pulses is missing
g. Open circuit or short circuit at the gate of rectifier thyristor
4) Fault Phenomenon: The equipment can start, but shuts down immediately after startup, and keeps repeating the startup cycle.
Analysis:
h. The leading angle is too small;
i. The load oscillation frequency is at the edge of the external excitation frequency
5) Fault Phenomenon: After the equipment starts, when the power rises to a certain value, overcurrent protection is easily triggered. Sometimes thyristor components are burned, and the same phenomenon remains after restarting
Analysis:
j. If overcurrent occurs at low voltage right after startup, it is because too small inverter leading angle prevents the thyristor from turning off reliably
k. Degraded heat dissipation performance of the water-cooled jacket for inverter thyristor
l. Poor contact of connecting wires in the tank circuit
6) Fault Phenomenon: There is no response when starting the equipment, and the phase loss indicator on the control board is on
Analysis:
Fast fuse blown
7) Fault Phenomenon: During operation, the DC current has reached the rated value, but the DC voltage and medium frequency voltage are low.
Analysis:
This phenomenon is not a fault of the medium frequency power supply, but is caused by too low load impedance
a. One or more series capacitors are damaged
b. Inter-turn short circuit occurs in the inductor 
8) Fault Phenomenon: During operation, both DC voltage and medium frequency voltage have reached the rated value, but the DC current is small and the output power is low.
Analysis:
This phenomenon is exactly opposite to the fault described in 7), and is caused by high load impedance
a. Insufficient compensation capacity of the load compensation capacitor
b. Excessive contact resistance at the connection nodes of the tank circuit, clean the dust
9) Fault Phenomenon: The equipment operates normally, the DC current indication is higher than normal. If the current is set to the rated value, the voltage will be too low, and the power meter indication is inconsistent with the product of the current indication and voltage indication
Analysis:
This phenomenon is usually caused by dirt and oxide layer on the shunt and its connecting wires, which increases contact resistance and leads to higher voltage generated on the shunt
10) Fault Phenomenon: The equipment operates normally, but there is no response when starting after shutdown, and there is no protection indication.
Analysis:
a. Medium frequency start switch is damaged
b. Fault in the protection circuit, usually it is a problem with the frequency sweep circuit integrated block NE556 on the circuit board
c. The given signal is interrupted in the given circuit

11) Fault Symptom: Thyristors are frequently burned out, and the newly replaced component also burns out after installation
Analysis:
You can refer to Fault E), additional instructions are as follows:
a. When the thyristor is reverse turned off, it withstands excessively high instantaneous spike voltage on reverse voltage. Check the RC snubber circuit.
b. Reduced insulation between load and ground, arcing to ground, or high voltage formed across the thyristor
c. Fault in the pulse trigger circuit: sudden loss of trigger pulse causes the thyristor to open
d. Load open circuit during equipment operation
e. Load short circuit during equipment operation
f. Fault in the protection system (protection failure)
g. Fault in the thyristor cooling water system
h. Reactor fault causes discontinuous current on the inverter side. Magnetic saturation of the reactor makes it lose the current limiting function, resulting in thyristor burnout
i. Overlarge inductance of the commutation inductor, or reduced insulation causes unstable current
12) Fault Symptom: When starting the equipment, after turning on the medium frequency start switch, the main circuit protection trips or overcurrent protection is triggered
Analysis: 
a. The power adjustment knob is at the maximum position, causing excessively large instantaneous current impact
b. Fault in the current regulator, especially damaged current transformer or open wiring. There is no current feedback suppression during startup, leading to excessively large current impact
13)  Fault Symptom: The medium frequency transformer is burned out, and it burns out again after replacement when the equipment is restarted
Analysis:
This phenomenon usually occurs on step-up load equipment, and is mainly caused by loose connection and open circuit of the discharge inductor. In step-up mode, the capacitor voltages of two groups are inconsistent. The high-voltage group discharges slowly, and starts charging again before discharge is completed, so DC charges will accumulate on capacitors and need to be released through the discharge inductor. If the discharge circuit is open, charges will be released through the medium frequency transformer, which will burn out due to its small capacity
14) Fault Symptom: The discharge inductor overheats or burns out in the step-up circuit
Analysis:
a. The inductance of the discharge inductor is too small
b. Inverter pulses are asymmetrical
c. The equipment operates with one inverter thyristor already burned out. At this time, the medium frequency output voltage waveform is distorted, causing extremely large current flowing through the discharge inductor, which leads to overheating or burnout
15) Fault Symptom: The equipment can be started, the frequency after successful startup is much higher than the original normal value, and sometimes it is hard to start
Analysis:
a. Inter-turn short circuit occurs in the load coil
b. Open circuit occurs at the terminal of load capacitor
16) Fault Symptom: The equipment is easy to start, but the voltage is prone to overvoltage during boosting, and sometimes overvoltage and overcurrent occur simultaneously
Analysis:
a. Excessively large inverter advance angle causes excessively high inverter spike voltage
b. There are loose connections, reduced insulation and arcing in the main circuit inside the power cabinet
c. There are loose connections, reduced insulation and arcing on the load coil or capacitors
d. There is a triggering fault of inverter thyristors, such as loose connecting wires or gate open circuit
17) Fault Symptom: The equipment can be started, but the voltage cannot be boosted; the reactor makes loud and dull noise, the voltage is unstable when rising, overcurrent or overvoltage protection is triggered sometimes, thyristors are burned sometimes, but the rectification system works normally
Analysis:
a. The reactor has too large inductance, leading to magnetic saturation, so it cannot perform the filtering function
b. Poor insulation of the reactor