A monitoring system for detecting an error of a harmonic filter in a high voltage direct current (HVDC) transmission system, wherein the harmonic filter is configured to be installed at a transmitting end of the HVDC transmission system to remove a harmonic wave that is generated in the process of converting an alternating current into a direct current or to be installed at a receiving end of the HVDC transmission system to remove a harmonic wave that is generated in the process of converting a DC into an AC, the monitoring system comprising: a harmonic filter comprising at least one capacitor (C1 to C3), at least one inductor (L1 to L3) and at least one resistor (R1 to R3) and configured to remove harmonic waves through a combination of the at least one capacitor (C1 to C3), the at least one inductor (L1 to L3), and the at least one resistor (R1 to R3); wherein the at least one capacitor (C1 to C3) includes a first capacitor that comprises a plurality of capacitor units (111 to 118), wherein the monitoring system further comprises: a monitoring sensor unit configured for sensing a plurality of voltages corresponding to respective capacitor units of the plurality of capacitor units (111 to 118); and characterised by further comprising a control unit configured for determining a voltage error of the plurality of capacitor units based on the sensing result received from the monitoring sensor unit, wherein the monitoring sensor unit includes a plurality of monitoring sensors, wherein each of the plurality of monitoring sensors corresponds to at least one capacitor unit of the plurality of capacitor units (111 to 118), and wherein at least one monitoring sensor among the plurality of monitoring sensors includes a variable resistor, and the at least one monitoring sensor is configured to sense the voltage of a different number of capacitor units by adjusting the variable resistor. a harmonic filter comprising at least one capacitor (C1 to C3), at least one inductor (L1 to L3) and at least one resistor (R1 to R3) and configured to remove harmonic waves through a combination of the at least one capacitor (C1 to C3), the at least one inductor (L1 to L3), and the at least one resistor (R1 to R3); wherein the at least one capacitor (C1 to C3) includes a first capacitor that comprises a plurality of capacitor units (111 to 118), wherein the monitoring system further comprises: a monitoring sensor unit configured for sensing a plurality of voltages corresponding to respective capacitor units of the plurality of capacitor units (111 to 118); and characterised by further comprising a control unit configured for determining a voltage error of the plurality of capacitor units based on the sensing result received from the monitoring sensor unit, wherein the monitoring sensor unit includes a plurality of monitoring sensors, a monitoring sensor unit configured for sensing a plurality of voltages corresponding to respective capacitor units of the plurality of capacitor units (111 to 118); and characterised by further comprising a control unit configured for determining a voltage error of the plurality of capacitor units based on the sensing result received from the monitoring sensor unit, wherein the monitoring sensor unit includes a plurality of monitoring sensors, wherein each of the plurality of monitoring sensors corresponds to at least one capacitor unit of the plurality of capacitor units (111 to 118), and wherein at least one monitoring sensor among the plurality of monitoring sensors includes a variable resistor, and the at least one monitoring sensor is configured to sense the voltage of a different number of capacitor units by adjusting the variable resistor.