6.7　DC Filter

6.7.1　General requirements

The DC filters（including HV pole line filters，neutral bus capacitors and/or electrode line filters）and their associated control and protection equipment shall reduce the harmonic currents in the HVDC bipolar lines and electrode lines to a level that will not impair the adjacent communication equipment.

The DC filters are usually of double-tuned or triple-tuned type.Their combination shall be determined based on the actual situation of the project after technical and economic comparison.The same design scheme should be used for filters in converter stations at both terminals for the same project.

Passive filters are usually used，which consist of filter arms that can be respectively switched.The number of filter arms at either terminal of either pole should be more than one.

Under all the specified operation modes，the circuit breakers used to switch filter arms shall be capable of switching in or switching off the branches without affecting the transmitted power on the connected pole.

The DC filters shall be such that no seasonal tuning is required.

Same or interchangeable parts shall be used as far as possible for easy maintenance and provision of spares，particularly the inductance components on the tuning branches.

The switching of DC filter branches shall not be limited by the ambient temperature，equipment temperature，AC system frequency，initial detuning，or the DC voltage within the range specified for the operation of converter.

During the resonance of converter station，as long as the converter station valve group operates continuously，the DC filter components shall not be subject to overload.

The performance evaluation of DC filters shall be based on the limit of equivalent interference current，the calculation of which is given in Appendix G.The harmonic current shall be calculated to the 50th order.

For any DC transmission power ranging from the minimum power to the rated DC power in either transmission direction，the equivalent interference current at any location on the DC line corridor and any point on the electrode line corridor at both terminals should not exceed the specified limit.The limit should be determined based on the length of the parallel section of the DC line and its neighboring open communication line，the vertical distance between the two lines，and the local soil resistivity.Without ascertaining the ambient conditions of the transmission line，the limit of the equivalent interference current during bipolar balanced operation can be selected within 1000mA-3000mA.

6.7.2　Performance design

6.7.2.1　Harmonic voltage

The harmonic voltages produced by converter shall include the fundamental and all 2nd-50th order harmonic voltages.The harmonic voltage shall be calculated under bipolar and monopolar metallic return and monopolar ground return operation modes.

Statistical method shall not be used in the calculation of the characteristic harmonic voltages.The characteristic harmonic voltages under the worst conditions which can last for more than 10 minutes must be calculated.

In all calculations of harmonic voltage，the effect of the ripple of DC current in valve，50Hz current in the DC pole line induced by parallel AC system，the power frequency negative sequence component of the AC bus voltage，and the DC current flowing in the AC grounding windings of the converter transformer shall be taken into account.

The existing negative sequence power frequency voltage in the AC system can be considered as 1% of the positive sequence voltage，the negative sequence voltage component produced by the HVDC transmission system shall be calculated subject to the minimum short-circuit level of the AC system，and these two shall be added arithmetically.

The weighted sum of the existing distortion of the AC bus voltage and the distortion produced by the HVDC transmission system shall be taken into account when calculating the harmonic voltage on the DC side.

In the calculation of harmonic voltages and resultant harmonic currents，the stray capacitances of the converter equipment，particularly at the multiples of three，such as 3rd，6th，9th，…，shall be taken into account.

For a particular operation mode，the maximum harmonic voltage for a single harmonic frequency can be determined，and the group of maximum harmonic voltages consisting of single maximum harmonics determined under various operation modes can be used to calculate the equivalent disturbing current.

6.7.2.2　Filter detuning

Detuning shall be considered for all filters and the detuning conditions that result in maximum voltage distortion shall be assumed.The detuning shall be considered under the simultaneous effect of frequency and temperature change，component fault and aging，and manufacturing error.

6.7.3　Rating design

The rated voltage of capacitors and reactors shall not be smaller than the sum of the fundamental and 2nd-50th harmonic voltages.The thermal stability current of the capacitors，reactors and resistors and the voltage used to determine the specific creepage distance shall respectively not be smaller than the root mean square of the fundamental and 2nd-50th harmonic current and harmonic voltages.

In determining the rating of the DC filter components，considerations shall be given that when any DC filter arm on any converter pole in any converter station is out of service，the bipolar operation is possible to transmit any power ranging from the minimum power to the 2h overload power.For all the other operation modes，the DC filter components shall also have a sufficient rating when one DC filter arm is out of service and the DC converter equipment is operating at any power level from the minimum power to the inherent maximum power of the given mode.

Allowances shall be made for possible current amplification resulting from resonances between DC filter arms.

In the calculation of the rating of DC filter components，a±10% error for the length of transmission line and electrode line shall be considered.In the calculation of the harmonic voltage and DC filter detuning，the functional relationship that the AC system voltage and frequency change along with the duration shall be considered.In addition，in the DC filter detuning calculation，the maximum damage of capacitor unit or component that still allows the continuous operation of the DC filter shall be considered.