6.6　AC Filter

6.6.1　General requirements

The AC filters and their associated control and protection equipment shall reduce the harmonic voltage on the AC bus connecting converter transformers and the harmonic current in the associated AC lines to a level that will not impair the equipment connected to the AC system in the station and adjacent communication equipment.

The AC filters shall meet the given requirements for reactive power，overvoltage and reactive power switching.

The AC filters shall be capable of keeping continuous operation without overload within the specified steady state and short-time variation range of voltage and frequency.

The AC filters can be single-tuned，double-tuned，triple-tuned，or high-pass type.Their combination shall be determined based on the actual situation of the project after technical and economic comparison.

No more than three types should be used in one converter station.

An AC filter usually consists of passive components such as capacitor，reactor and resistor.Same or interchangeable parts shall be used for these components as far as possible.

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

With reference to GB 14549，the indicators of harmonic interference on converter AC bus may be expressed by single harmonic distortion（Dn），total harmonic distortion（Deff）and telephone harmonic form factor（THFF）.The definitions of these indicators are given in Appendix F.The harmonic shall be calculated to the 50thorder.

The typical range of voltage distortion limit for converter AC bus is that Dn is usually 0.5%-l.5%，typically 1.0%.According to the system negative sequence voltage and background harmonic，different requirements can be applied to harmonics of different orders.For example，higher values are used for odd and low-order harmonics（3rd and 5th order）.The Deff is usually 1%-3%.The THFF is usually 1%-2%.

The AC filters shall meet the distortion limit requirements of AC bus voltage under the following conditions:

1　The HVDC transmission system operates under normal voltage，the transmission power is any power from the minimum power to the continuous overload power，and all AC filter sub-banks are in service or available for service.

2　For any DC transmission power from the minimum power to the continuous rating power（1.0p.u.）and normal DC voltage，one filter sub-bank is unavailable.In this case，if the unavailable AC filter sub-bank consists of low-order non-characteristic harmonic filters，the single harmonic distortion requirement for such non-characteristic harmonic can be waived.However，the maximum calculated value of the harmonic of this order shall be given，and the corresponding Deff and THFF shall be calculated using the maximum allowable distortion of this harmonic.

3　Within the specified range of reduced voltage operation，all the AC filter sub-banks shall be available from the minimum power to the maximum DC transmission power corresponding to the reduced voltage.

6.6.2　Performance design

6.6.2.1　Harmonic current produced by converter

The harmonic currents produced by converter shall include the fundamental and all 2nd-50th order harmonic currents under various operation modes.

Considering all combinations of power levels and voltage levels，the maximum current interval shall be 10% of the nominal current and the maximum voltage interval shall be 5% of the normal voltage.

The power frequency negative sequence voltage of the AC system shall be 1% of the positive sequence voltage.

All the calculation parameters shall be taken from the allowable steady state range to maximize the harmonic current produced by the converter.

6.6.2.2　Filter detuning

Detuning shall be considered for all filters and the detuning conditions that result in maximum voltage distortion shall be assumed.The following factors shall be considered for filter detuning:

1　The normal continuous power frequency variation at the station site.

2　The ambient temperature conditions at the station site and the component temperature rise variation due to change in load.

3　The maximum possible initial detuning due to design.

4　The maximum number of broken capacitors before detuning should be decided by following:

1）Under all operation modes and assuming that the frequency is within the normal continuous variation range，highest voltage of each capacitor lower than 1.4 p.u.，the predicted maximum number of broken capacitors.

2）95% of the predicted maximum number of broken capacitors before the pre-planned maintenance.

5　Detuning due to component aging.

6.6.2.3　AC system harmonic impedance

The AC system harmonic impedance shall be considered based on open circuit and any impedance in the given AC system harmonic diagram.

6.6.3　Rating design

6.6.3.1　General requirements

Under the ambient conditions and the AC system conditions of the converter station，the AC filters shall have sufficient rated capacity to suit the power levels required by all the operation modes of the HVDC transmission system.

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

In the calculation of the AC filter rating，the harmonic current produced on the AC side of the converter，filter detuning and AC system harmonic impedance shall，in addition to 6.6.2，further meet the following requirements.

6.6.3.2　Harmonic current

The power frequency negative sequence voltage of the AC system shall be 2% of the positive sequence voltage.

The effect of background harmonic shall be considered for the harmonic current in filter components.The harmonic currents and voltages due to background harmonic shall be added to the filter currents and voltages due to converter generated harmonic currents using the following equations:

Where In and Un are the harmonic current and voltage respectively in the filter and subscripts 1 and 2 respectively refer to the components caused by the converter and background harmonic in the AC system.The values of k are given in Table 6.6.3.2.

Considering that there are other harmonic current sources in the AC system，the 10th-50thharmonic currents flowing in each filter component shall be increased by 10% in addition to the harmonic current produced by the converter.

6.6.3.3　Filter Detuning

During abnormal frequency period the variation of frequency maximizes，and before the second-stage alarm of the filter，the capacitor unit or component is damaged to the maximum degree possible.

The frequency remains within the normal continuous range，the filter is about to be removed due to protection and the capacitor unit or component is damaged to the maximum degree possible.