6 Design Requirements
6.1 Rating
HVDC transmission systems shall be designed in strict accordance with the specified continuous rating，and meet requirements for overload，reduced voltage operation，and reversal operation.
6.1.1 Continuous operation
1 The continuous rating of the HVDC transmission system shall be guaranteed under the following conditions:
（1）Given normal variation range of converter AC bus voltages at both terminals.
（2）Given normal frequency variation range of AC system at both terminals.
（3）All ambient temperature conditions for both converter stations.
（4）All redundant equipment out of service.
2 Under all operation modes，the normal operating DC voltage of long-distance HVDC transmission system shall not exceed±2%-±3% of the rated voltage，considering tolerances and control errors of all equipment.The minimum allowable DC current shall not be greater than 10% of the rated current.
3 For a long-distance HVDC transmission system，the minimum resistance of DC line of each pole shall be considered in the calculation of rated DC power and voltage at the inverter.A back-to-back HVDC transmission system is usually in symmetrical design，with the sending and receiving terminals having the same rating.
4 For change of AC bus voltage and frequency，the rating of HVDC transmission system shall meet the following requirements:
（1）Over the extreme range of AC system frequency variation，there shall be no reduction in power transfer capability of the HVDC transmission system and converter equipment.
（2）Within the extreme continuous range of AC bus voltage and extreme frequency range，the HVDC transmission system shall be able to start up safely and operate continuously.Some degradation in DC system performance is acceptable for operation beyond the normal continuous voltage or frequency range.
（3）Where the converter AC bus voltage is lower than the normal minimum continuous operating voltage，but not lower than the extreme minimum continuous operating voltage，the transfer capability under DC bipolar or monopolar operation mode（the per unit value based on DC rating power）shall not be less than the minimum ratio of the actual AC bus operating voltage to the corresponding normal minimum continuous operating voltage at both terminals.
6.1.2 Overload
Where the ambient temperature is lower than the extreme temperature used in design，or the redundant cooling equipment is put into service，the HVDC transmission system shall have overload capability.For an HVDC transmission system designed based on continuous rating，without considering the limitations of DC voltage range and reactive power supply equipment，the system overload capability shall not be inferior to the following causing no significant increase in cost:
1 Under maximum ambient temperature and with the redundant cooling equipment in service，the continuous overload capability of the system is 1.05 times its rated power.
2 Under maximum ambient temperature and with the redundant cooling equipment in service，after a continuous overload operation，the 2h overload capability of the system is 1.1 times its rated power.
3 Under maximum ambient temperature and with the redundant cooling equipment in service，after 2h overload operation，the 3s overload capability of the system is 1.4 times its rated power，1.3 times for 5s，and 1.2 times for 10s.
6.1.3 Reduced voltage operation
When an HVDC transmission system transmits power from rectifier to inverter，usually either pole shall be able to operate continuously with the rated DC current under 70%-100% of the rated DC voltage，without increasing the cost of equipment，as long as AC bus voltages at both terminals are within the normal continuous operation range.However，the reactive power configuration will not be evaluated when DC voltage is below 80% of the rated DC voltage.
6.1.4 Reverse power operation
Reverse capability of HVDC transmission system shall not be less than 90% of the rating without considering reactive power compensation and increasing investment in equipment.
6.1.5 Calculation of main circuit parameters
The main circuit parameters involved in the calculation are:
1 DC voltage:
（1）The 12-pulse converter bridge DC voltage on the rectifier side and inverter side.
（2）The pole-to-ground DC voltage on the rectifier side and inverter side.
2 The DC voltage difference between the rectifier side and inverter side.
3 Voltage drop due to commutation impedance.
4 Overlap angle.
5 Reactive power consumption.
6 The phase-to-phase AC voltage and no-load DC voltage on the valve side of converter transformer，and the rated continuous AC current on the valve side.
7 Rated capacity of converter transformers.
When designing the range of the on-load tap changers of converter transformers at both terminals，the maximum variation range of DC line resistance of either pole shall be considered.
See Appendix B for the detailed calculation of main circuit parameters.