6.5　Reactive Compensation and Voltage Control

6.5.1　General provisions

The reactive compensation system shall include reactive compensation equipment at both converter stations and its associated control devices，such as AC filters and shunt capacitors，along with necessary circuit breakers，protection and control equipment.Shunt reactors shall be provided，if required，to meet reactive power balance requirements.

The control of reactive compensation equipment shall be automatic.The reactive power controllers can control all the reactive power supply and consumption equipment of the station，including the capacitor and reactor mounted on the third winding of auto-transformer.These controllers control the switching of AC filters，shunt capacitors and shunt reactors，and also reactive power consumption of DC converters according to applicable requirements.This ensures that the optimum combination of in-service reactive compensation equipment is realized under any DC transmission power and any DC operation modes.The controllers at both stations usually operate in the control mode of exchanging reactive power with the AC system.In addition，an optional mode shall be available to control the AC bus voltage as an alternative to the above mode.The reactive power control of the two converter stations shall be independent of each other.

The reactive compensation scheme shall meet the reactive power requirements of DC converters and AC system，and be well coordinated with DC control modes.In the scheme，capital expenditure，cost of loss，AC filter requirements，availability，reliability and maintainability shall be considered.

All the reactive compensation equipment shall operate normally and continuously at least under the following conditions:

1　The AC bus voltage is in the range of normal steady state operating voltage or extreme steady state operating voltage.

2　The AC system frequency is in the range of normal frequency variation and emergency frequency variation and their corresponding periods.

3　Normal variation of ambient temperature.

To meet the switching requirements，a circuit breaker shall be provided for each sub-bank.And for banks，circuit breakers should be provided in conjunction with the AC main electrical connection of converter station.

All switching operations for reactive power control shall be performed using circuit breakers，rather than a combination of circuit breakers and disconnectors.Disconnectors are used only for isolation.

It shall be possible to maintain each sub-bank and its circuit breaker while the rest sub-banks are energized.To do so，sufficient clearance and guardrails shall be provided.All circuit breakers used for switching shall be capable of switching off or switching in their corresponding sub-banks unrestrictedly.

Every circuit breaker used for switching shall be capable of disconnecting and re-energizing the subbank within 5 minutes.All necessary discharge equipment shall be provided.

In any DC transmission direction and under any transmission power，except the minimum combination of filters that must be switched in to meet the filter rating，any possible reactive compensation sub-bank shall be available for the AC bus.Under any temperature，frequency or AC bus voltage conditions，as long as the valve group operates continuously，the switching of any AC filter sub-bank and shunt capacitor sub-bank shall not be restricted.When the HVDC transmission system is totally down，nothing in its control and protection system shall restrict the switch-in of filter or shunt capacitor sub-banks.When a fault occurs on the HVDC transmission system，it is still possible to switch the AC filters，shunt capacitors or shunt reactors to control the AC system voltage.

All the shunt capacitor sub-banks at a converter station shall have the same nominal rating.If two or more shunt reactors are required at a station，they should also have the same nominal rating.

When calculating the reactive power produced by a filter or capacitor sub-bank，an assumption shall be made that the dielectric temperature of the capacitor is 25°C and all the capacitor elements are intact.

Related automatic control devices shall be provided for sub-bank switching.

The reactive consumption capability of DC converter equipment shall be used for reactive control.

6.5.2　Requirements for Supply and Absorption of Reactive Power

6.5.2.1　Reactive power consumption of converter

The calculation of reactive consumed by converter is given in Appendix C.

6.5.2.2　Capacity of reactive power supply equipment

The calculation of capacity of reactive power supply equipment is given in Appendix D.When configuring reactive power supply equipment for a converter station，the reactive power consumption of converter，Qdc，should be considered based on the rated bipolar transmission power.

6.5.2.3　Capacity of reactive power absorption equipment

The calculation of capacity reactive power absorption equipment is given in Appendix E.When configuring reactive power absorption equipment for a converter station，the reactive power consumption of converter，Qdc，should be considered based on 10% of the rated DC transmission power.

The DC converter equipment shall have a certain inherent reactive power absorption capability，which，during low DC power transmission，particularly during monopolar operation at 10%-20% of the rated monopolar power，reduces the reactive power flowing to the AC system.The reactive power absorption capability of converter equipment shall be a continuous and smooth function of the DC transmission power.

6.5.3　Maximum Bank and Sub-Bank Capacity

The total reactive power produced，when all capacitor sub-banks and AC filter sub-banks in a bank are switched in and none of the reactor sub-banks is switched in，should not exceed the capacity of the circuit breaker used to switch off the bank.

The sub-bank capacity shall be determined based on the requirements for switching and steady state regulation.

6.5.4　Switching requirements

6.5.4.1　Design conditions

1　Before switching，the transmission power ranges between the minimum power and the 2h overload in any direction.

2　Before switching，the AC system frequencies at both terminals remain within the steady state variation range.

3　Before switching，the AC bus voltages at both converter stations remain within the normal continuous operation range.

4　The ambient temperature remains within the specified range.

5　The short circuit level of the AC buses at both converter stations is no lower than the minimum value.

6.5.4.2　Sub-bank switching

1　When the AC filter sub-bank，shunt capacitor sub-bank or shunt reactor sub-bank is switched，the maximum change of the dynamic voltage on the converter AC bus shall not exceed 0.015p.u，which can be determined based on the location of converter station in system and grid operation conditions.

2　The switching of any sub-bank shall not result in a commutation failure.

3　The switching of any sub-bank shall not result in a DC control mode change or alter the DC power transfer level.This requirement shall be met after the DC system controls have responded but before the converter transformer tap-changers have operated.

4　The specified switching requirements can be met through the pre-control of firing angle or extinction angle.However，the increase or decrease of firing angle or extinction angle shall not result in commutation failure，the maximum dynamic change of DC voltage and current shall not exceed 0.05p.u.，and the DC transmission power or operation condition shall not engender any steady-stage change.

6.5.4.3　Removal of bank

1　The removal of bank can be deemed as a nonconventional means and shall not be used for reactive power control（only as a protection）.

2　All necessary measures，such as alarm，shall be provided to minimize the possibility of inadvertent removal of any reactive compensation bank.

3　The removal of a bank，either due to inadvertent act or intended protection，shall not result in monopolar or bipolar blocking.All the equipment shall have sufficient rated capacity and all necessary control functions to prevent overloading of the equipment.This may involve reducing the DC transmission power，however，step decrease of the power is not an option.The DC power may only be reduced at a rate not greater than 3% of the rating per minute.

4　After removal of a bank，it shall be possible to isolated the faulted equipment and put the other equipment in the bank back into service to recover the normal capacity of the HVDC transmission system as quickly as possible.

6.5.5　Steady state regulation requirements

The rated capacity of reactive compensation equipment sub-banks and reactive power absorption capability of DC converters shall meet the requirements for steady state regulation.

The HVDC transmission system shall be capable of regulating the reactive power exchange between the converter stations and the AC system to meet the reactive power supply and absorption requirements of the converter stations.In regulating the reactive power exchange，the deviation of actual exchange from scheduled exchange shall not exceed the maximum deviation specified for converter stations.Before the capacity of supply and absorption equipment is exhausted，under any specified operation modes and for any DC transmission power level between the minimum power and the rated power，the specified maximum reactive power deviation shall not be exceeded.The maximum reactive power deviation is usually set to be slightly greater than 1/2 of the maximum sub-bank capacity.This deviation shall not be considered when calculating the capacity of reactive power supply and absorption equipment.

Sub-bank switching and combination of sub-bank switching and DC converter control are an acceptable means to meet the following regulation requirements:

1　The steady state change of AC bus voltages at both converter stations caused by switching operation should not exceed 1% and 75% of the voltage for one step of the converter transformer on-load tap changer.

2　Within a cycle of DC load，i.e.，the DC transmission power level increasing from the minimum power to the rated power and decreasing back to the minimum power，each capacitor sub-bank or filter sub-bank may only be switched in or off for a maximum of once.

3　Binary switching of reactive power sub-banks is not allowed.

4　After any switching operation and before another required switching operation，the allowable change of the DC transmission power shall be at least 1.5 times the capacity of maximum reactive power sub-bank.

5　The DC voltage of rectifier，including all errors and dead zones，shall be maintained at a value not exceeding 2%-3% above the normal operating voltage.

The capacity of reactive compensation sub-banks and reactive power control strategies for DC converter equipment shall be checked and adjusted according to the static voltage characteristics of the converter AC bus to meet the voltage change requirements of switching and steady state regulation.

6.5.6　Temporary overvoltage control

6.5.6.1　Basic requirements

When permitted by the AC system，the HVDC transmission system shall be restarted and recovered to the transmission power before the fault as quickly as possible to minimize the effect of temporary overvoltage.If due to AC system restrictions the transmission power cannot be recovered or can only be partially recovered，the shunt capacitors and AC filters shall be removed to minimize the effect of temporary overvoltage.

In a network configuration which may result in self-excitation of generators close to the converter station，it has to be ensured that the reactive compensation equipment does not remain connected to the network.

The reactive compensation and control equipment shall be designed such that，at any power transmission level in either direction，the power frequency overvoltage on the converter AC bus greater than 1.3 times the maximum operating voltage caused by a complete or partial interruption of DC power transmission shall not last longer than 5 cycles，and the change of voltage amplitude shall not exceed 30%of the voltage before fault.

Within 500ms after the interruption of DC power，the power frequency overvoltage shall be reduced to 1.15 times the maximum operating voltage or lower，and be further reduced in 2s to 105% of the voltage before fault，or the voltage with all the capacitor sub-banks switched off.

6.5.6.2　Temporary overvoltage control

The requirements for limiting the temporary overvoltage shall be met without switching off shunt capacitors or introducing other reactive power control equipment，except under the following conditions:

1　In the case of delayed clearing of AC system fault or delay recovery of HVDC transmission system due to other reasons，if the power frequency overvoltage has to be reduced to 1.2 times the maximum operating voltage and it is allowed to recover only part of the DC transmission power after clearing of the AC system fault，the shunt capacitors and other reactive power equipment can be switched off using the overvoltage protection or other protections.

2　If the recovery of the HVDC transmission system fails，all or part of the reactive power equipment can be switched off to meet the specified requirements for power frequency overvoltage.

3　If all the AC lines connected to the AC bus quit，all the reactive power equipment shall be switched off immediately and the DC transmission shall be shut down.

4　If faults occur simultaneously on the two pole lines of the HVDC transmission system，all the reactive power equipment shall be switched off immediately.

All the necessary detection and control equipment shall be provided to operate the reactive compensation equipment，reduce the overvoltage，and distinguish temporary and permanent blocking of the HVDC transmission system.

The emergency current breaking capacity of bank and sub-bank switches and the capability of switching off capacitors under temporary overvoltage shall be optimized；the switches shall be capable of performing all switching operations；the temporary overvoltage is controlled using both sub-bank and bank switches.

6.5.6.3　Operation modes to be considered

In all of the following modes，the temporary overvoltage on the AC side shall meet the requirements:

1　Before fault，the converter AC bus voltage remains within the normal operating range；

2　In bipolar operation，the transmission power changes from 10% of the rated power to 2h overload；

3　In monopolar metallic return or ground return mode，the transmission power changes from 10% of the rated monopolar power to 2h overload.