6.4　Dynamic Performance

6.4.1　Basic response requirements

6.4.1.1　Response of current controller

1　The response of a DC current controller is usually determined by the parameters of the HVDC transmission system and AC system conditions，among other factors.For an HVDC transmission system with a line length below 1000km and a transmission power below 3000MW，if it is connected to a fairly strong AC system，the following criteria can be followed unless otherwise required:

（1）If the variation of current order does not exceed the DC current margin，the response time is not more than 30ms（considering the error in the current control circuit，it is allowable that the maximum variation of current order is smaller than the DC current margin by 2% of rated current）.

（2）If the variation of current order exceeds the DC current margin，the response time is not more than 70ms.

2　For an HVDC transmission system with a line length above 1000km and a transmission power above 3000MW，the following criteria can be followed:

（1）If the variation of current order does not exceed the DC current margin，the response time is not more than 90ms（considering the error in the current control circuit，it is allowable that the maximum variation of current order is smaller than the DC current margin by 2% of rated current）.

（2）If the variation of current order exceeds the DC current margin，the response time is not more than 110ms.

6.4.1.2　Response of power controller

The response of DC power controller shall be determined through system studies.For a strong or fairly strong AC system，the following criteria can be followed unless otherwise required:

1　The step response of power controller shall be adjustable within 100ms-9s.

2　When the HVDC transmission system is operating at any power level between the minimum and rated power，the response of DC power controller to either a step increase or step decrease in power order shall be such that 90% of the ordered change is achieved within 150ms after the change of setting value.This time shall include the time for confirmation of DC current order.

3　When a transient AC system disturbance gives rise to the variation of DC voltage，the response of DC power controller shall be 90% of the variation of power due to voltage variation and the power can be restored within 1s after initiation of the variation.

6.4.1.3　Response of extinction angle controller

The response characteristics of extinction angle controller shall match those of DC current controller and power controller.

6.4.1.4　Response of voltage controller

The DC voltage controller shall be adjusted to best coordinate the fast response to DC voltage setting and the sound commutation characteristics，thereby minimizing the risk of commutation failure.The DC voltage controller shall have a proper response time to meet the specified step response requirements of DC current control and power control.

6.4.2　Response after AC system fault

The response of HVDC transmission system after an AC system fault shall be determined through system studies.For a strong or fairly strong AC system，the following criteria can be followed unless otherwise required:

1　For AC system faults on the inverter side and rectifier side，the transmission power of the HVDC transmission system shall be recovered to 90% of the value before fault within respectively 140ms and 120ms from the instant the fault is cleared.

2　During recovery there shall be no commutation failure or continuous oscillation of DC current and voltage，and the actual recovery time of the HVDC transmission system shall be easily adjustable.

3　The same recovery time shall be achieved when the communications system is out of service.

6.4.3　Response after DC line fault

For a strong or fairly strong AC system，unless otherwise required，the following criteria can be followed for the response after a DC line fault:

1　The DC line fault protection system shall be able to detect the fault，de-energize the faulty pole line through its control，wait for some time for the air to deionize，and automatically attempt to recover the transmission power of the pole.A total communication system outage shall not in any way affect this line fault protection sequence.

2　Excluding the time for de-ionization，the time it takes from the beginning of the fault to recovery of 90% of the transmission power before the fault shall not exceed 120ms.This requirement shall be met for all DC line faults where the DC voltage reduces to 80% or lower of the normal DC voltage.

3　During DC line fault and de-ionization，to mitigate the effect on DC transmission power，the transmission power of the healthy pole will reach its temporary overload or 2h overload，and the required addition of 90% power shall be achieved within 80ms after the faulted pole is blocked.After recovery of the faulted pole，the total bipolar transmission power shall be able to restore to the pre-fault value smoothly with adjustable rate.The complete outage of DC communications system shall not in any way affect the power transfer from the blocked pole to the healthy pole.

6.4.4　Demonstration of HVDC system response

6.4.4.1　Demonstration items

All the control and protection functions which may affect the dynamic performance，including the time delay of communications system，shall be demonstrated in simulation of the HVDC transmission system.

The specified performance requirements shall be met at all power transfer levels from minimum to rated power in all operation modes.

The reactive power compensation shall be consistent with the general performance requirements and operation principles specified in 6.5.For demonstration and examination of response characteristics，the reactive power compensation capacity introduced before the change of setting or occurrence of other disturbances shall be consistent with that determined by reactive power controller.

Some operation conditions shall be studied assuming that a low order（2nd，3rd，4th or 5th）harmonic resonance exists in the AC filter and AC system to demonstrate the capability of the DC control system to operate normally in steady state and recover successfully after an AC system fault.The conditions of resonance can be achieved by suitably modifying the impedance of the equivalent AC system.

For AC systems of various strengths，besides in the very small mode that is possible in actual operation，the performance of DC control system shall be demonstrated assuming that the short-circuit current level on the inverter side reduces to the lower limit of the effective short-circuit ratio of the AC system of this strength specified in IEC 60919-3.

The system design shall demonstrate the dynamic performance in the following aspects at least:

1　The response characteristics of the HVDC transmission system controller to the step change of the power and current settings.

2　The response performance of the HVDC transmission system to various AC system faults on the rectifier side and inverter side，including the performance during fault and immediately after fault，reflecting all the related control and protection functions and actions.

3　The performance of the HVDC transmission system during and after the action of various protections，including DC line protection and station protection（proper fault type shall be selected to demonstrate the protection coordination，transient current and voltage characteristics，control actions for clearance of fault，and fault isolation control sequence initiated by protection actions）.

4　The steady state operating characteristics of the HVDC transmission system，including the performance during converter transformer on-load tap changer control and switching between DC control modes.

5　The pole startup and blocking sequence control.

6　Additional control performance.

7　The demonstration of DC control system performance during AC filter and shunt capacitor switching（including all the switching modes specified in 6.5.4）.

6.4.4.2　Demonstration methods

6.4.4.2.1　Response of DC current controller

The response of the DC pole current to the step increase or decrease of the current order shall be demonstrated when the DC transmission power level ranges between the minimum power and the rated power.Demonstration is also required for the DC control system with such a response to effectively meet the requirements for possible large signal（greater than 0.1p.u.）modulation through transient stability studies.

By injecting step signals into the current controller on the rectifier side to generate current step order，all the control parameters（α，γ，and DC voltage）shall be at their ratings before the step.Such signals shall be applied to specific points of open-loop control so that the current controller does not require a confirmation signal of current order change from the inverter.To avoid loss of current margin throughout the dynamic process，the current order shall be first increased then decreased.The response time considered does not include the allowable communications delay.

The current step response shall be demonstrated under all the normal control modes of the inverter.Before step，the inverter shall be operating steadily in a normal control mode.

With the other pole or converter units operating with 1.0p.u.of the rated DC power or out of service，the current step response of the tested pole or converter unit under the following conditions with the DC current order not greater than 0.1p.u.shall be demonstrated:

1　The current order steps from 0.92p.u.to 1.0p.u.

2　The current order immediately steps from 1.0p.u.back to 0.92p.u.after the step in 1.

The demonstration with a current order of 0.5p.u.shall be carried out in the same way and under the same conditions.

6.4.4.2.2　Response of DC power controller

The response of the DC power to the step increase or decrease of the power order，when the DC transmission power level ranges between the minimum power and the rated power，shall be demonstrated，so shall its response after an AC system disturbance.If necessary，as shown by studies，the DC system power controller shall be adjusted such that the HVDC transmission system has the characteristics of current control during transient AC system disturbances（which usually last less than 10 seconds）.

Before the step，the pole or converter unit shall be operating in a normal control mode and all the control parameters（α，γ，and DC voltage）shall be at their ratings.

The response characteristics of the power controller shall be demonstrated in all normal control modes of the inverter.

With one pole operating with 1.0p.u.of the rated DC power or out of service，the DC power step response of the other pole shall be demonstrated under the following conditions:

1　The power setting steps from 1.0p.u.to 0.9p.u..

2　Apply a power step from 0.9p.u.to 1.0p.u.immediately after dynamic process caused by 1.

3　The power setting steps from 0.9p.u.to 1.0p.u..

4　Apply a power step from 1.0p.u.to 0.9p.u.immediately after the dynamic process caused by 3.

The response of the power controller to the change of DC voltage after a permanent single-phase or three-phase earth fault on AC bus in rectifier or inverter shall be demonstrated.

6.4.4.2.3　Response of extinction angle controller

The response characteristics of the extinction angle controller to the step change of setting and the removal of reactive power sub-banks shall be demonstrated.

6.4.4.2.4　Response of DC voltage controller

The response characteristics of voltage controller to step change of setting and of measured control parameters during reactive sub-bank switching and removal of banks shall be demonstrated.

6.4.4.2.5　Response after AC system fault

The response characteristics of the HVDC transmission system after AC system fault on rectifier side and inverter side shall be demonstrated.

1　Where single-phase ground short circuit fault reduces the voltage on the faulted phase to 90%，70%，40%，20% or 0% of its normal value，the fault shall be cleared by the main or backup protection.

2　Where three-phase ground short circuit fault reduces the AC bus voltage to 90%，70%，40%，20% or 0% of its normal value，the fault shall be cleared by the main protection.

3　Where single-phase ground short circuit faults respectively occur at both terminals of a main AC line，the faulted phase shall be removed；if the reclosing fails，the three phases shall be removed.

4　Where single-phase ground short circuit faults respectively occur at both terminals of a main AC line，the reclosing succeeds.

5　Where three-phase ground short circuit faults respectively occur at both terminals of a main AC line，the three phases shall be removed.

Before fault，the entire HVDC transmission system shall be transporting the rated power.

6.4.4.2.6　Response after DC line fault

For long-distance HVDC transmission systems，the fault clearing and recovery characteristics of the DC line pole shall be demonstrated before fault when monopolar and bipolar rated power is transmitted.Faults shall be considered both at line terminals and at the line midpoint of one of the line poles.

The required response of the healthy pole or unit when one pole or unit is blocked shall be demonstrated as follows:

1　Both poles operate with 0.5p.u.of the rated power，and one pole is permanently blocked.

2　Both poles operate with 0.5p.u.of the rated power，a DC line fault occurs on one pole，and the recovery succeeds.

3　Both poles operate with 1.0p.u.of the rated power，and one pole is permanently blocked.

4　Both poles operate with 1.0p.u.of the rated power，a DC line fault occurs on one pole，and the recovery succeeds.

5　With one pole or unit lost，the response of the healthy pole or unit can be achieved in different transmission directions.

6.4.5　Converter operation during AC system fault

In either rectifier or inverter operation，when an AC system fault reduces the AC bus voltage to the following values which continue for their corresponding durations，the energy storage devices in all thyristor firing circuits shall have sufficient energy to continuously provide firing pulse to the thyristors so that the thyristor valve is safely turned on.

1　AC system single-phase ground fault，the voltage on the faulted phase reduces to zero for a duration of at least 0.7 seconds.

2　AC system three-phase ground short circuit fault，the voltage reduces to 30% of the normal value for a duration of at least 0.7 seconds.

3　AC system three-phase ground metallic short circuit fault，the voltage reduces to zero for a duration of at least 0.2 seconds.Immediately following the clearing of such faults and recovery of the commutation voltage，the valve firing circuit shall have sufficient energy to safely turn on the thyristors.No delayed recovery is allowed due to the need of charging the energy storage circuit.

The HVDC transmission system shall operate continuously and steadily when the three-phase mean voltage on the AC bus is higher than 30% of the normal voltage but lower than the extremely minimum continuous operating voltage for a duration of 1s caused by AC system fault.Under these conditions，the maximum DC current is determined by AC voltage and thermal stress limit of thyristor valves.

For severe AC system faults where the measured average three-phase rectifier voltage on the AC bus is 30% of the normal value or less，the recovery performance of the HVDC system shall be improved by maintaining，if possible，the DC current at a certain value by continuing firing the valves.If，in order to protect the HVDC equipment，the valve group has to be blocked with bypass pair put into service，the valve group shall be able to be deblocked within 20ms after the three-phase rectifier voltage on the AC bus recovers to 40% of its normal value.

6.4.6　DC circuit resonance

The DC smoothing reactor and DC filter design shall ensure that resonance at the fundamental frequency and the second harmonic of the fundamental frequency will not occur in the main circuit of the DC side.The deviation of primary series resonance frequency from the fundamental frequency and the second harmonic frequency shall be at least 15Hz for all connection and operation modes.The control system shall be able to provide positive damping for resonance produced in the DC circuit.