6.2　Design of DC Operation Mode and Control Mode

6.2.1　Operation mode

There are many possible operation modes for an HVDC transmission system.A combination of the following modes can be determined based on specific requirements of a given project.

1　Based on the power transmission direction，a system can be in forward or reverse power operation.

Based on the operating voltage，there are full-voltage and reduced-voltage operation mode.The range of voltage for reduced voltage operation depends on the specific project.While maintaining the current unchanged，the operating voltage of a single converter shall be optional within 70%-100%.However，an excessive voltage drop may increase the cost of equipment.Reduced voltage operation involves various conditions:

（1）One of the series converters out of service.

（2）The external insulation of DC equipment requires reduced voltage operation.

（3）The reactive power control requires reduced voltage operation.

2　Based on the type of connection，the operation modes can be classified as:

（1）Bipolar（or multiple valve unit）operation mode（including temporary grounding operation through converter station earth）.

（2）Monopolar（or single valve unit）metallic return operation mode.

（3）Monopolar ground return operation mode（including pole line parallel ground return）.

For HVDC transmission systems with multiple converters in series or parallel connection，the operation modes also include those with the converters in separate operation.

3　The test operation modes include open line test and zero-power test.

6.2.2　Operation control modes and basic operation control strategy

6.2.2.1　General requirements

In order to make the HVDC transmission system have the specified dynamic performance，the system design shall meet the following requirements:

1　Current controller，voltage controller，and extinction angle controller for the DC control system，and such functions as DC power control，converter transformer tap control and reactive power control shall be provided to meet the operation control requirements of the HVDC transmission system and realize an optimum operation performance.

2　Some special control and protection functions，such as VDCOL，commutation failure protection，and enhanced commutation during AC voltage disturbance，shall be designed and optimized to meet the specified response performance.

3　The control characteristics of rectifier and inverter shall be adjusted to the best coordination between the DC current and DC voltage response to meet the response requirements.

4　The control equipment of HVDC transmission system shall be designed based on the maximum communication delay on the main and backup communication system to meet the specified performance requirements.

5　Under all the specified operation modes，the designed DC control system shall meet the specified performance requirements.

6.2.2.2　Operation control modes

The most common operation control mode of DC system is power control.To realize the aforesaid operation modes，the HVDC transmission system usually requires the following control modes:

1　Bipolar（or multiple valve unit）power control.

2　Pole（or unit）power independent control.

3　Pole（or unit）current synchronous control.

4　Pole current emergency control.

5　Pole voltage control.

6　Reactive power/AC voltage control.

7　Pole reversal power control.

8　Open pole line voltage test control.

9　Zero-power test control.

6.2.2.3　Basic operation control strategies

The basic operation control strategy for the HVDC transmission system shall be properly determined according to the conditions of the AC system to which it is connected and the specific requirements of the project.

Generally，the DC current is controlled by rectifier through adjusting the firing angle，with the change of firing angle in a certain range through adjusting the on-load tap changer on rectifier side.The DC voltage is controlled by the on-load tap changer on the inverter side，with extinction angle control used for the inverter and a limit on minimum extinction angle set.

The converter transformer tap changers on rectifier side and inverter side may also be used only to compensate the change of AC voltage.On this basis，the rectifier may be used tocontrol the DC current and the inverter to control the DC voltage.When required，the extinction angle can be increased to reduce the DC voltage，thereby controlling the reactive power.

The optimum DC parameters of a back-to-back HVDC transmission system can be selected by developing a reasonable control strategy.When necessary，the DC voltage can vary in a large range to meet the requirements for reactive power control.The converter transformer tap changer on the inverter side is used only to compensate the AC voltage change on this side.

6.2.2.4　DC additional control

The quick controllability of DC system can be used to module the DC power and help the AC system increase its operational transient and dynamic stability.

6.2.2.4.1　Power run-back and run-up

It may be necessary to automatically reduce the DC transmission power when generation power loss occurs on the rectifier side or load rejection occurs on the inverter side.It also may be necessary to quickly increase the power of the HVDC transmission system when generation power loss occurs on the inverter side or load rejection occurs on the rectifier side.The power run-back and run-up level shall be determined through system studies.

The run-back and run-up also include quick reversal of power.

6.2.2.4.2　Frequency control

Applying frequency controls on AC system through HVDC transmission system usually involve the following cases:

1　An HVDC transmission system fed from remote generation.

2　An islanded or weak AC system connected to a strong AC system through HVDC transmission system.

3　AC systems connected through an HVDC transmission system.

In system design，the frequency control function shall be considered so that the AC system frequency can be controlled continuously under steady state conditions，or the AC system frequency control can be executed when it shifts beyond a given limit.

6.2.2.4.3　Damping of sub-synchronous oscillation

System studies shall be carried out to analyze the possibility of sub-synchronous oscillation between DC system and synchronous power generators in AC system，so that effective damping method can be brought out.Sub-synchronous oscillation usually occurs on the rectifier side.

6.2.2.4.4　Damping of low-frequency oscillation

A disturbance in the power system may result in electromechanical oscillations between generators or local networks.Unless effective measures are taken，these oscillations may lead to instability of grid and loss of synchronization of generators.Or if the system fails to provide sufficient damping，the oscillation may last a longer time.The power modulation function of the HVDC transmission system can be fully used to suppress these oscillations.The commonly used modulation modes include small signal modulation，large signal modulation，and reactive power modulation.

6.2.2.4.5　Voltage control

For weak AC systems，the change of load，switching operation or fault may cause change in the reactive power flow，result in significant fluctuation of voltage.Effective voltage control measures shall be provided to limit the sudden change of AC voltage within an acceptable range，for example，3% for frequently occurring fluctuation，and 10% for occasionally occurring fluctuation.

In addition，a significant change of load or load rejection may cause a very high temporary overvoltage，jeopardizing the safety of equipment.In system design，measures shall be taken to limit the temporary overvoltage to an acceptable amplitude and duration.

Usually converters，switchable reactive power equipment，SVCs or synchronous condenser can be used for voltage control.In the design of voltage control，full consideration shall be given to the voltage reactive power characteristics of such equipment and AC system，as well as the strength of AC system（usually described by effective short-circuit ratio）.