5.3　AC System Conditions

5.3.1　System overview

As different operation modes of AC and DC systems need to be considered in system design，it is necessary to identify the general conditions of related AC systems.The description of AC systems at both terminals shall detail system conditions in the commissioned year，rating level year and remote future year.These conditions mainly include schedule for putting the project into service，link between the regional network in which the converter station is located and the main network，connection scheme of the converter station（current and future），including number of outgoing circuits，line parameters，line length，and schedule of power plants related to the converter station（unit capacity，number of units）.

5.3.2　AC system conditions

The following system data shall be provided as input for analysis of main circuit parameters and performance verification of HVDC transmission system.

5.3.2.1　AC bus steady state voltage variation

The range of variation in steady state voltage of AC bus shall be provided，including normal operating voltage，range of normal continuous operating voltage，and range of extreme continuous operating voltage.

5.3.2.2　AC system frequency variation range

The normal operating and post-disturbance frequency，including the normal variation range of AC bus frequency，the variation range of post-disturbance frequency，the upper and lower variation limits after the fault is cleared.Usually an enveloping curve for the maximum variation of frequency is used to indicate these limits.See Fig.5.3.2.2.

5.3.2.3　Negative sequence and background harmonic voltage

The background negative sequence power frequency voltage and background harmonic voltage for AC system shall be provided.These voltages can be considered as a Thévenin's equivalent voltage source.

5.3.2.3.1　Negative sequence power frequency voltage

The background negative sequence voltages at power frequency of AC systems at both terminals are 1%of positive sequence voltages.The actual negative sequence power frequency voltage at converter AC bus shall be equal to the vector sum of the background negative sequence voltage and the negative sequence power frequency voltage produced by converter.The phase angle of the background negative sequence voltage relative to the phase angle of the positive sequence power frequency component shall be assumed to be the value which maximizes the magnitude of negative sequence power frequency voltage at the converter AC bus.

5.3.2.3.2　Background harmonic voltage

The AC system background harmonic voltage can be obtained by calculating the system harmonic power flow after measurement.All the third order harmonic voltages must be considered to be in positive sequence，and other orders of harmonic voltages can be considered in positive sequence，negative sequence，or a combination thereof，but their arithmetic sums shall be equal.The phase angle of the background harmonic voltage relative to the phase angle of the positive sequence power frequency component shall be assumed to be the value which maximizes the magnitude of the harmonic voltage at the converter AC bus.

5.3.2.4　AC bus short-circuit current level

The AC bus short-circuit current level shall be provided，including maximum three-phase，singlephase and minimum three-phase short-circuit current，corresponding short-circuit capacity（including the base voltage for calculating short-circuit capacity）and the ratio of system reactance to resistance.For projects involving multiple phases，the AC bus short-circuit current level shall be identified for each phase.

It is recommended to calculate the minimum short-circuit current as follows:under the minimum operation mode，assume that the circuit contributing the largest short-circuit current to the converter station is under maintenance，and at the same time assume that the circuit at the node adjacent to the converter station is under maintenance.

5.3.2.5　Fault clearing time

The fault clearing time for normal and backup protection can be selected from Table 5.3.2.5.

5.3.2.6　Time sequence for single phase reclosing

The single phase reclosing time sequence shall be provided，which can usually be selected from Table 5.3.2.6.

The values for fault clearing time given above are used for study on overvoltage and insulation coordination.The AC system fault clearing time to be used for study on transient stability shall be determined based on the actual system situation.

5.3.3　AC system equivalent

Where simulated analysis on an AC system equivalent is required，the application of system equivalent shall be defined so that each kind of equivalent is used only for its intended study purposes.

Usually，system equivalents mainly include those used for AC/DC simulation study，for reactive power switching and power frequency overvoltage study，for electro-magnetic transient performance of AC/DC system study，and for AC filter design.

5.3.3.1　System equivalent and model for AC/DC simulation study

5.3.3.1.1　Model

Operation modes of system equivalent for AC/DC simulation study shall be determined considering various typical operation modes in the commissioned year and design level year of the HVDC project.

The system equivalent for this purpose can be obtained by means of static equivalence.To ensure that the system equivalent shares the same or similar characteristics with the original network around the concerned converter station，the system equivalent shall be checked against the retained portion of the original network as follows:

1　If active and reactive power flow results are consistent.

2　If voltage levels at the retained nodes in the system equivalent are consistent.

3　If the deviation of bus short-circuit currents in the retained portion from the original network does not exceed 10%.

4　If the recovery characteristics of AC bus dynamic voltage are basically the same as the original network when an AC system fault occurs close to the converter station.

The system equivalent shall have static equivalent circuit composed of parts such as resistors，reactors and capacitors，etc.to represent the simulated system The harmonic impedance of such equivalent circuit shall be calculated based on 100% generator sub-transient reactance and 100% transformer leakage reactance of the whole original system.The positive sequence impedance of the system equivalent shall properly represent the power frequency impedance of the system under selected operation mode；its harmonic impedance shall properly represent the system harmonic impedance observed on the specified bus，including amplitude and phase，with frequency ranging 50Hz-500Hz.

5.3.3.1.2　Study contents

1　AC/DC system equivalents are mainly used for the following purposes:

（1）Evaluation of DC control and protection functions.

（2）Evaluation of AC/DC system performance for different DC system control modes.

（3）Evaluation of DC system performance when fault occurs on the DC side（such as converter station blocking，pole blocking，DC line fault，valve side winding fault）.

（4）Demonstration of DC system response in conformity to the specified response criteria.

（5）Demonstration of DC system transient response caused by switching of reactive power compensation banks and sub-banks.

（6）Study on interactions between DC system and local generators in case of disturbance.

（7）Test on subsystems of on-site control systems.

（8）Evaluation of DC system performance in case of AC bus voltage decrease and distortion resulting from severe AC system fault.

（9）Study on switching overvoltage and ferro-resonance on AC side and DC side.

（10）Study on transient overvoltage on DC side resulting from asymmetrical AC system faults.

2　The above-mentioned system equivalents shall not be used for the following purposes:

（1）Study on AC filters.

（2）The only means for insulation coordination study.

（3）Study on power frequency voltage.

（4）Demonstration of AC system static voltage regulation principles.

5.3.3.2　System equivalent for reactive power switching and power frequency overvoltage studies

Study on reactive power switching and power frequency overvoltage can be performed using an system equivalent obtained as follows:

First，use a network equivalence program to obtain the Thevenin equivalent impedance of the system equivalent viewed from the retained buss.

Then，demonstrate if the equivalence is valid by comparing the change of AC bus voltages obtained using the system equivalent and the stability model of the entire system.

The AC bus voltage can be obtained by regulating the voltage source but has to be kept within the range of extreme continuous operation voltage specified in 5.3.2.1，and other retained bus voltages must be kept in the range as well.

Such a system equivalent can be used to:

1　Analyze the voltage change on converter station AC bus resulting from switching of filter sub-bank or capacitor sub-bank.

2　Demonstrate the maximum overvoltage caused by load rejection of the HVDC transmission system.

This static equivalent model is not suitable for determining the thermal capacity of overvoltage control devices，as the static equivalents cannot reproduce the dynamic oscillation process of the entire system.The voltage obtained for 0.2s or a longer time using the equivalent model is much greater than the value obtained using stability analysis model of the entire system.

5.3.3.3　Equivalent impedance for AC filter performance analysis

This equivalent is used for AC filter design.Based on scanned harmonic impedance results，the 2nd to 50th harmonic impedances can be shown using a sector diagram or a subdivided sector diagram.For highcontent low-order harmonics（below 10th）and characteristic harmonics produced by converters，it is usually recommended to use a sector diagram to represent their corresponding system equivalent impedances，as shown in Fig.5.3.3.3-1；other harmonics can be presented by a single or a unified impedance circle，as shown in Fig.5.3.3.3-2.