1、 Abstract Urban grid system with high proportion of air conditioning load has the characteristics of unpredictability and uncontrollability.The sudden rise and drop of city load is becoming the norm.This has a negative impact on the system stability and the situation may get worse when fault occurs
2、in such areas.The reinforcement of city network is restricted by high short-circuit current level and limited transmission corridors,thus the system operators seek more flexible and controllable operation methods.This paper analyses the application of VSC-HVDC technology in city center systems,focus
3、ing on providing flexible power support for the connected areas while improving the characteristics of air conditioning load.The effect of VSC-HVDC on the city center system voltage stability is revealed and the capability of MMC for voltage support at different operating conditions is discussed tho
4、roughly.Index Terms Urban Grid,VSC-HVDC,Air conditioning Load,System Operating Characteristics.I.INTRODUCTION rban power grids are developing fast,and the network topology tends to be complex with concentrated loading in limited areas.The operation of urban power grid normally faces problems like un
5、even distribution of the power flow,overloading of the main transmission corridors,as well as the high short-circuit current in central areas.Meanwhile,the requirements of the power supply quality in central city are constantly increasing,which makes it a great challenge to provide high-quality and
6、reliable electricity.The emergence and the penetration of power electronic based technologies provide an effective technical solution for the development of urban power grids.IGBT based VSC(Voltage Source Converter)devices have superiority and flexibility in voltage and power control,and the latest
7、MMC(Modular-Multilevel Manuscript received June 29,2018.This work was supported by State Grid Sichuan Economic Research Institute.Authors are with State Grid Sichuan Economic Research Institute,Chengdu,China(phone:028 681-32142;e-mail:).Converter)technology provides reduced losses and harmonic injec
8、tions into the system.This makes the technology capable of providing flexible power supply to meet the required load characteristics in urban systems.Therefore,it is necessary to analyze the feasibility and economy of applying VSCs in the urban power grid.Air conditioning load is one of the troubles
9、ome loads in the urban grid areas due to its characteristics.The increasing number of city residents has stimulated the rapid growth of air conditioning loads which becomes an important influence factor for system stability.In literature 1,2,the key characteristics of a single air conditioning load,
10、including both static and dynamic behaviors,are revealed and validated via experimental tests.It is shown that the air conditioning load could deteriorate the voltage stability of the connected bus,and the aggregation of air conditioning load can cause dramatic impact on system voltage stability 3,4
11、.The application of MMC techniques in urban power grid are considered 5,6 as effective solutions to optimize the system power flow and improve voltage stability.However,its effect and capability in improving the urban system with large proportion of air conditioning load has not been analyzed thorou
12、ghly.The research carried out in this paper is to present an idea of the effectiveness of MMC on urban system voltage control as well as its power/reactive power capability under different operating points.This will give an idea on the selection of MMC capacity when designing such projects in urban
13、areas.The paper is organized as follows:section II provides an analysis of the characteristics of urban grid power supply and the influence brought by large proportion of air conditioning load;Section III discusses the operation and control of a grid connected MMC as well as its effect in improving
14、of the operating characteristics of the urban grid;Section IV looks further into the maximum capability of a MMC supporting the connected urban grid.Improvement of the Operating Characteristics of Urban Power Grid via MMC L.Shen,Y.Su,X.Ouyang,M.Zhu,Y.Wang,T.Li State Grid Sichuan Economic Research In
15、stitute U 2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804260000072 Page1/6 1412 II.CHARACTERISTICS OF URBAN GRID POWER SUPPLY A.Urban Grid Supply With the rapid growth of urban power grid load,the voltage level keeps increasing and the
16、network connection tends to be more complex.The power supply of urban power grid will be limited by various aspects such as substation capacity,stability,short-circuit current level and so on.At present,the generic problems appear in the development of urban power grid are mainly reflected in the fo
17、llowing aspects:a)Due to land resources constraints,new transmission corridors are very limited and thus the cost of installing new transmission lines in the city center increases.Instead of introducing high-voltage overhead lines into urban areas,power supply and distribution through underground ca
18、bles are preferred at this stage.As the capacitive charging current limits the maximum transmission distance of AC cables,reactive power compensation devices are often equipped at the line ends to increase the maximum transmission distance of AC cables.In such cases,the application DC transmission u
19、sing the XLPE cable which is lighter and cheaper becomes a more economic approach.b)Urban load demand tends to gather in multiple local areas within the city area,and the gathering locations vary rapidly with time.The uneven and variable load distribution can bring great pressure to the substations
20、and transmission lines in the system.Due to the lack of powerful control methods,the loading of transformers/lines can be very heavy while their nearby equipment are only lightly loaded.The temporary overloading situation may cause excessive investment for system operators to upgrade system equipmen
21、t.c)The system network in city center areas is closely connected to achieve high electricity supply reliability.In such cases,the system impedance is decreased,and the short-circuit current level is increased.With the development of urban grid topology and the further increase of load,the problem of
22、 short-circuit current will be aggravated,which threatens the safe and stable operation of urban power grid.d)Large generations are normally located far away from central loading areas,where the power supply can only rely on bulk power long distance transmissions.Therefore,voltage support in urban a
23、reas becomes crucial,especially in face of high proportion of temperature controlled load demand.These areas generally use load regulating transformers and shunt capacitors to improve the voltage profile.However,under low voltage conditions,the continuous action of the load voltage transformer can c
24、ause the voltage instability of the high voltage side and the reactive power output of the shunt capacitor will also decrease with the square of the voltage.The system voltage stability cannot be maintained actively.More effective and flexible voltage control are in demand.B.High Percentage of Air c
25、onditioning Load The increase of temperature controlled load becomes a key factor influencing urban grid power supply.Here we provide a detailed analysis of the temperature controlled load in the city areas which mainly consists of air-conditioners.Air conditioners mainly include compressor,condense
26、r,and evaporator,etc.Its load characteristics are mainly determined by the core compressor.Since the traditional fixed frequency air conditioners still occupies the market,this study will focus on the analysis of the fixed frequency air conditioners.Fixed frequency air conditioners cannot regulate i
27、ts output power continuously,as it operates at a fixed speed.The power adjustment is achieved through continuous start and stop to control the temperature.The accumulative effect of a large number of air conditioning devices has a strong effect on the system stability.In addition,as the frequency va
28、riation of the power grid is generally small,the frequency characteristics of air conditioners can be ignored and the focus is put on the voltage related load characteristics.Air conditioners used in commercial areas with large power ratings are usually equipped with low voltage protection.Under nor
29、mal operating state,the compressor has the greatest power consumption in the air conditioning load,and its characteristic is similar to an induction motor.The other parts of the air conditioner can be approximated by a static load.The research mainly focuses on the dynamics of urban grid system duri
30、ng normal operation and fault conditions with large percentage of air conditioning load.Therefore,three main characteristics of the air conditioning load are considered:(1)the dynamics during system disturbance,(2)machine starting and(3)low voltage protection mode.The dynamic characteristics of the
31、air conditioning load are described by an induction motor together with a ZIP model which represents the static part of the load.The accumulative effect of a large number of air conditioning loads is simulated by paralleling the combined models,as shown in Fig.1.The induction machine is represented
32、by the typical transient equivalent circuit equations as:01()()d d s s qs r qv v X X i vT(1)01()()q q s s ds r dv v X X i vT(2)1()()2r e mTTH(3)2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804260000072 Page2/6 1413where vd and vq are the
33、 stator voltages,X is the reactance of induction machine,p r is the slip speed and T0 is the open circuit time constant.The ZIP model for the static part of the air conditioning load is given as:20 1 2 3(1)pfP P p V p V p k f(4)20 1 2 3(1)qfQ Q q V q V q k f(5)As the frequency deviation in the urban
34、 system is small,its frequency dependency part can be neglected.C.Load characteristics of a single air conditioner Considering the case of a single air conditioner connecting to an infinite bus(Tload),utilizing the combined induction motor and ZIP model,its dynamic responses under system disturbance
35、 is analyzed.A self-clearing voltage disturbance is applied at the connecting bus and the load responses are shown in Fig.2-Fig.4.During small voltage disturbance,when the voltage of the connection bus is above 0.6 p.u.,the active power of the air conditioner acts as a constant power load,while obvi
36、ous variation is seen in the reactive power(Fig.2).Commercially used large air conditioners are generally equipped with low voltage protections.When subjected to a large voltage drop,where the voltage decreases below 0.6 p.u.(shown in Fig.3),the activation of low voltage protection mode.The air cond
37、itioner is removed from the system when the threshold voltage is reached with tis active and reactive power fast drop to zero to avoid further influence to the connected system.When starting,the maximum power is 4-7 times the rated power of the air conditioner.The starting process of air conditionin
38、g will produce a sudden rise of power,and usually return to a steady state within 1 second.The frequent starting/stopping of a large number of air conditioning loads will lead to sudden load changes and affects the system voltage,this is demonstrated in Fig.4.Fig.2 Dynamic responses of a single air
39、conditioner.Fig.3 low voltage protection mode.System BusMZIP+Induction MachineLow Voltage Bus M M Fig.1 Accumulated air conditioning load model.2018 China International Conference on Electricity Distribution Tianjin,17-19 Sep.2018CICED2018 Paper No.201804260000072 Page3/6 1414III.IMPROVEMENT OF THE
40、OPERATING CHARACTERISTICS OF URBAN POWER GRID USING MMC-HVDC It has been shown that VSC has many advantages in improving the controllability and stability of the connected system 7.The MMC based topology with fewer losses has further promoted the application of VSC，making it an effective solution fo
41、r the existing problems of urban power supply.The 500kV China ZhangBei Multi-terminal HVDC project under construction is an example of urban grid MMC application.The small footprint and the increased power rating of MMC-HVDC effectively resolve the dilemma of limited land resources in the city areas
42、.Instead of constructing large generation units around the city,clean and renewable energy can be transmitted through MMC-HVDC link,and the receiving end MMC can serve as a voltage supporting point in the urban network.Meanwhile,XLPE underground cable can be used for DC transmissions which make MMC-
43、HVDC an economic comparable solution to conventional AC transmissions.The black start capability and very small short-circuit current contribution also make MMC-HVDC suitable for interconnection between small city networks with high load density.A.Operation and Control of Grid Connected MMC In terms
44、 of voltage stability,the reactive power control in MMC may be used.In the urban power grid with larger air conditioning load,reactive power fluctuation is more significant as shown in section II.MMC control modes which are commonly seen include power control(P-Q mode),DC voltage and reactive power
45、control(Vdc-Q mode),active power and AC voltage control(P-Vac mode),and STATCOM operation mode.The STATCOM operation mode needs to open the DC switch under an uncharged condition,so that the DC switches are in separated positions.This cannot be transferred directly from other modes in an operating p
46、rocess,so the running state mode transformation is mainly among the other three modes,as presented in Fig.5.However,active power and DC side voltage is firstly guaranteed during operation,and then new set points for reactive power control are adjusted when needed.As the voltage stability of urban sy
47、stems is affected by the accumulated air conditioning load,the reactive power support provided by the grid connected MMC become useful.The reactive power control can be activated based on systems needs so that the MMC can provide transient support for system voltage regulation and recovery.During no
48、rmal operating conditions,the MMC can also act as a fixed capacitor to provide constant reactive power compensation for steady state voltage regulation.However,the reactive power provided by the MMC need to be within its capability,and satisfies the total apparent power rating limits.Alternatively,t
49、he MMC can be set to monitoring the voltage at the point of connection and provide dynamic reactive power support within the limits.The conversion process is presented in Fig.6.B.Simulation of a Grid Connected MMC with Air Conditioning Load The classical two-area system,as shown in Fig.7 is adopted
50、for analyzing the effect of air conditioning load and a grid connected MMC.The load connected at bus 7 is considered to be a static or an accumulated air conditioning load model which accounts for 50%of the total system load.The voltage and power at the load bus(bus 7)is monitored when the system is
