Control Strategies and Experimental Analysis on Primary Frequency Regulation for the Supercritical Power Unit

LIFushang，MA Yihe（State Grid Shandong Electric Power Research Institute，Jinan 250003，China）

·專題論述·

Control Strategies and Experimental Analysis on Primary Frequency Regulation for the Supercritical Power Unit

LIFushang，MA Yihe
（State Grid Shandong Electric Power Research Institute，Jinan 250003，China）

The most effective way to optimize the efficiency of conventional power plants and reduce the cost of per unit capacity is to improve the steam parameters combined with the development of large capacity units，thus as a result the large capacity units get rapid development.The drum boiler can notmaintain the natural circulation under supercritical pressure due to water and steam having the equal density under the critical parameters，and once-through boiler puts into practice instead. Rapid changes in load result in the main steam pressure fluctuations easily because of once-through boiler’s smaller heat storage capacity，which would affect the stability of the unit coordinated control system，and make it difficult to meet requirements of rapid load changes in participating in primary frequency control.Control strategies and experimental analysis on primary frequency regulation for the supercritical power unit are introduced in details on the basis of supercritical unit coordinated control features.

the supercritical power unit；once-through boiler；primary frequency regulation

0　Introduction

With the enlargement of network capacity and upping of user requirements for power quality，power grid frequency stability has attracted more concern. The sudden disturbance or accident in the power system tends to be in a very short period of time，with the addition of the relative laggard regulation of power grid dispatching system and automatic generation control（AGC）system being inadequate to meet the requirements of the stability of power grid，the effective implementation of primary frequency regulation（PFR）can make up for the shortage，realize the load on the frequency of fast response，maintain the frequency stability of power grid［1］.Regional power grid interconnection，large-scale wind power access，as well as high-power and long-distance transmission，the po tentialmagnitude of the disturbance of load and risks are expanding，measures must be taken to improve the unit’s PFR and grid AGC control response rate and adjustment ability，improve the response to sudden accidents or disturbance frequency stability［2］.

1　Control Strategies on Primary Frequency Regulation for the Supercritical Power Unit

1.1Characteristics of the supercriticalunitand it’s control system

With the development of electric power systems，600 MW，1 000 MWsuper（super）critical unit has become the main units of the power industry in our country.The supercritical unit refers to main steamsuperheateroutletpressuregreaterthan 22.129 MPa.The operation pressure is 24～25 MPa for currently running supercritical unit.In theory，supercritical parameters canimprove efficiency by 2%～2.5%，the ultra supercritical parameter can improve 4%～5%compared with thermal efficiency of subcritical thermal power generating unit.At present，the world’s advanced supercritical unit efficiency has reached about 47%～49%［3］.

The supercritical unit has great particularities in control system due to operation characteristics and operation mode of variable parameters，multivariable controlcharacteristicscomparedwithsubcritical drum boiler：

a）As a result of the nonlinear coupling between turbineandboiler，thedisturbanceof governing valves opening can lead to the power，pressure，temperature unexpected changes.

b）The pressure control is one of the most important controlled object for once-through boiler due to correlative coupling between controlled parameters，because the load and the feedwater flow is subject to the pressure change as well as the temperature.

1.2Frequency com pensation loop of unit coordinated control system

The coordination between power grid and plant means the units should response quickly to meet the load demand of power grid，which includes the grid AGC control and the primary frequency regulation control.The coordination between boiler and turbineis consideredacoordinatedunit，which mainly refers to the coordinated control of boiler and steam turbine，upgrading the response of the unit to load dispatching of power grid and the stability of unit operation.Steamturbine have a far different control characteristic to the unit load and pressurefromboiler，steamturbinepossessesa quickregulatingcharacteristictounit loadand pressure by controlling the opening of the governing valves，and the boiler’s regulating characteristic is muchslower.Therefore turbine-boiler coordinated control system（CCS）is introduced to realize the rapidity of load response and the stability of the pressure control with optimised control strategy.The coordinated control system contains the function of primary frequencycontrol，that is thefrequency correction circuits［4］.

The function of frequency correction circuit is to convert frequency offset signal to loaddeviation，then after limiter，maximum selector and minimum selector switch send respectively to the boiler control system and the turbine control system to rapidly response the demand of frequency regulation of the unit for power grid.Frequency correction circuit configurationdiagramis as shown in figure 1［5］. The frequency deviation between the field measured frequency signal restricted of dead band block 1 and the setting frequency value（50Hz）is converted into a corresponding load deviation signalΔMW. When CCS participation conditions of frequency calibration are not met，there is no activation signal of frequency calibration sent out，analogue switch set to’0’side，through the analog switch 7，speed limit block 6，multiplier 2，ΔMWand the constant 0 multiplied.W hen conditions are met by CCS participationoffrequencycalibration，throughanalog switch 7，speed limit 6，multiplier 2，andΔMW is multiplied by constant 1，CCS is normal in PFR.

Loaddeviationsignal amplitude is limitedby limiter 3 aiming to preventing the grid frequency deviation is too large，especially positive deviation is too large（that is，the gridfrequency is too lower than the setting value），as a result that the unit is overloaded and unstable for PFR.In addi-tion the deviation signal output and the minimum allowed by the unit possible deviation between the output and load command signal，in order to ensure that is controlled by frequency caused by load changes within the range of the unit safe operation allows.

Whenthegridfrequencychanges，the boiler output will varydependingontheturbineload change.Therefore，boiler load instruction should be changed based on frequency calibration signal.Considering the boiler follow load inertia，the instruction deviation is limited through the speed limiter 4 to avoid load shock to the boiler main control system，and the speed rate is set at 10%/s.

Figure 1 Unit coordinated control system frequency correction configuration diagram

1.3The frequency regulation circuit of a electro-hydraulic control system （DEH）

In the unit’s DEHcontrol loop，as shown in figure 2，speed deviation generated by the rotationspeedvalueandthepower gridfrequency value n，multiplied by the speed droop factor K is applied as the input signal to the power regulator.The speed deviation multiplied by K1，the speed ratio coefficient of feed forward loop，takes effects ontheoutput of thecontroller as well. The deadzone and clipping is set for the speed deviation to avoid unnecessary and excessive governing valve operation.

Loadcontrol deviationproducedby the actual load value and the unit load value N is applied to input power regulator，and load setting value takes effects on the output of the controller through the load ratio feed forward loop.

Figure 2 Primary frequency regulation control loop for DEH control system

2　The Process o f P rim a ry Frequency Regulation Test Response of a Unit

Figure 3 The process of primary frequency regulation test response of a unit

Main parameters of the unit as follows under steady operating condition：the unit load is 562.5 MW，the grid frequency is 50Hz，when the rotating speed is given a step change from 3 000 RPM （50 Hz）to 2 982 RPM for an example，the primary frequency control circuit takes effect，and the unit load falls fast to reach steady state value of 562.2 MW after thetransitiontimeof 7.43 s.Onthecontrary，when the rotatingspeedis givenastepchange from2 982 RPM to 3 000 RPM （50 Hz），the unit load rises fast to the steady-state value of 517.2MW after the transition time of 5.90 s.The primary frequency regulation response of the lag time，loadvariation and transitioning time are reached the de sign requirements，as shown in Figure 3 and Table 1.

Table 1 The parameters comparison before and after a primary frequency regulation response

3　Conclusion

For supercritical once-through boiler unit，the contradictionis very outstanding betweenrequirements of loadrapidly changing anda relatively small heat storage capacity of unit when participating in primary frequency modulation.To ensure the safe，stable operation of the unit，the steamturbine follow method（TF）for unit coordinated con-

trol systemis usually adopted.Primary frequency regulation fires at a time，the boiler master control command follows at a rapid rate，boiler water supply and fuel can quickly change respectively.It is possible for supplement or release heat timely to maintain the stability of steam pressure.As long as the control system design is reasonable，DEH and coordination function of primary frequency control system will be fulfilled together，and will not disturb the stability of the control system.

References

［1］YAO Jun，CHEN Weihe.Experimental study of primary frequencyregulationfor 900 MWsupercritical units［J］.East China Electric Power，2006，34（8）：84-87.

［2］CHEN Xiaoqiang，XIANG Jin，WEI Luping，et al.Perfor mance optimization study on primary frequency regulation of 1 000MW units［J］.Electric Power，2010，43（4）：63-66.

［3］WANG Jianqiang，ZHENG Weijian，TONG Xiaozhong，et al. Experimental study on enhancing the quality of AGC and primary frequency regulation based on heat systemcumulation ［J］.Electric Power，2014，47（9）：1-5，10.

［4］HUANG Wei-jian，ZHANG Xi，CHEN Shi-he，et al.Enhancingresponsespeedof primaryfequencyregulationin thermal power unit［J］.Electric Power，2011，44（1）：73-77.

［5］XIONG Shuyan，WANG Xingye，TIAN Jianyan，et al.Thermal power plant distributed control system［M］.Beijing：Science press，2000.

Accepted date：2016-02-29

LI Fushang（1968），senior engineer，is engaged in the thermal power engineering technology and scientific research work.

超臨界機組一次調頻的控制策略與試驗分析

李福尚，馬義河
（國網山東省電力公司電力科學研究院，濟南250003）

提高蒸汽參數與發展大容量機組相結合是提高常規火電廠效率及降低單位容量造價最有效的途徑，因此超臨界大容量機組得到迅速的發展。由于在臨界參數下汽水密度相等，在超臨界壓力下無法維持自然循環即不能采用汽包鍋爐，直流鍋爐成為唯一型式。直流爐的蓄熱能力比汽包爐要小得多，負荷的快速變化極易引起機前的壓力波動，影響到機組協調控制系統的穩定性，對參與一次調頻的負荷快速變化要求變得困難。在介紹超臨界機組協調控制特性的基礎上，重點對某類機組的一次調頻控制策略及試驗響應過程進行了詳細的分析與介紹。

超臨界機組；直流爐；一次調頻

TK323

A

1007－9904（2016）03－0017－04