CN104466952B - Method involving electric water heater in controlling stability of power grid frequency and frequency controller of electric water heater - Google Patents

Abstract

The invention relates to a method involving an electric water heater in controlling stability of the power grid frequency and a frequency controller of the electric water heater. The method comprises the steps of firstly, collecting the power grid frequency f(t) and the output water temperature T (t) of the electric water heater at the current moment t; secondly, designing the rated output water temperature allowance delta T of the electric water heater into a linear function changing along with the power grid frequency; thirdly, calculating the corrected rated output water temperature according to data obtained in the first step and the second step; fourthly, determining the work state of the next moment according to the result calculated out in the third step; fifthly; at the moment when t=t+1, executing the first step and starting next circulation. The frequency controller comprises a remote control module used for setting the coefficient of frequency modulation and the rated output water temperature, a frequency sensor module used for measuring the power grid frequency in real time, a temperature sensor module and a temperature control circuit module used for judging the work state of the electric water heater at the next moment.

Description

Electric heater participates in mains frequency and stablizes and control method and its frequency controller

Technical field

The invention belongs to power system frequency is stable and controls and household appliance control technology association area, specifically, It is related to a kind of electric heater participation power system frequency to stablize and control method and electric heater frequency controller.

Background technology

Power system frequency reflects the active power balance relation of power system Generation Side and electricity consumption side, and active power is uneven Weighing apparatus can lead to the appearance of frequency departure, and big frequency departure is for the safely, effectively operation of power equipment, and whole power train Safe and reliable, the economical operation of system are all unfavorable it is therefore desirable to being stablized to frequency and being controlled, by frequency bias control Within allowed band.Traditional main frequency is stable and control measure are to realize Generation Side by increasing/reducing unit output With electricity consumption side power-balance, thus realize by frequency bias control within allowed band.Although this mode of frequency regulation is overall On disclosure satisfy that the frequency modulation of power system requires, but have that unit response speed is slow, climbing time length, frequent movement are easy to wear, Frequency modulation high cost and the pollution weak point such as again.In addition, with the generation of electricity by new energy unit such as wind-force, solar energy in power system The increase of installation ratio, it makes to the intermittent impacts of electrical network power system frequency adjust and is faced with bigger challenge.

Different from traditional frequency regulative mode based on Generation Side, the frequency based on load side adjusts main consideration and passes through Adjust load side capacity and realize power-balance, and then realize frequency stable and control.The household electrical appliance power load distributing of load side is wide, Quantity is many, although the power of separate unit electrical equipment is less, its total capacity is big, disclosure satisfy that power system is wanted to frequency modulation spare capacity Ask.And electric heater, as the purely household electrical appliance load that can store heat energy, has easily controllable, on/off one Not the features such as short time does not interfere with users'comfort.Therefore, patent of the present invention is set about from electric heater, designs it and participates in electric power System frequency is stablized and control method and frequency controller.

Content of the invention

It is contemplated that overcoming the shortcomings of existing mode of frequency regulation and technology, developing a kind of electric heater and participating in power system Frequency stable and control method and electric heater Load-frequency Controllers, make electric heater participate in power system frequency and adjusted Cheng Zhong, effectively reduces the capacity requirement of frequency modulation standby generator sets, saves frequency modulation cost.In addition, there being a large amount of Wind turbines, too In the systems of new forms of energy unit such as sun energy generating set, intermittent unit output can be stabilized, improve Operation of Electric Systems stable Property.For reaching above-mentioned purpose, the technical solution used in the present invention is that design one kind makes electric heater participate in power system frequency Control method stably and in control process, specifically includes that

The first step: user is based on the side such as electrical network demand, the difference of users'comfort demand and electric heater degree of wear Considering of face independently sets coefficient of frequency modulation k with remote control_fTrigger TempNominal point t with temperature detect switch (TDS)^normalValue, design temperature Control switch triggering temperature adjustment circuit, calculates specified high and low temperature and triggers temperature, as shown in formula (1):

In formula, t^normalIt is by the specified leaving water temperature of user's sets itself on the premise of considering itself comfort level demand； Represent electric heater temperature detect switch (TDS) specified high and low temperature triggering temperature value, same it is contemplated that users'comfort Demand, sets temperature detect switch (TDS) specified high and low temperature triggering temperature value leaving water temperature desired with user and differs 2 in patent of the present invention Degree.

Second step: frequency sensor Real-time Collection mains frequency signal f (t), temperature sensor sampling electric heater water outlet Water temperature t (t) near mouthful.

3rd step: the mains frequency signal that electric heater temperature detect switch (TDS) collects according to second step, use in conjunction with the first step The coefficient of frequency modulation k of family sets itself_fThe value of temperature detect switch (TDS) triggering temperature after calculating temperature detect switch (TDS) triggering temperature adjustmemt amount and revising:

δ t=k_f×(f-f₀) (2)

$\begin{array}{c}{t}_{\mathrm{high}}=t_{\mathrm{high}}^{\mathrm{normal}}+\mathrm{\δt}\\ t_{\mathrm{low}}=t_{\mathrm{low}}^{\mathrm{normal}}+\mathrm{\δt}\end{array}---\left(3\right)$

In formula (2), k_fUnit be DEG C/hz, represent that in frequency electric heater when often changing unit hertz, hot water temperature becomes The centigrade changed, f represents the power system frequency signal that frequency sensor collects, f₀Represent that power system frequency is specified Value, present invention is mainly used in the domestic, electric water heater of CONTINENTAL AREA OF CHINA, frequency rated value is set to 50hz.

In formula (3), t_high、t_lowRepresent electric heater temperature detect switch (TDS) triggering temperature value after revising,Represent Electric heater temperature detect switch (TDS) triggering temperature value before correction.

4th step: electric heater temperature detect switch (TDS) next step action is determined according to the 3rd step result of calculation:

If water temperature t (t) is higher than that this moment temperature detect switch (TDS) high temperature triggers set point t in current time electric heater_high(t), Temperature detect switch (TDS) turns off, and electric heating tube stops heating；If water temperature t (t) is low less than temperature detect switch (TDS) this moment in current time electric heater Temperature triggering set point t_lowT (), temperature detect switch (TDS) is open-minded, and electric heating tube begins to warm up；Otherwise, temperature detect switch (TDS) working condition is constant, electricity Heating tube state is constant.

5th step: in the t=t+1 moment, return the first step, start subsequent cycle.

A kind of frequency controller that can control domestic, electric water heater load according to system frequency information, specifically includes that

Remote control: for the coefficient of frequency modulation k of oneself electric heater of user's sets itself_fThe specified leaving water temperature with electric heater t^normal.k_fUnit be DEG C/hz, represent hot water temperature's change in frequency electric heater when often changing unit hertz degree Celsius Number.Design hardware circuit makes high temperature rated trigger pointSpecified low temperature trigger pointWithBetween meet each 2 degree of difference, as formula (4):

$\begin{array}{c}{t}_{\mathrm{high}}^{\mathrm{normal}}=t^{\mathrm{normal}}+2\\ t_{\mathrm{low}}^{\mathrm{normal}}=t^{\mathrm{normal}}-2\end{array}---\left(4\right)$

Frequency sensor: for Real-time Collection mains frequency signal, and be passed along temperature control circuit；

Temperature sensor: for Real-time Collection electric heater delivery port water temperature t (t) and pass to temperature control circuit；

Temperature control circuit according to the mains frequency signal collecting, in conjunction with the coefficient of frequency modulation k of user's sets itself_fAnd electric hot water Device specified leaving water temperature t^normal, calculate leaving water temperature set point correction δ t and revised high temperature, low temperature trigger point, meter Calculate formula and see formula (5), (6):

δ t=k_f×(f-f₀) (5)

$\begin{array}{c}{t}_{\mathrm{high}}=t_{\mathrm{high}}^{\mathrm{normal}}+\mathrm{\δt}\\ t_{\mathrm{low}}=t_{\mathrm{low}}^{\mathrm{normal}}+\mathrm{\δt}\end{array}---\left(6\right)$

In formula (5), k_fUnit be DEG C/hz, represent that in frequency electric heater when often changing unit hertz, hot water temperature becomes The centigrade changed, f represents the power system frequency signal that frequency sensor collects, f₀Represent that power system frequency is specified Value, present invention is mainly used in the domestic, electric water heater of CONTINENTAL AREA OF CHINA, frequency rated value is set to 50hz.

In formula (6), t_high、t_lowRepresent electric heater temperature detect switch (TDS) triggering temperature value after revising,Table Show electric heater temperature detect switch (TDS) triggering temperature value before correction.

If water temperature t (t) is higher than that this moment high temperature triggers set point t in current time electric heater_high(t), positive displacement/speed Heated electric water heater temperature detect switch (TDS) turns off, and electric heating tube stops heating.If water temperature t (t) is less than this in current time electric heater Carve low temperature triggering set point t_lowT (), positive displacement/quick-heating electric water heater temperature detect switch (TDS) is open-minded, and electric heating tube begins to warm up；No Then, electric heater working condition is constant.

The technical characterstic of the present invention and effect: with conventional electric power system be based on Generation Side " for need move " frequency stable with Control measure are different, follow the thought of " need to be with for dynamic " based on the load side mode of frequency regulation of the present invention, by introducing user voluntarily The coefficient of frequency modulation setting, on the basis of ensureing users'comfort, makes the electric heater of power system load side participate in electric power System frequency, stably and in control process, can improve being actively engaged in property in Operation of Electric Systems for the Demand-side, and can save Generation Side frequency modulation spare capacity, improves generating set operational efficiency, reduces the environmental pollution producing in frequency-modulating process.

Brief description

Fig. 1 electric heater water temperature setting value is with mains frequency dynamic change figure；

Fig. 2 electric heater participates in mains frequency and stablizes and control assembly effect figure；

Fig. 3 electric heater frequency controller structure chart；

Fig. 4 positive displacement/quick-heating electric water heater frequency controller workflow diagram.

Specific embodiment

, it is contemplated that developing a kind of electric heater ginseng taking the electric heater that power consumption is stable, property of participation is strong as a example Stablize and control method and frequency controller with power system frequency, by the coefficient of frequency modulation of user's sets itself to electric heater The working condition (power on/off) of electric heating tube/heat control valve is controlled, thus the frequency participating in system is adjusted.This controller Without the support of communication system, effectively reduce the capacity to standby generator sets and energy storage device for the system frequency demand for control Demand, saves corresponding investment.Meanwhile, while supporting system frequency to adjust, taken into full account that terminal use's is comfortable with energy Degree, it is to avoid system " Secondary Disturbance " problem easily occurring in legacy frequencies control process.

1 electric heater frequency control model

Can be used as the stable electric heater model standby with control of power system frequency can be by its thermodynamic behaviour of expression Single order Differential Equation with Constant Coefficients (4) represents:

$\begin{array}{c}c\frac{\mathrm{dt}}{\mathrm{dt}}+g\×(t\left(t\right)-t_{\mathrm{out}})+{\mathrm{hw}}_d\left(t\right)(t\left(t\right)-t_{\mathrm{in}})=w\left(t\right)\×p\\ c=\mathrm{\ρ}{c}_{p}v,h=\mathrm{\ρ}{c}_p,g=a/r\end{array}---\left(1\right)$

In formula, t (t) represents the real-time water temperature (DEG C) in Water tank for electric water-heater；t_inRepresent inflow temperature (DEG C)；t_outTable Show storage tank external air temperature (DEG C)；w_dT () represents user water consumption (m³/s)；ρ represents water density (kg/m³)；c_pRepresent water Specific heat capacity (j/ DEG C of kg)；A represents Water tank for electric water-heater area (m²)；R represents water tank thermal resistance；P represents separate unit electric hot water Device rated power；W (t) represents the load operating conditions (0 represents that electric heating tube turns off, and 1 represents that electric heating tube is open-minded) of t, The value of any time w (t) can be represented by formula (2):

$w\left(t_{n+1}\right)=\left\{\begin{array}{c}0,t>t_{\mathrm{high}}\\ 1,t<t_{\mathrm{low}}\\ w\left(t_n\right),t_{\mathrm{low}}<t<t_{\mathrm{high}}\end{array}\right.---\left(2\right)$

T in formula_high、t_lowRepresent the temperature set-point of water heater, and be designed to the dynamic change with frequency value changes, Its dynamic change figure is shown in accompanying drawing 1, and corresponding change formula is:

$\begin{array}{c}{t}_{\mathrm{high}}=t_{\mathrm{high}}^{\mathrm{normal}}+\mathrm{\&delta;t}\\ t_{\mathrm{low}}=t_{\mathrm{low}}^{\mathrm{normal}}+\mathrm{\&delta;t}\end{array}---\left(3\right)$

In formula,Represent rated temperature set point, its specified water of electric heater with user's sets itself Warm t^normalBetween there is following mathematical relationship:

$\begin{array}{c}{t}_{\mathrm{high}}^{\mathrm{normal}}=t^{\mathrm{normal}}+2\\ t_{\mathrm{low}}^{\mathrm{normal}}=t^{\mathrm{normal}}-2\end{array}---\left(4\right)$

δ t represents electric heater temperature detect switch (TDS) triggering temperature change value, and it meets following number and mains frequency change between Relation:

δ t=k_f×(f-f₀) in (5) formula, f₀Represent system nominal frequency, i.e. 50hz；F represents system actual frequency； k_fRepresent the coefficient of frequency modulation of user's sets itself.

The operation principle of electric heater is to adjust temperature set-point by temperature control circuit, and then controls the logical of electric heating tube Power off or the action of heat control valve, keep electric heater delivery port water temperature in the scope [t allowing with this_low, t_high](t_low For electric heater delivery port water temperature lower limit, t_highFor the delivery port water temperature upper limit) interior change.Electric heater electric heating tube/heat control valve The switching of running status controlled by water temperature t real-time in electric heater.When water temperature t drops to t_lowWhen, electric heating tube energising adds Heat, raises water temperature, and power of electric water heater consumption is p it is assumed that this running status is " on ".When t rises to t_highWhen, electric heating tube Power-off, electric water heater storage tank and external environment carry out heat transfer, and t declines, power of electric water heater consumption be zero it is assumed that This running status is " off ".

2 electric heater FREQUENCY CONTROL polymerization models

The electric heater generally referred to electric heater condensate that frequency is adjusted will be participated in, when it reaches stable operation in electrical network State, that is, electric water heater water exit temperature existWhen being inside uniformly distributed, running status is the electric heater of " on " Proportion maintains dynamic equilibrium, and electric heater condensate has metastable power consumption.When power system frequency offrating When, participate in shown in the electric heater condensate power consumption such as formula (6) of FREQUENCY CONTROL:

$p^{\&prime;}=\left\{\begin{array}{c}0,\mathrm{\&delta;t}\&le;(t_{\mathrm{low}}-t_{\mathrm{high}})\\ p\&times;n\&times;\mathrm{on}\%+\frac{\mathrm{\&delta;t}}{t_{\mathrm{high}}-t_{\mathrm{low}}}\&times;p\&times;n\&times;\mathrm{on}\%,(t_{\mathrm{low}}-t_{\mathrm{high}})<\mathrm{\&delta;t}<0\\ p\&times;n\&times;\mathrm{on}\%+\frac{\mathrm{\&delta;t}}{t_{\mathrm{high}}-t_{\mathrm{low}}}\&times;p\&times;n\&times;\mathrm{off}\%,0\&le;\mathrm{\&delta;t}<(t_{\mathrm{high}}-t_{\mathrm{low}})\\ p\&times;n,\mathrm{\&delta;t}\&greaterequal;(t_{\mathrm{high}}-t_{\mathrm{low}})\end{array}\right.---\left(6\right)$

In formula, p' represents that electric heater condensate participates in power system frequency and controls power；N represent participation frequency stable with The electric heater quantity controlling；P electric heater rated power；On% (off%) represents that steady operational status are " on " (" off ") Electric heater ratio.

Fig. 2 describes certain electric heater condensate internal each electric heater each running status when mains frequency changes Change.The schematic diagram on Fig. 2 left side shows that mains frequency is that during rated value 50hz, electric heater condensate has 10 to be in " on " State, as mains frequency offrating 50hz, is dropped to as a example below 50hz by mains frequency, by formula (3), (4), (5) can Know that electric heater temperature detect switch (TDS) triggering temperature reduces, by Fig. 2 right figure it is also seen that being now in the electric heater of " on " state Become 7 by 10.Indicate the relation between electric heater working condition and power system frequency this schematic image. Power system frequency change impact electric heater temperature detect switch (TDS) triggering temperature spot changes, and then impact electric heater working condition becomes Change, finally affect in a certain region, to participate in the change of frequency stable and the power of electric water heater controlling.

3rd, electric heater frequency controller hardware configuration and its workflow

Its working method difference pressed by electric heater, can be divided into positive displacement, Instant heating type, fast heating type.Positive displacement is also called water storage Formula or heat storage type, are traditional electric heaters, are also the principal mode of existing electric heater；Instant heating type do not have water storage function, Cold water enters from water inlet, through excessive power electrical heating wire, will heat up at short notice and become hot water from delivery port outflow；Speed Hot type is also called half storage-type, and moisture storage capacity is little, and power is big, improves the utilization efficiency of heat energy of traditional electric water heater.Mesh Electric heater on front domestic market is mainly positive displacement, two kinds of fast heating type.The two all has automatic constant-temperature heat insulation function, can be Turn off a bit of time on the premise of not affecting users'comfort, and the regulation of leaving water temperature is all by temperature detect switch (TDS) control, therefore Patent of the present invention mainly considers that both electric heaters participate in power system frequency and stablize and control process.

The electric heater frequency controller hardware structure diagram that patent of the present invention proposes is as shown in Figure 3.Electric heater frequency control Device processed includes the modules such as remote control, temperature control circuit, frequency sensor, temperature sensor.Compared with common electric water heater controller, Patent of the present invention increased user sets itself coefficient of frequency modulation k in remote-controller function part first_fFunction, k_fUnit be DEG C/ Hz, in electric heater when representing that frequency often changes unit hertz, the centigrade of hot water temperature's change, is worth bigger, electric heater pair More sensitive in the reflection of frequency departure；Secondly, increased frequency sensor module, in order to real-time measurement mains frequency；In addition, it is warm Control circuit is no longer to only rely on temperature information to determine electric heating tube working condition, but considers mains frequency and electric hot water Device leaving water temperature, according to control method for frequency proposed by the present invention, controls electric heater working condition, reaches and both can guarantee that electric heating Hydrophone leaving water temperature and specified leaving water temperature deviation set by user, within allowed band, can participate in power system frequency again Stablize and control.

Electric heater frequency controller workflow is shown in Fig. 4, specifically includes following steps:

The first step: user is based on the side such as electrical network demand, the difference of users'comfort demand and electric heater degree of wear Considering of face independently sets coefficient of frequency modulation k with remote control_fTrigger TempNominal point t with temperature detect switch (TDS)^normalValue, design temperature Control switch triggering temperature adjustment circuit, calculates specified high and low temperature and triggers temperature, as shown in formula (8):

$\begin{array}{c}{t}_{\mathrm{high}}^{\mathrm{normal}}=t^{\mathrm{normal}}+2\\ t_{\mathrm{low}}^{\mathrm{normal}}=t^{\mathrm{normal}}-2\end{array}---\left(8\right)$

Second step: frequency sensor Real-time Collection mains frequency signal f (t), temperature sensor sampling electric heater water outlet Water temperature t (t) near mouthful.

3rd step: the mains frequency signal that electric heater temperature detect switch (TDS) collects according to second step, use in conjunction with the first step The coefficient of frequency modulation k of family sets itself_fThe value of temperature detect switch (TDS) triggering temperature after calculating temperature detect switch (TDS) triggering temperature adjustmemt amount and revising:

δ t=k_f×(f-f₀) (9)

$\begin{array}{c}{t}_{\mathrm{high}}=t_{\mathrm{high}}^{\mathrm{normal}}+\mathrm{\&delta;t}\\ t_{\mathrm{low}}=t_{\mathrm{low}}^{\mathrm{normal}}+\mathrm{\&delta;t}\end{array}---\left(10\right)$

In formula, t_high、t_lowRepresent electric heater temperature detect switch (TDS) triggering temperature value after revising；Expression is repaiied Just front electric heater temperature detect switch (TDS) triggers temperature value.

4th step: electric heater temperature detect switch (TDS) next step action is determined according to the 3rd step result of calculation:

If water temperature t (t) is higher than that this moment temperature detect switch (TDS) high temperature triggers set point t in current time electric heater_high(t), Temperature detect switch (TDS) turns off, and electric heating tube stops heating；If water temperature t (t) is low less than temperature detect switch (TDS) this moment in current time electric heater Temperature triggering set point t_lowT (), temperature detect switch (TDS) is open-minded, and electric heating tube begins to warm up；Otherwise, temperature detect switch (TDS) working condition is constant, electricity Heating tube state is constant.

5th step: in the t=t+1 moment, return the first step, start subsequent cycle.

Claims (5)

1. a kind of electric heater participation mains frequency is stablized and control method, it is characterized in that, comprises the steps:

The first step: user considers use based on electrical network demand, the difference of users'comfort demand and the electric heater degree of wear Remote control independently sets coefficient of frequency modulation k_fTrigger TempNominal point t with temperature detect switch (TDS)^normalValue, design temperature detect switch (TDS) triggering Temperature adjustment circuit, calculates specified high and low temperature and triggers temperature, as shown in formula (1):

$\begin{array}{c}{t}_{high}^{normal}=t^{normal}+2\\ t_{low}^{normal}=t^{normal}-2\end{array}---\left(1\right)$

In formula, t^normalIt is by the specified leaving water temperature of user's sets itself on the premise of considering itself comfort level demand； Represent electric heater temperature detect switch (TDS) specified high and low temperature triggering temperature value；

Second step: frequency sensor Real-time Collection mains frequency signal f (t), temperature sensor sampling electric heater delivery port is attached Near water temperature t (t)；

3rd step: the mains frequency signal that electric heater temperature detect switch (TDS) collects according to second step, in conjunction with user in the first step certainly The coefficient of frequency modulation k that row sets_fThe value of temperature detect switch (TDS) triggering temperature after calculating temperature detect switch (TDS) triggering temperature adjustmemt amount and revising:

δ t=k_f×(f-f₀) (2)

$\begin{array}{c}{t}_{high}=t_{high}^{normal}+\mathrm{\&delta;}t\\ t_{low}=t_{low}^{normal}+\mathrm{\&delta;}t\end{array}---\left(3\right)$

In formula (2), k_fUnit be DEG C/hz, represent taking the photograph of hot water temperature's change in frequency electric heater when often changing unit hertz Family name's number of degrees, f represents the power system frequency signal that frequency sensor collects, f₀Represent power system frequency rated value,

In formula (3), t_high、t_lowRepresent electric heater temperature detect switch (TDS) triggering temperature value after revising；

4th step: electric heater temperature detect switch (TDS) next step action is determined according to the 3rd step result of calculation:

If water temperature t (t) is higher than that this moment temperature detect switch (TDS) high temperature triggers set point t in current time electric heater_highT (), temperature control is opened Turn off, electric heating tube stops heating；If water temperature t (t) is less than temperature detect switch (TDS) low temperature triggering this moment in current time electric heater Set point t_lowT (), temperature detect switch (TDS) is open-minded, and electric heating tube begins to warm up；Otherwise, temperature detect switch (TDS) working condition is constant, electric heating tube State is constant；

5th step: in the t=t+1 moment, return the first step, start subsequent cycle.

2. method according to claim 1, set temperature detect switch (TDS) specified high and low temperature triggering temperature value desired with user go out Coolant-temperature gage differs 2 degree.

3. method according to claim 1, described frequency rated value is set to 50hz.

4. a kind of electric heater frequency controller, is characterized in that, comprising:

Remote control: for the coefficient of frequency modulation k of oneself electric heater of user's sets itself_fThe specified leaving water temperature with electric heater t^normal, k_fUnit be DEG C/hz, represent hot water temperature's change in frequency electric heater when often changing unit hertz degree Celsius Number, design hardware circuit makes high temperature rated trigger pointSpecified low temperature trigger pointWith t^normalBetween meet each 2 degree of difference, as formula (4):

$\begin{array}{c}{t}_{high}^{normal}=t^{normal}+2\\ t_{low}^{normal}=t^{normal}-2\end{array}---\left(4\right)$

Frequency sensor: for Real-time Collection mains frequency signal, and be passed along temperature control circuit；

Temperature sensor: for Real-time Collection electric water heater water exit temperature t (t) and pass to temperature control circuit；

Temperature control circuit according to the mains frequency signal collecting, in conjunction with the coefficient of frequency modulation k of user's sets itself_fWith electric heater volume Make coolant-temperature gage t^normal, calculate leaving water temperature set point correction δ t and revised high temperature, low temperature trigger point, calculate public Formula is shown in formula (5), (6):

δ t=k_f×(f-f₀) (5)

$\begin{array}{c}{t}_{high}=t_{high}^{normal}+\mathrm{\&delta;}t\\ t_{low}=t_{low}^{normal}+\mathrm{\&delta;}t\end{array}---\left(6\right)$

In formula (5), f represents the power system frequency signal that frequency sensor collects, f₀Represent power system frequency rated value；

In formula (6), t_high、t_lowRepresent electric heater temperature detect switch (TDS) triggering temperature value after revising；

If water temperature t (t) is higher than that this moment high temperature triggers set point t in current time electric heater_highT (), electric heater temperature control is opened Turn off, electric heating tube stops heating, if water temperature t (t) is less than the set point t of low temperature triggering this moment in current time electric heater_low T (), electric heater temperature detect switch (TDS) is open-minded, and electric heating tube begins to warm up；Otherwise, electric heater working condition is constant.

5. controller according to claim 4, described frequency rated value is set to 50hz.