An optimized control method for a primary frequency regulation based on an exergy storage correction of a thermodynamic system of a coal-fired unit, which determines an optimal primary frequency regulation control scheme suitable for a current state according to an exergy storage amount of each thermodynamic device of the thermodynamic system of the coal-fired unit during different transient processes, and corrects a primary frequency regulation control logic, comprising steps of:
- (1) acquiring a real-time exergy storage amount of each thermodynamic device of the thermodynamic system of the coal-fired unit, particularly comprising steps of:
- through pressure sensors, acquiring pressures of working fluids of each thermodynamic device of the thermodynamic system of the coal-fired unit;
- through temperature sensors, acquiring temperatures of the working fluids and metal heating surfaces of each thermodynamic device of the thermodynamic system of the coal-fired unit;
- looking up a calculation table of water and steam properties, and calculating the exergy storage amount of each thermodynamic device in arbitrary states, comprising exergy storage amounts of the working fluids and the metal heating surfaces;
- wherein:
- for an ith thermodynamic device, an exergy storage amount is calculated through formulas of:
- Exs,i=Ms·[u(Ps,i,Ts,i)−u0−T0·(s(Ps,i,Ts,i)−s0)];
- Exm,i=Mm·Cm[Tm,i−T0−T0·ln(Tm,i/T0)];
- Exw,i=Mw·[u(Pw,i,Tw,i)−u0−T0·(s(Pw,i,Tw,i)−s0)];
- Exi=Exs,i+Exm,i+Exw,i;
- in the formulas:
- Exs,i, Exm,i and Exw,i are respectively exergy storage amounts of steam, metal heating surfaces and feed water of the ith thermodynamic device, in unit of kJ;
- Ms, Mm and Mw are respectively mass of steam, metal heating surfaces and feed water of the ith thermodynamic device, in unit of kg;
- T0 is an ambient temperature, in unit of K;
- u0 is a corresponding internal energy under the ambient temperature and an ambient pressure, in unit of kJ/kg;
- s0 is a corresponding entropy under the ambient temperature and the ambient pressure, in unit of kJ/(kg·K);
- u(Ps,i, Ts,i) is an internal energy of steam, which is calculated through a steam pressure Ps,i and a steam temperature Ts,i, in unit of kJ/kg;
- s(Ps,i, Ts,i) is an entropy of steam, which is calculated through the steam pressure Ps,i and the steam temperature Ts,i, in unit of kJ/(kg·K);
- Cm is a specific heat capacity of metal heating surfaces of a heater, in unit of kJ/(kg·K), Tm,i is an average temperature of the metal heating surfaces of the heater, in unit of K;
- u(Pw,i, Tw,i) is an internal energy of feed water, which is calculated through a feed water pressure Pw,i and a feed water temperature Tw,i, in unit of kJ/kg;
- s(Pw,i, Tw,i) is an entropy of feed water, which is calculated through the feed water pressure Pw,i and the feed water temperature Tw,i in unit of kJ/(kg·K);
- (2) acquiring a maximum power output of various regulation schemes of the thermodynamic system of the coal-fired unit, wherein:
- the various regulation schemes of the thermodynamic system of the coal-fired unit for the primary frequency regulation comprises a high-pressure heater extraction steam throttling scheme, a high-pressure heater feed water bypass scheme, a low-pressure heater extraction steam throttling scheme and a low-pressure heater condensation water throttling scheme;
- during a transient operation process, an exergy storage amount of each regulation scheme involved in the primary frequency regulation in an initial state is a total energy storage amount of all thermodynamic devices of a corresponding subsystem;
- in the initial state, a total exergy storage amount Exj,a of a subsystem corresponding to a jth thermodynamic system regulation scheme is
- Ex
- j
- ,
- a
- =
- ∑
- i
- =
- 1
- N
-
-
- Ex
- j
- ,
- i
- ,
- a
- ,
- wherein j=1, 2, 3 and 4, respectively corresponding to the high-pressure heater extraction steam throttling scheme, the high-pressure heater feed water bypass scheme, the low-pressure heater extraction steam throttling scheme and the low-pressure heater condensation water throttling scheme;
- in the formula of
- Ex
- j
- ,
- a
- =
- ∑
- i
- =
- 1
- N
-
-
- Ex
- j
- ,
- i
- ,
- a
- ,
- Ex
- j
- ,
- i
- ,
- a
- is an exergy storage amount of an ith thermodynamic device of the subsystem corresponding to the jth thermodynamic system regulation scheme, in unit of kJ;
- N is a total number of all thermodynamic devices included in the subsystem corresponding to the jth thermodynamic system regulation scheme;
- in the different thermodynamic system regulation schemes, temperature values and pressure values of each thermodynamic device at the end of the primary frequency regulation are obtained according to principles as follows;
- for the high-pressure heater extraction steam throttling scheme, namely j=1, at the end of the primary frequency regulation, inside each high-pressure heater, a steam pressure is a drain pressure at an inlet of a deaerator, a steam temperature is a temperature of saturated steam corresponding to the drain pressure at the inlet of the deaerator, a feed water temperature is a temperature at an outlet of a feed water pump, a feed water pressure is a pressure at the outlet of the feed water pump, and a temperature of the metal heating surfaces is same to the steam temperature;
- for the high-pressure heater feed water bypass scheme, namely j=2, at the end of the primary frequency regulation, inside each high-pressure heater, a steam pressure is a pressure at a corresponding extraction steam outlet, a steam temperature is a temperature of saturated steam corresponding to the pressure at the corresponding extraction steam outlet, a feed water temperature is a temperature at an outlet of a feed water pump, a feed water pressure is a pressure at the outlet of the feed water pump, and a temperature of the metal heating surfaces is same to the steam temperature;
- for the low-pressure heater extraction steam throttling scheme, namely j=3, at the end of the primary frequency regulation, inside each low-pressure heater, a steam pressure is a drain pressure at an inlet of a condenser, a steam temperature is a temperature of saturated steam corresponding to the drain pressure at the inlet of the condenser, a temperature of condensation water is a temperature at an outlet of a condensation water pump, a pressure of condensation water is a pressure at the outlet of the condensation water pump, and a temperature of the metal heating surfaces is same to the steam temperature;
- for the low-pressure heater condensation water throttling scheme, namely j=4, at the end of the primary frequency regulation, inside each low-pressure heater, a steam pressure is a pressure at a corresponding extraction steam outlet, a steam temperature is a temperature of saturated steam corresponding to the pressure at the corresponding extraction steam outlet, a temperature of condensation water is a temperature at an outlet of a condensation water pump, a pressure of condensation water is a pressure at the outlet of the condensation water pump, and a temperature of the metal heating surfaces is same to the steam temperature;
- thereafter, an obtained total exergy storage amount Exj,b of the corresponding subsystem after acting of the jth thermodynamic system regulation scheme is
- Ex
- j
- ,
- b
- =
- ∑
- i
- =
- 1
- N
-
-
- Ex
- j
- ,
- i
- ,
- b
- ,
- wherein:
- j=1, 2, 3 and 4, respectively corresponding to the high-pressure heater extraction steam throttling scheme, the high-pressure heater feed water bypass scheme, the low-pressure heater extraction steam throttling scheme and the low-pressure heater condensation water throttling scheme;
- in the formula of
- Ex
- j
- ,
- b
- =
- ∑
- i
- =
- 1
- N
-
-
- Ex
- j
- ,
- i
- ,
- b
- ,
- Ex
- j
- ,
- i
- ,
- b
- is an exergy storage amount of the ith thermodynamic device of the subsystem corresponding to the jth thermodynamic system regulation scheme in an end state, in unit of kJ;
- N is the total number of all the thermodynamic devices included in the subsystem corresponding to the jth thermodynamic system regulation scheme;
- therefore, an exergy storage variation ΔExj under acting of the jth thermodynamic system regulation scheme is ΔExj=|Exj,a−Exj,b|, wherein:
- j=1, 2, 3 and 4, respectively corresponding to the high-pressure heater extraction steam throttling scheme, the high-pressure heater feed water bypass scheme, the low-pressure heater extraction steam throttling scheme and the low-pressure heater condensation water throttling scheme;
- an average exergy storage variation rate ΔĖxj under acting of the jth thermodynamic system regulation scheme is ΔĖxj=ΔExj/Tj, wherein:
- j=1, 2, 3 and 4, respectively corresponding to the high-pressure heater extraction steam throttling scheme, the high-pressure heater feed water bypass scheme, the low-pressure heater extraction steam throttling scheme and the low-pressure heater condensation water throttling scheme;
- in the formula of ΔĖxj=ΔExj/Tj, Tj is time required by the primary frequency regulation;
- and according to grid assessment requirements, Tj takes 60 seconds;
- a one-to-one correspondence exists between the average exergy storage variation rate and the maximum power output of the jth thermodynamic system regulation scheme that:
- ΔPj=ΔĖxj·ηj, wherein:
- j=1, 2, 3 and 4, respectively corresponding to the high-pressure heater extraction steam throttling scheme, the high-pressure heater feed water bypass scheme, the low-pressure heater extraction steam throttling scheme and the low-pressure heater condensation water throttling scheme;
- in the formula of ΔPj=ΔĖxj·ηj, ΔPj is the maximum power output of the jth thermodynamic system regulation scheme, in unit of kW;
- ΔĖxj is the average exergy storage variation rate under acting of the jth thermodynamic system regulation scheme, in unit of kW;
- ηj is an exergy storage conversion efficiency of the jth thermodynamic system regulation scheme;
- (3) generating an optimal primary frequency regulation control scheme suitable for a current operation state, particularly comprising steps of:
- according to a grid frequency deviation Δf of the current state and a current speed droop δ of the coal-fired unit, acquiring a maximum power regulation quantity ΔP required by a current frequency regulation through a formula of:
- ΔP=f1(Δf)=Δf/δ;
- comparing the maximum power output ΔPj under acting of the four regulation schemes with the maximum power regulation quantity ΔP required by the current frequency regulation, and judging whether it meets a condition of:
- ΔPj≥ΔP, wherein j is selected from 1,2,3 and 4;
- in the regulation schemes meeting the condition of ΔPj≥ΔP, generating the optimal primary frequency regulation control scheme k suitable for the current state, wherein a corresponding exergy storage conversion efficiency ηk should take a maximum value of the exergy storage conversion efficiencies of the four thermodynamic system regulation schemes;
- that is to say ηk meets a condition of:
- ηk=max{η1,η2,η3,θ4};
- (4) generating a primary frequency regulation control logic corresponding to the optimal primary frequency regulation control scheme, particularly comprising steps of:
- putting the determined optimal primary frequency regulation control scheme into a current primary frequency regulation control logic, wherein:
- the high-pressure heater extraction steam throttling scheme is to make an extraction steam pipe valve of each high-pressure heater involved in the primary frequency regulation control;
- the high-pressure heater teed water bypass scheme is to make a bypass pipe valve of each high-pressure heater involved in the primary frequency regulation control;
- the low-pressure heater extraction steam throttling scheme is to make an extraction steam pipe valve of each low-pressure heater involved in the primary frequency regulation control;
- and the low-pressure heater condensation water throttling scheme is to make a pipe valve of each low-pressure heater involved in the primary frequency regulation control;
- then, superimposing a regulation output ΔμPID obtained by a frequency difference of the primary frequency regulation in a PID (Proportion Integration Differentiation) controller to a corresponding control valve of the optimal scheme, and generating a new opening degree μnew of the valve through a formula of:
- μnew=μold+ΔμPID, wherein:
- in the formula, μold is a corresponding valve opening degree in the initial state;
- finally, forming a closed-loop optimized control logic for putting the optimal primary frequency regulation control scheme into the primary frequency regulation.