Method to optimize operation of a transformer cooling system, the corresponding system and a method to determine the VFD capacity

Abstract

The present application discloses a method to optimize operation of a transformer cooling system, the corresponding cooling system, and a method to determine the capacity of Variable Frequency Drives (VFD) that are used in the transformer cooling system. The method comprises: preprocessing the initial data input by user; collecting the on-line data, and calculating the optimized control command to meet the requirement of the transformer loss, top-oil temperature variation and noise; and executing the control actions by controlling a controllable switch and/or sending a control command to a VFD. Compared with the existing prior arts, the proposed solutions are much more intuitive and practical in the field of the cooling system.

Claims

1. A method to improve operation of a transformer cooling system, wherein the transformer cooling system comprises a central controller, a transformer, a plurality of motor-fan chains to cool down said transformer, a shared variable frequency drive (VFD) bus fed by a VFD, and an alternating current (AC) bus fed by an AC power source, both of the VFD bus and the AC bus being controlled by said central controller, and both of the VFD bus and the AC bus being shared by motor-fan chains, the method comprising: preprocessing an initial data input by a user; collecting an on-line data corresponding to a transformer loss associated with driving at least one motor-fan chain of the plurality of motor fan chains with at least one of the VFD bus or the AC bus; calculating a control command of the transformer cooling system to meet a requirement of a transformer loss by computing a control objective based on initial data and the on-line data, wherein the control command includes an instruction to selectively drive, based at least in part on the transformer loss, at least one motor-fan chain of the plurality of motor fan chains with at least one of the VFD bus or the AC bus; and executing control actions associated with the control command by selectively driving, based at least in part on the transformer loss, the at least one motor-fan chain with at least one of the VFD bus or the AC bus.

2. The method according to claim 1, wherein said calculating the control command is further based on at least one of a top-oil temperature variation of the transformer and a noise level of the transformer and a fan.

3. The method according to claim 2, wherein said calculating the control command is further based on a weighting factor of at least one of the transformer loss, the top-oil temperature variation, and the noise level.

4. The method according to claim 3, wherein said preprocessing further includes: collecting parameters of a transformer type, a transformer ratio, and a ratio of load losses at rated current to no-load losses; collecting parameters of a transformer thermal model; collecting parameters of a tap changer mid position, a step voltage and a present tap changer position; collecting parameters of a cooler type, a fan number and a power of a radiator; and collecting a relationship curve between a fan noise and a fan capacity.

5. The method according to claim 2 further comprising: calculating the top-oil temperature variation over time by the following equation: $D{}_o=\left\{{\left[\frac{1+{\mathrm{RK}}^2}{1+R}\right]}^x.Math.{}_{\mathrm{or}}.Math.\frac{100}{X_{\mathrm{cor}}}-\left({}_{\mathrm{oi}}-{}_a\right)\right\}.Math.\frac{\mathrm{dt}}{{}_o}$ wherein: D.sub.0 is the top-oil temperature variation; dt is the time; .sub.or is a top-oil temperature rise in a steady state at rated losses (K); R is a ratio of load losses at rated current to no-load losses; K is a load factor; .sub.o is an average oil time constant; .sub.oi is a top-oil temperature at prior time; .sub.a is an ambient temperature; and X.sub.cor is a rate of cooling in operation.

6. The method according to claim 2, further comprising: calculating the noise level of the transformer and the fan by the following equation: ${\mathrm{Lp}}_t=\{\begin{array}{cc}{\mathrm{Lp}}_{N1},& {\mathrm{Lp}}_{\mathrm{fan}}=0\\ {\mathrm{Lp}}_{N1}+101g\left[1+{10}^{-\frac{{\mathrm{Lp}}_{N1}-{\mathrm{Lp}}_{\mathrm{fan}}}{10}}\right],& {\mathrm{Lp}}_{N1}>{\mathrm{Lp}}_{\mathrm{fan}}\\ {\mathrm{Lp}}_{\mathrm{fan}}+101g\left[1+{10}^{-\frac{{\mathrm{Lp}}_{\mathrm{fan}}-{\mathrm{Lp}}_{N1}}{10}}\right],& {\mathrm{Lp}}_{\mathrm{fan}}>{\mathrm{Lp}}_{N1}\end{array}$ wherein: Lp.sub.t is a total noise level of the transformer and the fan; Lp.sub.fan is a fan noise; and Lp.sub.N1 is a transformer noise.

7. The method according to claim 2, wherein said preprocessing further includes: collecting parameters of a transformer type, a transformer ratio, and a ratio of load losses at rated current to no-load losses; collecting parameters of a transformer thermal model; collecting parameters of a tap changer mid position, a step voltage and a present tap changer position; collecting parameters of a cooler type, a fan number and a power of a radiator; and collecting a relationship curve between a fan noise and a fan capacity.

8. The method according to claim 2, wherein said on-line data further includes: a load current, a temperatures and a status of a cooler; and wherein calculating the control command further includes: calculating a cooling capacity required to meet said requirement; calculating a number of the motor-fan chains required to meet said cooling capacity; comparing the number of motor-fan chains required with an existing number of motor-fan chains in operation; and changing an operation solution in accordance with the comparison.

9. The method according to claim 1, wherein said preprocessing further includes: collecting parameters of a transformer type, a transformer ratio, and a ratio of load losses at rated current to no-load losses; collecting parameters of a transformer thermal model; collecting parameters of a tap changer mid position, a step voltage and a present tap changer position; collecting parameters of a cooler type, a fan number and a power of a radiator; and collecting a relationship curve between a fan noise and a fan capacity.

10. The method according to claim 9, wherein said preprocessing further includes: calculating a transformer copper loss; calculating a winding temperature; calculating a load current of different sides of a transformer; and calculating a power consumption of cooling system.

11. The method according to claim 1 wherein said on-line data further includes: a load current, temperatures and status of a cooler; and wherein said calculating further includes: calculating a cooling capacity required to meet said requirement; calculating a number of the motor-fan chains required to meet said cooling capacity; comparing the number of motor-fan chains required with an existing number of motor-fan chains in operation; and changing an operation solution in accordance with the comparison.

12. The method according to claim 11, wherein said changing the operation solution further includes: switching on the number of the motor-fan chains with utilization rate lower than a current utilization rate and driving a remainder of the motor-fan chains by the VFD with a calculated frequency; and switching off the number of the motor-fan chains with utilization rate higher than a current utilization rate and driving a remainder of the motor-fan chains by the VFD with a calculated frequency.

13. The method according to claim 11, wherein said changing the operation solution further includes: switching on the number of the motor-fan chains with utilization rate lower than a current utilization rate and driving a remainder of the motor-fan chains by the VFD with a calculated frequency; and changing the motor-fan chains driven by the VFD with the calculated frequency.

14. The method according to claim 1 further comprising: calculating the transformer loss under a load level for three-winding transformer by the following equation: $P_k^{}=\frac{1+\stackrel{\_}{{}_w}}{1+75}\left({}_1^2{P}_{k1N}+{}_2^2{P}_{k2N}+{}_3^2{P}_{k3N}\right)$ wherein: P.sub.K is the transformer loss; .sub.w is an average winding temperature; is a temperature factor; .sub.1, .sub.2, .sub.3 are load factors; and P.sub.k1N, P.sub.k2N, P.sub.k3N are winding losses at rated current.

15. The method according to claim 1 wherein said control actions further include: at least one of a start or stop of at least one of the motor-fan chains associated with the transformer cooling system; and a controllable switch operation associated with the transformer cooling system.

16. The method according to claim 1, wherein the initial data further includes: parameters and objectives of the transformer loss, a top-oil temperature variation of the transformer and a noise of the transformer, and wherein calculating the control command further includes: calculating a Net Present Value (NPV) curve versus a VFD capacity which shows a relationship between a saved energy of the transformer cooling system which can be attributed to the VFD and a VFD cost; calculating a VFD capacity limit for a pre-defined top-oil temperature variation; calculating a VFD capacity limit for a pre-defined noise; and determining a VFD capacity which has the highest NPV, and which is within limits of both top-oil temperature variation and noise.

17. The method according to claim 16, wherein determining the VFD capacity which has the highest NPV further includes: calculating the saved energy of the transformer cooling system which can be attributed to the VFD; calculating a capital cost of the VFD; evaluating a net present value (NPV) of the VFD considering both benefit and cost; and selecting the VFD capacity with the highest NPV.

18. The method according to claim 1, wherein each of said motor-fan chains is connected to a controllable switch configured to switch said motor-fan chains among connecting to said AC bus, connecting to said VFD bus, and disconnecting from power supplies.

19. The method according to claim 1, wherein said control actions further include: at least one of a start or a stop of at least one of the motor-fan chains; and a VFD frequency regulation.

20. A transformer cooling system comprising: a transformer; a shared variable frequency drive (VFD) bus configured to be fed by a VFD; and an alternating current (AC) bus configured to be fed by an AC power source, a plurality of motor-fan chains; a controllable switch switchable among a plurality of connection states, the plurality of connection states comprising: an AC connection state wherein at least one motor-fan chain of the plurality of motor-fan chains is connected to the AC bus; a VFD connection state wherein at least one motor-fan chain of the plurality of motor-fan chains is connected to the VFD bus; and a disconnected state wherein the plurality of motor-fan chains are disconnected from the AC bus and the VFD bus; and a central controller configured to: determine a transformer loss of the transformer; and generate control commands to cause the controllable switch to selectively switch among the plurality of connection states based on the transformer loss.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject matter of the invention will be explained in more details in the following description with reference to preferred exemplary embodiments which are illustrated in the drawings, in which:

(2) FIGS. 1A and 1B show an electrification scheme of the conventional transformer cooling system; in which FIG. 1A illustrates the structure of respectively installing VFD for each motor-fan chain, and FIG. 1B illustrates the structure of a plurality of motor-fan chains jointly driven by one VFD;

(3) FIG. 2 shows an electrification scheme of the transformer cooling system according to an embodiment of the present invention;

(4) FIG. 3 is the overall flow-chart for VFD capacity determination according to an embodiment of the present invention;

(5) FIG. 4 is the flow-chart for net present value calculation due to transformer efficiency improvement by installing different capacity of VFD in the cooling system according to an embodiment of the present invention;

(6) FIG. 5 is the main flow-chart for operation optimization of transformer cooling system according to an embodiment of the present invention;

(7) FIG. 6 illustrates a flow chart of parameters preprocessing procedures according to an embodiment of the present invention;

(8) FIG. 7 illustrates a flow chart of control command determination according to an embodiment of the present invention;

(9) FIG. 8 illustrates a flow chart of control command execution according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(10) Exemplary embodiments of the present invention are described in conjunction with the accompanying drawings hereinafter. For the sake of clarity and conciseness, not all the features of actual implementations are described in the specification.

(11) According to the first preferred embodiment, the electrical system design of the transformer cooling system is shown in FIG. 2, which consists of two power supply schemes for motor-fan loads, including an AC line supply and a VFD supply (e.g. VFD1 in FIG. 2).

(12) As shown in FIG. 2, one or more motor-fan chains can be connected to the VFD bus, the AC bus or disconnected from power supplies respectively through the controllable switches. That means, the motor-fan chains can only have one out of three statuses at one time: connecting to AC line, connecting to VFD, or disconnecting from power supplies.

(13) By coordinating the VFD and controllable switches, the start-up process of motor-fan loads can be optimized. As shown in FIG. 2, a motor-fan load can be switched to VFD for soft start. After completing the start-up process, it can be switched back to the AC line if it is operated at the rated output. In order to optimize the operation, the status information of VFD and controllable switches are all transmitted to a central controller. Besides these, the central controller also gets access to the real-time transformer load data, oil temperature and ambient temperature. With all these data, the controller performs the efficiency optimization calculation, top oil and its variation calculation, and noise level calculation of the whole transformer. After that, it will send out the control command to controllable devices, e.g. controllable switches for gross temperature regulation, and VFD for fine temperature regulation.

(14) According to the second preferred embodiment, the size of the VFD can be determined by techno-economic analysis to ensure best cost-effectiveness of the given type of transformer. The higher the VFD capacity is, the more accurate the temperature control will be, which can contribute to overall operation performance improvement. However, the cost of the VFD will also increase which will affect the business case. Meanwhile, different type of transformers have different cooling capacity requirement. The sizing of VFD should also take this into account. FIG. 3 shows the overall procedures for VFD capacity determination. Firstly, the parameters and the operation objectives, e.g. transformer loss, top-oil temperature variation and expected noise level will be input by the users; secondly, the NPV curve which shows the relationship between transformer loss and VFD capacity will be calculated; thirdly, the VFD capacity limitations to achieve the predetermined top-oil temperature variation and noise level requirements will be calculated; fourthly, the VFD capacity can be determined which has the highest NPV for transformer loss reduction, and meanwhile can fulfill the lifecycle and noise level requirement.

(15) FIG. 4 illustrates how to calculate the NPV curve versus VFD capacity through transformer system efficiency improvement. In FIG. 4, P.sub.VFD represents the rated capacity of the VFD; P.sub.VFD0 and P.sub.VFD represent the initial capacity and incremental capacity of VFD used for iteration By calculating the save energy loss through VFD, and the corresponding capital investment of VFD, the net present value curve can be obtained versus different VFD capacities.

(16) According to another preferred embodiment, the central controller performs the optimization calculation in real-time. The flowchart is shown in FIG. 5. Whenever the optimization result changes, the central controller will update the control commands for VFD and/or controllable switches respectively.

(17) Step 1: the first step of the flowchart is to preprocess the initial data input by user. The detailed information is shown in FIG. 6, where totally five groups of data will be collected as follows: 1) The transformer type, ratio, and ratio of load losses at rated current to no-load losses. The method uses them to calculate the copper loss. 2) Winding exponent, oil exponent, hot-spot to top-oil gradient, hot-spot factor, ambient temperature, average oil time constant, winding time constant, hot-spot-to-top-oil gradient at start, hot-spot-to-top-oil gradient at the rated current, top-oil temperature rise in steady state at rated losses, top-oil temperature rise at start, the load permissible in % of nameplate rating when all fans inoperative. The method uses them to calculate the hot-spot temperature which can be regarded as the winding temperature. 3) Tap changer mid position, step voltage, present tap changer position. The method uses them to calculate the load current of different sides. 4) Cooler type, fan number, the power of radiator. The method uses them to calculate the power consumption of cooling system. 5) Relationship curve between fan noise and fan capacity.

(18) After the preprocessing, all information except real-time data will be ready for calculation.

(19) Step 2: the second step, the central controller collects the load current, temperatures and the status of cooler. And then calculate the cooling capacity which can meet the requirements of transformer loss, top-oil temperature variation and/or transformer noise requirements. The detailed procedures for calculating winding loss, oil-temperature variation and noise are described from Section A to Section C; and the method to combine this three dimensional control objectives together using weighting factors are described in Section D.

(20) After the optimal cooling capacity is obtained by the central controller, the control strategy will lead to three possible operation solutions as shown in FIG. 7: if the number of fans required is greater, less than or equal to the number of existing fans in operation.

(21) If the number of fans required is nf_next, the number of existing fans is nf_prior, then

(22) n.sub.f=fix(n.sub.f_next)fiX(n.sub.f_prior);

(23) n.sub.VFD=n.sub.f_next(n.sub.f_prior+n.sub.f);

(24) If n.sub.f>0, switch on the corresponding number of fans; otherwise, switch off the corresponding number of fans. And the rest fans driven by VFD should change N.sub.VFD.

(25) When to increase or decrease percentage of transformer cooling, the central controller calculates the number of motor-fan chains needed, it is assumed that the number of motor-fan chains in operation is m.sub.1.Math.n.sub.1, the number calculated is m.sub.2.Math.n.sub.2, where m.sub.i is the integer number and n.sub.i is the percentage of cooling capacity which will achieved by VFD. The central controller gets the integer number of motor-fan chains by m.sub.2m.sub.1. The speed regulation of VFD can be calculated by n.sub.2. The priority of motor-fan chains depend on the utilization time. The central controller prioritizes the motor-fan chains according to the utilization time. Then, the central controller selects to start the motor-fan chain with lower utilization time, and selects to stop the motor-fan chain with higher utilization time.

(26) A. Basic Mathematics for Transformer Loss Calculation

(27) For three-winding transformer, the actual winding loss under specific load level is

(28) $\begin{array}{cc}{P}_k^{}=\frac{1+\stackrel{\_}{{}_w}}{1+75}\left({}_1^2{P}_{k1N}+{}_2^2{P}_{k2N}+{}_3^2{P}_{k3N}\right)& \left(1\right)\end{array}$

(29) Where,

(30) .sub.w: the average winding temperature;

(31) : temperature factor;

(32) .sub.1, .sub.2, .sub.3: load factor;

(33) P.sub.k1N, P.sub.k2N, P.sub.k3N: the winding loss at rated current;

(34) Assume n.sub.f equals to the total required cooling power divided by rated cooling power of each motor-fan chain P.sub.f, which consists of two parts: n.sub.r, which is the integer part, and n.sub.v, which is the decimal part.

(35) Assume n.sub.r is contributed by fans operated at rated speed; and n.sub.v is contributed by fans controlled by VFD operated at partial speed. The total power demand can be expressed as (2), where is the efficiency of the VFD.
P.sub.fans=n.sub.rP.sub.f+n.sub.vP.sub.f/(2)

(36) If all fans are at the same speed and all driven by VFDs, we have
P.sub.fans=n.sub.fP.sub.f/n(3)

(37) The transformer loss can be calculated as formula (4)
f.sub.1=P.sub.t=P.sub.k+P.sub.fans+C(4)

(38) Where, C is constant the power consumption of other parts.

(39) B. Basic Mathematics for Transformer Top-Oil Temperature Calculation

(40) The top-oil temperature variation over time dt is calculated by equation (5),

(41) $\begin{array}{cc}D{}_o=\left\{{\left[\frac{1+{\mathrm{RK}}^2}{1+R}\right]}^x.Math.{}_{\mathrm{or}}.Math.\frac{100}{X_{\mathrm{cor}}}-\left({}_{\mathrm{oi}}-{}_a\right)\right\}.Math.\frac{\mathrm{dt}}{{}_o}& \left(5\right)\end{array}$

(42) Then the difference between the top-oil temperature and a given value is f2,
f.sub.2=abs(.sub.oi+D.sub.o.sub.om)(6)

(43) Where,

(44) .sub.or: top-oil temperature rise in the steady state at rated losses (K);

(45) R: ratio of load losses at rated current to no-load losses;

(46) K: load factor;

(47) .sub.o: average oil time constant;

(48) .sub.oi: the top-oil temperature at prior time;

(49) .sub.a: the ambient temperature;

(50) .sub.om: the given value of top oil temperature;

(51) X.sub.cor: the rate of cooling in operation, which can be calculated by equation (7), where N is the rated current ratio of ONAN condition to ONAF condition;

(52) $\begin{array}{cc}{X}_{\mathrm{cor}}=\left(N+\frac{X}{100}\left(1-N\right)\right)100& \left(7\right)\end{array}$

(53) C. Basic Mathematics for Transformer Noise Level Calculation

(54) The transformer noise is Lp.sub.N1 at ON condition, and Lp.sub.N2 when all the fans are in operation at rated speed. The relationship between the noise Lp.sub.fan caused by fans and the proportion of fans X is shown in equation (8):
Lp.sub.fan=f(X)(8)

(55) So when the proportion of fans in operation is X, the total noise from the transformer and the fan is:

(56) $\begin{array}{cc}{f}_3={\mathrm{Lp}}_t=\{\begin{array}{cc}{\mathrm{Lp}}_{N1},& {\mathrm{Lp}}_{\mathrm{fan}}=0\\ {\mathrm{Lp}}_{N1}+101g\left[1+{10}^{\frac{{\mathrm{Lp}}_{N1}-{\mathrm{Lp}}_{\mathrm{fan}}}{10}}\right],& {\mathrm{Lp}}_{N1}>{\mathrm{Lp}}_{\mathrm{fan}}\\ {\mathrm{Lp}}_{\mathrm{fan}}+101g\left[1+{10}^{\frac{{\mathrm{Lp}}_{\mathrm{fan}}-{\mathrm{Lp}}_{N1}}{10}}\right],& {\mathrm{Lp}}_{\mathrm{fan}}>{\mathrm{Lp}}_{N1}\end{array}& \left(9\right)\end{array}$

(57) Wherein,

(58) Lp.sub.fan: the fan noise;

(59) Lp.sub.N1: the transformer noise.

(60) D. Objective Function with Weighting Factors

(61) When the cooling capacity varies, the variation of the loss f.sub.1, the top oil temperature f.sub.2 and the noise f.sub.3 are obviously different. In order to unify them, the maximum and minimum values of these three objectives f.sub.1min, f.sub.1max, f.sub.2min, f.sub.2max, f.sub.3min and f.sub.3max are calculated at each moment and put into the objective function shown in (10).

(62) By using weighting factors w.sub.1, w.sub.2, w.sub.3 for these three objectives, the objective function can be expressed as:

(63) $\begin{array}{cc}{f}_{\mathrm{obj}}=w_1\frac{f_1}{f_{1\max }-f_{1\min }}+w_2\frac{f_2}{f_{2\max }-f_{2\min }}+w_3\frac{f_3}{f_{3\max }-f_{3\min }}& \left(10\right)\end{array}$
where, w.sub.1+w.sub.2+w.sub.3=1

(64) With formula (10), the optimal cooling capacity for all three objectives can be calculated. Also, each of objectives can be met individually when set its weight to 1, and set other weights to 0.

(65) Step 3: the third step, after the control commands calculation, the central controller will execute the results by controlling the switches directly or sending the control command to VFD, as shown in FIG. 8, where the control actions includes the start and stop of fans, controllable switch operation, and VFD frequency regulation.

(66) To start the fan, the central controller switches the motor-fan which does not need VFD directly to AC lines. For the motor-fan chain will be driven by VFD, the control center switches it to VFD, and sends the speed regulation reference to VFD.

(67) To stop the fan, the central controller directly switches the motor-fan chains off-line.

(68) The central controller repeats the Step 2 and Step 3 in real-time.

(69) Advantages of the Method and System According to this Invention:

(70) This invention proposes a novel transformer cooling system and the corresponding operation method for optimal temperature control, which can improve the operation efficiency of the whole transformer with very limited capital investment on cooling system hardware upgrade, and meanwhile to extend the transformer lifecycle and lower the noise level of the transformer system.

(71) In this invention, the motor-fan loads of the cooling system will be controlled by one VFD selectively according to the temperature control requirement. For motor-fan loads needs to operate at rated power, they will connect to the AC bus directly. The temperature control will consider efficiency of the transformer windings and the cooling system together. Meanwhile, transformer top-oil temperature variation will be controlled in an coordinated way to extend the lifecycle. Furthermore, transformer noise level will be considered together in the cooling control in order to minimize the impact on the surrounding environment. With the proposed electrical design and the control method, the cooling system can be operated in an optimal way to achieve cost-effective efficiency improvement of the whole transformer.

(72) Though the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no means limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.