the invention will be detailed description with reference to the drawings and the embodiments as shown in the following : in fig1 , a , b , and c are the three units , p 1 , p 2 , p 3 and p 4 are the four photocouplers . the three - phase linkage apparatus is mainly composed of the said three units and four photocouplers , wherein , b is the mastering unit , a and c are the controlled units . in the circuit diagram of fig1 , s is the three - phase power supply , while l is the three - phase load . the six external terminals of the three units are connected to the three - phase power supply and the three - phase load respectively . in fig1 , b 2 is the common reference node of the internal control circuit of the mastering unit b , b 1 is the output terminal of the internal control circuit of the mastering unit b . when there are output signal in the internal control circuit of the mastering unit b , the main circuit bidirectional switching ( mcbs ) module in the mastering unit b is on , that is , the mastering unit b is on , otherwise , the mcbs module in the mastering unit b is off , or the mastering unit b is off . the mastering unit b also provides the short - circuit protection function . when it happens short - circuit fault in the external circuit of the mastering unit b , the short - circuit fault output signal on the secondary winding of the current mutual inductor via the bridge rectifying circuit enters into the short - circuit fault signal input terminal of the control circuit in the mastering unit b , result in the output signal of the internal control circuit disappears and the mcbs module is off , that is , the mastering unit b is from on into off . in fig1 , b 4 is the short - circuit fault signal input terminal of the internal control circuit of the mastering unit b , while b 3 is a suitable positive level node as compared with the common reference node b 2 . in fig1 , a 4 is the common reference node of the internal control circuit of the controlled unit a , a 3 is the short - circuit fault signal output terminal on the divider which is connected to the secondary winding of the current mutual inductor via the bridge rectifying circuit in the controlled unit a ; when it happens the short - circuit fault in the external circuit of the controlled unit a , this output terminal should have enough level short - circuit fault output signal ; a 1 is a suitable positive level node as compared with the common reference node a 4 , a 2 is the on - off signal input terminal of the internal control circuit of the controlled unit a , when this terminal gets the suitable level signal , the mcbs module in the controlled unit a is on , that is , the controlled unit a is on , otherwise , the mcbs module in the controlled unit a is off , that is , the controlled unit a is off . in fig1 , c 4 is the common reference node of the internal control circuit of the controlled unit c , c 3 is the short - circuit fault signal output terminal on the divider which is connected to the secondary winding of the current mutual inductor via the bridge rectifying circuit in the controlled unit c , when it happens the short - circuit fault in the external circuit of the controlled unit c , this output terminal should have enough level short - circuit fault output signal ; c 1 is a suitable positive level node as compared with the common reference node c 4 , c 2 is the on - off signal input terminal of the internal control circuit of the controlled unit c , when this terminal gets the suitable level signal , the mcbs module in the controlled unit c is on , that is , the controlled unit c is on , otherwise , the mcbs module in the controlled unit c is off , that is , the controlled unit c is off . as for the embodiment of fig1 , b 1 and b 2 are connected respectively to the light emitting diodes ( leds ) in the photocouplers p 1 and p 2 though current - limiting resistor r , while the terminal a 1 and a 2 and the terminal c 1 and c 2 are connected respectively to the photoconductive transistors in the photocouplers p 1 and p 2 , so when the internal control circuit of the mastering unit b has the output signal and the mastering unit b is on , the output signal of the internal control circuit of the mastering unit b on b 1 and b 2 causes the leds in the photocouplers p 1 and p 2 lightened and the photoconductive transistors in the photocouplers p 1 and p 2 present a low resistance state , and the suitable positive level on a 1 and c 1 can reach a 2 and c 2 through their respective photoconductive transistors , so that the on - off signal input terminals ( a 2 and c 2 ) of the internal control circuits of controlled units a and c can get a suitable level signal , and the mcbs modules of controlled units a and c are on , that is , the controlled units a and c are on . otherwise , when there is no output signal in the internal control circuit of mastering unit b and the mastering unit b is off , the leds in photocoupler p 1 and p 2 can not lighten , and the photoconductive transistors in photocoupler p 1 and p 2 present a high resistance state , and the suitable positive level on a 1 and c 1 can not reach a 2 and c 2 through their respective photoconductive transistor , so that the on - off signal input terminals of the internal control circuits of controlled units a and c can not get the signal , and the mcbs modules in the controlled units a and c are off , that is , the controlled units a and c are off . since via the signal transmissions of the photocouplers p 1 and p 2 , the three units a , b and c can have consistently - combined on - off actions and thus complete the linkage of the three - phase units . as for the embodiment in fig1 , it has short - circuit protection function apart from the three - phase units linkage function . the embodiment can also act as the three - phase solid - state short - circuit protector , or three - phase solid - state breaker , in other words , when the short - circuit fault happens on the external circuits of any of the three units , the three units a , b and c can have consistently - combined off actions . as the mastering unit b itself has short - circuit protection function , when the short - circuit fault happens in the external circuit of the mastering unit b , it can off itself , and can result in the units a and c are off through the above said on - off linkage function . terminals a 3 and a 4 , terminals c 3 and c 4 are connected respectively to the leds in the photocouplers p 3 and p 4 though the current - limiting resistor r , when the short - circuit fault happens in the external circuits of unit a and / or unit c , terminals a 3 and / or c 3 have enough level the short - circuit fault output signal , so that the leds in the photocouplers p 3 and / or p 4 lightened , and the photoconductive transistors in the photocouplers p 3 and / or p 4 presents a low resistance state ; and then the suitable positive level on the terminal b 3 can reach the short - circuit fault signal input terminal b 4 in the mastering unit b through the photoconductive transistors in the photocouplers p 3 and or p 4 , so that the mcbs module in mastering unit b is from cut - in to cut - off and the mastering unit b is off , and also the controlled units a and c are off at the same time through the on - off linkage function of the mastering unit b to the controlled units a and c . thus it can be seen , as for the embodiment in fig1 , when the short - circuit fault happens in the external circuit of any of the three units a , b and c , the three units a , b and c can have consistently - combined off actions , thus it completes the three - phase solid - state short - circuit protection function . fig2 is the first embodiment of the linkage apparatus with multi - units connected in series of this invention . in fig2 , there are five units , respectively d , e , f , g , and h , wherein , d is the mastering unit , the left are the controlled units ; as each unit has only two external terminals , so when the five units are connected in series , a new two - wire circuit is realized that has only two external terminals ( t 1 and t 2 ). in fig2 , there are four photocoupleres , respectively p 5 , p 6 , p 7 and p 8 , terminals d 1 and d 2 are the output terminals of the internal control circuit of the mastering unit d , d 1 and d 2 are connected to the four leds in the four photocouplers p 5 , p 6 , p 7 and p 8 through current - limiting resistance r ; terminals e 1 , f 1 , g 1 and h 1 are the suitable positive level nodes as compared with respectively four common reference nodes of the internal control circuits of the controlled units e , f , g and h ; e 2 , f 2 , g 2 and h 2 are respectively the on - off signal input terminals of the internal control circuits of the controlled units e , f , g and h ; terminals e 1 and e 2 , f 1 and f 2 , g 1 and g 2 , h 1 and h 2 are connected respectively to four photoconductive transistors in the four photocouplers p 5 , p 6 , p 7 and p 8 . as for the embodiment of fig2 , when the internal control circuit of the mastering unit d has the output signal , the mcbs module in the mastering unit d is on , that is , the unit d is on , and the output signal on the terminals d 1 and d 2 causes four leds in the photocouplers p 5 , p 6 , p 7 and p 8 lightened and four photoconductive transistors in the photocouplers p 5 , p 6 , p 7 and p 8 present a low resistance states ; therefore the suitable positive level of the terminals e 1 , f 1 , g 1 and h 1 can reach the terminals e 2 , f 2 , g 2 and h 2 through corresponding photoconductive transistor , so that the on - off signal input terminals e 2 , f 2 , g 2 , h 2 of the internal control circuits of the controlled units e , f , g , h get suitable level input signals at the same time , and the mcbs modules in the controlled units e , f , g and h are on , that is , the controlled units e , f , g and h are on . otherwise , when there is no output signal in the internal control circuit of the mastering unit d , the mcbs module in the mastering unit d is off , that is , unit d is off , and there is no output signal on the terminals d 1 and d 2 , the four leds in the photocouplers p 5 , p 6 , p 7 and p 8 can not lighten , and four photoconductive transistors in photocouplers p 5 , p 6 , p 7 and p 8 present a high resistance state , and the suitable positive level of the terminals e 1 , f 1 , g 1 and h 1 can not reach the terminals e 2 , f 2 , g 2 and h 2 through corresponding photoconductive transistor , so that the on - off signal input terminals of the internal control circuit of the controlled units e , f , g and h can not get the input signal , the mcbs modules in the controlled units e , f , g and h are off , that is , the controlled units e , f , g and h are off . thus it can be seen , through signal transfer of the photocouplers p 5 , p 6 , p 7 and p 8 , the five units d , e , f , g and h connected in series can have consistently - combined on - off actions and thus complete the linkage of the multi - units connected in series , which is heightened the working voltage . fig3 is the second embodiment of the linkage apparatus with multi - units connected in series of this invention . in fig3 , there are five units , respectively j , k , m , n , and q , wherein , j is the mastering unit , the left are the controlled units ; as each unit has only two external terminals , so when the five units are connected in series , a new two - wire circuit is realized that has only two external terminals ( t 3 and t 4 ). in fig3 , there are four photocouplers , respectively p 9 , p 10 , p 11 and p 12 , terminals j 1 and j 2 are the output terminals of the internal control circuit of the mastering unit j , j 1 and j 2 are connected to the led in the photocoupler p 9 through current - limiting resistance r ; terminals k 3 and k 4 are the output terminals of the internal control circuit of the controlled unit k , k 3 and k 4 are connected to the led in the photocoupler p 10 through current - limiting resistance r ; terminals m 3 and m 4 are the output terminals of the internal control circuit of the controlled unit m , m 3 and m 4 are connected to the led in the photocoupler p 11 through current - limiting resistance r ; terminals n 3 and n 4 are the output terminals of the internal control circuit of the controlled unit n , n 3 and n 4 are connected to the led in the photocoupler p 12 through current - limiting resistance r ; terminals k 2 , m 2 , n 2 , q 2 are the suitable positive level nodes as compared with respectively the common reference nodes of the internal control circuits of the controlled units k , m , n and q ; k 1 , m 1 , n 1 and q 1 are respectively the on - off signal input terminals of the internal control circuits of the controlled units k , m , n and q ; terminals k 1 and k 2 are connected to the photoconductive transistor in the photocoupler p 9 , terminals m 1 and m 2 are connected to the photoconductive transistor in the photocoupler p 10 , terminals n 1 and n 2 are connected to the photoconductive transistor in the photocoupler p 11 , and terminals q 1 and q 2 are connected to the photoconductive transistor in the photocoupler p 12 . as for the embodiment of fig3 , when the internal control circuit of the mastering unit j has the output signal and the mcbs module in the mastering unit j is on , that is , unit j is on , what &# 39 ; s more , the output signal on the terminals j 1 and j 2 causes the led in the photocoupler p 9 lightened and the photoconductive transistor in the photocoupler p 9 presents a low resistance state ; and the suitable positive level of the terminal k 2 can reach the terminal k 1 through the photoconductive transistor in the photocoupler p 9 , so that the on - off signal input terminal k 1 of the internal control circuit of the controlled unit k have a suitable level signal , the output terminals k 3 , k 4 of the internal control circuit of the controlled unit k has the output signal , the mcbs module in the controlled unit k is on , that is , the controlled unit k is on ; what &# 39 ; s more , the output signal on the terminals k 3 and k 4 causes the led in the photocoupler p 10 lightened this moment and the photoconductive transistor in the photocoupler p 10 presents a low resistance state ; and the suitable positive level of the terminal m 2 can reach the terminal m 1 through the photoconductive transistor in the photocoupler p 10 , so that the on - off signal input terminal m 1 of the internal control circuit of the controlled unit m get suitable level signal , the output terminals m 3 , m 4 of the internal control circuit of the controlled unit m has the output signal , the mcbs module in the controlled unit m is on , that is , the controlled unit m is on ; furthermore , the output signal on the terminals m 3 and m 4 causes the led in the photocoupler p 11 lightened this moment and the photoconductive transistor in the photocoupler p 11 presents a low resistance state ; and the suitable positive level of the terminal n 2 can reach the terminal n 1 through the photoconductive transistor in the photocoupler p 11 , so that the on - off signal input terminal n 1 of the internal control circuit of the controlled unit n get suitable level signal , the output terminals n 3 , n 4 of the internal control circuit of the controlled unit n has the output signal , the mcbs module in the controlled unit n is on , that is , the controlled unit n is on ; at the same time , the output signal on the terminals n 3 and n 4 causes the led in the photocoupler p 12 lightened and the photoconductive transistor in photocoupler p 12 presents a low resistance state ; and the suitable positive level of the terminal q 2 can reach the terminal q 1 through the photoconductive transistor in the photocoupler p 12 , so that the on - off signal input terminal q 1 of the internal control circuit of controlled unit q have suitable level signal , the output terminals of the internal control circuit of the controlled unit q have the output signal , the mcbs module in the controlled unit q is on , that is , the controlled unit n is on ; otherwise , when there is no output signal in the internal control circuit of mastering unit j , and the mcbs module in the mastering unit j is off , that is , the unit j is off , and there is no output signal on the terminals j 1 and j 2 , the led in the photocoupler p 9 can not lighten , and the photoconductive transistor in the photocoupler 9 presents a high resistance state , and the suitable positive level of the terminal k 2 can not reach the terminal k 1 through the photoconductive transistor in p 9 , so that the on - off signal input terminal k 1 of the internal control circuit of the controlled unit k can not get the signal , the mcbs module in the controlled unit k is off , that is , the controlled unit k is off ; and then the led in the photocoupler p 10 can not lighten , and the photoconductive transistor in the photocoupler 10 presents a high resistance state ; and the suitable positive level of m 2 can not reach m 1 through the photoconductive transistor in p 10 , so that the on - off signal input terminals m 1 of the internal control circuit of the controlled unit m can not get the signal , the mcbs module in the controlled unit m is off , that is , the controlled unit m is off ; in the same way , the leds in the photocouplers p 11 and p 12 can not lighten , and the photoconductive transistors in the photocoupler p 11 and p 12 present a high resistance state , the controlled units n and q are both off . thus it can be seen , through signal transfer of the photocouplers p 9 , p 10 , p 11 and p 12 , the five units j , k , m , n and q connected in series can have consistently - combined on - off actions and thus complete the linkage of the multi - units connected in series , which is heightened the working voltage . fig4 is the third embodiment of the linkage apparatus with multi - units connected in series of this invention . in the fig4 , there are five units , respectively u , v , w , x and y , wherein , u is the mastering unit , the left are controlled units ; as each unit has only two external terminals , so when the five units are connected in series , a new two - wire circuit is realized that has only two external terminals ( t 5 and t 6 ). in fig4 , terminals u 1 and u 2 are the output terminals of the internal control circuit of the mastering unit u , the terminals u 1 and u 2 are connected to the light emitting module le through the current - limiting resistance r ; op 1 , op 2 , op 3 and op 4 are four fibers ; pc 1 , pc 2 , pc 3 and pc 2 are four photoconductive transistors ; terminals v 2 , w 2 , x 2 , and y 2 are the suitable positive level nodes as compared with respectively the common reference nodes of the internal control circuit of the controlled units v , w , x and y ; terminals v 1 , w 1 , x 1 and y 1 are respectively the on - off signal input terminals of the internal control circuit of the controlled units v , w , x and y ; terminals v 1 and v 2 are connected to the photoconductive transistor pc 1 , terminals w 1 and w 2 are connected to the photoconductive transistor pc 2 , terminals x 1 and x 2 are connected to the photoconductive transistor pc 3 , while terminals y 1 and y 2 are connected to the photoconductive transistor pc 4 . as for the embodiment of fig4 , when the internal control circuit of the mastering unit u has output signal and the mcbs module in the mastering unit u is on , that is , unit u is on , what &# 39 ; s more , the output signal of the terminals u 1 and u 2 causes the light emitting module le lightened , and the light shines four photoconductive transistors pc 1 , pc 2 , pc 3 and pc 4 through light propagation of four fibers op 1 , op 2 , op 3 and op 4 , so that the four photoconductive transistors pc 1 , pc 2 , pc 3 and pc 4 present a low resistance states ; and the suitable positive level of the terminals v 2 , w 2 , x 2 , and y 2 can reach the terminals v 1 , w 1 , x 1 and y 1 through the four photoconductive transistors pc 1 , pc 2 , pc 3 and pc 4 , therefore the on - off signal input terminals v 1 , w 1 , x 1 , y 1 of the internal control circuits of controlled units v , w , x and y can get suitable level input signal , the mcbs modules in the controlled units v , w , x and y are on , that is , the controlled units v , w , x and y are on ; otherwise , when the internal control circuit of the mastering unit u has no output signal and the mcbs module in the mastering unit u is off , that is , unit u is off , what &# 39 ; s more , there is no output signal on the terminals u 1 and u 2 , the light emitting module le can not lighten and the four photoconductive transistors pc 1 , pc 2 , pc 3 and pc 4 present a high resistance states ; and the suitable positive level of the terminals v 2 , w 2 , x 2 , and y 2 can not reach v 1 , w 1 , x 1 and y 1 through the four photoconductive transistors pc 1 , pc 2 , pc 3 and pc 4 , so that the on - off signal input terminals v 1 , w 1 , x 1 , y 1 of the internal control circuits of the controlled units v , w , x and y can not get the input signal , the mcbs modules in the controlled units v , w , x and y are off , that is , the controlled units v , w , x and y are off . thus it can be seen , through signal transfer of the light emitting module le and four fibers op 1 , op 2 , op 3 , op 4 and the four photoconductive transistors pc 1 , pc 2 , pc 3 , pc 4 , the five units j , k , m , n and q connected in series can have consistently - combined on - off actions and thus complete the linkage of the multi - units connected in series , which is heightened the working voltage . fig5 is the first embodiment of three - phase high voltage multi - units linkage apparatus of this invention . in fig5 , due to the large number of the units and their related devices involved , simplified diagram will be adopted here for illustration . in fig5 , a 1 , a 2 , a 3 . . . to an , b 1 , b 2 , b 3 . . . to bn , c 1 , c 2 , c 3 . . . to cn are respectively made up of n units into the three linkage apparatuses with multi - units connected in series , s is the three - phase power supply , l is the three - phase load . as each of the three linkage apparatuses is a two - wire circuit apparatus which has only two external terminals , it is the same with the embodiment of fig1 , the six external terminals of the three linkage apparatuses are respectively connected to the three - phase power supply and three - phase load . as shown in the fig5 , the solid line and the arrow indicate the route and direction which transmit on - off control signal through photoelectric coupling , while the dotted line and the arrow indicate the route and direction which transmit the short - circuit fault signal through photoelectric coupling . in fig5 , b 1 is the mastering unit , others are the controlled units . the transmission of the on - off control signal from b 1 to a 1 and b 1 to c 1 , and the transmission of the short - circuit fault signal from a 1 to b 1 and c 1 to b 1 , they are the same with the embodiment of fig1 . the transmission of the on - off control signal from b 1 to b 2 , b 7 , b 12 , b 17 , etc . is the same with the embodiment of fig4 , this is a way using the non - metal fibers to transmit signals between units with many units in between will not cause generally voltage breakdown . the transmission of the on - off control signal from a 1 to a 2 , a 7 , a 12 , a 17 , etc . and c 1 to c 2 , c 7 , c 12 , c 17 , etc . also adopts the same method as used in the embodiment of fig4 . the left transmission of the on - off control signals , for example , from a 2 to a 3 , a 4 , a 5 , etc ., from b 7 to b 8 , b 9 , b 10 , b 11 , etc ., from c 17 to c 18 , c 19 , c 20 , c 21 , etc ., adopts the same method as used in the embodiment of fig2 . as for the embodiment of fig5 , the transmission of the on - off control signal starts from the mastering unit b , reaches to all the other units along the solid line and arrow level by level to several levels , thereby consistently - combined on - off actions are realized . when there is short - circuit fault on the external circuit of a 1 or c 1 , the short - circuit fault signal can be transmitted from a 1 or c 1 to b 1 , as b 1 itself has short - circuit protection function , thus when any a phase external circuit of the three - phase external circuits has short - circuit fault , all the units can be consistently - combined “ off ”, therefore , this embodiment has the three - phase high voltage solid - state short - circuit protection function . fig6 is the second embodiment of three - phase high voltage multi - units linkage apparatus of this invention . there is only one difference of this embodiment from the embodiment of fig5 , that is , the transmission of the on - off control signal of the last level , for example , from a 2 to a 3 , a 4 , a 5 , etc . from b 7 to b 8 , b 9 , b 10 , b 11 , etc . from c 17 to c 18 , c 19 , c 20 , c 21 , etc , adopts the same method as used in the embodiment of fig3 . as the signal is transmitted between two adjacent units , the voltage breakdown of the photocoupler will not occur . as for the others of the fig6 , it is the same with that of the fig5 please refer to fig5 .