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Power-Line Carrier System Line Traps J. D. MOYNIHAN ASSOCIATE MEMBER AIEE fT ^HE PROPER application of line traps in power-line -L carrier systems requires the consideration of the needs of the system and the properties of the line traps. In the past, insufficient information was generally given or, in many cases, known, to realize the possible resonant condi- tions that could exist in an installation. Recent investi- gations have brought to light many of these pitfalls. Line traps are used to insert additional impedance at carrier frequencies in a line. This impedance may be desired to prevent solid grounding of the carrier signal during some fault conditions for carrier relaying, or the impedance may be desired to reduce the attenuation due to the signal reaching a bus and being divided among the several lines terminating at the bus. Because of these uses for line traps, the general feeling has been "the higher the impedance, the better the trap." As a result, com- paratively high-Q line traps have been universally used. When keyed carrier operation is desired, the carrier having no modulation at any time, it is acknowledged that an extremely high-Q line trap is desirable. However, when a band of frequencies such as a modulated carrier occupies is used, then the trap impedance across the band of fre- quencies used must be considered. Only the carrier-frequency characteristics are to be considered here. The power-frequency and mechanical characteristics are considered of minor importance in so far as the carrier-frequency operation is concerned. 1. High-Q Line Traps. The general characteristics of line traps are expressed in a variety of ways. However, the presently available low-inductance (less than 0.5 millihenry) high-Q line traps, irrespective of the manner of stating their characteristics, are essentially the same. The line trap has a minimum of 400 to 500 ohms im- pedance at ± 5 per cent off of the resonant frequency over the band of 50 to 150 kilocycles. The maximum im- pedance at the resonant frequency is quite high. Usually not stated, but of interest, is the value of Q which is in the vicinity of 75 or 80. It must be remembered that for a 2-frequency trap, the maximum impedance at each of the two resonant frequencies is approximately one-half that of the same line trap tuned for a single frequency. This type of line trap is ideally suited to narrow band channels. This includes keyed carrier for relaying telemetering, supervisory control, and any of the other on-off functions when operated on a keyed carrier channel. This trap is also applicable for 1- or 2-frequency shift type of channels when utilized for any of the foregoing intelligence functions. Since the impedance varies quite radically over any appreciable bandwidth, the losses can vary considerably. This can cause attenuation of side- bands if this type of trap is used for trapping a modulated carrier. If the termination behind the line trap happens to be essentially reactive and of the proper magnitude, part of the reactive component of the trap can be effec- tively cancelled. This increases the attenuation even more over a portion of the desired sideband. 2. Low-Q Line Traps. The low-inductance low-Q line trap has a lower maximum impedance at its center frequency than does the high-Q line trap just considered. Due to the lower impedance at center frequency, the attenuation of the center frequency signal is higher than that of a low-inductance high-Q line trap. The ad- vantages of a low-Q trap of this type is the broad-band impedance and resistance characteristics. At the ±5- per-cent frequency points, the impedance has decreased to about 0.7 of the center frequency impedance and the resistive component has decreased to about 0.5 of the center frequency resistance as compared to 0.13 and 0.016, re- spectively, for the high-Q line traps. The advantage in using the lower Q inductance is that the resistive component of its impedance at ± 5 per cent or less is sufficiently high so that the reactive characteristics of the device, bus, or line behind the trap will not ap- preciably affect the performance of the trap. In many installations the trap termination is reactive in nature and under certain conditions series resonance of the line trap and its terminating equipment just off center fre- quency can cause severe loss of signal unless a sufficiently large resistive component of impedance is available in the trap. From the characteristics of the line traps, both high and low Q, the following conclusions can be drawn: 1. Line traps should have a low ratio of resistance at resonance to resistance away from resonance for the desired bandwidth. 2. A considerable improvement in percentage modula- tion or effective frequency deviation will result with low-Q line traps. 3. Loss of sidebands due to resonant conditions will not be as severe with low- as with high-Q line traps. 4. Trapping of each frequency-shift channel is more uniform for a group of frequency-shift channels operating adjacent to one another with low-Q traps. It should not be inferred that a low-Q line trap should be used in all applications. The high-Q line trap has very definite advantages in narrow-band applications, such as carrier relaying, and should be considered in these applica- tions. By judicious consideration of trap requirements and trap characteristics, the optimum trap performance can be obtained. Digest of paper 52-275, "Application of Line Traps to Power-Line Carrier Systems,** recommended by the AIEE Committee on Carrier Current and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Fall General Meeting, New Orleans, La., October 13-17, 1952. Scheduled for publication in AIEE Trans- actions^ volume 71, 1952. J. D. Moynihan is with Westinghousc Electric Corporation, East Pittsburgh, Pa. April 1953 Moynihan—Power-Line Carrier System Line Traps 319

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Page 1: Power-line carrier system line traps

Power-Line Carrier System Line Traps J . D . M O Y N I H A N

A S S O C I A T E M E M B E R A I E E

f T ^ H E P R O P E R a p p l i c a t i o n of l ine t r a p s in p o w e r - l i n e -L c a r r i e r sys tems r e q u i r e s t h e c o n s i d e r a t i o n of t h e n e e d s

of t h e sys tem a n d t h e p r o p e r t i e s of t h e l ine t r a p s . I n t h e pas t , insufficient i n f o r m a t i o n w a s g e n e r a l l y g iven or , i n m a n y cases, k n o w n , t o r ea l i ze t h e possible r e s o n a n t c o n d i ­t ions t h a t c o u l d exist in a n ins ta l l a t ion . R e c e n t inves t i ­ga t ions h a v e b r o u g h t t o l i gh t m a n y of t he se pitfalls .

L i n e t r a p s a r e used t o inse r t a d d i t i o n a l i m p e d a n c e a t ca r r i e r f requencies in a l ine . T h i s i m p e d a n c e m a y b e des i red t o p r e v e n t solid g r o u n d i n g of t h e c a r r i e r s igna l d u r i n g s o m e faul t cond i t i ons for c a r r i e r r e l ay ing , o r t h e i m p e d a n c e m a y b e des i red t o r e d u c e t h e a t t e n u a t i o n d u e t o t h e s ignal r e a c h i n g a b u s a n d b e i n g d i v i d e d a m o n g the several l ines t e r m i n a t i n g a t t h e b u s . Because of t hese uses for l ine t r a p s , t h e g e n e r a l feel ing h a s b e e n " t h e h i g h e r t h e i m p e d a n c e , t h e b e t t e r t h e t r a p . " As a resul t , c o m ­p a r a t i v e l y h i g h - Q l ine t r a p s h a v e b e e n un ive r sa l ly u s e d . W h e n k e y e d c a r r i e r o p e r a t i o n is des i red , t h e c a r r i e r h a v i n g n o m o d u l a t i o n a t a n y t i m e , i t is a c k n o w l e d g e d t h a t a n e x t r e m e l y h i g h - Q l ine t r a p is de s i r ab l e . H o w e v e r , w h e n a b a n d of f requencies s u c h as a m o d u l a t e d c a r r i e r o c c u p i e s is used , t h e n t h e t r a p i m p e d a n c e across t h e b a n d of fre­q u e n c i e s u s e d m u s t b e c o n s i d e r e d .

O n l y t h e c a r r i e r - f r e q u e n c y cha rac te r i s t i c s a r e t o b e cons ide red h e r e . T h e p o w e r - f r e q u e n c y a n d m e c h a n i c a l charac te r i s t i c s a r e c o n s i d e r e d of m i n o r i m p o r t a n c e in so far as t h e ca r r i e r - f r equency o p e r a t i o n is c o n c e r n e d .

1. High-Q Line Traps. T h e g e n e r a l cha rac t e r i s t i c s of l ine t r a p s a r e expressed in a v a r i e t y of w a y s . H o w e v e r , t h e p resen t ly a v a i l a b l e l o w - i n d u c t a n c e (less t h a n 0.5 mi l l i hen ry ) h i g h - Q l ine t r a p s , i r r espec t ive of t h e m a n n e r of s t a t ing the i r charac te r i s t i c s , a r e essent ia l ly t h e s a m e . T h e l ine t r a p has a m i n i m u m of 400 to 500 o h m s i m ­p e d a n c e a t ± 5 p e r c e n t off of t h e r e s o n a n t f r equency ove r t h e b a n d of 50 to 150 ki locycles. T h e m a x i m u m i m ­p e d a n c e a t t h e r e s o n a n t f r equency is q u i t e h i g h . U s u a l l y n o t s t a t ed , b u t of in te res t , is t h e v a l u e of Q w h i c h is in t h e vic ini ty of 75 or 80. I t m u s t b e r e m e m b e r e d t h a t for a 2- f requency t r a p , t h e m a x i m u m i m p e d a n c e a t e a c h of t h e t w o r e s o n a n t f requencies is a p p r o x i m a t e l y one -ha l f t h a t of t he s a m e l ine t r a p t u n e d for a s ingle f r equency .

T h i s t y p e of l ine t r a p is idea l ly su i t ed to n a r r o w b a n d channe l s . T h i s i nc ludes k e y e d c a r r i e r for r e l a y i n g t e l eme te r i ng , superv i so ry c o n t r o l , a n d a n y of t h e o t h e r on-off funct ions w h e n o p e r a t e d o n a k e y e d c a r r i e r c h a n n e l . T h i s t r a p is also a p p l i c a b l e for 1- o r 2 - f requency shift t y p e of c h a n n e l s w h e n u t i l ized for a n y of t h e fo rego ing in te l l igence funct ions . S ince t h e i m p e d a n c e var ies q u i t e r ad i ca l l y over a n y a p p r e c i a b l e b a n d w i d t h , t h e losses c a n v a r y cons ide rab ly . T h i s c a n cause a t t e n u a t i o n of s ide­b a n d s if th is t y p e of t r a p is used for t r a p p i n g a m o d u l a t e d ca r r i e r . I f t h e t e r m i n a t i o n b e h i n d t h e l ine t r a p h a p p e n s to b e essent ial ly r e a c t i v e a n d of t h e p r o p e r m a g n i t u d e ,

p a r t of t h e r e a c t i v e c o m p o n e n t of t h e t r a p c a n b e effec­t ive ly c a n c e l l e d . T h i s inc reases t h e a t t e n u a t i o n even m o r e o v e r a p o r t i o n of t h e de s i r ed s i d e b a n d .

2 . Low-Q Line Traps. T h e l o w - i n d u c t a n c e l o w - Q l ine t r a p h a s a l o w e r m a x i m u m i m p e d a n c e a t its c e n t e r f r e q u e n c y t h a n does t h e h i g h - Q l ine t r a p j u s t cons ide red . D u e t o t h e l o w e r i m p e d a n c e a t c e n t e r f r equency , t h e a t t e n u a t i o n of t h e c e n t e r f r e q u e n c y s igna l is h i g h e r t h a n t h a t of a l o w - i n d u c t a n c e h i g h - Q l ine t r a p . T h e a d ­v a n t a g e s of a l o w - Q t r a p of th is t y p e is t h e b r o a d - b a n d i m p e d a n c e a n d res i s t ance cha rac t e r i s t i c s . A t t h e ± 5 -p e r - c e n t f r e q u e n c y po in t s , t h e i m p e d a n c e h a s dec reased to a b o u t 0.7 of t h e c e n t e r f r e q u e n c y i m p e d a n c e a n d t h e resis t ive c o m p o n e n t h a s d e c r e a s e d to a b o u t 0.5 of t h e c e n t e r f r e q u e n c y r e s i s t ance as c o m p a r e d t o 0 .13 a n d 0 .016, r e ­spect ive ly , for t h e h i g h - Q l ine t r a p s .

T h e a d v a n t a g e i n u s i n g t h e l o w e r Q i n d u c t a n c e is t h a t t h e resis t ive c o m p o n e n t of its i m p e d a n c e a t ± 5 p e r c e n t o r less is sufficiently h i g h so t h a t t h e r e a c t i v e charac te r i s t i cs of t h e d e v i c e , b u s , o r l i ne b e h i n d t h e t r a p will n o t a p ­p r e c i a b l y affect t h e p e r f o r m a n c e of t h e t r a p . I n m a n y ins t a l l a t ions t h e t r a p t e r m i n a t i o n is r e a c t i v e in n a t u r e a n d u n d e r c e r t a i n c o n d i t i o n s series r e s o n a n c e of t h e l ine t r a p a n d its t e r m i n a t i n g e q u i p m e n t j u s t off c e n t e r fre­q u e n c y c a n c a u s e s eve re loss of s igna l unless a sufficiently l a r g e resis t ive c o m p o n e n t of i m p e d a n c e is ava i l ab l e in t h e t r a p .

F r o m t h e cha rac t e r i s t i c s of t h e l ine t r a p s , b o t h h igh a n d low Q , t h e fo l lowing conc lus ions c a n b e d r a w n :

1. L i n e t r a p s s h o u l d h a v e a l o w r a t i o of res i s tance a t r e s o n a n c e t o r e s i s t ance a w a y f rom r e s o n a n c e for t h e des i red b a n d w i d t h .

2 . A c o n s i d e r a b l e i m p r o v e m e n t i n p e r c e n t a g e m o d u l a ­t i on o r effective f r e q u e n c y d e v i a t i o n wil l resu l t w i t h l o w - Q l ine t r a p s .

3 . Loss of s i d e b a n d s d u e t o r e s o n a n t cond i t i ons will no t b e as severe w i t h low- as w i t h h i g h - Q l ine t r a p s .

4. T r a p p i n g of e a c h f requency-shi f t c h a n n e l is m o r e un i fo rm for a g r o u p of f requency-shif t c h a n n e l s o p e r a t i n g a d j a c e n t to o n e a n o t h e r w i t h l o w - Q t r a p s .

I t s h o u l d n o t b e in fe r red t h a t a l o w - Q l ine t r a p shou ld b e used in al l a p p l i c a t i o n s . T h e h i g h - Q l ine t r a p h a s ve ry def in i te a d v a n t a g e s i n n a r r o w - b a n d app l i ca t i ons , such as c a r r i e r r e l a y i n g , a n d s h o u l d b e c o n s i d e r e d in these a p p l i c a ­t ions . By j u d i c i o u s c o n s i d e r a t i o n of t r a p r e q u i r e m e n t s a n d t r a p cha rac t e r i s t i c s , t h e o p t i m u m t r a p p e r f o r m a n c e c a n b e o b t a i n e d .

Digest of paper 52-275, "Application of Line Traps to Power-Line Carrier Systems,** recommended by the AIEE Committee on Carrier Current and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Fall General Meeting, New Orleans, La., October 13-17, 1952. Scheduled for publication in AIEE Trans-actions^ volume 71, 1952.

J . D . Moynihan is with Westinghousc Electric Corporation, East Pittsburgh, Pa.

April 1953 Moynihan—Power-Line Carrier System Line Traps 319

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