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TECHNICAL BRIEF

CABLE GROUP 29brief,5 May, 1998CENTRILIFT, A Baker Hughes company !2210 E. L. Anderson Boulevard, Claremore, OK 74017 !Phone (918) 341-9600, ! FAX (918) 342-0176

FLAT CABLE BALANCE

Most electrical power is delivered over a balanced, three-phase AC system. This means

that each of the three phases is connected to (mostly) equivalent electrical ci rcui ts. Insome cases, one circuit will not match exactly the other two circuits and draw a currentdifferent from the average and th is leads to a voltage imbalance.

NEMA defines imbalance as:

% Voltage Imbalance = Max deviation from the average x 100average voltage

Voltage imbalance may result from mismatches anywhere in the supply circuit . Thisincludes the supply itself, the power cable and the motor windings.

Studies have shown that for normal depths (below 8000 feet) a flat cable imbalance hasless effect than is likely found in normal variations in power supplies and the motorwind ings. Only in very deep wells, where the cable losses are a significant portion of

the total losses, do cable imbalance effects need to be considered.

CONCLUSIONS

Factors that affect the system imbalance are:

Pre-exist ing l ine (power supply) voltage imbalance

Load (motor) impedance

Cable impedance imbalance (that varies wi th length and current)

RECOMMENDATIONS

Several methods exist to reduce imbalance condi tions:

Rotating the three phases until balanced current condi tions exist Adjust transformer taps to achieve balance, this can be used in conjunct ion with

rotation of phases

Use round cable where clearance is available, consider using cable protectors if

tight condition exists

Use a mixed str ing where appropriate (i.e., top round and bot tom f lat)

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Use a higher voltage motor (for a given HP load) thus reducing the current loading

on the cable

Do a phase transpos ition splice at 1/3 intervals of the total cable length. Phase

transposition allows each of the three conductors to spend an equal amount of timein the center posit ion where the imbalance is created.

TRANSPOSITION SPLICE

A B C

B C A

C A B

The transposi tion splice is equally spaced at 1/3 intervals of the total length of thecable. This option may reduce overall cable reliability by having two extra splices in thestring of cable. The cable is also larger at the transposit ion, which causes a size

problem and the possibili ty of damage. This area requires cable protectors over the twosplices.

No single criterion exists for recommendation maximum lengths for flat cables. Anyrecommendation needs to account for the effect on specific motor sizes, normal

variation of conductor resistance (1%), conductor size and kV rating.

With known or estimated voltage imbalance values, the AutographPCprogram can

quickly estimate the operating motor temperature for proper size of motor and cablesystems.

Thomson H. WallaceThomson H. WallaceThomson H. WallaceThomson H. Wallace Walter DinkinsWalter DinkinsWalter DinkinsWalter Dinkins

Product Development Manager Senior Staff Applications& Product Engineer

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