Figure 1. 125 VDC system key diagram

According to the ANSI/IEEE 946

Contents [hide]

1 Introduction

2 Voltage Considerations

3 Available Short-Circuit Current

4 Calculation Approach

5 Partial Fault Currents

5.1 Short-Circuit Current from Batteries

5.2 Short-Circuit Current from DC Motors/Generators

5.3 Short-Circuit Currents from Chargers

6 References

Introduction

Scope of the IEEE 946-1992: This recommended practice provides guidance for the design of the DC

auxiliary power systems for nuclear and non-nuclear power generating stations. The components of the

DC auxiliary power system addressed by this recommended practice include lead-acid storage batteries,

static battery chargers and distribution equipment. Guidance for selecting the quantity and types of

equipment, the equipment ratings, interconnections, instrumentation, control and protection is also

provided.

This recommended practice is intended for nuclear and

large fossil-fueled generating stations. Each

recommendation may or may not be appropriate for

other generating facilities; e.g., combustion turbines,

hydro, wind turbines, etc. The AC power supply (to the

chargers), the loads served by the DC systems, except

as they influence the DC system design, and engine

starting (cranking) battery systems are beyond the

scope of this recommended practice.

For more informations please refer to the standard

itself IEEE 946-1992 .

Voltage Considerations

The nominal voltages of 250, 125, 48, and 24 are

generally utilized in station DC auxiliary power systems.

The type, rating, cost, availability, and location of the

connected equipment should be used to determine

which nominal system voltage is appropriate for a

specific application. 250 VDC systems are typically used

to power motors for emergency pumps, large valve operators, and large inverters. 125 VDC systems are

typically used for control power for nest relay logic circuits andthe closing and tripping of switchgear

circuit breakers. 48 VDC or 24 VDC systems are typically used for specialized instrumentation.

Figure 2. Recommended voltage range of 125 V and 250 V DC (nominal) rated components (for designs in which the

battery is equalized while connected to the load)

Available Short-Circuit Current

For the purpose of determining the maximum available short-circuit current (e.g., the required interrupting

capacity for feeder breakers/fuses and withstand capability of the distribution buses and disconnecting

devices), the total short-circuit current is the sum of that delivered by the battery, charger, and motors

(as applicable). When a more accurate value of maximum available short-circuit current is required, the

analysis should account for interconnecting cable resistance.

Calculation Approach

As defined in "Industrial power systems data book" [2], there are two calculation ways to acquire the

fault current:

1. Approximation Method: All the network is converted into the equivalent impedance (Req, Leqare

used for the time constant) and the system voltage is being used for the fault current calculation:

2. Superposition Method: The fault current is calculated for each source individually, while other,

not observed sources, are being shorted out (with their internal resistances). The voltage for each

partial current is the rated voltage of the source. The total current is the sum of the partial currents.

This approach shall be described in following articles.

Partial Fault Currents

Short-Circuit Current from Batteries

The current that a battery will deliver on short-circuit depends on the total resistance of the short-

circuit path. A conservative approach in determining the short-circuit current that the battery will deliver

at 25°C is to assume that the maximum available short-circuit current is 10 times the 1 minute ampere

rating (to 1.75 V per cell). For more than 25°C the short-circuit current for the specific application should

be calculated or actual test data should be obtained from the battery manufacturer. The battery nominal

voltage should be used when calculating the maximum short-circuit current. Tests have shown that an

increase in electrolyte temperature (above 25°C) or elevated battery terminal voltage (above nominal

voltage) will have no appreciable effect on the magnitude of short-circuit current delivered by a battery.

The internal battery resistance is calculated using:

Where EB is the battery rated voltage and I8hrs is the 8-hour battery capacity.

The maximum (or peak) short-circuit current is:

RBBr is the sum of the battery internal resistance RBand the line resistance RBr up to the fault location.

The initial maximum rate of rise of the current at t=0 s is as follows:

The time constant is calculated as:

The sustained short-circuit current is calculated using:

And the fault current from the battery for the time t:

Short-Circuit Current from DC Motors/Generators

DC motors, if operating, will contribute to the total fault current. The maximum current that a DC motor

will deliver to a short-circuit at its terminals is limited by the effective transient armature resistance (r'd)

of the motor. For DC motors of the type, speed, voltage, and size typically used in generating stations,

rd is in the range of 0.1 to 0.15 per unit. Thus, the maximum fault current for a short-circuit at the motor

terminals will typically range from 7 to 10 times the motor’s rated armature current. Therefore, it is

conservative to estimate the maximum current that a motor will contribute to a fault as 10 times the

motor’s rated full load current. When a more accurate value is required, the short-circuit contribution

should be calculated, using specific rd data for the specific motor, or actual test data should be obtained

from the motor manufacturer. For additional accuracy, the calculation should account for the resistance

Figure 3. Typical short-circuit characteristic of DC motor/generator

of the cables between the

motor and the fault. A

complete expression for

the short-circuit current

is:

Where: ia per-unit current, e0 is the internal emf prior short-circuit (p.u.), rd steady-state effective

resistance of machine (p.u.), r'd transient effective resistance of machine (p.u.). The frequency is 60 Hz.

Typically, for motors e0=0,97 p.u., and for generators e0=1,03 p.u.

The machine electrical parameter are to be calculated in case when no additional data is known for

observed machine. Normally, it is more practical to use the real machine data given by the manufacturer.

The machine inductance is derived from the following equation:

Where P is the pole number, nn nominal speed, UMnominal voltage and IM nominal current. Cx depends on

the machine type: Cx=0,4 is for motors without pole face windings, Cx=0,1 is for motors with pole face

windings, Cx=0,6 is for generators without pole face windings, and Cx=0,2 is for generators with pole

face windings.

The base resistance of the machine is derived from:

Then the transient resistance in Ohms is derived from:

The peak short-circuit current in Amps:

Or in p.u.:

The initial rate of rise of the current is:

The first 2/3-time constant of rise is:

And the second 1/3-time constant of rise is:

The total time constant is:

The armature circuit decrement factor is:

The field circuit decrement factor is:

Short-Circuit Currents from Chargers

The maximum current that a charger will deliver into a short-circuit, coincident with the maximum battery

short-circuit current, is determined by the charger current-limit circuit. The current-limit setting is

adjustable in most chargers and may vary from manufacturer to manufacturer. Thus, the maximum

current that a charger will deliver on short circuit will not typically exceed 150% of the charger ampere

rating.

The initial sustained short-circuit current (or quasi steady-state current) is given by:

The factor K2 is taken from the diagram of sustained fault current factor versus rectifier terminal voltage,

zC is the commutating impedance per unit and IR is the rated rectifier current. The commutating

impedance includes AC side impedance with transformer (RC and XC).If the commutating impedance is in

per-unit value then it should be converted.

Figure 4. Peak fault current factor as a function of system

constants

Figure 5. Sustained fault current vs rectifier terminal voltage

Conversion of zC (p.u.) to ZC (Ohms):

Case of double-way rectifier, equation

is:

Case of double-wye rectifier:

The current Ida is used to determine

equivalent rectifier resistance and

inductance on the DC side, which are

then given by:

Where Eda is the assumed voltage at the

rectifier terminals during the fault and

equals e0 (p.u.) x System Voltage

(Volts).

If the fault current is calculated using

the superposition method, then the

following relations are used:

When: Then:

When: Then:

The sustained value of the fault current is:

The rectifier terminal voltage is:

The rate of rise fault current is:

The peak current is given as:

Where the factor K1 is taken from the diagram and is in function of K3 and K4, which are calculated as

follows, for the full-wave bridge connected rectifier:

Note: The value Eda = edaED should be within 10% of the calculated value Edc, the rectifier terminal

voltage under sustained short-circuit current. The iterative process is repeated until the desired

tolerance is achieved.

K1 - peak fault current factor

K2 - sustained fault current factor

K3 - reactance constant (used to determine K1)

K4 - resistance constant (used to determine K1)

Index "RBr" refers to the combined resistance of the rectifier and the branch up to the fault location

References

1. IEEE 946-1992: IEEE Recommended Practice for the Design of DC Auxiliary Power Systems for

Generating Stations

2. Industrial power systems data book, General Electric, 1956