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Short Circuit Calculations using SIMARIS design software

M Prakash ­ Senior Manager, Business Development - LV Controls & Distribution, Siemens Ltd

Introduction Today, business processes ­ be it manufacturing or services ­ require reliable power supply. Proper designing of the electrical power system is of paramount importance to achieve a reliable power supply. While designing an electrical power system has never been easy, it has never been as complex as it is today. Multiple sources like transformers, generators and various options like parallel operation of a number of generators and automatic load sharing has increased the complexity of the power system design. For example, calculating the short circuit currents at various levels in the distribution feeder is important for the switchgear and conductor (cable/busbar) selection. The short circuit currents varies greatly every time a different number of generators operate in parallel. Calculating these manually for such varying combinations can be very time consuming besides the errors that can creep in. Further, it is difficult to do these calculations every time there is a change of load or source, which is very common during the design phase. In such cases, SIMARIS design software from Siemens can be of great help. All the calculations can be done with ease and can be recalculated effortlessly whenever there is a change in the project. Short Circuit Currents As mentioned earlier, it is important to calculate the short circuit currents for the selection of switchgear and conductors. However, it is not enough if we calculate only the maximum short circuit currents at various levels as is the practice conventionally. It is also important to calculate the minimum short circuit current at various levels. Before we discuss the importance of these, let us look at some of the important definitions. Definitions Some important definitions of short circuit currents, which are relevant to our discussions, are listed below. The IEC standard for Calculation of Short Circuit Currents in three-phase AC systems, IEC 60909 Part 0 (2001-07) defines the following terms: Prospective (available) short-circuit current Current that would flow if the short circuit were replaced by an ideal connection of negligible impedance without any change of the supply Symmetrical short-circuit current r.m.s. value of the a.c. symmetrical component of a prospective (available) short-circuit current, the asymmetrical component of current, if any, being neglected Initial symmetrical short-circuit current Ik" r.m.s. value of the a.c. symmetrical component of a prospective (available) short-circuit current, applicable at the instant of short circuit.

Peak short-circuit current ip Maximum possible instantaneous value of the prospective (available) short-circuit current Steady state short-circuit current Ik r.m.s. value of the short circuit current which remains after the decay of the transient phenomena. Typical Short Circuit Current Calculation Consider the following example for which the short circuit current at the fault location has to be calculated.

S"kQ max = 350 MVA S"kQ min = 250 MVA

11 kV / 433 V 630 kVA %Z­5% X/R Ratio ­ 4.84

Cable 1 Length = 125 m 3 x 150 mm2 Armoured Al R / Km = 0.249 X / Km = 0.082

Cable 2 Length = 75 m 3 x 50 mm2 Armoured Al R / Km = 0.770 X / Km = 0.086

M

In order to calculate the short circuit current we must calculate the total impedance till the point of short circuit as below Total impedance till the point of Short Circuit = Source impedance HT cable impedance Transformer impedance Busbar impedances LT cable impedance + + + +

Then we use the following formula to arrive at the short circuit current

Un (Rated Voltage) Short-circuit current Ik" =

3

. Zk

where Zk - the total impedance = Rk + jXk and Rk is the total resistance Xk is the inductive reactance. Maximum and Minimum Short Circuit Currents However, it must be noted that at any given point in the installation, short circuit current can have a maximum and minimum value. The maximum value (r.m.s) of steady-state short circuit current that can occur at any given point in an installation is referred to as the Ikmax and similarly, the minimum value is referred to as the Ikmin. While it is essential to calculate the Ikmax which determines the breaking capacity of the switchgear to be used, calculating the Ikmin is also important. Ikmin is required to determine the setting of the short circuit releases of the circuit breakers. To elaborate, since Ikmin is the smallest short circuit current that can occur at a given point in the installation, the setting of circuit breaker releases installed at that point should be sufficient to sense the fault and trip the circuit. The point can be elucidated with the following example Example: Consider a 2000 amp Air Circuit Breaker (ACB) with the following release being installed at a place which has an Ikmax of 36 kA.

G

In = 2000A Ikmax = 36kA

Circuit Breaker rated current (In) = 2000A Parameter L (Long time setting) Ir S (Short-time delayed short circuit setting) I (Instantaneous short circuit setting)

Range 0.4 to 1 x In 1.5 to 11 x In 12 times In

Set Value 1 (2000A) 11 times (22kA) Fixed (24kA)

In this case if the calculated Ikmin value is 18 kA, then for the above setting of release the circuit breaker would not see this as a short circuit but rather as an overload and trip according to the inverse time-current (i-t) characteristics. This is not desirable as the short circuit current is allowed to flow for a longer duration before the circuit breaker trips. The appropriate setting of `S' release in this case should have been 9 times or lesser. Determinants of short circuit currents So how does short circuit have multiple values (Ikmax, Ikmin) at the same point on a network? The short circuit currents depend on the following factors: · · · · · Type of source Type of fault The voltage levels and The impedance values Synchronous and asynchronous machines like Induction Motors in the circuit

Let us discuss them one by one. Type of source The type of source is one of the determinants of the magnitude of short circuit. If a feeder is fed from two different sources e.g. a transformer and a generator, the fault level can have varying magnitude. For a given kVA rating, transformers generally feed more short circuit currents owing to their low impedance than synchronous generators which have higher Xd" (Sub-transient reactance). In the earlier example the Ikmax of 36kA could be when the feeder is being fed by the transformer and the Ikmin could be when the feeder is being fed from generator, as shown below.

G

G

In = 2000A Ik = 36kA Short-circuit currents when transformer is feeding

In = 2000A Ik = 18kA

Short-circuit current when Generator is feeding

Similarly the magnitude of short circuit currents will also increase if more sources are operated in parallel as the effective impedance of the sources connected in parallel reduces. A designer has to calculate the Ikmax and Ikmin values for all such combinations which can be cumbersome. SIMARIS design software from Siemens provides options for defining various operating modes as shown below and can calculate these values considering all the operating conditions. Screenshot of a sample operating modes defined in SIMARIS design software

Type of fault The next factor influencing the magnitude of short-circuit currents is the type of fault. The short-circuit condition can be of various types. IEC 60909 part 0 (2001-07) describes the four major type of faults as illustrated below.

Three-Phase Short Circuit L1 L2 L3 I" k3 L1 L2 L3 I" k2 Phase-to-Phase Ungrounded Fault

Phase-to-Phase Grounded Fault L1 L2 L3 I" k2E2 L1 L2 L3

Phase-to-Ground Fault

I" k1

Three-phase short circuit current is a symmetrical fault and is the easiest of the fault currents to calculate. Though this constitutes only a small portion of all the faults that happen practically, calculating this fault is important as in most of the cases this causes the maximum short-circuit currents (Ikmax). When short-circuit current calculations are being done manually, often only a three-phase short circuit current is calculated owing to its simplicity. However, in order to determine the Ikmin, it is essential to calculate all possible types of short-circuit including other 3 types of short circuits depicted above. These short-circuits are asymmetry in nature and require calculations involving Zero-sequence and Positivesequence components. Hence when a designer is designing a system, these shortcircuit currents are generally ignored. SIMARIS design software automatically calculates all these short-circuit currents and arrives at the Ikmax and Ikmin . A screen shot from SIMARIS design software showing all the calculates short circuit currents for a distribution feeder is shown below

Screenshot from SIMARIS design showing various short-circuit currents

Voltage Levels Voltage is another factor that influence the short-circuit current as we all know from the formula which was mentioned earlier. Un (Rated Voltage) Short-circuit current Ik" =

3

. Zk

Where Zk (Impedance till the point of short-circuit) = Rk + jXk IEC 60909-0 (2001-2007) allows for the tolerance in voltage while calculating Maximum short-Circuit current Ikmax and Minimum short-circuit current Ikmin . This is discussed later in this paper.

Impedance Values The other major determinant of the short-circuit currents is of course the impedance till the point of the short-circuit as mentioned in the above equation. As seen from the above equation Zk consists of two parts, the resistance part (Rk) and the inductive reactance (Xk) part. Since the resistance part can vary with the temperature IEC 60909-0 (2001-07) describes the conditions for calculating Ikmax and Ikmin which is described subsequently. Synchronous and asynchronous machines like Induction Motors in the circuit IEC 60909-0 (2001-07) describes that when calculating the initial symmetrical shortcircuit current Ik", the peak short-circuit current ip, and the steady-state short-circuit current Ik. Synchronous compensators are treated in the same way as synchronous generators. If synchronous motors have a voltage regulation, they are treated like synchronous generators. If not, they are subject to additional considerations. Similarly, medium-voltage and low-voltage asynchronous motors also contribute to the initial symmetrical short-circuit current Ik", to the peak short-circuit current ip, and, for unbalanced short circuits, also to the steady-state short-circuit current Ik. Further, it states that the contribution of asynchronous motors in low-voltage power supply systems to the short-circuit current Ik" may be neglected if their contribution is not higher than 5 % of the initial short-circuit current Ik" calculated without motors. Again, all the above conditions are automatically taken care of when the designer uses SIMARIS design software. Conditions for calculating Ikmax and Ikmin according to IEC60909 - 0 (2001-07) Having understood the importance of calculating the Maximum and Minimum values of short-circuit currents let us look at the conditions for calculating these values as described in IEC60909 - 0 (2001-07) Maximum Short Circuit Current Ikmax · Introduce voltage factor cmax, as per table 1 of IEC 60909-0 2001-07 (also furnished below for reference), if there are no national standards to be applied ­ This is done to take care of the positive voltage tolerance while calculating the Ikmax Resistance RL of lines (overhead lines and cables) are to be introduced at a temperature of 20 °C ­ At lower temperature the resistance would be lower and the short-circuit currents would be higher Motors, if present in the circuit, are to be considered for calculating Ikmax Consider the maximum short circuit power (in MVA) of the system infeed (Source Fault Capacity) or If substitute impedances ZQ for simulating system infeed are used, select the lowest short- circuit impedance

·

· · ·

Minimum Short Circuit Current Ikmin · Introduce voltage factor cmin, as per table 1 of IEC 60909-0 2001-07 (also furnished below for reference). This is done to take care of the negative voltage tolerance while calculating the Ikmin Resistance RL of lines (overhead lines and cables) are to be introduced at a higher temperature ­ At higher temperature the resistance would be higher and the short-circuit currents would be lower R L =[1+ (e - 20 °C)] ·R L20 where RL20 is the resistance at a temperature of 20 °C e is the conductor temperature in degrees Celsius at the end of the short-circuit period; is the temperature co-efficient of resistance of the conductor Motors, if present in the circuit, are to be neglected (considered switched off) Consider the minimum short circuit power (in MVA) of the system infeed (Source Fault Capacity) or If substitute impedances ZQ for simulating system infeed are used, select the highest short- circuit impedance

Voltage Factor c for calculating maximum short circuit minimum shor circuit currents cmax currents cmin 1.05(1) or 1.10(2) 1.1 0.95 1

·

· · ·

Nominal Voltage Un

Low Voltage 100V upto 1000V (IEC 60038, Table I) Medium Voltage >1 kV upto 35 kV (IEC 60038, Table III) High Voltage >35 kV (IEC 60038, Table IV)

1) For low voltage network with a tolerance of +6 % 2) For low voltage networks with +10 %

Conclusion: In practice, it is difficult to observe all these conditions, particularly for a large project involving various sources of supply and multiple levels of distribution. SIMARIS design considers all the above conditions specified in the IEC 60909 ­ 0, including the contribution of induction motors to the short-circuit currents in the calculation, thus making it reliable and accurate. Apart from the short circuit calculations described above, SIMARIS design provides the following functionality which is vital for power system design · · · · Load flow analysis including currents and power factor at various feeders in the distribution Energy balance report highlighting the apparent, active and reactive power enabling the designer to optimize the power requirement Voltage drop calculations at various points in the network Selectivity graphs for optimum setting of circuit breaker releases

· · ·

Suggestions for protection against personal safety considering the type of connection to earth (e.g. TN-S, TT, IT etc) Selection of Switchgear, Cables sizes etc. Output in the form of pdf and dxf files (autocad compatible)

SIMARIS design with the above features, along with its `easy-to-use' user interface makes power system design Easy, Safe and Fast.

References: · International Electrotechncial Commission (IEC) Standard, IEC 60909-0, First edition 2001-07, Short-circuit currents in three-phase system ­ Part 0: Calculation of currents Electrical Installations Handbook - Third Edition by Gunter G Seip Switching, Protection & Distribution in Low-Voltage Networks ­ 2 Published by Siemens, AG

nd

· ·

revised edition 1994,

The author can be contacted at [email protected]

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