Short-circuits Lecture 1 Introduction and Basic Definitions

Transcription

1 Short-circuits Lecture 1 Introduction and Basic Definitions Prof. Désiré Rasolomampionona Prof. dr hab. Jan Machowski

2 Outline of the lecture Introduction Definitions Basics Useful notions

3 INTRODUCTION The normal operating conditions of an electric power system are occasionally disrupted because of faults. Analysis of power systems usually implies the computation of network voltages and currents under a given set of conditions. Under many circumstances we tend to ignore the unbalanced operation in the system and unbalanced operation is always present.

4 Need for power system fault analysis Short-circuit analysis is carried out in electrical power utility systems, industrial power systems, commercial power systems and power station auxiliary systems. Other special applications are in concentrated power system installations on board military and commercial ships and aircraft. Short-circuit calculations are generally performed for a number of reasons. These are briefly described in the next sections.

5 Health and safety considerations Short-circuit fault analysis is carried out to ensure the safety of workers as well as the general public. Power system equipment such as circuit-breakers can fail catastrophically if they are subjected to fault duties that exceed their rating. Other equipment such as busbars, transformers and cables can fail thermally or mechanically if subjected to fault currents in excess of ratings.

6 Health and safety considerations In addition, to ensure safety, short-circuit fault analysis is carried out and used in the calculation of rise of earth potential at substations and overhead line towers. Other areas where fault analysis is carried out are for the calculation of induced voltages on adjacent communication circuits, pipelines, fences and other metallic objects.

7 Short-circuit basics Electric power systems in industrial plants and commercial and institutional buildings are designed to serve loads in a safe and reliable manner. One of the major considerations in the design of a power system is adequate control of short-circuits or faults as they are commonly called. Short-circuits are caused by faults in the insulation of a circuit, and in many cases an arc ensues at the point of the fault. Prolonged duration of arcs, in addition to the heat released, may result in transient overvoltages that may endanger the insulation of equipment in other parts of the system

8 Short-circuit basics Clearly, the fault must be quickly removed from the power system, and this is the job of the circuit protective devices the circuit breakers and fusible switches. A short-circuit current generates heat that is proportional to the square of the current magnitude, I 2 R. The heat that is generated by high short-circuit currents tends not only to impair insulating materials to the point of permanent destruction, but also exerts harmful effects upon the contact members in interrupting devices. Other important effects of short-circuit currents are the strong electromagnetic forces of attraction and repulsion to which the conductors are subjected when short-circuit currents are present.

9 Short-circuit basics The strong electromagnetic forces that high short-circuit currents exert upon equipment can cause deformation in rotational machines, transformer windings, and equipment bus bars, which may fail at a future time. Deformation in breakers and switches will cause alignment and interruption difficulties. Modern interconnected systems involve the operation in parallel of large numbers of synchronous machines, and the stability of such an interconnected system may be greatly impaired if a short-circuit in any part of the system is allowed to prevail.

10 Design, operation and protection of power systems Short-circuit current calculations are made at the system design stage to determine the short-circuit ratings of new switchgear and substation infrastructure equipment to be procured and installed. System reinforcements may be triggered by network expansion and/or the connection of new generating plant to the power system. Routine calculations are also made to check the continued adequacy of existing equipment as system operating configurations are modified.

11 Design, operation and protection of power systems calculations of minimum short-circuit currents are made and these are used in the calculation of protection relay settings to ensure accurate and coordinated relay operations. In transmission systems, short-circuit currents must be quickly cleared to avoid loss of synchronism of generation plant and major power system blackouts. Maximum short-circuit current calculations are carried out for the design of substation earth electrode systems. Short-circuit analysis is also carried out as part of initial power quality assessments for the connection of disturbing loads to electrical power networks.

12 Design, operation and protection of power systems part of initial power quality assessments for the connection of disturbing loads to electrical power networks. These assessments include voltage flicker, harmonic analysis and voltage unbalance. Other areas where short-circuit analysis is carried out is in the modification of an existing system or at the design stage of new electrical power installations such as a new offshore oil platform, new petrochemical process plant or the auxiliary electrical power system of a new power station. The aim is to determine the short-circuit rating of new switchgear and other substation infrastructure equipment that will be procured and installed.

13 Nature of fault Characteristics of power system faults A fault on a power system is an abnormal condition that involves an electrical failure of power system equipment operating at one of the primary voltages within the system. Generally, two types of failure can occur. The first is an insulation failure that results in a short-circuit fault and can occur as a result of overstressing and degradation of the insulation over time or due to a sudden overvoltage condition. The second is a failure that results in a cessation of current flow or an open-circuit fault.

14 Normal types of short-circuits are: Fault due to lightning Tree limbs falling on the line Wind damage Insulation deterioration Vandalism Characteristics of power system faults

15 Some definitions Definitions short-circuit - According to IEC , a short-circuit is the accidental or intentional conductive connection through a relatively low resistance or impedance between two or more points of a circuit which are normally at different potentials. short-circuit current: According to IEC , a short-circuit current results from a short-circuit in an electrical network. It is necessary to differentiate here between the short-circuit current at the position of the short-circuit and the transferred shortcircuit currents in the network branches

16 Some definitions Definitions Initial symmetrical short-circuit current: This is the effective value of the symmetrical short-circuit current at the moment at which the short-circuit arises, when the short-circuit impedance has its value from the time zero. Initial symmetrical short-circuit apparent power: The shortcircuit power represents a fictitious parameter. During the planning of networks, the short-circuit power is a suitable characteristic number

17 Some definitions Definitions Peak short-circuit current: The largest possible momentary value of the short-circuit occurring Steady state short-circuit current: Effective value of the initial symmetrical short-circuit current remaining after the decay of all transient phenomena. DC aperiodic component: Average value of the upper and lower envelope curve of the short-circuit current, which slowly decays to zero

18 Definitions Some definitions Symmetrical breaking current: Effective value of the short-circuit current which flows through the contact switch at the time of the first contact separation Equivalent voltage source: The voltage at the position of the short-circuit, which is transferred to the positive-sequence system as the only effective voltage and is used for the calculation of the short-circuit currents. Superposition method: The superposition method considers the previous load of the network before the occurrence of the shortcircuit. It is necessary to know the load flow and the setting of the transformer step switch.

19 Definitions Some definitions Voltage factor: Ratio between the equivalent voltage source and the network voltage U N, divided by 3 Equivalent electrical circuit: Model for the description of the network by an equivalent circuit Far-from-generator short-circuit: The value of the symmetrical AC periodic component remains essentially constant.

20 Some definitions Definitions Near-to-generator short-circuit: The value of the symmetrical AC periodic component does not remain con- stant. The synchronous machine first delivers an initial symmetrical short-circuit current which is larger than twice the rated current of the synchronous machine. Positive-sequence short-circuit impedance: The impedance of the positive-sequence system as seen from the position of the short-circuit Negative-sequence short-circuit impedance: The impedance of the negative-sequence system as seen from the position of the short-circuit.

21 Some definitions Definitions Zero-sequence short-circuit impedance - The impedance of the zero-sequence system as seen from the position of the shortcircuit. Three times the value of the neutral point to ground impedance occurs here. short-circuit impedance: Impedance required for calculation of the short-circuit currents at the posi- tion of the short-circuit Negative-sequence short-circuit impedance: The impedance of the negative-sequence system as seen from the position of the short-circuit.

22 Dictionary of IEEE Standards Terms Useful notions 30 cycle time: The time interval between the time when the actuating quantity of the release circuit reaches the operating value, and the approximate time when the primary arcing contacts have parted. The time period considers the ac decaying component of a fault current to be negligible. arcing time: The interval of time between the instant of the first initiation of the arc and the instant of final arc extinction in all poles.

23 Dictionary of IEEE Standards Terms Useful notions armature: The main current carrying winding of a machine, usually the stator. armature resistance: R a The direct current armature resistance. This is determined from a dc resistance measurement. The approximate effective ac resistance is 1.2R a. asymmetrical current: The combination of the symmetrical component and the direct current component of the current.

24 Dictionary of IEEE Standards Terms Useful notions available current: The current that would flow if each pole of the breaking device under consideration were replaced by a link of negligible impedance without any change of the circuit or the supply. breaking current: The current in a pole of a switching device at the instant of the arc initiation. Better known as interrupting current.

25 Dictionary of IEEE Standards Terms Useful notions clearing time: The total time between the beginning of specified overcurrent and the final interruption of the circuit at rated voltage. In regard to fuses, it is the sum of the minimum melting time of a fuse plus tolerance and the arcing time. In regard to breakers under 1000 V, it is the sum of the sensor time, plus opening time and the arcing time. For breakers rated above 1000 V, it is the sum of the minimum relay time (usually 1/2 cycle), plus contact parting time and the arcing time. Sometimes referred to as total clearing time or interrupting time.

26 Dictionary of IEEE Standards Terms Useful notions close and latch: The capability of a switching device to close (allow current flow) and immediately thereafter latch (remain closed) and conduct a specified current through the device under specified conditions. close and latch duty: The maximum rms value of calculated shortcircuit current for medium- and high-voltage circuit breakers during the first cycle with any applicable multipliers for fault current X/R ratio.

27 Dictionary of IEEE Standards Terms Useful notions Often the close and latching duty calculation is simplified by applying a 1.6 factor to the calculated breaker first cycle symmetrical ac rms short-circuit current. Also called first cycle duty (formerly, momentary duty). close and latch rating: The maximum current capability of a medium or high-voltage circuit breaker to close and immediately thereafter latching closed for normal-frequency making current.

28 Dictionary of IEEE Standards Terms Useful notions direct axis: The machine axis that represents a plane of symmetry in line with the no-load field winding. quadrature axis: The machine axis that represents a plane of symmetry in the field that produces no magnetization. This axis is 90 degrees ahead of the direct axis.

29 Bibliography IEEE Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems, 2008