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TRANSIENT ANALYSIS STUDIES

Powertech Engineering Services

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Transient Analysis Studies

Summary

Electrical transient power system performance studies are completed for clients with Permanent or

Temporary Power Supplies to assess if running under various operational scenarios meets the

system Design Criteria and National/State Technical Rules Criteria. It is important to assess at least

the following electrical system operations for transient responses:

• Largest Motor Start Event

• Largest Switching Event

• Largest Load Rejection Event

• Variable Speed Drive Performance due to Switching Events

Transient surge voltage effects internal to the plant from lightning, switching and internal reflections

are reviewed separately in an Insulation Coordination Assessment.

Transient studies can be completed using either: ETAP, Digsilent Power Factory or SKM Power

Tools. There are two approaches to transient response testing using either:

• Full Network Modelling

• Stand-Alone Modelling

Full network modelling links the facility model with the network providers model which provides full

network response dynamic transient analysis because all cable and equipment responses are

included in the analysis. This approach requires strict model control in conjunction with the upstream

network provider so any changes must be tracked. This can add minor accuracy benefits over the

stand-alone method but becomes time consuming to manage and costly to implement.

Alternatively, the model can be confined within the facility electrical distribution system to the terminal

connection points at the Main Switchboard. The results of the transient analysis in this case will be

determined by the influence of the upstream minimum and maximum short circuit currents which

provide the input system impedances with respect to the upstream X/R ratio. This is all that is really

required to represent an upstream substation. These parameters are provided by the power supply

provider whether it be a local generation system or a network provider. This technique reduces

reliance on minor changes to the upstream model. It only relies on the upstream currents and X/R

ratio to provide a reasonably accurate transient response.

Maximum transient responses occur on a low impedance system with respect to a higher impedance

source. The mode of operation providing low system impedance in proportion to source impedance

is at maximum system load. Therefore, this is generally the only analysis mode required and reduces

costs by saving on unnecessary operational scenarios.

The transient analysis study demonstrates that the step voltage change on the buses remain within

acceptable limits. The studies also confirm that the system will operate in compliance with the

clauses as stated in Technical Rules for: step changes in voltage levels resulting from switching

operations, voltage stability and fault types to be studied.

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Table of Contents

1.0 Transient Study Methodology ..................................................................................................... 4

1.1 Overview ...........................................................................................................................................4

1.2 Operation Mode ...............................................................................................................................4

1.3 Network or Generation Configuration ..............................................................................................4

1.4 Unbalanced Load Flow Analysis ........................................................................................................4

1.5 Short Circuit and Fault Studies ..........................................................................................................4

1.6 Transient Stability Simulation and Analysis ......................................................................................4

1.7 Largest Motor Start Event ................................................................................................................5

1.8 Largest Transformer Energisation Event...........................................................................................5

1.9 Largest Load Rejection Event ............................................................................................................5

1.10 Assessment of Infrequent Switching Operations on Existing Plant Variable Speed Drive

Performance. ................................................................................................................................................7

1.10.1 Assessment of VSD drive on Starting of DOL Drive. ..................................................................7

1.10.2 Assessment of VSD drive on Transformer Energisation Switching Event ..................................7

1.10.3 Assessment of VSD drive on Load Rejection Event. ..................................................................7

1.10.4 Performance of VSD Drive ........................................................................................................7

2.0 Results Expected from A Typical Transient Study ......................................................................... 8

2.1 Performance of Transient Event Results ...........................................................................................8

2.1.1 Motor Start Event .........................................................................................................................8

2.1.2 Transformer Energisation Switching Event. ................................................................................10

2.1.3 Largest Load Rejection Event ......................................................................................................11

2.1.4 Assessment of Infrequent Switching Operations on Variable Speed Drives. ..............................12

3.0 Final Notes ............................................................................................................................... 15

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1.0 Transient Study Methodology

Power System Study software is used for the simulation of transient events. Using Electromagnetic

Transient Algorithms applied to the power system model dynamically, the transient responses of the

electrical power system can be analysed. The dynamic network model is used for electromagnetic

and electromechanical transients under balanced and unbalanced network conditions.

1.1 Overview

The following steps are followed when performing electromagnetic transient simulations:

• Selection of operation mode

• Selection of network configuration

• Simulation of unbalanced load flow and 3-phase maximum fault

• Simulation of initial conditions and transient event.

1.2 Operation Mode

The operating mode will produce transient response to switching operations of selected loading

scenarios.

1.3 Network or Generation Configuration

The transient simulation should be carried out for network or generation configurations in terms of

number of units and whether bus ties are opened or closed.

1.4 Unbalanced Load Flow Analysis

The electromagnetic transient evaluation of switching events uses plant running load. Transient

analysis requires an unbalanced load flow to provide a detailed representation of component

unsymmetrical characteristics.

1.5 Short Circuit and Fault Studies

Short circuit analysis is run under the chosen operating mode and network/generation configuration

to configure initial set-points for the transient analysis.

1.6 Transient Stability Simulation and Analysis

Transient stability simulation is completed to ensure the Design and Technical Rules acceptance

criteria requirements are met for the following events as a minimum:


• Largest Transformer

Energization Event

• Short Circuit Event


• VSD Performance

• Transient Surge Voltage Effects

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1.7 Largest Motor Start Event

The transient analysis of the largest motor start event is completed to determine the following

acceptance criteria are met:

• Power frequency voltage variation of the plant is in accordance with Electrical Design

Criteria.

• Power frequency voltage variation of plant at the point of connection meets Technical

Rules.

Acceptance criteria for power frequency voltage variation as per the Electrical Design Criteria and

the Technical Rules at the point of supply may show for example:

• Step changes in voltage levels resulting from switching operations shall not exceed the

limits given.

• The minimum steady state voltage on the transmission network will be 90% of nominal

voltage and the maximum steady state voltage will be 110 % of nominal voltage.

In addition, the Electrical Design Criteria may state that for the point of supply to the facilities require:

• Step change voltage limits during routine switching (e.g. capacitor switching,

transformer tap changing, motor starting) shall be +/-5%, and for infrequent switching

events (e.g. tripping of generators, loads and other faults) shall be +6% to -10%.

• To meet the acceptance criteria for maximum voltage drop during a motor starting

event as per the Technical Rules, requires that for a routine switching event (i.e. motor

starting) at less than the supply voltage, the step change voltage limits shall be within

+/- 5% (max) of the nominal voltage at the Point of Connection.

1.8 Largest Transformer Energisation Event

Energising an unloaded transformer via a long cable may cause excessive over-voltages on the

secondary side due to a resonant overvoltage phenomenon and therefore may cause under-voltage

at the transformer primary.

A transient analysis for transformer energisation event may be completed to determine the following

acceptance criteria are met:


Criteria.


Rules.

1.9 Largest Load Rejection Event

The transient analysis for load rejection is completed by selecting an appropriate and large plant

load and switching it off during normal operation. To determine the response, the following

acceptance criteria shall be met:


Criteria.

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Rules.

A suitable acceptance criteria for temporary over voltage as per the Electrical Design Criteria and

Technical Rules would be:

• Temporary AC over-voltages should not exceed the time duration limits given in Figure

1.1.

• In addition, the Electrical Design Criteria may state that for the point of supply to the

facilities: “For infrequent switching events (e.g. tripping of generators, loads and other

faults) shall be +6% to -10%”.

For example, to meet the acceptance criteria for maximum temporary over voltage during tripping of

loads as per the Technical Rules may require that for a load rejection event (i.e. tripping of load) at

less than certain voltages, the highest acceptable temporary over voltage for duration of 0.2 seconds

shall be no more than 1.5 p.u. at the Point of Connection. The studies would be performed on the

basis that the load is switched and not tripped from an electrical fault.

Figure 1-1 Temporary Overvoltage Limit

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1.10 Assessment of Infrequent Switching Operations on Existing Plant Variable

Speed Drive Performance.

The transient response is completed to determine the performance of the variable speed drive.

1.10.1 Assessment of VSD drive on Starting of DOL Drive.

A simulation is completed to demonstrate the performance of VSD drives during the switching

operation of the largest motor start.

1.10.2 Assessment of VSD drive on Transformer Energisation Switching Event

A simulation is completed to demonstrate the performance of a VSD drive during the switching

operation of a large transformer which is unloaded.

1.10.3 Assessment of VSD drive on Load Rejection Event.

A simulation is completed to demonstrate the performance of VSD drives during a large load

rejection by tripping major bus systems.

1.10.4 Performance of VSD Drive

The bus voltages at the terminal of the input rectifier and inverter output are monitored to evaluate

a change above the acceptable limits during a switching event.

VSD overvoltage at the inverter output can occur when a transient is applied to the input and the

motor cable exceeds a certain length. The longer the motor cable is, the higher the overvoltage that

occurs. This negative effect is amplified when using a shielded cable. However, the overvoltage is

limited to twice the DC bus voltage. A transient response is completed to determine the performance

of the variable speed drive.

A transient analysis is required to assess the largest routine and infrequent switching event which

transferred to the VSD input and therefore determine the performance response at the output of the

VSD. Generally, there are no specific requirements identified in Technical Rules or Design Criteria

(with respect to this test).

The acceptance criteria are therefore based on evaluation of bus voltages at the terminal of the

rectifier input and invertor output. It is therefore determined whether a step change on bus voltages

are above the acceptable limits during a switching event.

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2.0 Results Expected from A Typical Transient Study

2.1 Performance of Transient Event Results

This section will describe typical results of a large facility with good outcomes with respect to

transient response. The following events are demonstrated:


• Largest Transformer

Energization Event


• VSD Performance

• Transient Surge Voltage Effects

• Short Circuit Event

2.1.1 Motor Start Event

2.1.1.1 Dual Feeder Network Configuration

The outgoing feeder for the motor was simulated to switch on at 0.1s in a Dual Feeder Network

configuration. The instantaneous current p.u of the Motor is shown in below.

Figure 2-1 Motor Phase Current

Figure 2-1 shows the motor switch event at occurring at 0.1 sec. The switching event crosses at the

negative cycle of the sine wave and therefore a negative DC offset results. This creates a peak

current of 9.6 p.u. with a DC offset value of 4.4 p.u. The start current stabilises to a peak value of

5.1 p.u. and after 1.6 seconds the peak value is 1.0 p.u. where the motor is running at full speed.

This switching event was simulated to show the largest DC offset and worst case current transient.

This is important to ensure we capture the effect on the voltage response due to this current spike.

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Figure 2-2 Motor Bus Terminal Voltage

Figure 2-2 represent the motor bus terminal voltage. The motor starts at 0.1 seconds where the

motor bus voltage is 0.765 p.u. for the transient switching period which runs for several cycles. Most

of the bus voltage during starting is 0.97 p.u. and returns to 1.07 p.u. once the motor is running at

full speed.

It can be seen on Figure 6-2 that the voltage response is stable due to the current spike. The voltage

response does dip below 0.85 p.u. for several cycles but recovers within 200ms. Motor stalling is not

established during this start sequence due to this narrow voltage dip.

Figure 2-3 33kV Switchboard Voltage

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To meet the requirements of the Technical Rules, the voltage on the main switchboard during the

switching event was evaluated. As shown in Figure 2-3, the transient voltage response at the 33kV

main switchboard caused a voltage drop of 0.932 p.u. which then recovered to the normal

operational value of 0.948 p.u. which is well within the Technical Rule criteria.

2.1.1.2 Single Feeder Network Configuration

The outgoing feeder for a large motor was simulated to switch on at 0.1s in the Single Feeder

Network configuration. The results showed no discernible difference to the Dual Feeder

arrangement.

2.1.2 Transformer Energisation Switching Event.

This test examined the largest transformer with the longest cable in an unloaded condition.

The Transformer energisation transient analysis was conducted to identify what effect the expected

peak current value and decay time that would result for the remainder of the network.

It is important to understand that a large transformer is always switched on in an unloaded condition.

All motor and VSD loads are not connected in this initial state. When a transformer is switched on,

the individual loads will be started one by one as per the process start-up sequence.

The transformer energisation transient response is demonstrated for Dual and Single Network

Feeder configurations.

2.1.2.1 Dual Feeder Network Configuration - Transformer Energisation

Transformer energisation transient response for the Dual Feeder Network configuration was

reviewed by closing the transformer primary circuit breaker. Figure 2-4 below shows the energisation

switching event of a large transformer.

As shown in Figure 2-4, the transformer is energised at 0.02 seconds. This causes a sharp and

narrow inrush current of 0.001 p.u. at the point of energisation. The magnitude is small as the

transformer is unloaded and therefore magnetisation is minimal. DC offset is not noticeable because

the transformer secondary is open circuit.

The transformer is then de-energised at 0.04 seconds and at the transformer primary terminals,

demagnetisation voltage occurs for a period of 0.01 seconds. During de-energisation at 0.04

seconds, no current spikes occur as cable capacitance and circuit breaker quenching absorb any

potential current transients.


switching event is evaluated. As shown in Figure 2-4, the transient voltage response at the 33kV

main switchboard caused a voltage drop of 1.0 p.u. which then remained at the normal operational

value of 1.0 p.u. which is well within the Technical Rules.

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Figure 2-4 Dual Feeder Transformer Energisation

The step change during the energisation of the transformer event is within the acceptable limit of ±

10% during switching events as may be defined in the Technical Rules.

2.1.2.2 Single Feeder Network Configuration - Transformer Energisation

Transformer energisation transient response for the Single Feeder Network configuration was

reviewed by closing the transformer primary circuit breaker. The results showed no discernible

difference to the Dual Feeder arrangement.

The step change during the energisation of the transformer event is within the acceptable limit of ±

10% during switching events as may be defined in the Technical Rules.

2.1.3 Largest Load Rejection Event

This test will examine the largest load rejection event as a result of tripping a major feeder supply.

Load rejection is achieved by tripping of a major feeder supply on the main switchboard. The

maximum load demand as per the load flow study for this feeder needs to be significant. The load

rejection transient response is demonstrated in this section for Dual and Single Network Feeder

configurations.

2.1.3.1 Dual Feeder Network Configuration – Load Rejection

A sample load rejection transient response for a Dual Feeder Network configuration was reviewed

by tripping of a large feeder supply.

As shown in Figure 2-5, the Ship Loader is tripped at 0.02 seconds. DC offset is not noticeable

because an open circuit condition is initiated. At the feeder, a demagnetisation voltage occurs for a

period of 0.002 seconds. During de-energisation at 0.04 seconds, no current spikes occur as cable

capacitance and circuit breaker quenching absorb any potential current transients. Modulation found

in the current waveform prior to tripping is caused by VSD rectifier pulses.

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switching event is evaluated. As shown in Figure 2-5, the transient voltage response at the main

switchboard caused a voltage drop of 1.0 p.u. which then remained at the normal operational value

of 1.0 p.u. which is well within the Technical Rules criteria.

The voltage waveform for all phases on the buses at the point of connection remain within acceptable

limits during the switching event as defined in the voltage stability requirement in for Temporary Over

Voltages of Technical Rules. The results demonstrate that during a switching event, switching OFF

(tripping) of largest load on the system, the voltages at the system remain stable and are within

acceptable limits.

Figure 2-5 Dual Feeder Load Rejection

2.1.3.2 Single Feeder Network Configuration – Load Rejection

Load rejection transient response for the Single Feeder Network configuration was reviewed by

tripping the Ship Loader circuit breaker. The results show no discernible difference to the Dual

Feeder arrangement. The results demonstrate that during a switching event, switching OFF

(tripping) of largest load on the system, the voltages at the system remain stable and are within

acceptable limits.

2.1.4 Assessment of Infrequent Switching Operations on Variable Speed Drives.

The transient performance of a VSD drive was analysed against the routine tests described in the

previous sections. A drive was chosen that represented a large drive with a long output cable length

providing worst case dv/dt conditions and therefore transient effects on the input of the drive.

2.1.4.1 Assessment of VSD drive on Starting of Large DOL Drive.

The simulation was completed to demonstrate the performance of the VSD drive during the switching

operation of the largest motor start. The circuit breaker of the DOL motor was switched on.

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2.1.4.2 Assessment of VSD drive on Transformer Energisation Switching Event

The simulation was completed to demonstrate the performance of the VSD drive during the switching

operation of the transformer. The breaker of transformer was switched on.

2.1.4.3 Assessment of VSD drive on Load Rejection Event.

The simulation was completed to demonstrate the performance of the VSD drive during a large load

rejection of by tripping of a large system load.

2.1.4.4 Performance of VSD Drive

The bus voltages at the input terminals of the rectifier are shown in Figure 2-6 and the output

terminals of the inverter are shown in Figure 2-7. were monitored to evaluate any change above the

acceptable limits during the switching event.

Figure 2-6 and Figure 2-7 are shown as a single response representing all the tests and not redrawn

6 times as no discernible difference in response occurred for the following tests:

• single network arrangement – starting of DOL

• dual network arrangement – starting of DOL

• single network arrangement – transformer energisation

• dual network arrangement – transformer energisation

• single network arrangement – load rejection

• dual network arrangement – load rejection

Figure 2-6 VSD Input Rectifier Waveform

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Figure 2-7 VSD Inverter Output Waveform

The waveform includes the harmonic contribution with phase voltages and current remain stable

during the switching event. The details of Total Harmonic Distortion on each bus is provided in a

harmonic study. In this case the performance of the large VSD drive during switching events were

found to be satisfactory.

2.1.5 Short Circuit Event

This test examines the short circuit response as a result of fault occurring within the power system.

The following types of short circuit response are tested to determine the stability of the power system:

• A three -phase to earth fault cleared by protection system;

• A single phase to earth fault cleared by protection system;

• A single phase to earth fault cleared with auto reclosure onto a persistent fault.

2.1.5.1 Dual Feeder Network Configuration – Short Circuit Event

A sample short circuit transient response for a Dual Feeder Network configuration was analysed by

simulating a bolted three phase fault at the motor terminals of a 6.6kV 450kW D.O.L motor

As shown in Figure 2-8, a short circuit event is introduced at the motor terminal at 0.1 sec. The

feeder protection relay trips the breaker on an instantaneous set value which is shown in Figure 2-

8 by opening the breaker and clearing the fault at 0.2s.

The voltage on the 6.6kV Switchboard drops 0. 417p.u. for a duration of 90ms and then upon

clearance of the fault recovers back to 1.0p.u.

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Figure 2-8 Bus Voltage Response for Short Circuit Event

3.0 Final Notes

Transient analysis is essential for any power system to ensure the bus voltages remain within their

design limits and that over or under voltage transients are not leading to equipment degradation.

Equipment degradation can lead to cable or other equipment failures. Uncontrolled transients can

also cause false tripping leading to unplanned outages.

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