Unit - 5

Modelling and Simulation of Protection Schemes

5.1 Current Transformer Modelling

Current transformer (CT) is a bridge between the primary and secondary equipment in power system, utilized for transferring large currents proportionally to currents with small amplitudes for secondary measuring and monitoring equipment and relay protective equipment. In case of a short-circuit incident, as the power system capacity and the voltage level increase, the short-circuit current will reach tens of times or even hundreds of times of CT rated current, leading to CT saturation

In that situation, CT cannot transfer the primary current correctly, which causes  mal-operation of protection equipment and greater errors of fault location equipment. In response to this, manufacturers open a small air gap on the core of CT, which is about one-thousandth of the total length of the magnetic circuit. The air-gapped CT can guarantee that the errors are within the permissible range of the relay protection in the steady state and the transient state.

Currently, there are large number of papers on the closed-core CT modeling. However, there are only a few studies have been accomplished on modeling air-gapped CT.

The air-gapped CT model is established on the PSCAD/EMTDC platform, where the real-time magnetization curve is generated by hysteresis loop compressing method, and the real-time excitation inductance is calculated by using the arc tangent function to fit the limiting hysteresis loop. This model can simulate the saturation characteristics of air-gapped CT, but the simulation of the core is not accurate enough.

Jiles-Atherton (JA) Theory. This model can accurately simulate the effect of air gap on the magnetization process. However, the parameters such as regional coupling coefficient, thermal coefficient, regional flexibility, direction coefficient, which are required for the JA theoretical model, are often hard to accurately obtained and can only be approximated.

Equivalent Circuit of CT

To  begin  with,  equivalent  circuit of  a  CT  is  not  much  different from  that  of  a regular transformer  (fig1).  However,  a fundamental  difference  is  that while  regular  power  transformers are  excited  by  a  voltage  source, a  current  transformer  has  current source excitation. Primary winding  of  the  CT  is  connected  in series  with  the  transmission  line. The  load  on  the  secondary  side  is the  relaying  burden  and  the  lead wire  resistance.

Figure1: Equivalent Circuit of CT

Total  load  in  ohms  that  is introduced  by  CT  in  series  with the  transmission insignificant  and  line is  hence,  the connection  of  the  CT  does  not alter  current  in  the  feeder  or  the power  apparatus  at  all.  Hence from  modeling  perspectives  it  is reasonable  to  assume  that  CT primary  is  connected  to  a  current source.  Therefore, the CT equivalent circuit  will  look  as shown  in  fig  2.  The  remaining steps  in  modeling  are  as  follows: As  impedance  in  series  with  the  current  source  can  be  neglected,  we  can  neglect  the  primary  winding resistance  and leakage  reactance  in  CT  modeling.

For  the  convenience  in  analysis,  we  can  shift  the  magnetizing  impedance  from  the  primary side  to  the secondary  side  of the  ideal  transformer.

Figure 2: Modelling of a CT

After  application  of  the  above steps,  the  CT  equivalent  circuit is  as  shown  in  the  Fig  3. The  secondary  winding resistance and leakage reactance  is  not  neglected  as  it will  affect  the  performance  of CT.  The  total  impedance  on  the secondary  side  is  the  sum  of relay  burden, resistance impedance and of  lead wire leakage  secondary winding.  Therefore, the voltage developed in the secondary winding depends upon these parameters directly. The  secondary  voltage  developed  by  the  CT  has  to  be  monitored  because  as  per  the  transformer  emf equation,  the  flux  level  in  the  core  depends  upon  it.  The transformer emf equation is given by,

E2 = 4.44 fN2 m

Where  is  the  peak  sinusoidal flux  developed  in  the  core.  If  corresponding  to  this  flux  is above  the  knee  point,  it  is  more or  less  obvious  that  the  CT  will saturate.  During saturation, CT secondary winding cannot replicate the primary current accurately and hence, the performance of the CT deteriorates.

Thus,  we  conclude  that  in practice,  while  selecting  a  CT  we should  ascertain  that  it  should not  saturate on  the  sinusoidal currents  that  it  would  be subjected  to.

Use  of  numerical  relays  due  to  their  very  small  burden  vis-a-vis  solid  state  and  electromechanical  relays, improves  the  CT  performance.  CT is to be operated always in closed condition.  If  the  CT  is  open  circuited, all  the  current  Ip/N,  would  flow  through  Xm.  This  will  lead  to  the  development  of  dangerously  high  level  of voltage  in  secondary  winding  which  can  even  burn  out  the  CT.

The  equivalent  circuit  of  a  CT  can be further simplified by  transferring  the  current  source to the secondary side. Thus,  the  equivalent  circuit  of  the  CT  is  as  shown  in  fig.

Figure 3:  Final equivalent ZB circuit of CT

Equivalent circuit of Saturated CT 

One  of  the  major  problems  faced  by  the  protection systems  engineer  is  the  saturation  of  CT  on  large  ac currents  and  dc  offset  current  present  during  the transient.  When  the  CT  is  saturated,  primary  current source  cannot  be  faithfully  reflected  to  the  secondary side.  In  other  words,  we  can  open  circuit  the  current source  in  Fig. 4.  Also, the  magnetizing  impedance  falls down  during  saturation.  Then  the  transformer  behaves more  like  an  air  core  device,  with  negligible  coupling between  the  primary and  secondary  winding.  The  high reluctance  due  to  the  air  path  implies  that  the magnetizing  impedance  (inductance)  falls  down.  The corresponding equivalent circuit is shown in fig. 5.

Figure – 4 CT equivalent circuit during saturation

Classifications of CT 

The  CTs  can  be  classified  into  following  types:

      Measurement  CTs

      Protection  CTs

A  measurement  grade  CT  has  much  lower  VA  capacity  than  a  protection  grade  CT.  A  measurement  CT  has to  be  accurate  over  its  complete  range  e.g.  from  5%  to  125%  of  normal  current.  In  other  words,  its magnetizing  impedance  at  low  current  levels.  (and  hence  low  flux  levels)  should  be  very  high.  Due  to  non-linear  nature  of  B-H  curve,  magnetizing  impedance  is  not  constant  but  varies  over  the  CT's operating  range.  It  is  not  expected  to  give  linear  response  (secondary  current  a  scaled  replica  of  the primary  current)  during  large  fault  currents.

In  contrast,  for  a  protection  grade  CT,  linear  response  is  expected  up  to  20  times  the  rated  current.  Its performance  has  to  be  accurate  in  the  range  of  normal  currents  and  up to  fault  currents.  Specifically,  for protection  grade  CT's  magnetizing  impedance  should  be  maintained  to  a  large  value  in  the  range  of  the currents  of  the  order  of  fault  currents.

When  a  CT  is  used  for  both  the  purposes,  it  has  to  be  of  required  accuracy  class  to  satisfy  both  accuracy conditions  of  measurement  CTs  and  protection  CTs.  In  other  words,  it  has  to  be  accurate  for  both  very small  and  very  large  values  of  current.  Typically,  CT  secondary  rated  current  is  standardized  to  1A  or  5A (more  common).

However,  it  would  be  unreasonable  to  assume  that  the  linear  response  will  be  independent  of  the  net burden  on  the  CT  secondary.  For  simplicity,  we  refer  to  the  net  impedance  on  the  secondary  side (neglecting  magnetizing  impedance)  as  the  CT  burden.  It  is  quite  obvious  that  the  driving  force E2 required  to  drive  the  primary  current  replica  will  increase  as  this  burden  increases.  If  this  voltage  exceeds the  designer's  set  limits,  then  the  CT  core  will  saturate  and  hence  linear  response  will  be  lost.  Hence, when  we  say  that  a  CT  will  give  linear  response  up  to  20  times  the  rated  current,  there  is  also  an  implicit constraint  that  the  CT  burden  will  be  kept  to  a  low  value.  In  general,  name-plate  rating  specifies  a  voltage limit  on  the  secondary  (e.g.,  100  V)  up  to  which  linear  response  is  expected.  If  the  CT  burden  causes  this voltage  to  be  exceeded,  CT  saturation  results.

IEEE Classification

ANSI/IEEE standards classify  CTs  into  two  types:

      Class  T  CT

      Class  C  CT

Class T CT’s

Typically,  a  class  T  CT  is  a  wound  type  CT  with  one or  more  primary turns  wound  on  a  core.  It is associated with high leakage flux in the core. Because  of  this,  the  only  way  to  determine its performance  is  by  test.  In  other  words, standardized  performance  curves  cannot  be  used with  this  types  of  CTs.

Figure 6 shows one such experimentally calibrated curve for a CT. The letter ‘B'  indicates the  burden  in  ohms  to  which  the  CT  is  subjected.  It is  seen  that  when  burden  is  less  than  say  0.1 ohms,  CT  meets  the  linear  performance  criterion. However,  as  the  burden  increases  to  0.5  ohms,  the corresponding  linearity  criteria  is  not  met  till  the end.  At  4  ohms  burden,  there  is  significant deviation  from  the  linear  response.

A  general  rule of  thumb  is  that,  one  should  try  to  keep  the  CT burden  as  low  as  possible.

Fig 5

Ratio Error:  CT  performance  is  usually  gauged  from  the  ratio  error.  The  ratio  error  is  the  percentage deviation  in  the  current  magnitude  in  the  secondary  from  the  desired  value.  In  other  words,  if  the current  measured  in  the  secondary  is  Is,  true  or  actual  value  is  Ip/N,  where  N  is  nominal  ratio  (e.g.  N for  a  100:5  CT  is  20)  and  Ip  is  the  primary current  then  ratio  error  is  given  by

When the CT is not saturated ratio error

Is a  consequence  of  magnetizing  current  IE 

Since

Therefore, % ratio error is equal to .

When  the  CT  is  saturated,  coupling  between  primary  and  secondary  is  reduced.  Hence, large ratio errors are expected in saturation.  The current in the secondary is also phase shifted.  For  measurement  grade CTs,  there  are  strict  performance  requirements  on  phase  angle  errors  also.  Error  in  phase  angle measurement  affects  power  factor  calculation and  ultimately  real  and  reactive  power  measurements.  It is  expected  that  the  ratio  error  for  protection  grade  CTs  will  be  maintained  within .

Class C CT’s

Letter designation  'C'  indicates  that  the  leakage  flux  is  negligible.  Class C  CTs  are  the  more  accurate  bar type  CTs.  In  such  CTs,  the  leakage  flux  from  the  core  is  kept  very  small.  For  such  CTs,  the  performance can  be  evaluated  from  the  standard  exciting  curves.  In addition, the ratio error is maintained within for standard operating conditions.

For  such  CTs,  voltage  rating  on  the  secondary  is  specified  up  to  which linear  response  is  guaranteed.  For  example,  a  class  C  CT  specification  could  be  as  follows:  200:5  C  100. The  labeling  scheme  indicates  that  we  are  dealing  with  a  200:5  class  C  CT  which  will  provide  linear response  up  to  20  times  rated  current  provided  the  burden  on  the  secondary  is  kept  below [100/(5 x 20) = 1] ohm. Similarly, a corresponding  class  T  CT  may be  labeled  as  200:5  T  100.

For  class  C  CTs,  standard  chart  for E2  versus  excitation  current (IE) on  the  secondary  side  is  available. This  provides  the  protection  engineer  data  to  do  more  exact  calculations  refer  fig. 7  e.g., in determining  relaying  sensitivity.

Figure 6

5.2 Potential Transformer Modelling 

Potential transformers are also known as voltage step-down transformers or voltage transformers or instrument transformer, in which the voltage of a circuit is reduced to a lower voltage for measurement. The electromagnetic device used for the transformation of the higher voltage of the circuit to the lower voltage is called a potential transformer. The output of a low voltage circuit can be measured through voltmeters or wattmeters. These are capable of increasing or decreasing the voltage levels of a circuit, without a change in its frequency and windings. The working principle, construction of a potential transformer is similar to the power transformer and conventional transformer.

Potential Transformer Circuit Diagram

The potential transformer consists of primary winding with more turns and secondary winding with less number of turns. The high input AC voltage is given to the primary winding (or connected to the high voltage circuit to measure). The lower output voltage is taken across the secondary winding by using a voltmeter. The two windings are magnetically coupled to each other without any connection between them.

Construction of a Potential Transformer

Potential transformers are constructed with high quality to operate at low flux density, low magnetic current, and minimized load. When compared to a conventional transformer, it uses large conductors and an iron core. It can be designed in the form of a core type and shell type to ensure the highest accuracy. Usually, core type potential transformers are preferred to transform the high voltage to lower voltage.

It uses co-axial windings to reduce the leakage reactance. As the potential transformers are operated at high voltages, the high voltage primary winding is divided into small sections turns/coils to reduce the insulation cost and damage. The phase shift between an input voltage and output voltage should be monitored carefully to maintain a lower voltage by varying the load. Windings covered with vanish cambric and cotton tape to reduce the insulation cost.

Hard fiber separators are used to separate the coils. Oil-filled bushings are used to connect the high voltage potential transformers (above 7KV) to the main lines. The primary winding of a potential transformer has a large number of turns whereas secondary winding has fewer turns. The multimeter or voltmeter is used to measure the lower output voltage.

Potential Transformer Working

The potential transformer connected to the power circuit whose voltage should be measured is connected between the phase and the ground. That means the primary winding of a potential transformer is connected to the high voltage circuit and the secondary winding of a transformer is connected to a voltmeter. Due to the mutual induction, the two windings are magnetically coupled to each other and work on the principle of electromagnetic induction.

The decreased voltage is measured across the secondary winding with respect to the voltage across the primary winding using multimeter or voltmeter. Due to the high impedance in the potential transformer, the small current flows through the secondary winding and operates similarly to the ordinary transformer with no or low load. Hence these types of transformers operated at a voltage range of 50 to 200VA.

According to the convention transformer, the transformation ratio is

V2 = N1/N2

‘V1’= voltage of the primary winding

‘V2’ = voltage of the secondary winding

‘N1’= number of turns in the primary winding

‘N2’= number of turns in the secondary winding

The high voltage of a circuit can be determined by using the above equation.

Types of Voltage or Potential Transformers

Based on the function of a potential transformer, there are two types,

These are available in single or three-phase and operate with the highest accuracy. These are used to operate and control measuring devices, relays and other devices. Based on the construction, there are

Electromagnetic Potential Transformers

These are similar to the primary transformer, where primary and secondary windings are wounded on a magnetic core. It works on a voltage of above or below 130KV. The primary winding is connected to phase and the secondary winding is connected to ground. These are used in metering, relay and high voltage circuits.

Capacitive Potential Transformers

These are also known as capacitive potential dividers or coupling type or bushing type capacitive potential transformers. The series of capacitors are connected to the primary winding or secondary windings. The output voltage across the secondary winding is measured. It is used for power line carrier communication purposes and it is more costly.

5.3 Simulation of transients using Electro-Magnetic Transients (EMT) programs

Nature of Transients in Power Systems 

For the purpose of the present discussion, transients in power systems may be classified into two categories: electromagnetic transients and electromechanical transients.

Switching operations and faults produce step changes in the voltage and current waveforms. On long transmission lines and in transformer and reactor windings, there may be multiple reflections of generated transients. Resonances in the power network create additional frequencies in the waveforms during these phenomena. Another source of electromagnetic transients is lightning. Such transients may be classified as electrical or electromagnetic transients. The effect of these transients is to introduce high-frequency components in the signals. These transients dissipate within a short time and the waveforms then return to a quasi-steady-state condition. The frequencies of signals produced by electromagnetic transients can be summarized as shown below:

The harmonic phenomena indicated in Fig. Are typically produced by power electronic devices. It should be noted that the harmonic frequencies are multiples of the prevailing network frequency, which may be different from the nominal power frequency. (It should be remembered that phasor estimate performed with sampling rates keyed to nominal power frequency do not eliminate harmonics of off-nominal power system frequency.)

Network resonances are caused by shunt and series capacitors, and line charging capacitances interacting with various reactances in the network. Winding resonance phenomena are characteristic of transformer, generator, and reactor windings. Faults may produce some very high-frequency components, especially if arcing is involved in the fault.

Simulation of transients

Simulation of electromagnetic transients in modern power systems is widely used for the determination of component ratings such as insulation levels and energy absorption capabilities, in the design and optimization process, for testing control and protection systems and for analyzing power systems in general.

The simulation tools or methods for electromagnetic transients fall into the category of EMT-type (or EMTP-type) tools. Such tools are designed to study the power system at a very high precision level by trying to reproduce the actual time-domain waveforms of state variables at any location in the system. The power system is modeled at the circuit level in phase domain and with the representation of all wires and all required components. As for control systems, they are usually represented using block-diagrams. In the time-domain approach there are no inherent limitations in studying harmonics, nonlinear effects and balanced or unbalanced networks.

EMT-type simulation tools are classified into two main categories (families): off-line and real-time. The purpose of an offline simulation tool is to conduct simulations on a generic computer. Although an off-line tool must be designed to be highly efficient using powerful numerical methods and programming techniques, it does not have any time constraints and can be made as precise as possible within the available data, models, and related mathematics.

Real-time simulation tools are capable of generating results in synchronism with a real-time clock. Such tools have the advantage of being capable of interfacing with physical devices and maintaining data exchanges within the real-time clock. The capability to compute and interface within real-time, imposes important restrictions on the design of such tools.

It is feasible to apply EMT-type programs to study transient stability or even small signal stability problems. EMT-type programs can produce more precise simulation results for such studies due to inherent modeling capabilities to account for network nonlinearities and unbalanced conditions. Frequency dependent and voltage dependent load models can be also incorporated. The main disadvantages, especially in off-line tools, remain the computational speed and requirements for data. In EMT-type programs the network equations are solved in time-domain and not with phasors as in transient stability solution methods, which is the main explanation for reduced computational speed.

The building blocks that constitute an EMT-type program are shown in Fig. 1. This figure is labeled as “ultimate” since some of the presented modules or internal features are still at the research stage. Generally speaking, the simulation of a given electrical network is based on the solution of a system of equations

Ax = b.                                                        (1)

The unknown variables found in the vector x are usually voltages and currents. Matrix A is used to express topological constraints and component equations, and vector b represents known quantities. All network component models must participate in matrix A and vector b. Although there are several methods for the solution of nonlinear models, the most generic approach is to convert matrix A into a Jacobian and solve (1) through an iterative process. Equation (1) is solved using sparse matrices which provide the capability to solve very large systems efficiently.

Graphical User Interface (GUI)

The first entry level to the simulation process is the graphical user interface or data input. Graphical user interfaces with various levels of flexibility and visualization capabilities allow basically drawing the circuit diagram of the simulated system and entering all the appropriate data for selected models. An example of GUI based design is shown in Fig. 2. Modern GUIs are based on the hierarchical design approach with subnetworks and masking. Subnetworks allow simplifying the drawing and hiding details while masking provides data encapsulation. The design of Fig. 2 is using several subnetworks. The 230-kV network is interconnected with a 500-kV network evacuated with all its details into the subnetwork shown in Fig. 2. In a hierarchical design, subnetworks can also contain other subnetworks. Subnetworks can be also used to develop models. The 3-phase transformers shown in Fig. 2 are based on the interconnection of single-phase units. The synchronous machine symbols are also subnetworks containing the load-flow constraints, machine data and also voltage regulator and governor controls subnetwork, as shown in Fig. 2.

Initialization

The importance of initialization can be illustrated through the simple example of Fig. 3. The presented waveforms are the voltages at the receiving end of an arrester protected transmission line for two simulation cases. The first case is without any initial conditions and second case is with automatic initialization from steady-state solution. Even if frequency dependent line models (increased damping over constant parameter models) are used, the transients without initialization require more than 100 ms for attaining the actual steady-state response. This will have dramatic computing time consequences on large systems.

A complex subject in automatic initialization is the initialization of systems with power electronics switching devices. It is not obvious to automatically predict commutation patterns in a given operating mode and initialize state variables for harmonic waveforms. A programmed initialization method should find steady-state conditions in significantly less computing time that the brute force approach. In some cases, such as wind generation installations with power electronics devices connected on the rotor side, the best approach is to start with mean-value models or tricked equivalents and to switch onto actual commutating functions after establishing steady state operation. To complete the picture, it is important to mention that initialization concerns also the control system diagrams. It is usually a more complex, but essential feature, since, for example, initialization of synchronous machine variables without initialization of its controls can become worthless. Fully automatic methods do not yet exist, but backward propagation of variables in control blocks from specified initial condition variables is a practical option. In the lack of an automatic initialization, some programs are based on blocking the machine speed for forcing the steady-state, but such methods require additional knowledge on operating conditions and extra user intervention. Some programs also offer a snap-shot feature which allows preserving the steady-state solution conditions (after all time-domain transients have decayed) for successive studies. This option assumes that there are no changes in the saved case.

Statistical and Parametric Methods

The Statistical methods are for simulating with random data and evaluating worst case overvoltages or other probabilities for network variables. A new trend in power system applications is to provide Parametric study options. These options can incorporate arbitrary solution search rules through statistical and/or systematic data laws. Such methods are capable to modify and manipulate data using data scripting languages with full access to visualization and analysis functions. Parametric and statistical studies are particularly useful for estimating failure risks due to lightning and switching events or for evaluating performance limits for controllers.

External Interface

Modern applications have some means of interfacing with external packages or code. The interfacing methods are either object oriented or capable of calling Dynamic Link Libraries (DLLs) or both. Such interfaces are important since they provide a simple interoperability and expandability path. An important user-defined type modeling application is the connection of advanced controllers or relay models available in actual programming language codes.

Time-Domain Module

The time-domain module is the heart of an EMT-type program. It starts from 0-state (all devices are initially deenergized) or from given automatic or manual initial conditions and computes all variables as a function of time. Since component models may have differential equations, it is needed to select and apply a numerical integration technique for their solution. Since many electrical circuits result in a stiff system of equations, the chosen numerical integration method must be stiffly-stable. Such a need excludes explicit methods. In the list of implicit numerical integration methods, the most popular method in industrial applications remains the trapezoidal integration method. It is a polynomial method that can be programmed very efficiently. If an ordinary differential equation is written as

Then the trapezoidal integration solution is given by

The terms found at constitute history terms and all quantities at time-point are also related through network equations. The integration time-step can be fixed or variable. The fixed (set by the user) approach has several advantages in power systems. It avoids the time consuming reformulation of system equations and programming issues related to the models. In the case of transmission line models, for example, it is necessary to maintain history buffers for interpolating for propagation delays. The time-step variability will affect the buffer sizes continuously thus slowing down the computations. Fixing the size for the smallest time-step will create memory problems for large cases.

Control Systems

The simulation of control system dynamics is fundamental for studying power system transients. The development of control system solution algorithms based on the block-diagram approach has been initially triggered by the modeling of synchronous machine exciter systems. It was then extensively used in HVDC applications. Control elements can be transfer functions, limiters, gains, summers, integrators and many other mathematical functions. In many applications the block-diagram approach is also used to build and interface user-defined models with the built-in power system components.

OFF-LINE SIMULATION TOOLS

Off-line simulation tools are available on generic computer systems on which they can be easily installed and integrated within the working environment and operating system of the user computer.

The first nodal analysis tool used for power systems was named Electromagnetic Transients Program (EMTP). For historical reasons, the programs available in this category are called EMTP-type tools. The nodal analysis method simply accounts for the equilibrium of current injections at each node by using the nodal admittance matrix Y: Yv = i (4)

The vector of unknown voltage is v(t) and the vector of current injections is i(t). The nodal admittance matrix is also time-dependent since most applications model ideal switches which require inserting or deleting rows and columns. The fact that (4) is real means that it contains only resistances in the symmetrical admittance matrix Y.

This is achieved by applying the trapezoidal integration method through which all branch models with differential equations are given the Norton equivalent resistive companion model for a given integration time-step. The time-step can be fixed or variable. It becomes embedded in Y and each change of time-step requires the complete reformulation of Y.

The most widely used and available packages in power system applications are: ATP, EMTDC and EMTP-RV. These tools are all based on the fixed time-step trapezoidal integration method using (4).

5.4 Relay Testing

Inverse definite minimum type relay (IDMT):-  

IDMT operation

The    IDMT    relay    works    on    the    induction    principle,    where    an   aluminium    or    copper disc  rotates  between  the  poles  of  an  electromagnet  and  a  damping  magnet.  The fluxes  induce    eddy    currents    in    the    disc    which    interact    and    produce    rotational  torque.  The disc  rotates  to  a  point  where  it  operates  a  pair  of  contacts  that  break  the  circuit  and remove the  fault  condition.

The effect of plug setting   

The    plug    setting    (p.s.)    of    the    relay    changes    the    number    of    turns    in    the    exciting coil.  The  winding  of  the  coil  is  provided  with  seven  taps,  which  are  brought  to  the  front panel  and  the  required  tap  is  selected  by  a  push-in  plug.  With  the  plug  in  the  first  position (0.5),  the  whole  of  the  coil  is  utilised,  and  the  relay  is  most  sensitive.  In  the  seventh position  (2.0),  only  a  quarter  of  the  coil  is  utilised  and  hence  four  times    more    current    is      required        to        operate        the        relay.        The        seven        plug    positions  are  marked  0.5, 0.75,    1,    1.25,    1.5,    1.75,    and    2.0    (or    50%,    75%,    100%,125%,150%,175%,200%).  Should    the    plug    be  removed  altogether,  the  relay  automatically  defaults  to  the  2.0  or 200%  setting.   The   effect  of  altering  the   plug  setting   is  that for   a    given  current,  the  greater  the  plug  setting, the  longer  the time  of operation. 

The effect of  Time  Multiplier  (TM) setting

The  TM  (time  multiplier)  setting  of  the  relay  adjusts  the  "backstop"  of  the  rotating  disc. The    time    of    operation    is    proportional    to    the    distance    through    which    the        disc    must    rotate        in        order        to        operate        the        contacts.        With        the        time    multiplier   set    to    one,   the  backstop    is    as    far    back    as    it    can    go    (),    and    the        disc        has    to      move        through        its    maximum    travel    in    order    to    operate    the  contacts.    If    the  time  multiplier   is  set  to  zero  then   the  backstop  is   positioned  so that  the contacts are permanently  closed.    The    effect    of    altering    the    time    multiplier    setting    is    that    for    a given   current,  the  greater the  time multiplier  setting, the longer the  time of  operation .

Relay Characteristics

a.  Normal Inverse

      3.0 sec  relays  -  i.e. 3.0 sec. At ten times pickup with  TMS  of 1.0

      1.3 sec  relays  -  i.e. 1.3 sec. At 10 times pickup

b.  Very  Inverse  relays

c.  Extremely  Inverse  relays

O/C  relaying  is  very  well  suited  to  distribution  system  protection  for  the  following reasons:-  

      It is basically  simple and inexpensive  

      Very  often  the  relays  do  not  need  to  be  directional  and  hence  no  PT  supply  is required.  

      It  is  possible  to  use  a  set  of  two  O/C  relays  for  protection  against  inter-phase faults and  a  separate  O/C  relay  for  ground faults.  

Pick-up Setting  

For  coordination  of  the  inverse  time  O/C  relays,  the  pickup  current  and  time  dial  setting are  to  be  chosen.  The  pickup  of  the  relays  must  be  choosen  such  that  it  will  operate  for all  short  circuits  in  its  own  line  and  provide  backup  for  adjoining  lines,  keeping  in  view of  maximum full  load current.

O/C relay Pickup setting =  I  max. Load  

E/F  Relay

Pickup setting  =  20%  of  rated current.  

For    the    E/F    relay,   the    load    current    is    not    a    factor    in    the    selection    of    pickup settings and  is normally  set at 20%  of rated current.  

Time Settings  

The    actual    operating    time    of    the    O/C    &    E/F    relays    can    be    varied    by    proper selection of  the ‘Time  Dial Setting’ which is selectable  from 0.1 to 1.0.   Time  dial  settings  are  to  be  chosen  by  having  proper  coordination  and  gradation  in  the system.  Gradations  between  successive  relays  are  obtained  by  ‘Selective  time  interval’ which is usually  set between 0.3 to 0.4 Sec.  

The  operating  time  of  various  types  of  IDMT  relays  are  in  the  sketches.  Also  can  be obtained by  the  formulae: 

Calculation example for relay:

For remote bus fault, lets assume a fault current of 3000 Amp through the protected element
Assuming C.T. Ratio on protection line

Pickup Setting for relay (Plug Setting)

Actual time of operation for relay is generally set to grade with the down side system.
Assume time setting required

Actual Time of Operation (ATO): TMS (For Normal Inverse)

Set Time Dial (TMS) for relay

Actual Time of Operation (ATO) for O/L relay

(With TMS:0.1)

Relay Testing  

Primary    and    secondary    current    injection    tests    are    normally    conducted    to    check  the  operation    of    breaker    and    their    protective    relays/devices.    The    protective  devices  installed    vary    from    circuit    to    circuit    depending    on    the    protection    needs  and  philosphy    but    typical    relays/devices    include    overload,    over    current,    reverse  power, earth fault, differential protection, etc.   

Primary Injection Test

Primary  injection  testing  normally  involves  injecting  the  actual  current  required  to operate    a    protective    device    power    through    the    circuit    breaker.    Primary    injection testing    normally    requires    specialist    injection    sets/test    rings    and    measurement equipment  (particularly  for  high  power  and  MV  and  above)  and  can  be  extremely arduous    where    the    circuit    breaker    interrupts    large    currents,    shortening    its    life    or requiring  repair  after.  In  many  cases,  primary  injection  testing  is  only  conducted  by specialists. Testing  and  research  of  this  form  is  certainly  carried  out  by  circuit  breaker manufacturers.  

The  PI  test is usually  performed by  injecting  a  current at low voltage  (say  5  -10   V)  from a  purpose  built transformer with high  current capable secondary  winding.  

The  current  is  passed  through  the  breaker  or  busbar  section  as  appropriate.  The magnitude  of  current  injected  is  generally  not  considered  important  so  long  as  it  is    above  the    minimum    operating    current    determined    by    the    protection    relay  settings.    This  test    is    also    carried    out    to    ensure    the    C.T    ratio    of    current  transformers.  If  this  test  is carried  out  after  C.T  secondary  wiring  is  completed  it  ensures    not    only    the    correct  ratio    of    C.Ts    but    also    the    correctness    of    the    entire  C.T    secondary    wiring  comprising    protection    and    metering    portions.    The    testing  equipment    consists    of    a  loading    (injection)    transformer,    controlled    by    a    variable  transformer  to  get  the required  current on the  primary  side  of the  C.T under test.

For    carrying    out    the    ratio    test    on    C.Ts,    Current    is    passed    through    the    primary windings    of    the    standard    C.T    and    C.T    under    test.    The    ratio    of    the    C.T    can    be determined by  comparing  the  currents in ammeters A1 and A2.  

Thus    Primary    injection        is    carried    out    as    a    test    to    determine    the    integrity    of    the whole    secondary    protection    circuit    including    CTs,    CT    leads    and    control    cubicle wiring.    In    other    words    it    proves    that    the    CB    trips    in    response    to an    over  current. The  test  is  performed  after  secondary  injection  tests  and  CT  ratio  tests  and  when  all  the secondary  test  links  are  closed  and  ready  for  service.  Hence  it  is  often  the  last  test perfomed in the  commissioning  process. The  purpose of PI  test can be  summarized as :  

      to ensure  wiring  connection of  protection system is correct 

      to check CT ratios and polarities 

      to check Overcurrent relays  and   Earth  Fault  relays  are  set to correct setting 

      to check Direct Acting  Tripping  element of the main MCCB.  

Secondary Injection Test

Secondary    injection    testing    is    normally    different    to    primary    injection    testing because    it    is    normally    conducted    when    the    circuit    breaker    is    closed    but    is    not carrying    any    current    through    its    main    poles.    Secondary    injection    tests    are performed  by  injecting  currents  into  the  relay  terminals  to  determine  that  the  relay  is  operating   correctly   and   in  accordance   with  its   settings.   These   tests  include injecting  currents    of    various  magnitudes  from  minimum    operating  current    all  the  way    up    to    10 or    20    times    minimum    operating    current    and    measuring    the    relay  operating    time.  This    generally    involves    disconnection    of    the    protective    device  from    it's    normal    CT  and    connection    to    a    specialist    test    set    that    can    inject    and  measure/record    the  required    operating    signal    directly    into    the    protective    device  relay    to    cause    it    to  operate    the    circuit    breaker.    The    advantage    of    secondary  injection  testing  is  that  the circuit  breaker  does  not  have  to  interrupt  large  current  and    only    low    voltage    signals  are    injected    to    operate    the    device.    A    perceived  disadvantage    of    secondary    injection  testing    is    that    the   actual    operation    of   the  'whole'    system    is    not   tested   but   this   may  be    compensated    by    the    fact    that    the  circuit    breaker    has    operated    without    having    to  interrupt    a    large    current    and    the  circuit  breaker  type  has  tested  and  rated  by  its manufacturer.  However,  specialist  equipment  and  knowledge  is  still  required,  including significant  knowledge  of  the  actual  protection  scheme  and  philosophy.  Furthermore, disconnecting  of  CT  can  also  lead  to  potential  danger.  For  this  reason,  secondary injection  testing  is  also  often  conducted  by  specialists.  In  other  words,  it  is  not  something that is  jumped into without significant experience  and knowledge.  

C.T Polarity Test

Each    current    transformer    should    be    individually    tested    to    verify    that    the    polarity markings    on    the    primary    and    secondary    windings    are    correct.    The  figure  shows  the test  unit  for  this.  The  ammeter  ‘A’  is  a  robust,  moving  coil,  permanent  magnet  centre zero  type  instrument.    A    low    voltage    battery    is    used    to    energise    the    primary  windings  through    a    single    pole    push    button.    On    closing    the    push-button,   with  above    C.T  ammeter    markings,    the    ammeter    should    give    a    positive    flick,    indicating  correct polarity  of the  C.T.

Open-Secondary Circuits   

Secondary  circuits  of  CT's  must  not  be  open  while  primary  current  flows.  Extreme care  should    be    taken    to    avoid    breaking    the    secondary    circuit    while    primary  current    is  flowing.    If    the    secondary    is    open-circuited    the    primary    current    raises  core  flux density  to  saturation  and  induces  a  high  voltage  in  the  secondary  which  can  endanger human  life,  and  can  damage  connected  apparatus  and  leads.  If  it  is  necessary  to  change secondary  conditions  while  primary  current  is  flowing,  the  secondary  terminals  must  be short-circuited  while  the  change  is  being  made.  It  is  recommended    that    the    secondaries  of    all    current    transformers    be    kept    short-circuited  at  all  times  when  not  installed  in  a circuit  such  as being  held in stock or being  transported.

References:

1. Wu, Y.H., Dong, X.Z. & Mirsaeidi, S. Modeling and simulation of air-gapped current transformer based on Preisach Theory.

2. J. Mahseredjian, V. Dinavahi and J. A. Martinez, "Simulation Tools for Electromagnetic Transients in Power Systems: Overview and Challenges," in IEEE Transactions on Power Delivery, vol. 24, no. 3, pp. 1657-1669, July 2009

3. Y. G.Paithankar and S. R. Bhide, “Fundamentals of power system protection”, Prentice Hall, India, 2010.

4. A. G. Phadke and J. S. Thorp, “Computer Relaying for Power Systems”, John Wiley & Sons, 1988.

5. A. G. Phadke and J. S. Thorp, “Synchronized Phasor Measurements and their Applications”, Springer, 2008.