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Home / Technical Articles / The facts about switchyards, incoming and outgoing feeder connections, CTs and VTs

Switchyard Design Facts

What do you need to know about designing a switchyard? Well, to simplify, you need a lot and it’s not that simple. Switchyards need to be designed with respect to the foreseeable voltage stress, switching, breaking and short-circuit withstand capability, loading under normal and emergency conditions taking account of the requirements of load dispatch management, operational security, and supply reliability.

Switchyard arrangement and connection of incoming and outgoing feeders
Switchyard arrangement and connection of incoming and outgoing feeders

However, this technical article does not deal with design details but switchyard design facts, so let’s start. As you already know, switchyard consists of many elements, and it would take many words and hours to name them all, but roughly we can group them into few groups:

  1. Electrotechnical equipment such as switchgear, current and voltage transformers, surge arresters, insulation joints, armatures.
  2. Mechanical structural parts such as conductors, bars and pipes for busbars and gantries, partly seen as electrotechnical equipment as well.
  3. Secondary devices such as measuring and protection transformers and transducers, protective relays, coupling for remote control, batteries and so on.
  4. Civil engineering structures such as buildings, foundations, fire-extinguishing equipment and fences.

The documentation of switchyards becomes extensive when account is taken of the multiplicity of items of equipment, their interdependency and their importance. Knowledge of the appropriate standardized symbols according to the different parts of IEC 60617 is therefore necessary.

Table of Contents:

  1. Circuit Breakers and Switches
  2. Incoming and Outgoing Feeders
  3. Current Transformers
  4. Voltage Transformers

1. Circuit Breakers and Switches

The different types of breakers and switches to be used in switchgears are described in different parts of IEC 60947 and IEC 60890 for low-voltage installations as well as in EN 50052 and EN 50064 for high voltage installations.

Let’s remind ourselves of basics:

Fact #1Circuit-breakers have a switching capability for switching on and off any kind of current up to the rated current, that is, load current and short-circuit currents.

Fact #2Circuit-breakers installed in overhead systems should have the capability of operating sequences for successful and unsuccessful autoreclosing.

Fact #3Load-break switches are capable of switching load currents under normal operating conditions, but have no capability for switching short-circuit currents.

Fact #4Disconnecting switches can be operated only under no-load conditions. Currents of busbars without load and no-load currents of transformers with low rating can be switched on and off as well. Interlocking with the circuit-breaker is necessary.

Fact #5Earthing switches are used for earthing of equipment. The combination of earthing switch with disconnecting switch is common.

Fact #6Fuses are installed in LV and MV systems only. They interrupt currents of any kind by the melting of a specially designed conductor and must thereafter be replaced. Combination of fuses with disconnecting switches can be found especially in LV systems.

Circuit-breakers are named according to the method of arc quenching they use. Vacuum circuit-breakers are nowadays installed in MV systems. In the HV and EHV range outdoor circuit-breakers are operated with compressed air or sulfur hexafluoride (SF6). Circuit-breakers in gas-insulated switchgear (SF6-isolated) are of the SF6 type.

Recommended reading (paper, PDF): Fundamentals of High Voltage Circuit Breakers, Switching Stresses and Failure Modes

Fundamentals of High Voltage Circuit Breakers, Switching Stresses and Failure Modes
Fundamentals of High Voltage Circuit Breakers, Switching Stresses and Failure Modes (on photo: 345KV SF6 3PH Circuit Breaker; credit: wolfartistic.deviantart.com)

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2. Incoming and Outgoing Feeders

Incoming and outgoing feeders in switchgear are equipped with circuit-breakers and disconnection and earthing switches. Current and voltage transformers for the connection of protection and measurement devices are usually installed at each feeder in HV switchyards.

The current transformer is placed at the busbar side of the voltage transformer in order to detect short-circuits of the voltage transformer by the protection device. Installations without voltage transformers in each feeder are also found: in this case the voltage transformer is placed at the busbar.

In addition, the feeders are equipped with surge arresters and coupling devices for frequency carrier signals depending on the requirements of the switchgear.

A typical arrangement of the individual devices of feeder arrangement in a HV switchyard is outlined in Figure 1.

Typical feeder arrangement in a HV switchyard
Figure 1 – Typical feeder arrangement in a HV switchyard: (a) Overhead line feeder with double busbar; (b) transformer feeder

Where:

  1. Busbar disconnecting switch,
  2. Circuit-breaker,
  3. Feeder disconnecting switch,
  4. Earthing switch,
  5. Current transformer,
  6. Voltage transformer,
  7. Capacitive voltage transformer with coupling for frequency carrier signal,
  8. Blocking reactor against frequency carrier signals.

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3. Current Transformers

Current transformers are used for the connection of protection and measuring devices. In order to avoid damage by high voltages, current transformers are not allowed to be operated without load and must not be protected by fuses on the secondary side.

One terminal of the current transformer on the secondary side is to be grounded for definition of ground potential. Current transformers are named according to the intended purpose (M: Measurement; P: Protection) and accuracy. For measuring purposes, accuracy classes 0.1, 0.2 and 0.5 are used.

The total measuring error remains below 0.1%, 0.2% and 0.5% if the current transformer is operated in the range of 100–120% of the rated current. Current transformers with accuracy classes 1, 3 and 5 (accuracy 1%, 3% and 5%) are used for operational measurements.

Measuring (instrument) current transformers should be saturated in case of overcurrent in order to protect the attached measuring devices,  whereas protection current transformers should have sufficiently small errors while operated with overcurrent. This is defined by the overcurrent factor n of the current transformer according to IEC 60044, see also IEC 61869.

The rated overcurrent factor nr defines multiples of the rated current at rated load Sr and power factor cosϕ = 0.8 of the load (burden), for which the error remains below the value defined in the accuracy class. For measuring purposes, overcurrent factors of nr < 5 are sufficient, while the overcurrent factor for protection purposes should be nr > 5–10.

Current transformer (CT) saturation calculator (MS Excel Spreadsheet .xls):

Current transformer (CT) saturation calculator
The spreadsheet ‘CT Saturation Calculator’ is intended to provide quick indication not only of whether or not a CT will saturate in a particular application, but also an accurate indication of the actual waves-shape of the secondary current so that the degree of saturation as a function of time is apparent.

The total error is indicated according to IEC 60044 as for the accuracy class, for example:

  • 40 VA 5 P10 Rated load (burden) 40 VA:
    • Protection current transformer P;
    • Total error below 5% with the rated overcurrent factor nr = 10
  • 30 VA 0.5 M5 Rated load (burden) 30VA:
    • Measuring current transformer M;
    • Total error below 0.5% with rated overcurrent factor nr = 5.

The total error is given by the geometric addition of magnitude error Fi and the phase-angle error δi, if the instantaneous values of the primary and secondary current are taken into account for the determination of the total error.

According to IEC 60044, the phase-angle error for a current transformer of the class 1M5 shall thereby remain below 60 minutes (one degree). The rated load (burden) of a current transformer is to be selected in such a way that the load of the connected devices and the losses of the connecting cables on the secondary side are covered.

With reduced burden SB, the overcurrent factor increases in accordance with the following equation:

Reduced CT burden SB

Error of a current transformer for different overcurrent factors and different burden
Figure 2 – Error of a current transformer for different overcurrent factors and different burden

Figure 2 represents the error of a current transformer for different rated overcurrent factors and different burden. Current transformers have to withstand the expected short-circuit currents. The withstand capability is determined by the rated short-time current Ithr and the rated peak short-circuit current ipr.

The rated short-time current Ithr is the r.m.s.-value of that current in the primary winding, permitted for one second during short-circuit of the secondary winding. The rated short-time current must be higher than or equal to the thermally relevant short-time current Ith. The rated peak short-circuit current ipr should be larger than 1.8 times the peak value of the rated short-time current Ithr.

Apart from the measuring error of the current transformer, the transient characteristic of short-circuit currents with maximal DC component and the magnetic remanence flux remaining in the core after disconnection of short-circuits are of significance.

In automatic reclosing it is possible to switch into the still-existing short-circuit, with the consequence that the remaining remanence flux in the current transformer leads to a very high magnetizing current, driving the core into saturation and thus impairing the performance.

Current transformer nameplate
Figure 3 – Current transformer nameplate

IEC 60044 additionally defines the current transformer class PR with special requirements on the remanence flux. The ratio of remanence flux to saturation flux (remanence factor Kr) may not exceed 10%.

Short-circuit currents with maximal DC component must be transferred by the current transformer with a low error. The maximum flux caused by the short-circuit current with maximal DC component must be lower than the saturation flux. This can be achieved by means of larger cross-section of the iron core and by smaller secondary burden, including larger cross-sections for the connection cables.

An increased transformation ratio with reduced burden must be avoided with respect to difficulties with protective excitation. Requirements for the transient characteristic of current transformers are defined in IEC 60044. The requirements are realized by a linearization of the iron cores: four categories are defined, as specified in Table 1.

Table 1 – Categories of current transformers and their parameter according to DIN EN 50482 and DIN EN 61869.

CategoryImplementationFiδiKrTs
P
  • Iron closed core
  • Total error defined for symmetrical current on primary side
  • Remanence flux not limited
 – – –
TPS
  • Iron closed core
  • Low residual flux
  • Remanence flux not limited
  • To be used for differential protection
±0.25%Some seconds
TPX
  • Iron-closed core
  • Defined limits for error in magnitude and phase-angle
  • Remanence flux not limited
  • Suitable for automatic reclosure
±0.5%±30′~0.8Some seconds
TPY
  • Small air-gap to limit remanence flux
  • Remanence flux ≤10% of saturation flux
  • Suitable for automatic reclosure only if time-constant is smaller than reclosing time
±1.0%±60′~0.10.1s up to 1s
TPZ
  • Large air-gap (linear core)
  • Remanence flux negligible
  • Protective devices remain in excitation for a long time
±1.0%±18′~060ms

The transfer from primary to secondary of the fundamental current (50Hz or 60 Hz) is achieved adequately with these current transformers; the transfer of the DC component is less with increasing linearization of the core.

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4. Voltage Transformers

Voltage transformers are used for the measurement of line-to-line (phase-to-phase) voltages and line-to-earth (phase-to-neutral) voltages. Voltage transformers are constructed either as inductive transformers (Un = 1–765kV) or as capacitive transformers (Un ≥ 60 kV). Capacitive voltage transformers are suitable for the connection of frequency carrier signals.

Voltage transformers are named according to their intended application (M: Measurement; P: Protection) and their accuracy class. One has to take into account error in magnitude FU and phase-angle error δU. The indicated error margins are to be guaranteed with secondary burden of 25% and 100% of the rated burden at power factor cosϕ = 0.8.

The accuracy of measuring voltage transformers (class M) is to be guaranteed in the voltage range between 0.8 × Un and 1.2 × Un, and for protective applications the voltage should be 1.0 × Un.

Error in phase-angle and magnitude of a voltage
Figure 4 – Error in phase-angle and magnitude of a voltage

Voltage transformers with classes 0.1, 0.2 and 0.5 (error less than 0.1%, 0.2% and 0.5% within the range 0.8–1.2 × Un) are used for exact measurement purposes. For operational measurement, voltage transformers with accuracy class 1 and 3 (error less than 1% and 3%) are used.

For protection purposes, voltage transformers 3P and 6P are used, having voltage error less than 3% and 6% and a phase-angle error less than 120 minutes and 240 minutes respectively in the voltage range 0.8–1.2 × Un.

Figure 4 indicates the dependence of the phase-angle error and the error in magnitude with different burden for a voltage transformer:

Voltage transformer burden

Single-pole isolated voltage transformers are exposed to increased voltages of varying duration depending upon the neutral earthing of the system. The rated voltage factor indicates multiples of the nominal system voltage.

The voltage transformer is allowed to be operated for times of 30 s, 4h or 8h. Voltage transformers with a rated voltage factor 1.5/30s may be used only in systems with low impedance earthing (earth-fault factor δ ≤ 1.4); those having a rated voltage factor of 1.9/30 s or 1.9/4 h or 1.9/8 h can be installed in systems with all kinds of neutral earthing, that is, isolated neutral, resonance earthing and low impedance earthing.

The factor 1.9 corresponds approximately to the maximal permissible voltage of the system (power frequency).

Capacitive voltage transformers, which are used in high-voltage transmission systems in particular, show on the secondary side transient oscillations arising from short-circuits, which exhibit decaying amplitudes up to 10% of the system nominal voltage with frequencies up to 10 Hz in the case of short-circuits at zero voltage crossing.

These transient oscillations downgrade the performance of distance protection relays, especially with respect to the direction decision.

The problem can be solved, however, by small load of the voltage transformer and/or by installation of a filter circuit. Furthermore, ferroresonances can occur with capacitive voltage transformers.

Recommended reading: Connection schematics of voltage transformers for protective applications

The Essentials Of Voltage Transformers (Advanced Theory and Practice)
The Essentials Of Voltage Transformers (Advanced Theory and Practice)

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Source: Power System Engineering by Jürgen Schlabbach and Karl-Heinz Rofalski

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

2 Comments


  1. khin san myint
    Oct 22, 2020

    May I know how to calculate the stacking factors of 3-stepped and 4-stepped transformer cores?
    I found the dimension drawings but I cannot reach the right answers. The name of the original text book is not shown.


  2. Shambu Kumar T.V.
    Oct 21, 2020

    Very informative article.
    Since I am in the job of training staff in the Facility Management field, may I request if some more basic understanding with photos of the Sub-Station components could be shared to help the budding engineers in the FM field.

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