By Edvard
Specifying MV Switchgear
When specifying and planning medium voltage switchgear for a substation, functions and influencing factors must be matched and an economically efficient solution must be found among the offerings of manufacturers.
For this there is no simple recipe with an unambiguous solution for engineers, simply because:
1.The tasks of a switchgear substation can vary greatly
2.Many influencing factors are interdependent
3.The same influencing factors and requirements can be weighted differently by different manufacturers.
Generally, a switchgear substation MUST provide a high level of safety so that both operator protection and fault-free network operation is ensured. It must meet the requirement of touch protection and exclude the possibility of maloperation.
If a fault occurs nevertheless, its impact on the fault location should be limited and not entail personal injury.
Contents:
1.Distribution level of a MV switchgear
2.Standards for the Design and Installation of MV Switchgear
3.Configuration Parameters
4.Medium-Voltage Switchgear Design:
-①.Operator protection
-②.Expandability
-③.Installation site
-④.Operating and maintenance areas
-⑤.Accessibility of compartments
-⑥.Service continuity during work in progress
-⑦.Busbar systems
Single busbar
Double busbar
-⑧.Internal arc classification (IAC)
-⑨.Estimation of pressure effects according to Pigler
-⑩.Finite-elements-simulation of pressure load under conditions of arcing
1. Distribution level of a MV switchgear
In analogy to distribution grids, switchgear can be assigned to the primary or secondary distribution level:
Primary Distribution – What is characteristic for the primary distribution level are high load and short-circuit currents and high-end secondary features of the switchgear with regard to protection, measuring, and (remote) control.
At the primary distribution level (Figure 1) you will find the main substation, where energy is fed in with a higher voltage and transformed to the medium-voltage level.
The switchgear is almost completely equipped with circuit-breakers. They switch large consumers, mostly in industrial plants, or cable rings which feed switchgear at the secondary distribution level.
Secondary Distribution – At the secondary distribution level, the switchgear is equipped with switches or a mixture of switches and circuit-breakers, where the proportion of switches
clearly dominates.
The currents are lower, short-circuit protection is often ensured by the assigned circuit breaker at the primary distribution level. The requirements placed on secondary features are usually lower.
Figure 1 – Structure of the voltage and power distribution levels
Generally speaking, typical forms of substation are:
Consumer substation
The consumer substation from which the energy is distributed at the fed-in line voltage (medium voltage).
A load transfer switch (coupling) in the substation can form the property border between the supply company and the customer if the customer wants to develop his switchgear part independently. In that case, measuring and metering equipment for billing will also be available.
Figure 2 – Consumer substation
Secondary unit substation
The substation, also called secondary unit substation, where the energy is transformed from medium into low voltage and distributed as such.
In industrial plants, substations are often installed in the production centres which are also load centres. Therefore, these substations are called load-centre substations.
For very compact-built substations which are not accessible, the designation ‘small’ or ‘compact substation’ has become popular.
Figure 3 – Compact substation (on photo: New gas-insulated medium-voltage switchgear, 8DJH Compact, for secondary distribution systems up to 24 kV; credit: Siemens)
2. Standards for the Design and Installation of MV Switchgear
The standards distinguish between two main groups of medium-voltage switchgear:
1.Factory-assembled, type-tested plants with:
Metal enclosure in accordance with IEC 62271-200 (VDE 0671-200)
Moulded-plastic enclosure in accordance with IEC 62271-201 (VDE 0671-201)
2.On-site or workshop-built switchgear in accordance with IEC 61936-1 (VDE 0101-1), as it is rarely built nowadays.
In the following, we will describe the metal-enclosed, type-tested medium-voltage switchgear in accordance with IEC 62271-200 (VDE 0671-200), since both mouldedplastic enclosed and on-site built, i.e. workshop-built plants are manufactured significantly less frequently.
The high manufacturing and testing expense often amortise only if high quantities are produced and the production is standardised accordingly. The technical data must be verified by type tests. The manufacturing quality is monitored by routine tests.
3. Configuration Parameters
The selection parameters for the configuration of switchgear can be distinguished as follows:
Pre-defined
For example connection to earth, grid voltage, grid frequency, neutral-point connection, ambient conditions, peak short-circuit current.
Conditionally selectable
For example insulation level, neutral-point connection, overvoltage protection, short-circuit duration, type of operating area, plant design.
Any selection
For example switchgear type, switching devices and their operating mechanisms, busbar circuitry, compartments and partitions, operational availability, accidental arc qualification
Table 1 gives an overview of the configuration parameters and characteristics which may play a part in the planning. The most important aspects are presented in more detail below.
– Overview of the rated values and selection parameters for the confguration of medium-voltage switchgear
Primary rated values
| Selection parameter | Determinants |
| Ur – Rated voltage
Rated insulation level: Ud – Short-duration power-frequency withstand voltage Up – Lightning impulse withstand voltage |
Line voltage
Insulation coordination: Neutral-point connection Cable / overhead line grid “Critical” consumers Overvoltage protection Altitude Environmental influences (pollution) |
| Rated withstand capacity:
Ip – Peak current IK – Short-time current tK – Short-circuit duration Rated switching capacity: Ima – Short-circuit making current Isc – Short-circuit breaking current |
Grid characteristics
Consumers and power quality Grid protection, response times Selectivity criteria |
| Ir – Rated operating current
Busbar Feeder circuits |
Load (feeder circuit), power to be distributed (busbar)
Ambient temperature Reserves /service continuity |
Busbar circuit
| Selection parameter | Determinants |
| Single / Double busbar
Bus sectionalizer/ busbar coupler, longitudinal (BCL) Switch-over (BCL) using switch or circuit-breaker Busbar coupler, transversal (BCT) (double busbar) |
System confguration
Grid protection, response times, selectivity criteria Reserves /service continuity, switch-over times Operational procedures Embedded or in-plant power generation, emergency power supply Power quality (unsteady loads) Operational procedures |
| Double busbar with common connection
Two single busbar systems |
Frequency of busbar switch-over
Interlockings, switching fault protection Installation (spatial) |
Switching device
| Selection parameter | Determinants |
| Circuit breaker
Switch Contactor HV HRC fuse |
Operating current and switching task
Switching capacity (fault currents) Switching frequency Grid protection, selectivity requirements |
Design and panel type
| Selection parameter | Determinants |
| Circuit breaker panel
Switch panel Type of construction: Extendable panels Block type |
Primary rated values
Switching devices Operating current, switching capacity Grid protection Numerical ratio of switch panels to circuit-breaker panels Operational workflows and handling Conditions of installation Transportation and mounting Expandability, electrical / mechanical reserves |
Insulation medium
| Selection parameter | Determinants |
| Air (AIS)
Gas (GIS) |
Room climate: temperature cycling, humidity, pollution, salt, aggressive gases
Type of operating site Place of installation (spatial requirements) Fire protection requirements (fire load) Altitude Switching frequency and switch lifetime |
Disconnector
| Selection parameter | Determinants |
| Withdrawable unit/truck
Disconnector (fixed mounted) |
Switching frequency
Service life of components Operational |
Encapsulation
| Selection parameter | Determinants |
| Degree of protection (IP in accordance with IEC 60529, VDE 0470-1) |
Environmental conditions
Personal safety Type of operating site Building |
| Internal arc classification (IAC):
A or B (type of accessibility) F/L /R (classified sides) IA, tA (arc fault current and duration) Pressure relief duct |
Compartments and partitions
| Selection parameter | Determinants |
| Category of service continuity (compartment partitioning LSC – loss of service continuity)LSC 1LSC 2LSC 2ALSC 2B |
Operational procedures:
Operating, working Maintenance requirements Servicing and maintenance (service life of components) Operating company instructions Personnel qualification Shock-hazard protection during work in progres Switchgear space requirements |
| Accessibility and access control using:
Interlocking Work instruction + locking Tools Non-accessible switchgear compartment |
|
| Partition class:
PM (partition of metal) PI (partition of insulating material) |
Outgoing feeder components
| Selection parameter | Determinants |
| Cable connection:
Termination: conventional / plug Number of cables Conductor cross sections |
Operating and short-circuit current
Switch task: Cable / overhead line Altitude |
| Surge arrester | |
| Voltage transformer:
Earth fault winding (if required) Current transformer Number and data of cores Summation current transformer |
Grid protection
Metering, counting Control Neutral earthing |
| Earthing switch:
Class E0, E1 or E2 |
Operational procedures |
Busbar components
| Selection parameter | Determinants |
| Measuring transducer
Earthing switch Class E0, E1 or E2 Surge arrester |
Grid protection and measurement
Operational procedures |
Secondary equipment
| Selection parameter | Determinants |
| Protection relays
Equipment for control, interlocking, and switching fault protection Equipment for metering, counting, measured-value transducers Equipment for monitoring and communication Motorized drives Voltage test system Damping resistor (for voltage transformer) |
Grid parameters, protective equipment
Grid operation, integration into (industrial) processes and operational procedures Electromagnetic compatibility |
4. Medium-Voltage Switchgear Design
Gas-insulated switchgear should be used for the medium voltage consumer substation. The advantages of gas-insulated switchgear are:
-1.Lower space requirements (up to approx. 70 % savings with 30 kV) compared to air-insulated switchgear
-2.Smaller transportation size and consequently easier shipping
-3.Increased reliability of operation due to hermetically sealed primary switchgear section (adverse impact such as from contamination, small animals, contact, condensation are excluded due to the encapsulation)
-4.Maintenance-free primary section (no lubrication and readjustment necessary)
-5.Better eco balance than air-insulated switchgear referred to the entire system life cycle
4.1. Operator protection
-1.The gas-insulated switchgear is safe to touch thanks to its earthed metal enclosure
-2.HV HRC fuses and cable terminations are only accessible if branch circuits are earthed
-3.Operation is only possible if the enclosure is fully sealed (and any doors closed)
-4.A maintenance-free pressure absorption system, laid out as “special cooling system” reduces pressure-related and thermal impacts of an arc fault so that personnel and building will be safe (Figure 4).
Figure 4 – Room layout for switchgear with pressure relief downward (left) and with pressure absorption duct (click to expand)
4.2. Expandability
The switchgear should be extendible with a minimum time expense. A modular system with ordering options for busbar extensions on the right, left or both sides provides the best prerequisite for this:
Individual panels and panel blocks can be mounted side-by-side and extended as desired – no gas work required on site
Low-voltage compartment (cubicle) is available in two heights, wired to the switchgear panel by means of plug connectors
All panels can be replaced at any time
4.3. Installation site
The medium voltage switchgear is usable as indoor installation in accordance with standard IEC 61936-1 (VDE 0101-1).
A distinction is made between:
-1.Switchgear types in locations with no access from the public, outside closed off electrical operating areas.
Switchgear enclosures can only be removed with the aid of tools and operation by ordinary persons must be prevented.
-2.Closed electrical operating areas: A closed electrical operating area is a room or location used solely for the operation of electrical switchgear and is kept locked.
Access is only granted to electrically skilled persons and electrically instructed persons. Ordinary persons are allowed only when accompanied by electrically skilled or instructed persons.
Figure 5 – Installation of switchgear in accordance with standard IEC 61936-1 (VDE 0101-1)
4.4. Operating and maintenance areas
These are corridors, connecting passages, access areas, transportation and escape routes.
Corridors and access ways must be sufficiently dimensioned for work, operation and transportation of components and must have a minimum width of 800 mm.
Corridor width must not be obstructed by equipment protruding into the corridor, such as permanently installed drives or switchgear trucks in disconnected position.
The width of the escape route must be at least 500 mm, even if removable parts or fully open doors protrude into the escape route.
Switchgear panel or cubicle doors should close in the direction of escape.
For mounting and maintenance work behind enclosed units (stand-alone) a passage width of 500 mm is sufficient.
A minimum height of 2,000 mm below ceilings, covers or enclosures, except for cable basements is required.
Exits must be arranged in such a way that the length of the escape route inside the room does not exceed 20 m in case of rated voltages up to 52 kV. This requirement does not apply to walk-in busbar or cable conduits or ducts.
For installations with a rated voltage up to 52 kV, the length of the escape route inside the room must not exceed 20 m (40 m for installations above 52 kV).
Fixed ladders or similar facilities are permissible as emergency exits in escape routes.
4.5. Accessibility of compartments
The IEC 62271-200 (VDE 0671-200) standard for metal-enclosed switchgear distinguishes between accessibility level A for authorized personnel and accessibility level B for unlimited access (also for the general public).
In addition to this, the opening possibilities of a compartment are distinguished, which influences the accessibility, and thus the availability, of a switchgear.
A gas-insulated switchgear is also available as a type with:
Non-accessible compartment
It must not be opened. Opening such a compartment could destroy it and impair functioning of the switchgear.
Medium-voltage switchgear are further differentiated according to 3 opening types:
Interlock-controlled accessible compartment
An interlock in the the panel grants access when live parts are isolated and earthed. Opening the switchgear under normal operating conditions or for maintenance, for example to replace HV HRC fuses, is possible.
Process-dependent accessible compartment
Access is described through instructions of the operating company, and a lock shall ensure safety of access during normal operation and maintenance.
Tool-dependent accessible compartment
Tools and precise work instructions are needed to open the compartment, for example including a safety note. This kind of accessibility shall not be usable during normal operation or maintenance
4.6. Service continuity during work in progress
IEC 62271-200 (VDE 0671-200) specifies categories of operational availability (LSC, loss of service continuity) of the functional units of a switchgear. They describe which parts must be put out of operation during the opening process of an accessible switchgear compartment.
Accessibility of switches and terminals is categorized according to Table 2 below:
| Category of service continuity | When an accessible compartment of the switchgear is opened… | Type of construction | |
| LSC 1 | The busbar and therefore the complete switchgear must be isolated | No partition plates within the panel, no panel partitions to the adjacent panels | |
| LSC 2 | LSC 2A | Only the supply cable must be isolated. The busbar and the adjacent panels can remain in operation | Panel partitions and isolating distance with compartmentalization to the busbar |
| LSC 2B | The supply cable, the busbar, and the adjacent panels can remain in operation | Panel partitions and isolating distance with compartmentalization to the busbar and the cable | |
Figure 6 (below) shows some examples for the different categories of service continuity:
Figure 6 – Example for the service continuity (LSC) of medium voltage switchgear
4.7. Busbar systems
The following aspects play a part when choosing a single or double busbar:
1.The number of outgoing and incoming feeders
2.Separate operation of parts of the installation required
3.Operability of certain installation parts required during maintenance work in progress
4.Switch-over of consumers to different feed-in sections
5.Non-interruptible switch-over required
Single busbar
A single busbar is sufficient for most supply tasks, even if this supply task consists of two incoming feeders. It is straightforward and easy to handle, which reduces the likelihood of switching faults.
When fault-affected switching operations happen, circuit-breakers only must be operated. If the wrong breaker should be operated inadvertently, this would not have any safety-relevant consequences in the switchgear, since circuit-breakers are capable of making and breaking all load and short-circuit currents, even under earth-fault and other fault conditions.
In case of more intense branching (rule of thumb: more than five feeders), the single busbar can be subdivided once or several times, with its own feed-in in every section.
Disconnectors or switch disconnectors at the interruption points create bus sectionalizers, whereas circuit-breakers create longitudinal busbar couplers (BCL). A BCL makes sense if the busbar sections are to be operated as alternately separated or coupled.
Double busbar
Reasons for using a double busbar can be, for example:
1.Two or more feed-in points must always be operated separately (for example because there are different suppliers, or embedded power generation is used separate from the public grid).
2.Consumers with disturbing perturbations on the grid are separated from consumers placing high requirements on the power supply quality.
3.Consumers classified according to importance and assigned to service continuity requirements placed on the grids.
4.Limited short-circuit strength of already installed equipment requires a subdivision into two subsystems with switch-overs for load balancing in case of varying power demand
Apart from the first example, examples two to four allow the use of a transversal busbar coupler (BCT), which permits changing busbars without interrupting the energy flow (Figure 7).
Figure 7a – Duplicate busbar with bus sectionalizer and busbar coupler, transversal (BCT)
Figure 7b – Gas-insulated switchgear NXPLUS C (double-busbar)
4.8. Internal arc classification (IAC)
A successful type test of medium voltage switchgear also requires an internal arcing fault classification IAC in accordance with IEC 62271-200 (VDE 0671-200).
The classification distinguishes as follows:
Accessibility:
A – access for qualified personnel only
B – public access (meaning a testing under tightened conditions)
Qualified, accessible sides of the switchgear:
F – Front
L – Lateral
R – Rear
Test current and duration
Example: – Internal arc classification: IAC AR BFL 25 kA 1 s
The specification means that the rear side may only be accessed by qualified personnel, whereas the front and lateral sides may be accessed by anybody. The internal arcing test was made with a test current of 25 kA for a duration of 1 s.
Note: Medium-voltage switchgear are generally tested for accessibility of Type A. Only complete, factory-assembled stations (transformer /load-centre substations) are tested for Type B.
Testing normal switchgear for conformance with Type B doesn’t make sense, since they will always be built into an additional station housing in public spaces.
Considering the hazards involved in the occurrence of an arcing fault, the following aspects should be noted when configuring on the basis of the IEC 61936-1 (VDE 0101-1) standard:
1.Protection against operator faults, for instance ensured by the following measures:
Switch disconnectors instead of disconnectors
Make-proof switches
Locking devices
Unambiguous key locks
2.Keep operating aisles as short, high and wide as possible.
3.Use sealed encapsulations or covers instead of encapsulations with openings or meshed wire.
4.Deploy installations which are arcing-fault-tested instead of installations in open design (e.g. installations in accordance with IEC 62271-200; VDE 0671-200).
5.Bleed off arc gases into a direction away from the operator personnel, and if required, out of the building.
6.Use current-limiting devices.
7.Ensure very short tripping times from fast-acting relays or devices that respond to pressure, light or heat.
8.Operate the installation from a safe distance.
9.Prevent the re-energization by use of non-resettable devices which detect internal equipment faults, incorporate pressure relief and provide an external indication.
According to this, the operating room must always be included in the protective measures to be taken against the effects of an arcing fault:
1.A calculation of the dynamic pressure load on the operating room, from which an architect or structural engineer may recognize the stress on building structures, is recommended.
2.The operating room must be equipped with pressure relief openings of sufficient cross section or with a pressure relief duct.
Siemens provides two calculation methods as a service to establish rough guidance values for the calculation of the room size and /or pressure relief openings during the planning phase.
4.9. Estimation of pressure effects according to Pigler
A simple method provides the estimation according to F. Pigler for rooms up to 50 m³. Data on the room volume, the area of the free relief cross section and the short-circuit current to be tested are entered into a matrix.
This supplies a simple curve progression for the overload pressure (see Figure 8).
Figure 8 – Example of stationary excess pressures resulting from internal arcing faults
4.10. Finite-elements-simulation of pressure load under conditions of arcing
Although the incidence of an internal fault (arc fault) is very unlikely in type-tested air- or gas-insulated switchgear, the consequences of such an arcing fault may be severe for the operating personnel as much as for the room itself.
For this reason, appropriate measures in relation to the room situation must be provided for pressure relief, such as pressure relief outlets, ducts, absorbers or coolers. Possibly this must already be considered during the installation and room planning stage.
With the aid of ultra-modern finite element methods, pressure calculations can be performed in the entire three-dimensionally mapped space over the entire burning time of the accidental arc.
Some manufacturers of MV switchgears, such as Siemens – offers the service of a numerical calculation on the basis of a 3D volume model, where the real installation of the switchgear, pressure development, reflection, and arrangement of the pressure relief openings is taken account of.
Figure 9 – Contour plotting of a simulation at the point 0.1 s
Various pressure load scenarios can be calculated for specific switchgear types, short-circuit currents, and installation sites. Thus the customer benefits from extended planning security and a cost-optimised solution.
The flow conditions are defined as boundary conditions. Firstly, these are the switchgear steel sheets and secondly, the absorber sheets to be penetrated. At last, the pressure relief openings in the switchgear room are defined. But the model also allows to calculate a fully enclosed room, or factor in pressure relief openings with a pre-defined response pressure. As a result, the model yields the pressure rise and the flow conditions at any point of the finite elements grid over time.
Additionally, the pressure distribution on the walls can be shown as a contour plotting at a certain point in time (Figure 9).
Note: Typically, the overpressure caused by an arcing fault, when assuming the same room volume, is significantly higher for air-insulated switchgear than for metal-enclosed, gas-insulated switchgear.