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:
- The tasks of a switchgear substation can vary greatly
- Many influencing factors are interdependent
- 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.
- Distribution level of a MV switchgear
- Standards for the Design and Installation of MV Switchgear
- Configuration Parameters
- Medium-Voltage Switchgear Design:
- Operator protection
- Installation site
- Operating and maintenance areas
- Accessibility of compartments
- Service continuity during work in progress
- Busbar systems
- Internal arc classification (IAC)
- Estimation of pressure effects according to Pigler
- Finite-elements-simulation of pressure load under conditions of arcing
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.
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
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.
Generally speaking, typical forms of substation are:
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.
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.
The standards distinguish between two main groups of medium-voltage switchgear:
- 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)
- On-site or workshop-built switchgear in accordance with IEC 61936-1 (VDE 0101-1), as it is rarely built nowadays.
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.
The selection parameters for the configuration of switchgear can be distinguished as follows:
For example connection to earth, grid voltage, grid frequency, neutral-point connection, ambient conditions, peak short-circuit current.
For example insulation level, neutral-point connection, overvoltage protection, short-circuit duration, type of operating area, plant design.
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.
Tables: – Overview of the rated values and selection parameters for the confguration of medium-voltage switchgear
Primary rated values
Design and panel type
Compartments and partitions
Outgoing feeder components
Gas-insulated switchgear should be used for the medium voltage consumer substation. The advantages of gas-insulated switchgear are:
- Lower space requirements (up to approx. 70 % savings with 30 kV) compared to air-insulated switchgear
- Smaller transportation size and consequently easier shipping
- 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)
- Maintenance-free primary section (no lubrication and readjustment necessary)
- Better eco balance than air-insulated switchgear referred to the entire system life cycle
- The gas-insulated switchgear is safe to touch thanks to its earthed metal enclosure
- HV HRC fuses and cable terminations are only accessible if branch circuits are earthed
- Operation is only possible if the enclosure is fully sealed (and any doors closed)
- 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).
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
The medium voltage switchgear is usable as indoor installation in accordance with standard IEC 61936-1 (VDE 0101-1).
A distinction is made between:
- 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.
- 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.
- 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.
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).
A gas-insulated switchgear is also available as a type with:
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
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:
The following aspects play a part when choosing a single or double busbar:
- The number of outgoing and incoming feeders
- Separate operation of parts of the installation required
- Operability of certain installation parts required during maintenance work in progress
- Switch-over of consumers to different feed-in sections
- Non-interruptible switch-over required
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.
Reasons for using a double busbar can be, for example:
- 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).
- Consumers with disturbing perturbations on the grid are separated from consumers placing high requirements on the power supply quality.
- Consumers classified according to importance and assigned to service continuity requirements placed on the grids.
- 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).
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:
- 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:
- Protection against operator faults, for instance ensured by the following measures:
- Switch disconnectors instead of disconnectors
- Make-proof switches
- Locking devices
- Unambiguous key locks
- Keep operating aisles as short, high and wide as possible.
- Use sealed encapsulations or covers instead of encapsulations with openings or meshed wire.
- 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).
- Bleed off arc gases into a direction away from the operator personnel, and if required, out of the building.
- Use current-limiting devices.
- Ensure very short tripping times from fast-acting relays or devices that respond to pressure, light or heat.
- Operate the installation from a safe distance.
- 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:
- 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.
- 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.
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).
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.
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.
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.
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.
Reference // Planning of Electric Power Distribution – Technical Principles by Siemens