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close this book Village Electrification
close this folder Part 3: Electrical control systems
View the document 1. Fundamental principles of electrical control systems
View the document 2. A concept for standardized electrical control systems
View the document 3. Specific problems with isolated, rural electrification schemes

2. A concept for standardized electrical control systems

2.1 Categories of Complexity

2.2 Standardized Designs

2.2.1 Standard Electrical Control System Category A

2.2.2 Standard Electrical Control System Category B

2.2.3 Standard Electrical Control System Category C

2.3 Components

2.3.1 Components for Standard Systems

2.3.2 Additional Equipment

2.3.3 Sources of Origin

2.4 Summary and Recommendations

Electrical control systems for small hydro schemes have been standardized worldwide to a large extent. However, those standards are designed for plant capacities above I MW. For schemes with capacities below 100 kW these standards cannot be applied directly, but have to be simplified and optimized with respect to the specific technical and economical requirements.


Table 8 Definition of categories of complexity for MHP schemes

Remarks: Instead of hydraulic governors electronic load controllers could be used without changing the complexity table.

The major reason BYS have not installed any load controllers to date is simply because all concerned schemes suffer from a lack of water - most plants have daily storage ponds - and minimization of discharge is therefore a must.

The common factor of the two sub-classes of category C is synchronisation (either of the two units or of one single unit to the grid). As there are only minor differences between these two cases they shall further on no longer be distinguished.

Even within the MHP range, the degree of control, protection and instrumentation can vary considerably. Determining factors for the degree of automation are:

- type of operation (isolated mode or connection to a grid)

- number of units

- type of speed governing system (manual or automatic control)

- capacity of plant

- category of operator (state-owned or private) - main consumers (domestic or industrial load)

- particular wishes of owners

- cost of electrical components

The optimal electrical system tends to provide high reliability, safety and user friendliness at competitive prices through equipment and design that is standardized and kept as simple and cheap as possible (but neither simpler nor cheaper than that!).

In connection with MHP schemes a concept that supports achieving these goals is based on the following elements:

-split of schemes into three categories of complexity

-a standardized design for each of the three respective electrical systems

-criteria for the selection of adequate components

2.1 Categories of Complexity

As a basis for the determination of the appropriate electrical system, BYS have divided its hydro power projects within Nepal into three categories of complexity.

2.2 Standardized Designs

In accordance with the above defined categories of complexity, BYS has worked out three standardized electrical systems. These will be outlined based on the corresponding single line diagrams on the following pages. For symbols, abbreviations of components and a summary refer to Table 9 and 10.

Apart from the automatic speed and voltage control of the generating set, the electrical control functions for run-up, voltage build-up, load control etc. are of manual type (push buttons) for all categories.

Electrical control systems based on a programmable logic controller (PLC), providing fully automatic control would form another category, characterized by a much higher level of automation. These systems are of limited relevance in this context and shall not be included here.

2.2.1 Standard Electrical Control System Category A


Fig 2 Single line diagram for the control circuit of category A. The load control is optional.

This system (see Fig 2) represents the basic level of automation and may well represent the absolute minimum requirements for an isolated electrification scheme. Besides a fuse/ circuit breaker to limit the output current and an over voltage relay, which deexcites the generator to limit the line voltage, this system has to be manipulated completely manually by an operator. Instrumentation is marginal, only three values are displayed: The (line) voltage, current and frequency. An hour meter is, however, highly recommended. The operation hours not only determine energy and power output but also the cycle of equipment servicing.

In case the load is simple e.g. does not vary, this might prove an adequate and very economic solution. In case of varying loads, which will be the case in even very simple distributions with several customers, the stability of this system will very soon reach its limits. It is not feasible either to keep a skilled operator all day long on the controls to regulate even moderate fluctuations. An automatic load controller can improve this situation (see part generators, IMAG).

As this system has no synchronisation gear, it cannot run parallel to a grid if a synchronous generator is used.

2.2.2 Standard Electrical Control System Category B


Fig 3 Single line diagram for the control circuit of category B. The earth fault relay is optional.

This category (see Fig 3) represents the majority of the realized rural electrification projects. Compared to category A it provides first of all an automatic speed regulation through a hydraulic governor. It is equipped with additional automatic safety controls like over current, under voltage and (optional) earth fault relay which all de-exile the generator.

Besides this all operations are manual and a skilled operator is needed.

This system is equipped with a substantially improved instrumentation: power, energy and operation time are displayed. This allows a simple recording of production/consumption, which is the base for any efficient energy management (water management), consumption prediction and tariff structures.

Still no synchronisation gear is added limiting this system's use to isolated operation.


Table 9 Abbreviation for the electrical symbols in the circuit diagrams (Fig 2, 3 & 4)


Fig 4 single line diagram for the control circuit of category C. Control panel I and 2 are identical.


Table 10 Summary of the three categories AB&C

2.2.3 Standard Electrical Control System Category C

The design of this electrical system (see Fig 4) is fairly complete regarding protection and instrumentation. Still, it is a manual control system with a semiautomatic paralleling procedure, without any provision for fully automatic control through a PLC.

2.3 Components

2.3.1 Components for Standard Systems

Some more details on components that are either of importance for control or that have been neglected so far.

Breakers / Over Current Protection:

To switch off the generator and disconnect it from the grid, there are two suitable alternatives:

* Moulded Case Circuit Breakers (MCCB). They are remote controllable (for category C systems).

Air Break (power) Contactor (ABC) in combination with thermal overload relays and HBC-fuses or with OCRs.

For very small, normally single phase systems (A), miniature circuit breakers (MCBs) will also serve for this purpose.

Control push buttons should be fitted to provide manual overriding signals for remote controlled breakers in category C systems.

Over-/ Undervoltage Protection:

Protective relays are normally available in different versions:

* single-phase or three-phase

* under- and overvoltage separate or combined

The best solution will mainly be determined by price

and availability.

Instrumentation Transformers:

Ampmeters should never be used with the full load current flowing through. Instead use always current transformers (CT) and standard 5A ampmeters. In rare cases (e.g. synchronisation equipment) PTs might be required.

Synchronizers:

Today's synchronizers are mostly fully automatic, switching the incoming breaker. Still, some simpler non-automatic versions are available which prevent

only from manual closing of the breaker under invalid conditions. As operators of rural electrification schemes are mostly not very experienced, strict use

* of automatic synchronizers is certainly the best solution. It could provide manual control for speed and voltage instead of the fully automatic adjustment through the synchronizers control signals. BYS has always followed this semi-automatic approach with its twin unit schemes.

2.3.2 Additional Equipment

The standard designs are fairly basic and cost-effective solutions (at least categories A and B). Additional components that aim to improve safety and reliability can, however, be incorporated at extra cost. Also the option to operate the station at least part-time unattended is provided. Especially in state run rural electrification schemes, there is a clear tendency towards a higher level of automation.

Typical additional features are:

Speed Monitoring Systems: with at least two independent subsystems for emergency overspeed triggering (e.g. first stage: electro mechanical sensor with speed-indicator and over speed relay; second stage: mechanical overspeed-switch).

Pressure Monitoring Systems: normally simple pressure gauges for penstock/turbine inlet and governor pressure.

- Shutdown-Solenoids: for automatic /emergency shutdown of governor or inlet valve.

- DC-Supply System: to provide an uninterruptible supply for the (DC) control system. It is based on lead acid or alcaline batteries.

- Status & Fault-lndicatorPanels: together with DC control systems.

2.3.3 Sources of Origin

In most developing countries one will have to distinguish between two groups of components: components that are locally available and components that have to be imported.

Components that generally fall into the first group are: instruments, switches, push-buttons, auxiliary relays, fuses, CTs, terminals, cables, glands...

Imported components are typically: protective relays, synchronisers, PLCs...

There are some components, which can fall in one group or the other group (medium-technology): MCCBs, ABCs... It is advisable, however, to avoid experiments with locally produced protective relays.

2.4 Summary and Recommendations

A set of standardized electrical control systems has been described above. In practice, however, it might not always be easy to relate a given project clearly to one of the categories specified and to choose the respective standard system, mainly for three reasons: * the classification does take into account neither the type/skill of consumer nor operator. * individual requirements/ requests of the customers might force a redesign to anon-standard system.

components might not be available at all or only at prohibitive costs.

To provide assistance for cases where either none of the proposed standardised designs seems to fit properly or simply to visualize the design and selection process, a list of design/ selection criteria shall be given hereunder.