Small and Micro HPP: Electromechanical

Components and their ControlPART 2

Speed increasers• When the turbine and the generator operate at the same

speed and can be placed so that their shafts are in line, direct coupling is the right solution; virtually no power losses are incurred and maintenance is minimal.

• In many instances, particularly in the lowest power range, turbines run at less than 400 rpm, requiring a speed increaser to meet the 1 000-1 500 rpm of standard alternators. Turbine manufactures will recommend the type of coupling to be used, either rigid or flexible although a flexible coupling that can tolerate certain misalignment is usually recommended. In the range of powers contemplated in small hydro schemes this solution is always more economical than the use of a custom made alternator.

GeneratorsGenerators transform mechanical energy

into electrical energy. Nowadays only three-phase alternating

current generators are used in normal practice. Depending on the characteristics of the network supplied, the producer can choose between:

- Synchronous generators - Asynchronous generators.

Asynchronous generators IG are simple squirrel-cage induction motors with

no possibility of voltage regulation and running at a speed directly related to system frequency. They draw their excitation current from the grid, absorbing reactive energy by their own magnetism.Adding a bank of capacitors can compensate for the absorbed reactive energy. IG cannot generate when disconnected from the grid because are incapable of providing their own excitation current (without an external source of reactive energy).

Synchronous generators• SG are equipped with a DC excitation system

(rotating or static) associated with a voltage regulator, to provide voltage, frequency and phase angle control before the generator is connected to the grid;

• SG supply the reactive energy required by the power system when the generator is tied into the grid.

• Synchronous generators can run isolated from the grid and produce power since excitation is not grid-dependent.

• Synchronous generators are more expensive than asynchronous generators and are used in power systems where the output of the generator represents a substantial proportion of the power system load.

• Asynchronous generators are cheaper and are used in large grids where their output is an insignificant proportion of the power system load. Their efficiency is 2 to 4 per cent lower than the efficiency of synchronous generators over the entire operating range.

• In general, when the power exceeds 5000 kVA a synchronous generator is installed.

working voltage of the generator• The working voltage of the generator varies with its

power. • The standard generation voltages are:• 430 V up to 1400 kVA • 6000/6600 V for bigger installed power. • Generation at 430 V allows the use of standard

distributor transformers as outlet transformers and the use of the generated current to feed into the plant power system.

• Generating at medium voltage requires an independent transformer MT/LT to supply the plant services.

VSG relying on Power Electronics

• Recently, variable-speed constant-frequency systems (VSG), relying on Power Electronics, in which turbine speed is permitted to fluctuate widely, while the voltage and frequency are kept constant and undistorted, have entered the market.

• This system can even “synchronise” the unit to the grid before it starts rotating.

• The key to the system is the use of a electronic converter in conjunction with a IG.

• Unfortunately its cost price is still rather high (but see Wind Generators).

Generator configurations • Generators can be manufactured with horizontal

or vertical axis, independently of the turbine configuration. E.G.:A vertical axis Kaplan turbine turning at 214 rpm is directly coupled to a custom made 28 poles alternator.

• A flywheel is frequently used to smooth-out speed variations and assists the turbine control.

• When the generators are small, they have an open cooling system, but for larger units it is recommended to use a closed cooling circuit provided with air-water heat exchangers.

synchronisation • The mains supply defines the frequency of the stator

rotating flux and hence the synchronous speed above which the rotor shaft must be driven.

• On start-up, the turbine is accelerated up to 90-95% of the synchronous speed of the generator, when a velocity relay close the main line switch. The SG passes immediately to hyper-synchronism and the driving and resisting torque are balanced in the area of stable operation.

• The IG case the turbine is accelerated up to 105% of the synchronous speed and the driving and resisting torque are balanced in the area of stable operation at the mains frequency.

SG Voltage regulationExciters for synchronous generator: The exciting current for the synchronous generator can be supplied by a small DC generator, known as the exciter, to be driven from the main shaft. The power absorbed by this dc generator amounts to 0.5% - 1.0% of the total generator power. Nowadays a static exciter usually replaces the DC generator, but there are still many rotating exciters in operation.

Voltage regulation: Asynchronous generators

• An asynchronous generator needs to absorb a certain power from the three-phase mains supply to ensure its magnetisation.

• The IG can receive its reactive power from a separate source such as a bank of capacitors.

• Due to cost issues, a lot of effort was made over the years to improve the IG control for making it suitable to equip stand-alone MHPs.

• Recent advances in Power Electronics and μP technologies are offering the possibility to favorably solve the IG control problem.

• When the load changes, control is necessary to maintain the produced electric energy in an acceptable range.

• A variable load is affecting mainly two electric energy parameters: the voltage and the frequency.

Turbine controlTurbines are designed for a certain net head and

discharge. Any deviation from these parameters must be compensated for, by opening or closing control devices such as the wicket-vanes or gate valves to keep constant, either the outlet power, the level of the water surface in the intake or the turbine discharge.

In schemes connected to an isolated net, the parameter to be controlled is the runner speed, which control the frequency. The generator becomes overloaded and the turbine slows-down. In this case there are basically two approaches to control the runner speed:

• by controlling the water flow to the turbine; • keeping the water flow constant and adjusting the

electric load by an electric ballast load connected to the generator terminals.

Stand alone SHP and MHP

IG based systems

Variable load is affecting two electric parameters: the voltage and the

frequency.• The voltage can be controlled through the

excitation current by means of the excitation circuit. The excitation current is flowing through highly inductive circuits; consequently, the circulated power is the reactive power.

• The frequency is basically related to the active power of the load circuit.

• The dependence between the induced EMF and frequency is affected by the nonlinear characteristic of the magnetic circuit. The interdependence between the two parameters control has also to be considered.

IG Voltage control

• The voltage control is requiring a reactive power balance in the network.

• The performance of the IG voltage control circuit is highly influenced by the structure of the excitation circuit.

• In simple terms the voltage is regulated by the means of the self exciting capacitors in such cases. By controlling the reactive power, VAR, through the capacitors value, such controllers are known as VAR controllers.

AiABu

AuA

B

C

Modern VAR controllers employ micro-controllers for measuring the voltages and currents, compute the required value for the capacitors and switch

properly the electronic switches. The capacitance is varied in very fine amounts. The capacitance will

be varied considering different Power Factors.

The PWM inverter, the inductances play the role of capacitor emulator.

IG LOAD

EMULATOR CONTROL

Va

Vb

Gate Driving Signals

C

VcaVcbVcc

Vdc

Vdc

Frequency Control

To keep constant the frequency it is necessary to keep the active power constant at its rated value. As the loads of a MHP are variable by nature, the equilibrium cannot be maintained:

• a variable dump load must be added into the circuit, in order to maintain the total load constant.

• The balance can be reached by adjusting the input mechanical power, too. However, such a solution implies a water turbine speed governor. The mechanical adjustment is slow; it produces high mechanical and hydraulic stress and is less reliable.

the dump load

• There are different solutions. Basically, a power electronic controlled resistor is employed as dump load.

Automatic control

of Small hydro power plants

Automatic controlSmall hydro schemes and MHP are normally unattended and operated

through an automatic control system. Because not all power plants are alike, it is almost impossible to determine the extent of automation that should be included in a given system, but some requirements are of general application :

a) All equipment must be provided with manual controls and meters totally independent of the programmable controller to be used only for initial start up and for maintenance procedures.

b) The system must include the necessary relays and devices to detect malfunctioning of a serious nature and then act to bring the unit or the entire plant to a safe de-energised condition.

c) Relevant operational data of the plant should be collected and made readily available for making operating decisions, and stored in a database for later evaluation of plant performance, SCADA.

Automatic control 2d) An intelligent control system should be included to allow

for full plant operation in an unattended environment.e) It must be possible to access the control system from a

remote location and override any automatic decisions.f) The system should be able to communicate with similar

units, up and downstream, for the purpose of optimising operating procedures.

g) Fault anticipation constitutes an enhancement to the control system. Using an expert system, SCADA, fed with baseline operational data, it is possible to anticipate faults before they occur and take corrective action so that the fault does not occur.

Automatic control 3The system must be configured by modules:• An analogue-to-digital conversion module for measurement of

water level, wicket-gate position, blade angles, instantaneous power output, temperatures, etc.

• A digital-to-analogue converter module to drive hydraulic valves, chart recorders, etc.

• A counter module to count generated kWh pulses, rain gauge pulses, flow pulses, etc.

• a “smart“ telemetry module providing the interface for offsite communications, via dial-up telephone lines or radio link.

• modular software allows for easy maintenance.

This modular system approach is well suited to the widely varying requirements encountered in hydropower control, and permits both hardware and software to be standardised. Cost reduction can be realised through the use of a standard system.

Automatic control 4• Automatic control systems can significantly reduce the

cost of energy production by reducing maintenance and increasing reliability, while running the turbines more efficiently and producing more energy from the available water.

• With the tremendous development of desktop computers, their prices are now very low. The new programming techniques -Visual Basic, Delphi etc- assist the writing of software using well-established routines; the GUI interfaces, that every body knows thanks to the Windows applications; everything has contributed to erase the old aura of mystery that surrounded the automatic control applications.

Environmental impact• Electricity production in SHP and MHP is environmentally rewarding.• In sensitive areas, local impacts are not always negligible. The

significant global advantages of small hydropower must not prevent the identification of burdens and impacts at local level nor the taking of necessary mitigation actions.

• On the other hand because of their economic relevance, thermal plants are authorised at very high administrative levels, although some of their impacts cannot be mitigated at present. A small hydropower scheme producing impacts that almost always can be mitigated is considered at lower administrative levels, where the influence of pressure groups - angling associations, ecologists, etc.- is greater.

• It is not difficult to identify the impacts, but to decide which mitigation measures should be undertaken it is not easy, because these are usually dictated by subjective arguments. It is therefore strongly recommended to establish a permanent dialogue with the environmental authorities as a very first step in the design phase.

Economic Analysis

• The estimation of the investment cost constitutes the first step of an economic evaluation. For a preliminary approach the estimation can be based on the cost of similar schemes .

• In its recent publications ESHA analyses the cost of the different components of a scheme -weir, water intake, canal, penstock, power-house, turbines and generators, transformers and transmission lines.

graph sumarising data for schemes developed

Economic Analysis• A cost estimate is essential for economic analysis.• it is necessary as a second step, to make a preliminary

design including the principal components of the scheme.

• Based on this design, budget prices for the materials can be obtained from suppliers. Such prices cannot be considered as firm prices until specifications and delivery dates have been provided. This will come later, during the actual design and procurement process.

• Anyhow , a sound economic analysis must be performed regarding the ultimate economic efficiency of the SHP or MHP.

Conclusions: the advantages of Small and MHP

• It is the most concentrated energy source between the RES;

• The available energy is predictable;• The available power is continuously on

demand;• Only limited maintenance is required;• It is a long-lasting technology and its

performances are still object of improvement.

The main disadvantages

• It is a very site-specific technology;

• So it requires a great deal of experience from the designer, not available always;

• The useful power is limited by the available flow. It is not possible to expand the power.