Fig 1 - General layout of a hydropower plant

A hydroelectric plant converts the potential energy of water into electricity by the use of flowing water.

This water flows in water streams with different slopes giving rise to different potential for creating heads (size of fall), varying from river to river.

The capacity (power) of a plant depends on the head (change in level) and flow as a result of the hydrology in the catchment area of a river.

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Medium and high head schemes:

This type of plant typically uses weirs to divert water to the intake. From there it is led to the turbines via a pressure pipe or penstock. An alternative to penstocks, which in many cases is more economic, relies on a canal with reduced gradient running alongside the river. The canal carries the water to the pressure intake, and then, in a short penstock, to the turbines.

Categories of heads of the streams:

Low head schemes:

This kind of project is appropriate to river valleys, particularly in the lower reaches. Either the water is diverted to a power intake with a short penstock, or the head is created by a small dam, complete with integrated intake, powerhouse and fish ladder.

What are the main types of hydro schemes?

There are three main categories of hydroschemes, as described bellow by IEC (International Electrotechnical Commission):

• Run-of-river hydro plants use the river flow as it occurs, the filling period of its reservoir being practically negligible. The majority of small hydropower plants are run-of-river plants because of the high construction cost of a reservoir.
• Pondage hydro plants are plants in which the reservoir permits the storage of water over a period of a few weeks at most. In particular, a pondage hydro plant permits water to be stored during periods of low load to enable the turbine to operate during periods of high load on the same or following days. Some small hydropower plants fall into this type, especially high head ones with high installed capacities (> 1.000 kW).
• Reservoir hydro plants are plants in which the filling period of the reservoir is longer than several weeks. It generally permits water to be stored during high water periods to enable the turbine to operate during later high load periods. As the operation of these plants requires the construction of very large basins, practically no small or micro hydropower plant is of this type.

What are the typical characteristics of small-sized hydro schemes?

• Micro hydropower plants up to 100 kW
• Mini hydropower plants up to 500 kW
• Small hydropower plants up to 10,000 kW*

Micro-hydro power schemes normally only support investment in large reservoirs if these are built for other purposes in addition to hydropower (e.g. water abstraction systems, flood control, irrigation networks, recreation areas). Nevertheless, there are ingenious solutions for linking and fitting the turbine in waterways designed for other purposes, e.g. schemes integrated with an irrigation canal or a water abstraction system.

Below are a few examples of several possible applications of small, mini and micro hydropower plants.

Schemes integrated with an irrigation canal

EXAMPLE-1 The canal is enlarged to the extent required to accommodate the intake, the power station, the tailrace and the lateral bypass. The scheme should include a lateral bypass to ensure an adequate water supply for irrigation, should the turbine be shut down. This kind of scheme should be designed at the same time as the canal, because the widening of the canal in full operation is an expensive option.

EXAMPLE-2 The canal is slightly enlarged to include the intake and spillway. To reduce the width of the intake to minimum, an elongated spillway should be installed. From the intake, a penstock running along the canal brings the water under pressure to the turbine. The water, once through the turbine, is returned to the river via a short tailrace. As fish are generally not present in canals, fish passes are usually unnecessary.

Schemes integrated in a water supply system

Drinking water is supplied to a city by conveying the water from a headwater reservoir via a pressure pipe. Usually in this type of installation, the dissipation of energy at the lower end of the pipe at the entrance to the Water Treatment Plant is achieved through the use of special valves. The fitting of a turbine at the end of the pipe, to convert this otherwise lost energy to electricity, is an attractive option, provided that waterhammer, which would endanger the pipe, is avoided.
Waterhammer overpressures are especially critical when the turbine is fitted on an old pressure pipe.

Micro hydropower plants at sluice systems

The installation of a small hydropower plant in a sluice system along large rivers can be an interesting multi-purpose use of existing structures dedicated to other purposes. The exploitation for hydroelectric purposes of the head created by a sluice system allows the production of energy by a renewable energy source without further significant environmental impacts. An interesting and recent example of this application is given by a pilot project where a 26 kW turbine unit of four parallel 6.5kW propeller turbines has been inserted in an old sluice constructed for agricultural purposes at Niemieryczow in Poland.

Micro hydropower plants on river stabilization ramps

This rather unusual application is very interesting from an environmental point of view. Ramps are often constructed to stabilize the river course, particularly on fast flowing mountain rivers. The artificial head created by the ramps or by a series of check dams can be exploited for hydroelectric production. Installed power is however generally small since the flows and heads are generally low. Nevertheless this application represents an opportunity to meet the twin objectives of river protection and the use of a renewable energy source for energy production at the same time.

Micro hydropower plants at bed load barriers

Bed load barriers create an artificial head in the watercourse which can be exploited for energy production.

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How to choose a site from a technical point of view?

Apart from the environmental issues, which will be discussed in detail in later chapters, a MHPP should consider the following three main issues if it is to be economically feasible.

Relevant aspects for site evaluation:

What does the power of an MHPP depend on?

The amount of electricity generated is the result of the head and the flow rate at a specific site. The power generated also depends on the turbine generator efficiency and pressure losses at the intake and penstock. Moreover, other constraints, such as environmental issues and fisheries, may oblige the developer to leave a minimum flow in the watercourse. It should also not be forgotten that the available energy depends on the day-to-day and year-to-year variations of the flow. The impact of these variations could be very significant, so careful measurements should be made.

How to estimate the power availability in a site:

What parameters are used in selecting a hydro turbine?

Head, flow and power are the three main technical aspects in selecting a turbine. There are five main turbine types and each might be appropriate to certain physical conditions at each site.
Turbines can be grouped in two main categories:

Action turbines (or impulse turbines)

These only use the speed of the water, i.e. only use kinetic energy. This type of turbine is appropriate for high heads (75 meters to >1000 meters) and small flows.

The most popular such turbines are Pelton Wheels, which have a circular disc or runner with assembled vanes or double-hollow spoons. There are also other models like the Turgo side injection turbine, and the Ossberger or Banki Mitchell double propulsion turbines (these are further described in the text as “crossflow on Banki Mitchel turbine”).

Reaction turbines

This kind of turbine takes advantage of the water speed, and the pressure maintains the flow when contact takes place. The most frequently used are Francis and Kaplan turbines. Usually they have four basic elements: the casing or shell, a distributor, a pad, and the air intake tube.

There are two distinct groups: radial turbines (Francis type) are suitable for operating on sites with a medium head and flow and axial turbines (Kaplan and Propeller type) are appropriate for operation with low heads and high and low flows. Both action and reaction turbines may be used in MHPP.

What are the differences between the turbines?

Pelton turbine: is a typical high head turbine, which can also be used for medium heads, with power ranging from 5 kW to large sizes. This is an easy to use action-type turbine with a high efficiency curve and it has a good response to variations in flow.

Cross Flow or Banki Mitchell turbines: are mainly used at sites where there is low installed power. In general their overall efficiency (around 75-80%) is lower than conventional turbines. They have a good response to variations in flow, which makes them appropriate for work where there is a wide range of flows. They have the advantage of simplicity and ease of maintenance and repair. They are a tried and proven technology which can exploit sites that cannot otherwise be used economically and where, therefore, their limited efficiency is not relevant. They are suitable for low to high head sites from 1 m to 200 m head with flows over 100 l/s.

Francis Turbines: are single regulated turbines more appropriate to use with higher heads given their efficiency.

Propeller turbines: have the advantage of running at high speeds even at low heads. Kaplan Turbine are high efficiency propeller-type turbines, very advanced and consequently quite expensive in investment and maintenance. Their response to different ranges of flow conditions is very good.More is said about propeller turbines in the next chapter.

Pelton turbine	Cross Flow or Banki Mitchell turbines
Francis Turbines	Propeller turbines (Kaplan Turbine)

SOURCE: GUIDE LINES FOR MICRO HYDROPOWER DEVELOPMENT – European Commission

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Substation, Its Function And Types

An electrical sub-station is an assemblage of electrical components including busbars, switchgear, power transformers, auxiliaries etc.

These components are connected in a definite sequence such that a circuit can be switched off during normal operation by manual command and also automatically during abnormal conditions such as short-circuit. Basically an electrical substation consists of No. of incoming circuits and outgoing circuits connected to a common Bus-bar systems. A substation receives electrical power from generating station via incoming transmission lines and delivers elect. power via the outgoing transmission lines.

Sub-station are integral parts of a power system and form important links between the generating station, transmission systems, distribution systems and the load points.

MAIN TASKS

…Associated with major sub-stations in the transmission and distribution system include the following:

1. Protection of transmission system.
2. Controlling the Exchange of Energy.
3. Ensure steady State & Transient stability.
4. Load shedding and prevention of loss of synchronism. Maintaining the system frequency within targeted limits.
5. Voltage Control; reducing the reactive power flow by compensation of reactive power, tap-changing.
6. Securing the supply by proving adequate line capacity.
7. Data transmission via power line carrier for the purpose of network monitoring; control and protection.
8. Fault analysis and pin-pointing the cause and subsequent improvement in that area of field.
9. Determining the energy transfer through transmission lines.
10. Reliable supply by feeding the network at various points.
11. Establishment of economic load distribution and several associated functions.

TYPES OF SUBSTATION

The substations can be classified in several ways including the following :

1. Classification based on voltage levels, e.g. : A.C. Substation : EHV, HV, MV, LV; HVDC Substation.
2. Classification based on Outdoor or Indoor : Outdor substation is under open skv. Indoor substation is inside a building.
3. Classification based on configuration, e.g. :
   • Conventional air insulated outdoor substation or
   • SF6 Gas Insulated Substation (GIS)
   • Composite substations having combination of the above two
4. Classification based on application
   • Step Up Substation : Associated with generating station as the generating voltage is low.
   • Primary Grid Substation : Created at suitable load centre along Primary transmission lines.
   • Secondary Substation : Along Secondary Transmission Line.
   • Distribution Substation : Created where the transmission line voltage is Step Down to supply voltage.
   • Bulk supply and industrial substation : Similar to distribution sub-station but created separately for each consumer.
   • Mining Substation : Needs special design consideration because of extra precaution for safety needed in the operation of electric supply.
   • Mobile Substation : Temporary requirement.
     NOTE :
   • Primary Substations receive power from EHV lines at 400KV, 220KV, 132KV and transform the voltage to 66KV, 33KV or 22KV (22KV is uncommon) to suit the local requirements in respect of both load and distance of ultimate consumers. These are also referred to ‘EHV’ Substations.
   • Secondary Substations receive power at 66/33KV which is stepped down usually to 11KV.
   • Distribution Substations receive power at 11KV, 6.6 KV and step down to a volt suitable for LV distribution purposes, normally at 415 volts

SUBSTATION PARTS AND EQUIPMENTS

Each sub-station has the following parts and equipment.

1. Outdoor Switchyard
   • Incoming Lines
   • Outgoing Lines
   • Bus bar
   • Transformers
   • Bus post insulator & string insulators
   • Substation Equipment such as Circuit-beakers, Isolators, Earthing Switches, Surge Arresters, CTs, VTs, Neutral Grounding equipment.
   • Station Earthing system comprising ground mat, risers, auxiliary mat, earthing strips, earthing spikes & earth electrodes.
   • Overhead earthwire shielding against lightening strokes.
   • Galvanised steel structures for towers, gantries, equipment supports.
   • PLCC equipment including line trap, tuning unit, coupling capacitor, etc.
   • Power cables
   • Control cables for protection and control
   • Roads, Railway track, cable trenches
   • Station illumination system
2. Main Office Building
   • Administrative building
   • Conference room etc.
3. 6/10/11/20/35 KV Switchgear, LV
   • Indoor Switchgear
4. Switchgear and Control Panel Building
   • Low voltage a.c. Switchgear
   • Control Panels, Protection Panels
5. Battery Room and D.C. Distribution System
   • D.C. Battery system and charging equipment
   • D.C. distribution system
6. Mechanical, Electrical and Other Auxiliaries
   • Fire fighting system
   • D.G. Set
   • Oil purification system

An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be “planned” or “unplanned”. A transmission line or other component may need to be deenergized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.

Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.

There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.

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ABB - General about motors

Modern electrical motors are available in many different forms, such as single phase motors, three-phase motors, brake motors, synchronous motors, asynchronous motors, special customised motors, two speed motors, three speed motors, and so on, all with their own performance and characteristics.
For each type of motor there are many different mounting arrangements, for example foot mounting, flange mounting or combined foot and flange mounting. The cooling method can also differ very much, from the simplest motor with free self-circulation of air to a more complex motor with totally enclosed air-water cooling with an interchangeable cassette type of cooler.

To ensure a long lifetime for the motor it is important to keep it with the correct degree of protection when under heavy-duty conditions in a servere environment. The two letters IP (International Protection) state the degree of protection followed by two digits, the first of which indicates the degree of protection against contact and penetration of solid objects, whereas the second states the motor’s degree of protection against water.
The end of the motor is defined in the IEC-standard as follows:

• The D-end is normally the drive end of the motor.
• The N-end is normally the non-drive end of the motor.

Note that in this handbook we will focus on asynchronous motors only.

Squirrel cage motors

In this chapter the focus has been placed on the squirrel cage motor, the most common type of motor on the market. It is relatively cheap and the maintenance cost is normally low.

There are many different manufacturers represented on the market, selling at various prices. Not all motors have the same performance and quality as for example motors from ABB. High efficiency enables significant savings in energy costs during the motor’s normal endurance. The low level of noise is something else that is of interest today, as is the ability to withstand severe environments.

There are also other parameters that differ. The design of the rotor affects the starting current and torque and the variation can be really large between different manufacturers for the same power rating. When using a softstarter it is good if the motor has a high starting torque at Direct-on-line (D.O.L) start. When these motors are used together with a softstarter it is possible to reduce the starting current further when compared to motors with low starting torque. The number of poles also affects the technical data. A motor with two poles often has a lower starting torque than motors with four or more poles.

Voltage

Three-phase single speed motors can normally be connected for two different voltage levels. The three stator windings are connected in star (Y) or delta (D). The windings can also be connected in series or parallel, Y or YY for instance. If the rating plate on a squirrel cage motor indicates voltages for both the star and delta connection, it is possible to use the motor for both 230 V, and 400 V as an example.

The winding is delta connected at 230 V and if the main voltage is 400 V, the Y-connection is used. When changing the main voltage it is important to remember that for the same power rating the rated motor current will change depending on the voltage level. The method for connecting the motor to the terminal blocks for star or delta connection is shown in the picture below.

Power factor

A motor always consumes active power, which it converts into mechanical action. Reactive power is also required for the magnetisation of the motor but it doesn’t perform any action. In the diagram below the active and reactive power is represented by P and Q, which together give the power S.

The ratio between the active power (kW) and the reactive power (kVA) is known as the power factor, and is often designated as the cos φ. A normal value is between 0.7 and 0.9, when running where the lower value is for small motors and the higher for large ones.

Speed

The speed of an AC motor depends on two things: the number of poles of the stator winding and the main frequency. At 50 Hz, a motor will run at a speed related to a constant of 6000 divided by the number of poles and for a 60 Hz motor the constant is 7200 rpm.

To calculate the speed of a motor, the following formula can be used:

n = speed
f = net frequency
p = number of poles

Example:
4-pole motor running at 50 Hz

This speed is the synchronous speed and a squirrel-cage or a slip-ring motor can never reach it. At unloaded condition the speed will be very close to synchronous speed and will then drop when the motor is loaded.

The difference between the synchronous and asynchronous speed also named rated speed is ”the slip” and it is possible to calculate this by using the following formula:

s = slip (a normal value is between 1 and 3 %)
n₁ = synchronous speed
n = asynchronous speed (rated speed)

Table for synchronous speed at different number of poles and frequency:

Torque

The starting torque for a motor differs significantly depending on the size of the motor. A small motor, e.g. ≤ 30 kW, normally has a value of between 2.5 and 3 times the rated torque, and for a medium size motor, say up to 250 kW, a typical value is between 2 to 2.5 times the rated torque. Really big motors have a tendency to have a very low starting torque, sometimes even lower than the rated torque. It is not possible to start such a motor fully loaded not even at D.O.L start.

The rated torque of a motor can be calculated using the following formula:

M_r = Rated torque (Nm)
P_r = Rated motor power (kW)
n_r = Rated motor speed (rpm)

Slip-ring motors

In some cases when a D.O.L start is not permitted due to the high starting current, or when starting with a star-delta starter will give too low starting torque, a slip-ring motor is used. The motor is started by changing the rotor resistance and when speeding up the resistance is gradually removed until the rated speed is achieved and the motor is working at the equivalent rate of a standard squirrel-cage motor.

In general, if a softstarter is going to be used for this application you also need to replace the motor.

The advantage of a slip-ring motor is that the starting current will be lower and it is possible to adjust the starting torque up to the maximum torque.

SOURCE: ABB – SOFTSTARTER HANDBOOK

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Current Switching with High Voltage Air Disconnector

In the paper are presented results of switching overvoltages investigations, produced by operations of air disconnector rated voltage 220 kV. Measurements of these switching overvoltages are performed in the air-insulated substation HPP Grabovica on River Neretva, which is an important object for operation of electric power system of Bosnia and Herzegovina.

Investigations of operating of air disconnector type Centre-Break were performed in order to determine switching overvoltage levels that can lead to relay tripping in HPP Grabovica. During operations of disconnector (synchronization or disconnecting of generator from network) malfunctions of signalling devices and burning of supply units of protection relays were appeared. Also, results of computer simulations using EMTP-ATP [1] are presented.

I. INTRODUCTION

Switching operation in power stations and substations, highvoltage faults and lightning cause high levels of high frequency overvoltages that can be coupled with low voltage secondary circuits and electronic equipment unless they are suitably protected. The function of high-voltage air-break disconnectors is to provide electrical isolation of one part of the switchgear.

Disconnector’s standards define a negligible current interrupting capability (≤0.5 A) or a voltage between the contacts if it is not significantly changed. These values of currents include the capacitive charging currents of bushing, bus bars, connectors, very short lengths of cables and the current of voltage instrument transformers. Disconnector’s contacts in air-insulated substations (AIS) are moving slowly causing numerous strikes and restrikes between contacts.

When the contacts are closed, the capacitive charging current flowing through the contacts ranges from 0.017×10^-3 to 1.1×10^-3 A/m for voltage levels 72.5 – 500 kV [2], depending on the rated voltage and length of bus, which is switched.

Strikes and restrikes occur as soon as the dielectric strength of the air between contacts is exceeded by overvoltage. The distance between contacts, the contacts geometry and relative atmospheric condition defines the overvoltage at the instant of strike. Every strike causes high-frequency currents tending to equalize potentials at the contacts. When the current is interrupted, the voltages at the source side and the loading side will oscillate independently. The source side will follow the power frequency while the loading side will remain at the trapped voltage. As soon as the voltage between contacts exceeds the dielectric strength of the air, at that distance the restrike will occur, and so on. Successive strikes occurring during the closing and opening operations of the off-loaded bus by the disconnector are shown in Fig. 1 a and b, respectively.

When closing takes place, the first strike will occur at the maximum value of the source voltage. Its values can be positive or negative. As the time passes a series of successive strikes will keep occurring at reduced amplitude, until the contacts touch. The highest transient overvoltage therefore occurs during the initial pre-arc, Fig.1 a. When the disconnector opening, restrikes occur because of the very small initial clearance between the contacts. At the transient beginning, the intervals between particular strikes are on the order of a millisecond, while just before the last strike; the period can reach about one half of cycle at power frequency, Fig. 1 b.

Fig. 1. The voltage due to the disconnector switching a) Disconnector closing, b) Disconnector opening 1-source side voltage, 2- load side voltage

During the switching time of operations of disconnectors at HPP Grabovica up to 500 restrikes were registered. In paper [3] there are up to 5000 restrike registered during switching operation of the disconnector. The maximum value of voltages and maximum value of the wave front increasing will take place at the maximum distance between contacts. For the purpose of the investigation of the insulation strength and induction of electromagnetic interferences (EMI), the most important are the first few strikes during the closing operation or the last few strikes during the opening operation. Each individual strike causes a travelling wave with the basic frequency on the order 0.5 MHz (330 kHz-600 kHz). Very fast transient overvoltage due to the closing operation of the disconnector at the load side of the test circuit is shown in Fig.2.

Fig. 2. Very fast transient overvoltage due to the closing operation Channel 1- source side voltage Channel 2-load side voltage

These high-frequency phenomena are coupled with the secondary circuits as a result of various mechanisms. The strongest interference is exerted by the stray capacities between the high-voltage conductors and the grounding system, followed by the metallic link between the grounding system and the secondary circuits.

High-frequency transient current flowing in the grounding system generates potential differences, every time when a strike occurs between disconnector’s contacts. In large secondary circuits, the potential differences are in the form of longitudinal voltages between the equipment inputs and the equipment enclosures.

Depending on the type of secondary circuits used and the way they are laid, differential voltages may also occur. Such a coupling mechanism has a special effect on the secondary circuits of instrument transformers, and particularly on the connected instruments, since these circuits are always galvanically linked to the grounding system. Another factor, which cannot be discounted, is the linking of these circuits to the primary plant via the internal capacities of the instrument transformers [4].

Interference levels in secondary circuits of air-insulated substations during switching disconnectors depend on following parameters:

• The transient voltages and currents generated by the switching operation;
• The voltage level of the substation;
• The relative position of the source of disturbances and susceptor;
• The nature of the grounding network;
• The cable type (shielded or unshielded);
• The way the shields are grounded.

There are two main modes of coupling secondary circuits with primary circuits [3, 5]:

1. Electromagnetic or EM coupling, which can be split into three sub-categories; inductive, capacitive and radiative. The most important source of EM coupling is the propagating current and voltage waves on bus bars and power lines during high-voltage switching operations by disconnectors;
2. Common impedance coupling, as a result of coupling caused by the sharing of a lumped impedance common to both the source and susceptor circuits.

Common mode voltages, i.e., voltages measured between conductors and local ground, represent the main parameter used for assessing equipment immunity. The difficulty of comparing data comes from the fact that different authors performed measurements at different places (some measurements were made at the closest point to the disconnector being operated whereas others made measurements in the vicinity of the auxiliary equipment, i.e. in the relay room). Little information is available about the grounding practice of the neutral conductor in CT or VT circuits, the quality and grounding of the sable shields as well as how the measurements have been performed. Therefore, the measured levels have to be analyzed very carefully before comparison and drawing any conclusions [5]. Results of up to date measured common mode voltages at secondary circuits of CVT, CT and VT are presented in the paper [5]. There are maximum levels of the common mode voltages ranging from 100 V_peak up to 2.5 kV_peak in the shields of the secondary circuits cables of the CT and VT. Results show that measured values of the common mode voltages at CT/CV secondary circuits, 220 kV ratings, range from U_cm=0.32 kV_peak [6] up to U_cm=0.85 kV_peak [7].

Results shown in paper [3] are for measured common mode voltages from 3-4 kV during switching operation by disconnector in 150 kV switchgear up to 6-10 kV at 400 kV switchgear.

II. RESULTS OF EXPERIMENTAL MEASUREMENTS ON SITE

The last ten years of extensive analysis of disconnector and circuit breakers generated EMI measurements that have confirmed that disconnector operation with off-loaded busbar is the most important and typical source of interference in secondary circuits of substations. Measurements of switching overvoltages generated during disconnector operation in the air insulated substation HPP Grabovica on the river Neretva were performed. HPP Grabovica is an important object for operating of electric power system of Bosnia and Herzegovina. Investigations of operating of air disconnector type Centre-Break were performed in order to determine switching overvoltage levels that can lead to relay tripping in HPP Grabovica [8].

During operations of disconnector (synchronization or disconnecting of generator from network) malfunctions of signalling devices and burning of supply units of protection relays were appeared. Malfunctioning of auxiliary circuits were manifested by tripping relay of differential protection of the generator, phase ’4′- signalization on relay box ‘ZB I‘ and signalling ‘fire’ in 35 kV control panel.

At the same time sparking between primary terminals of the current transformer (CT) was occurred. Malfunctioning of
signalling circuits were lower (not eliminated) with installing shielded cables. Also, independent of switching operation of air insulated disconnectors, during synchronization of generator AG1 on network, it’s happened that one of the pole of 220 kV circuit breaker failures. In this case generator AG1 worked in motor regime. Because of that, HPP Grabovica plans to install circuit breakers on generator’s voltage (10,5 kV) [9].

The field tests were performed at the test circuit at HPP Grabovica, Fig. 3.

Fig. 3. The considered test circuit VT-voltage transformer (220/√3/0.1/√3/0.1/√3 kV), CT-current transformer (200/1/1 A), CVD-capacitive voltage divider, CB-circuit breaker with two interrupting chambers and parallel capacitors (SF6 220 kV, 1600 A), Dc- disconnector (220 kV, 1250 A), MOSA-metal oxide surge arrester (Ur=199,5 kV, 10 kA), PT-power transformer (64 MVA, 242/10,5±5% kV, YD5), AG1- generator 1 (64 MVA, 10,5±5% kV)

The recorded wave shape of the overvoltage at the load side is shown in Fig. 4. The overvoltage factors at busbar, k, were recorded up to 1.16 p.u. with the dominant frequency of considered transient f_d equal to 0.536 MHz. Common mode voltages, U_cm, at VT were up to 708 V_peak, with dominant frequency equal to 1.31 MHz.

Fig. 4. Waveshape of the overvoltage Channel 1-voltage at CVD; ch 1 (2.5 V/div), probe 1x100, ratio 455 Channel 2-voltages at secondary of VT; ch 2 (5 V/div), probe 1x100

III. MODELING OF THE TEST CIRCUIT

Computer simulations were performed on the model of test circuit containing elements drawn in Fig. 5. Overvoltages at busbars were calculated during disconnector closing operations, for the same substation layout on which measurements were carried out.

Fig. 5. Model of the test circuit Arc-4 Ω; stray-200 pF; connection tube Z=370 Ω; CVD-R=300 Ω, C=1 nF; VT-500 pF; CB-2 capacitors, each C≅2 nF, (capacitance of open contacts, each C≅20 pF), Ccb=100 pF; CT-500 pF; MOSA-100 pF; connection wire Z=440 Ω; PT-3.5 nF

The waveshape of simulated overvoltage surge at load side is given in Fig. 6. The difference between magnitudes of measured and simulated overvoltages is 5 %. The dominant frequency of simulated overvoltage is 0.620 MHz. Comparison between results of measured and calculated overvoltages certified a good agreement of obtained values.

Fig. 6. Waveshape of simulated overvoltage surge

When the Capacitive Voltage Divider (CVD) was excluded, there were higher values of calculated overvoltages (15% higher on amplitude and 6 % on frequency). Capacitive divider due to primary resistor equal to 300 W and primary capacitance equal to 1 nF influences on overvoltage at the same measurement point causing attenuation and damping of transient overvoltrages. In order to reduce EMI in secondary circuits the best way is to reduce sources of interference emission during switching of air insulated disconnector.

One of the ways of reducing is to install disconnecting circuit breakers. Substation disconnectors isolate circuit breakers from rest of the system during maintenance and repair. The maintenance requirements for modern SF6 high voltage circuit breakers are lower than maintenance demands made on disconnectors, which means one of reasons for disconnectors removed. Installing disconnecting circuit breaker there are no needs for switching operation of disconnectors. With disconnecting circuit breakers it is still possible to isolate the line, but low maintenance requirements means it is no longer necessary to isolate the circuit breaker. The disconnecting breaker had to be designed to safety lock in the open position, and to meet all voltage withstanding capabilities and safety requirements of disconnectors.

Another way of reducing sources of interference emission is to install circuit breaker without parallel capacitors to contacts. This suggestion is based on analyses performed on three circuit models:

1. Model of CB with two breaking chambers and paralel capacitors and VT on netvork side of CB;
2. Model of CB with two breaking chambers and without paralel capacitors and VT on netvork side of CB
3. Model of CB with two breaking chambers and without paralel capacitors and VT on generator side of CB

Magnitudes of simulated overvoltages are presented in Table I. Voltages are measured in point of connection of VT, CT and PT.

TABLE I - MAGNITUDES OF SIMULATED OVERVOLTAGES

Overvoltages on generator side of 220 kV CB during switching of disconnectors could be up to 320 V in the case of installing instrument voltage transformer (VT) on generator side of CB without parallel capacitors (near instrument current transformer CT). This case causes installing of circuit breaker at generator’s voltage (10,5 kV) for synchronization of generator to network (better conditions for synchronization). This solution of installing circuit breakers on generator’s voltage resulted from problems have occurred during synchronization of generatror with current 220 kV CB.

IV. CONCLUSION

Switching overvoltages due to disconnector operations have been analysed on the existing 220 kV AIS on HPP Grabovica. Measurements and calculations were conducted on the characteristic points in AIS, in order to determine the level of the EMI.

The result of measurements has shown that high frequency voltages on busbars occur with amplitudes up to 1.16 p.u. (233 kV_peak) and the dominant frequencies up to 0.6 MHz. The difference between magnitudes of measured and calculated overvoltages is 5 % and 15.6 % on frequency. Measured common mode voltages at secondary circuits were from 430 V up to 708 V. CVD influences on overvoltages at the same measurement point on busbars causing attenuation and damping of transient overvoltages.

Comparison of the transient computer simulations with field measurements showed that calculations could be used for
assessment of the transient overvoltages due to disconnector switching. In order to reduce EMI in secondary circuits, it is suggested to install switching modules and disconnecting circuit breakers [10] or to install circuit breakers without parallel capacitors to contacts.

AUTHORS: Salih Carsimamovic, Zijad Bajramovic, Miroslav Ljevak, Meludin Veledar, Nijaz Halilhodzic

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