report of frequency control in power system with renewable resources

8/8/2019 Report of Frequency Control in Power System With Renewable Resources

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UNIVERSITY OF BRITISH COLUMBIA

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

Frequency Control in Power Systemwith Renewable Resources

EECE 561 Final Report

Alternative Energy Sources

Instructor: Dr. William G. Dunford

Author: Shahrzad Rostamirad

7/26/2010


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TABLE OF CONTENTS

TABLE OF CONTENTS....................................................................................................................... 2

LIST OF ILLUSTRATIONS................................................................................................................... 31. INTRODUCTION ....................................................................................................................... 4

2. BASICS OF FREQUENCY CONTROL AND PROTECTION ............................................................. 5

2.1 FREQUENCY AND POWER BALANCE ................................................................................ 5

2.2 FREQUENCY CONTROL MECHANISM ............................................................................... 8

2.2.1 SPEED GOVERNOR .................................................................................................... 8

2.2.2 AUTOMATIC GENERATION CONTROL ..................................................................... 10

2.3 UNDERFREQUENCY PRIMARY PROTECTION .................................................................. 123. DYNAMIC FREQUENCY CONTROL .......................................................................................... 14

3.1 DEMAND MATCHING: LOAD FOLLOWING ..................................................................... 14

3.2 DEMAND MATCHING: FREQUENCY RESPONSE.............................................................. 17

3.3 CURRENT STANDARDS ON RE FREQUENCY RESPONSE .................................................. 18

3.3.1 GRID CONNECTION POLICIES .................................................................................. 18

3.3.2 ISLANDING POLICIES ............................................................................................... 19

4. FUTURE OF FREQUENCY PROTECTION IN POWER SYSTEM .................................................. 204.1 FRQUENCY RESPONSE CAPABILITIES OF RE ................................................................... 21

4.2 VARIABLITY OF RE ........................................................................................................... 22

4.3 VIRTUAL POWER PLANT ................................................................................................. 23

5. CONCLUSION ......................................................................................................................... 26

6. REFERENCES ........................................................................................................................... 27


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LIST OF ILLUSTRATIONS

Figure 1: Equivalent circuit of a synchronous generator [2] .......................................................... 5

Figure 2: Phasor diagram of synchronous generator [2] ................................................................ 5

Figure 3: Change in phasor diagram when electrical real power increases ................................... 7

Figure 4: The frequency versus power curve of a generator [2] .................................................... 8

Figure 5: Governor speed-power characteristics of two generator units 1 and 2 [5] .................... 9

Figure 6: Governor frequency-power characteristics for different speed references [5] ............ 10

Figure 7: Demand curve of England and Wales [4] ...................................................................... 15

Figure 8: Ontario demand curve at July 20th [12] ........................................................................ 16

Figure 9: Electricity Generation of Canada in 1999 ...................................................................... 16

Figure 10: Timescale of frequency response [4] ........................................................................... 17

Figure 11: Anticipatory - stage 1 of Combined Power Plant operation [19] ................................ 24

Figure 12: Fine Tuning - stage 2 of Combined Power Plant operation [19] ................................. 24

Table 1: 59.1 Hz Load Shedding Plan as required by WECC [9] .................................................... 12

Table 2: Abnormal frequency minimum performance [15] ......................................................... 18

Table 3: Distributed generation and storage elements of the Combined Power Plant [19] ........ 23


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1. INTRODUCTIONIn North America, the current issues with developing Renewable Energy (RE) problem is not the

methods of power generation because the technology, equipment and methods are all

available. The problem is, in fact, mechanism of sharing the generated electrical power.

According to Small-Scale RE Systems, Grid-Connection and Net Metering: Overview of Canadianexperience in 2003, two obstacles that prevent widespread deployment of RE are identified as:

[1]

1- Inconsistency in policies and standards2- Reluctance of utilities to share the lines

While many technical and safety factors need to be considered prior to integration of a

generator to the system such as operating voltage, frequency, substation grounding and

insulation, this report sheds some light on one major concern which is the frequency response

of the system after the introduction of RE resources.

Statements, such as distributed generators free ride on the transmission grid operator or the

regulatory body to maintain system frequency or REs endanger reliability and stability of the

grid, are heard every day. However, the question is whether they are accurate and what can be

done to improve the system. [2]

The first chapter of the report, which provides technical background on the system frequency

and its control mechanism, in fact clarifies the meaning of the above stereotypical statement.

The second chapter introduces the demand matching concept and its timelines in which

generators need to respond to load variations. Furthermore, it summarizes two majorstandards, one is local and the other is international, on integration requirements of RE

generators. The third chapter illustrates the capabilities of different renewable energy

resources to sustain system frequency control and the concept of Virtual Power Plant to further

expand their potential limits.


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2. BASICS OF FREQUENCY CONTROL AND PROTECTIONIn a conventional power system, it is the synchronous generator that converts mechanical

power of water or steam to AC electrical power. This part of the report reviews the operation of

synchronous generator and auxiliary control such as governor as they affect frequency of the

system. This chapter also illustrates the function of automatic generation control (AGC) toreturn the frequency of the system back to normal.

2.1 FREQUENCY AND POWER BALANCEA synchronous generator circuit is consisted of two subsystems of rotor (or field) and stator (or

armature) winding. Rotor circuit, a coil fed by a DC source, is attached to prime mover and

rotates it. This rotation induces AC voltages of EA on the armature circuit, which is connected to

the power grid via terminal voltage (VT). The equivalent circuit of synchronous generators is

shown below. [2]

Figure 1: Equivalent circuit of a synchronous generator [2]

The phasor representation of the synchronous generator stator circuit is simplified in the

following graph (assuming inductive load and lagging power factor).

Figure 2: Phasor diagram of synchronous generator [2]

EA

RA

jXA

LF

RF

VF VT

EA

VT

jXIA

RAIA

jXAIA

a

P

a Q


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The electrical output power (both active and reactive) of a generator, which is the amount of

power consumed by the load, determines the armature current and terminal voltage of the

generator [2]:

Se= Pe + jQe = 3 VT IA*

On the other hand, mechanical power depends on the torque of the prime mover (Tm), which is

the product of force applied by the source of energy (water or steam) times the radius of prime

mover. Power is related to torque by T = P x .

Since both electrical and mechanical torques try to accelerate the rotor but in the opposite

direction, the accelerating torque (or power) of a generator could be written as the product of

the rotor mass moment of inertia (J) and acceleration of rotor angle () [3]:

To better understand the connection between rotor angle, synchronous frequency and

frequency of the system, it is important to further discuss rotor angle. The above rotor angle

(), which is the angle of the rotor with respect to an arbitrary stationary axis, could be

represented as a relative angle () versus a reference axis which rotates at the synchronous

speed (e.g. 60 Hz) of:

By combining the above two formulas and considering that angular speed is the rate of change

of the angle, the following equation is concluded:

The above formula, swing equation, shows when electrical power increases, the frequency of

the system will drop until the electrical power returns to its initial value or governor adjusts the

mechanical power (this is explained in the next section) to be equal to electrical power [3]. The

amount of change in frequency also depends on angular momentum of the generator,,,

which is related to rotors resistance to changes to rotation [3].

Another way of illustrating the relation between frequency and real power is to use phasor

diagram of stator. First of all, the internal voltage of a synchronous generator (EA), which is

induced by revolution ofrotors flux (), changes with the speed of the machine:


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When the speed of the rotor drops, the induced voltage, , decreases. Due to drop in

frequency, the armature reactance (Xl) decreases. The terminal voltage is assumed to remain

constant; the armature current increases more significantly (as well as its angle) due to increase

in electrical output power. The following figure shows the phasor diagram of a generator after a

sudden increase in electrical power which led to drop in frequency (rotor speed).

Figure 3: Change in phasor diagram when electrical real power increases

In summary, a boost in load real power demand forces the electrical output power of the

feeding synchronous generator to increase. According to their mass moment of inertia,

generators speed (frequency) will initially drop to meet the extra demand. However, after that,

governor will sense the change and adjust the mechanical power to match electrical power

which was not discussed in this part of the report [4]. In the next section the functions of speed

governor as well as Automatic Generation Control (AGC) in maintaining the speed of the system

are discussed.

VT

EA1

EA2

jXIA1

jXIA2

a

P1

a

P2

Cause Result

Pe P1 < P2

EA2 < EA1

XA2 < XA1

IA1 < IA2


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2.2 FREQUENCY CONTROL MECHANISMLoad of the power system is constantly changing and the primary responsibility of the electric

grid is to fulfill the consumers demand at all time. As discussed in the previous section, if the

mechanical power does not match electrical demand of the consumer (either higher or lower),

the frequency starts to deviate; excess variation of frequency is harmful to customer load, thegenerators and in the severe cases it will lead to instability.

2.2.1 SPEED GOVERNORIn order to arrest the frequency change, most synchronous generators are equipped with speed

governor, which automatically adjusts output power of the prime mover accordingly. The speed

sensor of governor detects the change in system frequency and sends a signal which activates a

control mechanism to adjust the valves of prime mover that allows more or less flow (e.g.

water and steam). For example, when a decrease in system frequency occurs due to increase in

demand, the governor senses the change and opens the gates of prime mover to allow more

fuel to flow. As a result, the mechanical output of the prime mover increases and governor

stops the frequency from further decreasing. [5]

The amount of change in frequency that will cause the governor to move the valves of prime

mover from close to entirely open condition is called Speed droop. The following graph shows

the frequency versus output power characteristics of a synchronous generator with governor

control. The slope of the graph represents the speed droop of the system. [5][2]

Figure 4: The frequency versus power curve of a generator [2]


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Since there is usually more than one generator connected to the power system, it is important

to understand how output power of each prime mover changes with frequency change. The

amount of change in output of different generators depends on the speed droop (or slope of

frequency-power curve) of that generator. The following figure shows the frequency-power

characteristics of different generators connected to the same grid; unit 1 and 2 generators havespeed droops of 5% and 2% respectively and are operating at the equilibrium system frequency

of 100% and power output of 50%. To examine the response of each generator, the load is

increased by 35%; as a result of that, frequency of the system begins to drop. As frequency

decreases, governor of each generator gradually increases the output power according to the

frequency. At the frequency of 99.5%, the cumulative extra generation of both generators is

equal to 35%. Unit 1 and 2 generate additional power of 10% and 25% due to their differing

speed droop. The new frequency of the system is 99.5%. [5]

Figure 5: Governor speed-power characteristics of two generator units 1 and 2 [5]


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2.2.2 AUTOMATIC GENERATION CONTROLAs the preceding example and figure illustrated, the governor only stops frequency from

changing and does not return the frequency back to normal; this is the job of Automatic

Generation Control (AGC). The AGC (at dispatch center) or in some cases the plant control

operator send a speed reference signal to modify the speed settings of the governor control

mechanism to allow more flow through prime mover. In other words, the speed reference

signals parallel-shift the droop characteristics of governor as per following figure. [5][3]

Figure 6: Governor frequency-power characteristics for different speed references [5]

It is important to note that for governor and AGC control to be effective in adjusting the mains

frequency, the generators or at least some of them need to operate below their maximum

output.

So far, this report has illustrated the change in frequency when generation and demand of the

system do not match; furthermore, the function of speed governor and AGS system in restoring


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system frequency by adjusting generation was explained. However, in todays power system,

where generators might be operating very close to their rated power and there is little reserve

available, it is more difficult to correct underfrequency problems. Since the generation level

cannot be raised any further, the only way to restore the system frequency back to normal is by

disconnecting some loads. Load shedding which is the primary protection againstunderfrequency is the subject of the next section and will be described in full details. [6]


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2.3 UNDERFREQUENCY PRIMARY PROTECTIONIf generation and customer load do not match and governor and AGC do not manage to restore

the balance, the frequency will continue to drop. Major decline in system frequency will

damage generating equipment and transmissionfacilities, endanger reliability of the

interconnected systems and lead to total system collapse. This is why power utilities hasadopted underfrequency load shedding (UFLS) programs. [7]

Load shedding schemes are designed to disconnect enough customers to restore the balance

and correct the frequency. Since the frequency drop is a measure of severity of system

overload, automatic UFLS relays disconnect blocks ofnon critical load (excluding emergency

services and Olympic venues!) as the frequency drops. [6]

Western Electricity Coordinating Council (WECC), which coordinates and promotes reliability in

western interconnection, requires all the involved utilities to implement 59.1 Hz Plan as a

minimum standard. Following table illustrates 59.1 Hz plan as required by WECC. [9]

Even though exact balance between demand and supply is not always possible, the major

power utilities such as UK keep the frequency within the permissible range of 0.6 Hz (59.4-

60.6 Hz) at 95% of the time. WECC requires utilities to shed at least 5.3% of load within 14

cycles since frequency reaches 59.1 Hz. If frequency continues to decay, the next blocks of load

should be disconnected. [4][9]

Table 1: 59.1 Hz Load Shedding Plan as required by WECC [9]

Load Shedding Block % of Customer Load

Dropped

Pickup (Hz) Tripping Time (Cycle)

1 5.3 59.1 14

2 5.9 58.9 14

3 6.5 58.7 14

4 6.7 58.5 14

5 6.7 58.3 14

Most standard entities including Western Electricity Coordinating Council require all pre-

determined load shedding blocks, one or more distribution feeders, to trip before the

frequency reaches 57 Hz. Besides major damages to turbine, this frequency level will impair the

auxiliary equipment of the plant which will cause the output power to fall. Thus, protective

devices will trip the generators out instantaneously. This will worsen the generation/demand

mismatch and the system will collapse in minutes. Due to the possibility of cascading outage,

interties are allowed to disconnect even before the frequency reaches 57 Hz and isolate the

problematic region. [7][8]


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Another way of regulating demand, which is similar to load shedding but more voluntarily, to

meet generation level by power utilities is Load Curtailment. Due to prior agreement between

the utility and (usually industrial or commercial) customer, the electricity is offered at a lower

rate but the customer is required to disconnect all or some of its load upon the utilitys request.

Depending on the severity of generation shortfall and their agreement, load curtailment takesdifferent forms; for example, the amount of notification time and load might be different. In

some cases, a remotely controlled switch at the connection point of customer to grid might be

installed for faster response and more control. [10]

This chapter illustrated the problem of abnormal frequency in bulk electric system, the

importance of maintaining the frequency within the normal range via speed governor,

automatic generation control, load shedding and load curtailment schemes. Next chapter

discusses the dynamic frequency control mechanism and the related standards and policies

under which renewable energy resources can integrate with the grid.


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3. DYNAMIC FREQUENCY CONTROLAs discussed in previous section, power utilities try to design and operate the power system

such that its frequency remains within narrow range of 59.4 Hz to 60.6 Hz. This is because,

beside the reliability risk that significant drop in frequency impose on the grid (cascading

outage for example), the abnormal frequency has different negative impacts on customersload and electrical power equipments.

Because speed of motor determines to the system frequency, it must remain constant for

customers motors that drive a device for example pump of washing machine and conveyor of

assembly line. Change in System frequency also affects electronic devices that use mains

frequency to time different processes. Similar to auxiliary equipment of generation plant,

equipment of electric power grid is not immune from damages caused by abnormal frequency.

Transformers for examples may be overloaded due to significant drifts of frequency. [4]

To ensure the system frequency within the normal range, the study of system operation (ordemand matching) was traditionally divided into two timescales. One timescale, which is called

frequency response, captures the changes over seconds to minute. The second timescale is

for variations from ten minutes to hours and is called load following. This is the responsibility

of the operators of the system to ensure there is enough generating capacity to meet the

varying demand via generation scheduling and reserve during load following and frequency

response timescales respectively. [4]

3.1 DEMAND MATCHING: LOAD FOLLOWINGThis section describes the load following timescale. According to the Energy Dictionary, the

definition of load following is the practice of power utilities in communicating the demand

requirements with generation facilities to produce neither too little nor too much power and

meet the moment by moment demand of the system.

Over the years, utilities have developed accurate load forecasting methods which modify

demand patterns of previous years, months and days based on factors such as weather

condition. The demand variations are also watched in real time to find the trend for the

following hours and minutes to come. Accordingly to the expected demand, economic and

technical considerations, operators schedule different generators to match hour-to-hour and

daily variations of load. [4]

Varieties of generators both conventional and renewable have different characteristics which,

in fact, define their functions in the power system. One of these characteristics based on which

power utilities put the generators to work is their flexibility to change their output power; this

is called load following capability. Although customer demand changes throughout the day,

there is a portion of this load that is always present and is referred to as the base load. Thus


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inflexible generators, which are not capable of load following, operate at their maximum

output power to supply the base load. The more flexible ones will meet the changing demand

especially during peak hours. These two types of generators are called base load power plant

and loadfollowing power plant. [4][11]

In order to represent the contribution of different generators to meet the aggregate load, the

demand curve (consumers demand over a period of time) is divided into multiple layers. The

following figure demonstrates the demand of curve of England and Wales power system and

the share of various generators in meeting the demand. [4]

Figure 7: Demand curve of England and Wales [4]

Inflexible generators of nuclear and natural gas which take multiple hours to reach full output

from cold are placed at the bottom layers of the demand curve to supply base load. Coal

generators, however, are more flexible and can change their output power from zero to full

capacity in minutes; coal generation along with hydroelectric and renewable is put at the top

layers as the load following power plants.


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Figure 8: Ontario demand curve at July 20th [12]

Just for the comparison, a typical demand curve of Ontario (preceding figure) as well as

distribution of electricity resources in Canada (following figure) are given in this section. Similar

to the UK demand curve, the Ontario daily demand curve can be layered based on the pie

diagram of Canada electricity generation. In that case, the demand curve for Canada would be

very similar with a difference that hydroelectric will be at the top supplying most of demand,

close to 60%. This allows the grid to take on more variations in demand since hydroelectric is

the most flexible among the conventional generations. On the other hand, fossil Fuel and

nuclear plants operate as the base load generating plants based on economical factors. [4][13]

Figure 9: Electricity Generation of Canada in 1999


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3.2 DEMAND MATCHING: FREQUENCY RESPONSEIn addition to load following timescale, demand could change suddenly and substantially and

the power grid must be able to response fairly quickly in matters of seconds to minute. This

response which is referred to as the frequency response divides into two sections: 1.

Continuous service; 2. Occasional service.

During continuous service timescale, modest frequency change, inertia and governor of

generators maintain the frequency. Inertia, which is based on Newtons First Law of Motion, is

the tendency of an object in motion (e.g. rotating turbine) to resist acceleration.

The occasional service or reserve timescale, which is for incidents of significant frequency

excursions, is divided to primary and secondary subcategories. Primary reserve requires very

fast generators that can increase their output within 10 seconds from incident and maintain

that for 20 seconds. For secondary reserve, generators are not as fast and response after 30

seconds but they must maintain their response for longer time of 30 minutes. Occasionalservice is performed by AGC system. [4]

Figure 10: Timescale of frequency response [4]

Depending on the characteristics of different generators, they are planned to participate in

frequency response actions. By assigning different droop ratio to each generator, operatorsdictate the contribution of each generator. Traditionally, partly loaded hydro, pumped storage

and flexible coal generator were used for fast response. Next chapter will discuss the

capabilities of renewable energy (RE) resources to participate in load following and frequency

response actions. But first, in the next section, the policies and local standards that these new

electricity resources need to follow to participate in demand matching function. [4][13]


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3.3 CURRENT STANDARDS ON RE FREQUENCY RESPONSEWhile the previous section discussed the demand matching process from frequency response

to load following, this section summarizes some of the standards and policies about frequency

control for transmission and distribution connected generators.

3.3.1 GRID CONNECTION POLICIESPolicy of BC Hydro, which also complies with IEEE 1547 standard on Distributed Generation

Interconnection, regarding speed governor is that all generator rated above 1.0 MVA need a

speed governor or have a similar performance to hydraulic and steam turbine-generator units.

Generators smaller than 1 MVA are discounted from the above criterion because they have

little impact on system operation. These governors must operate at droop of 5% or in the case

of smaller generators (< 69kV), they should be able to operate at isochronous as well. For

power generators (PG) to regulate system frequency and improve stability of the grid, BC Hydro

requires the unstrained operation for all speed governors. [15][16]

As described in previous chapter, droop of 5% means 5% drop in frequency is necessary to

move turbine control valve from fully close to fully open. In Isochronous, which is zero droop,

governor keeps the frequency constant regardless of power required by load (as long as it is

within generation capabilities of the generator). Isochronous setting is desirable for islanding

situation but not grid connected condition. That is because small fluctuations in system

frequency will cause major changes in output power of that generator. [5]

Table 2: Abnormal frequency minimum performance [15]

Underfrequency Limit (Hz) Overfrequency Limit (Hz) Minimum Time

60.0-59.5 60.0-60.5 Continuous

59.4-58.5 60.6-61.5 3 minutes

58.4-57.9 61.6-61.7 30 seconds

57.8-57.4 7.5 seconds

57.3-56.9 45 cycles

56.8-56.5 7.2 cycles

Less than 56.4 Greater than 61.7 Instantaneous

Power system disturbances cause frequency oscillations which may be temporary and damp

out after a short time. Power generators (PG) (and customer loads) are protected againstextended off nominal frequencies via under/over-frequency relays. In power generators that

are connected to the grid through power electronic convertors, the protection function of their

internal microprocessor can be programmed instead of external relays. However, they and

abnormal frequency relays need to be set up such that they do not trip the generator

prematurely before upstream protection or load shedding relays. As discussed previously,

sudden changes in feeder loading may lead to voltage phase-angle swings which could trip PGs


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if their abnormal frequency relays do not have sufficient time delays. For extensive off-nominal

frequency situation, these power generators are not allowed to trip out earlier than specified in

the preceding. [15][16][17]

In addition to continuous frequency response, large generation plant (> 69 kV) may be required

to connect to automatic generation control (AGC) system of the dispatching centre to

participate in wholesale transmission services. Due to local consideration even smaller

generators may be mandated to be dispatchable. [16]

3.3.2 ISLANDING POLICIESFollowing operation of a circuit breaker, power system divides into groups of isolated loads and

generators; this situation is called islanding and generators that continue supplying the local

load are called islanded. Without getting into details about different methods of detecting

islanding situation, one of the major methods is by measuring frequency deviation; this method

relies on the high probability of unbalance between generation and demand of island. IEEE1547 standard claims that problem with unplanned/unintentional islanding is that it expose line

crew to live lines that workers may believe to be de-energized or it will delay the restoration

time as the crew trying to ensure islanding of the generator is not a problem. This seems to be a

minor issue since line crew deal with this problems everyday and communication between

operators of the distributed generation plant and the control centre of the grid (or accurate

documentation about location of isolating breakers) should solve the issue. However BC hydro

has other concerns such as possibility ofdamage to customers load due to abnormal

frequency and voltage, voltage flicker, increased harmonic generation or miscoordination of

protection devices ifa fault happens during islanding. Thus BC Hydro requires the powergenerators to automatically disconnect from system when islanding occurs. This is because

after an islanding condition, load and generation become unbalanced; in the case of induction

generator for example, overvoltage and/or overfrequency may occur due to resonance

between generator inductance and self-excitation capacitor or sudden drop in load which leads

to decrease in slip. However, they allow the generators to continue their operation if their

control (e.g. governor) managed to establish a new equilibrium (balanced load situation) in

the island. For more details about considerations of the utility and generation plant in

operation in islanding situation, please refer to reference 15 about BC Hydro Distribution Power

Generator Islanding Guidelines. [15][16][17][18]

In summary, power generators, including renewable energy resources, are allowed by BC Hydro

to operate in islanding condition as long as sufficient coordination and analysis with the utility

has been done and the islanded generator managed to maintain frequency and voltage within

the limit during the islanding situation.


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4. FUTURE OF FREQUENCY PROTECTION IN POWER SYSTEMTo fulfill the essential objective of the electric system in meeting the growing electricity

demand while respecting international agreements on minimizing environmental impacts of

electricity generation, the form of electric system is changing from centralized generation

sourced from fossil and nuclear fuel to more distributed generations from Renewable Energy(RE) resources. The portion of electrical power generated by renewable energy resources is

identified as penetration: [4]

[Instantaneous or average] penetration =

The current average penetration of RE resources in the World is 11%. This number, however, is

guaranteed to incline due to increasing capacity of renewable power; for example, the World

capacity of wind power has been growing at 30% over the last ten years. With all the

international pressure for replacement of fossil fuel based generation with renewable, many

believes that by the end of this century, the main supplier of electricity will be RE resources.

With increasing penetration ratio of renewable energy resources, many are anxious to know

the impact of these newly integrated resources on the power grid and on its main responsibility

of the continuous power balance. [4]

As discussed in previous chapter, in order for a generator to participate in dynamic frequency

control it must be flexible and predictable. For frequency response period, the generator must

be able to response within few seconds from when demand fluctuates. For load following

timeline though, the generator needs to maintain its output for up to 20 minutes which means

its source of energy needs to be predictable. As a result, this chapter discusses the frequencyresponse capability and variability of different RE resources.

Demand matching capabilities of RE resources could potentially play an important role in parts

of the World with dominating inflexible generation units and even in Canada, as the World

leader of hydroelectricity with fast frequency control. For example, during low demand days of

year when very few conventional units are supplying the grid to provide adequate levels of

response and reserve, the connected RE resources such as wind and solar might be highly

available to supply variable load. [4][13]


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4.1 FRQUENCY RESPONSE CAPABILITIES OF REEarly technologies of wind power were based on simple fixed speed induction generators with

little control over dynamic performance. Similar to conventional generation plants, the kinetic

energy stored in rotor inertia and turbine will assist in maintaining mains frequency by slowing

down as long as it remains within their operating limits. [4][14]

Wind power technologies have progressed significantly, over the recent years, because of the

development of variable speed wind turbines with significant control technologies. Although

these wind turbines are usually programmed to maximize their power production due to

economical consideration, they are able to provide frequency response via output power

control mechanism. Besides primary and secondary service timelines, the variable speed wind

technologies can contribute to continuous service (high frequency response) by providing

greater inertial energy than conventional plant of the same size. [4][14]

Due to low energy density, energy consumption and cost of transportations of biomass, biofuelgenerating plants are relatively very small and low efficient. Although biofuel plants are

theoretically capable of continuous and occasional frequency response, they are practically too

inefficient and small that are almost always operated at their full capacity to supply base load.

[4]

Small and medium sized hydrogenation plants without major storage capacity are considered

intermittent. Due to relatively high opportunity cost associated with spilling water, governor

control equipment and real time monitoring, they currently do not participate in frequency

regulation services. However, small or medium hydro scheme based on synchronous generator

are perfectly capable of continuous and occasional frequency response as long as economical

constraints allow. [14]

Although large photovoltaic panels do not have any mechanical inertia, due to their electronic

interface to grids, they can response very fast. Because of their relatively high efficiency and

predictability, they are likely to be partly loaded for continuous and occasional frequency

response. [4]

Although large tidal scheme have many advantages such as high-predictability, due to their high

capital cost there are not commercialized yet. Besides economical constraints, tidal power is

available in few places in the World for only few hours a day. Thus, partly loaded tidal schemes

to participate in frequency control are unlikely. [4]

In summary, currently all RE power plants are fully load due to high initial cost of these

technologies, many of them are expected to be partly-loaded and take part in frequency

regulation in future.


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4.2 VARIABLITY OF REAnother argument against usage of renewables for load following purposes is their variability.

Although variation level of a renewable resource is location specific and needs to be studied

individually, this section will try to shed some light on this problem by providing some general

information about RE variation. [4]

Small hydroelectric unit without storage varies very little in minute by minute basis but

substantially over hourly, daily and time of the year due to sudden rainfall. Although minute by

minute changes might affect the capability of a small hydroelectric in load following (not so

much on fast frequency response), having multiple small hydroelectric units which are

geographically dispersed will smooth out the changes. [4]

Since radiation and temperature impacts the output of photovoltaic panels, their output vary

hourly and even minute by minute due to clouds. However, similar to small hydroelectric

change in PV systems is rather slow and their geographical spread will alleviate the short-rangechanges. [4]

Speed of Wind is continuously changing annually, seasonally, with passing weather (synoptic),

daily and second by second (turbulence) among which synoptic has the most effect on speed

variation. However, its variation which takes multiple days is slow enough for the operators of

power system. Although turbulences can have significant impact on wind speed, their

aggregation decreases the problem. [4]

Although wind over the surface of water is the source of wave power and its variation is very

similar to wind power, the change in electrical output of wave power is slow due to itsconversion process. Output of biomass, which is a storable fuel, does not vary over time. Both

biomass and wave power (to some extend) are considered predictable. [4]

With the advanced technologies in weather forecast, the output power of all the renewable

energy resources is predictable to varying degrees. As discussed earlier aggregation of many

renewable energy units (of the same type or different types) will help smooth their variation

and their integration into power system. Thus, with high penetration of renewable resources in

future, they will contribute to frequency control services of the grid more.


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4.3 VIRTUAL POWER PLANTDue to concerns over variability and unpredictability of renewable energy based generators,

they are penalized in electricity wholesale markets for example by reducing their market value.

In response to this market distrust, the concept of Virtual Power Plant was introduced

recently by the Institute for Solar Energy Supply Technology of the University of Kassel inGermany. [4]

The pioneer project on Virtual Power Plant (VPP), which is a cluster of distributed generators

operated together by a central control mechanism, is the Combined Power Plant which is

carried out by the University of Kassel with cooperation of many companies in renewable

energy sector. The objective of this project was not only to illustrate the reliability of RE

generation but to prove that full electricity supply from renewable sources is possible. [19]

With constantly varying RE generation and demand, it seems very difficult to maintain the

reliability of a system that is fully supplied by renewable resources. However, as discussed inprevious section, the combination of renewable energy sources balances out their fluctuations.

The Combined Power Plant, which links and runs 36 RE resources around Germany (table 3), is

as reliable as a conventional large-scale power station. [20]

Table 3: Distributed generation and storage elements of the Combined Power Plant [19]

RE Type Numbers Total Capacity

Wind Turbines 11 12.6 MW

Solar Power Plants 20 5.5 MW

Biogas (CHP) units 4 4.0 MW

Pumped Storage 1 1.06 MW for 80 Hours

The central control unit operates in two steps of anticipatory control and fine tuning. In

anticipatory step, the central control is provided by load profile or demand forecast as well as

forecast of wind strength and hours of sun. According to this information, central control

decides how much wind turbines and solar modules can generate at each instant and its surplus

and shortage against demand. Then, control unit plans how to spread and balance out these

shortages and surpluses using biogas CHP and pump storage (figure 11). [20]


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Figure 11: Anticipatory - stage 1 of Combined Power Plant operation [19]

During the real time operation, despite accurate weather forecast, there are small deviations inactual generation and demand which will be handled by fine tuning mechanism. As following

figure illustrates, the central control unit of Combined Power Plant constantly receives

information on output of each RE generation and storage plants and commands an increase in

their output as necessary. [20]

Figure 12: Fine Tuning - stage 2 of Combined Power Plant operation [19]

Virtual Power Plant works on the principle that aggregate of the small RE generation plants

perform more reliably such that it makes a power system with 100% perturbation of renewable

energy sources possible.

However, the concept of Virtual Power Plant (VPP) is not adopted by power utilities and still in

research phase. One of the most-talked-about issues with VPP is that depending on the


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circumstances of the geographical location and its local resources, the generators that take part

in the cluster may have to be geographically dispersed; this jeopardizes the existing

requirements for local control of power and voltage. This topic is in fact the topic of many

ongoing research projects. [4]

One of the components of VPP is the energy storage element, which refills and drains during

low and high demand periods. Although pumped hydro and biogas, which are rather

conventional storage technologies, was used in the Combined Power Plant project, electrical

storage takes many different forms: large hydro, compressed air, hydrogen, batteries, flywheel,

super capacitors and heat/cold store. However, economical feasibility of storage devices is

doubtful due to high capital cost, high operation and maintenance costs and low efficiency of

these devices. Inexpensive efficient storage devices are also the topic of ongoing research

projects. [4]

Other major VPP projects include FENIX, Flexible Electricity Network to Integrate the eXpectedenergy revolution, FortZED and Xcel Energys SmartGridCity. FENIX is European collaborative

project that focuses on integrating wholesale electricity generation from wind farms and co-

generation. FortZED and Xcel Energy s SmartGridCity are taking place in Colorado and dispatch

the aggregate of RE resources as one big generation plant and optimize the system via demand

management techniques. [22]


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5. CONCLUSIONCurrent issues preventing the full implementation of Renewable Energy (RE) resources are not

their technologies but concerns over their integration with existing power grid. Because the

main responsibilities of the electrical power grid is to balance demand and supply of electricity

and thus preservation of the mains frequency, the objective of this report was the frequencyregulation and RE resources.

The first chapter of the report explained how power imbalance effects speed of generators and

system frequency. Higher generation than demand increases the frequency and vice versa. It

also described the function of speed governor, which is stopping frequency variations,

Automatic Generation Control, which is restoring frequency back to normal, and load shedding

mechanism as the primary protection against underfrequency.

The second chapter re-emphasised the importance of maintaining frequency via demand

matching. It also divided load matching mechanism to two timelines of frequency responsewithin seconds to ten minutes and load following within ten minutes to hours of the incident.

Based on the flexibility and availability of generators, they are assigned to contribute to the

actions of one or both these timelines. At the end, IEEE 1547 and BC Hydro standards for

frequency controls of integrated generators in grid connected and islanded situations were

discussed.

The third chapter explained the penetration of RE resources is growing such that at the end of

the century they might become the dominant source of electricity. All RE resources are

technically capable of fast frequency response but some vary and are unpredictable. Their

aggregate smooth out fluctuations and is used with the help of central control in a so called

Virtual Power Plant (VPP). VPP is not without problems and is still in research phase.

To sustain and fulfill the main responsibility of electric power system in balancing demand and

generation, renewable energy resources are green and competent candidate. By resolving

problems such as unpredictability and continuous variation, the power grid will be fed only

from renewable energy resources.


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