Module Overview (indicative) Part I the integration of renewable Power into Power system

Unit 1: Introduction to distributed generation (Week 1 - 2)

Unit 2: Power flow with distributed generation scheme (Week 2-3)

Unit 3: Voltage magnitude variations, faults and stability (Week 3-4)

Unit 4: Protections and power quality(Week 4-5)

Unit 5: Reactive Concerns and future power system (Week 5)

Revision of Unit 1-5 (Week 12)

Part II Renewable Sources- Unit 6-11

Unit Overview

In this unit, you will learn :

● The concepts of renewable power generation
● The evolution of power system
● The concept of Distributed Generation (DG)
● How the hosting capacity against overloading can be improved
● The technical impact that DGs can bring into a power system

Introduction to Renewable Power

“Renewable” power is probably one of the most boosted word worldwide, as it is supposed to be able to offer clean, endless and “free” power to humans.

From electrical power grid aspect, how to maximize the use of renewable power remains a challenge worldwide.

As we know, the major types of renewable can be categorized as
– Wind
– Solar
– Hydro
– Biomass – Etc.

What the renewable power generations could be like ?

The typical types of renewable and distributed generation can be summarized as follows:

Before study how the above renewable power generations can affect the modern power systems, let us have a look at our modern power grid is like.

Centralized Generation Vs. Distributed Generaion

Modern power systems are mainly backed by those large power plants and the power are dispatched by network operators. This kind of operational pattern is called “centralized control”. On the other hand, renewable sources in most cases are naturally dispersed (considering the geographical, weather/climate conditions).

If we still generate renewable power in a centralized way, we may experience:
– Cost on transmission system
– High transmission loss
– Easier to control due to aggregation
– Rigid design in construction

Alternatively, if the generation are produced in a small-scale, it becomes distributed. Distributed renewable power generation can bring about
– No cost on transmission system
– Hardly any losses on transmission level
– Hard to control due to large number of small capacities
– Flexibility in construction

Why centralized renewable generation is costly?

How the power system is evolved?

- Use the power system in UK as an example

The Evolvement of Power System: take UK as an example 1

In early days of electricity supply, each town/city in UK had its own power station which supplied the needs of its area.

Standardization was not evident; many different voltages and frequencies were used throughout the country.

1914–1918, there were 600 independent power stations in use.

Heavy demands by war industry showed inadequacies of system

1926 : 126 of the largest, most efficient power stations selected and connected by a grid of HV transmission lines covering UK

Frequency standardized at 50 Hz.

Remaining power stations closed down and local supply authorities obtained their electricity in bulk from the grid, via suitable substations.

System voltage was 132 kV and supply freq.50 Hz

April 1948, electricity supply industry was centralized

1957, Central Authority ’ responsible for generation of electricity was renamed CEGB.

Since then, electricity industry has become privatized

CEGB being replaced by the National Grid Company,

“National Grid” Buys, at the lowest price, generated electricity from such generation companies such as:
– National Power
– PowerGen
– Nuclear Electric
– French Electrique and Scottish Hydro Electric

Electricity boards have become Distribution Network Operators (DNO)

(DNOs) buy electrical energy from the National Grid Company to distribute to their own consumers.

Power System Structure

National Grid Today: from Generation to Customer

Transmission systems generally include overhead lines, underground cables and substations. They connect generation and interconnectors to the distribution system.

National Grid owns and operate the transmission network in England and Wales.

National Grid operates but do not own the Scottish networks.

National Grid also share a joint venture with Scottish Power to construct and operate an interconnector to reinforce the GB transmission system between Scotland and England and Wales.

Generation is the production of electricity from fossil fuel, nuclear power stations, renewable sources e.g. wind and solar.

National Grid do not own or operate any electricity generation in the UK.

Transmission grids are often interconnected so that energy can flow from one country or region to another. This helps electrical energy supply for citizens and society across the region.

Interconnectors also allow power suppliers to sell their energy to customers in other countries and provide electrical support between the interconnected grids.

Great Britain is linked via interconnectors with France, Ireland, Northern Ireland and the Netherlands.

National Grid own part of the interconnectors with France and the Netherlands.

National Grid is also now entering the construction phase for two new interconnectors, between the UK and Belgium and the UK and Norway and other parts of Europe in plan

Distribution systems carry lower voltages than transmission systems over networks of overhead lines, underground cables and substations.

Distribution systems transport electricity from the transmission network, and deliver it to consumers at a lower voltage they can use.

National Grid do not own or operate electricity distribution networks in the UK.

The supply of electricity involves buying electricity and selling it on to customers.

It also involves customer services, billing and the collection of customer accounts.

National Grid do not sell electricity to consumers in the UK.

Introduction to Distributed Generation

Distributed Generation

Distributed Generation (DG) is the generation from a power station that is embedded in a Distribution Network. This means that any generation that is connected to the downstream of distribution substation can be categorized as DG.

Nowadays, DGs are mostly renewable power generations

Question: Which ones of the followings are Distributed Generation?
– 1. Centralized power plant using coal
– 2. 40- MW PV arrays with direct connection to a transmission line
– 3. Off-shore windfarm
– 4. Wind turbines connecting to 35 kV bus in a substation supplying a small town
– 5. Roof-top PV system

Click here for the Answer

Other Terms for Distributed Generation

The concept of “Distributed Generation” has been developed for a few decades. In the history of electrical power engineering, it has been referred to as some other “names”. Some of them are still being used:

Embedded Generation (traditionally often refers to those synchronous generator)

Decentralized Generation

Dispersed generation (to distinguish it from central generation)

Distributed Generation (DG: more popular nowadays)

Advantage of Conventional Grid

Although DG has demonstrated many advantages, why our current power grid is so far mainly centralized?

One of the major advantage of centralized system is the ease and simplicity of control: conventional centralized control means that Large generating units can be made efficient and operated with small number of personnel.

So far, the most reliable and mature power generation plants are still based on fossil fuels and the technology has been implemented and optimized for decades. Those plants of large scale can be dispatched at any time.

Since the location of end users are normally far away from the generator, bulk power can be transported over long distance with limited electrical losses.

Inter-connecters are build to enable bulk power transmission crossing countries.

As is demonstrated above, the interconnected HV transmission network allows generator reserve requirements to be minimized so the large plants can support the grid together.

Driving towards DG

Since the modern power system as UK stems from some form of “decentralization” in early day, why DG is coming back?

The environmental concerns is global – Greenhouse Gas Emissions from Fossil Fuels (Kyoto Protocol) is the common concern worldwide. DG is naturally suitable for dispersed renewable power generations.

Diversification of energy sources: different types of energy sources can be the backup for each other so failure of one type does not extend to the complete outage in short term or energy depletion in long term

Technological Development: wind, Solar , Wave & Biofuels technologies are much more developed than they used to be, which makes the corresponding usage available.

New Government Policy – the regulations and policies over the utilities is demanding higher reliability against blackout, Low carbon emission, competitively Priced. Renewable DG can contribute to these aspects.

The use of DG can also bring about flexibilities, such as

Availability of modular generating plant: this mean ease of instalment and eventually leads to off-the-shelf solution in a plug-and-play manner.

Ease of finding sites for smaller generators: small scale also means small footprint. DG will not require a significant area of land to be installed as any central power plant always does.

Short construction time and lower capital costs per plant: for a modular DG, the construction time is much quicker than large plants and the cost can be more affordable to private owners with flexibility on the choice of scale.

Generation closer to the load which may reduce resistive losses during power delivery, which means that both the utilities and the owner can benefit from DG in terms of operational cost.

Common Attributes of DG

Not centrally planned:

since DG is supposed to be of small scale and operate in a plug-and- play manner, the number of DGs can be tremendous within a power network. In addition, power generation is dependent on the climate and environment conditions, hence unlikely to be planned by the power system operator.

Usually operated by Independent Power Producers (IPPs) or consumers:

Due to the ownership and large numbers: the DG is usually operated by the end users rather than the major utilities or generation company

Not centrally dispatched

In most cases, DGs are not dispatched by central operators due to the small scale and numbers.

Connected to the distribution system (Medium or low voltage system): in UK, DG is supposed to be connected to 33 kV level and below.

Normally smaller than 50 MW

DGs are connected to a distribution network, considering the network operating limits, it is usually less than 50 MW.

Technical Aspects of DG to a Power System

Concerns

Power system is probably the most complex engineering system in human history.

By introducing DG, it brings about a lot of changes including the following aspects:
• Protection
• Operation
• Voltage control • Reliability
• Stability
• Power quality

Direction of Power flow

Conventional distribution system is of unidirectional power flow : the flow of both active power (P) and reactive power (Q) are commonly from the higher to the lower voltage levels.

Distribution system with distributed generation: The power flows may be in either direction - bidirectional power flow

DG’s Interface to the Grid

For all kinds of equipment or appliance, when connected to a power grid, the connecting point has to follow certain restrictions.

PCC (Point of Common Coupling): the connection point of energy sources to the grid

PCC of a DG is normally defined at the primary/secondary side of the interface transformer .

Interface Technology for Distributed Generations

Although the DG are all supposed to be connected to a power system through PCC, the technology to be used between the renewable source and the grid varies. A general summary is made as follows:

The power conversions using power electronics technologies has become one of the enabling technologies in renewable power generations!

DG and Voltage Variations

For all kinds of power equipment or house appliance, they have to operate with a power supply of a predefined range of voltages.

The power generation from DG can bring about significant voltage rise in a distribution power network, which had never been expected when the power systems were initially designed.

How DG can affect the voltage profile and if the impact can be mitigated has to be considered.

DG and Network Fault Level

In a modern power system, when a short fault happens, it is expected that the fault current flow into the faulty point from known sources, such as generator in a central plant or from the upstream network.

The maximum amount of the fault current, which is the fault level in per unit, at all the possible fault point are ideally estimated before operational and protective schemes are designed.

The connection of DG after a pre-defined automatic protective scheme has been made can bring about “surprise” to those systems: unexpected fault current may “confuse” the automations and made the wrong decision causing unnecessary outages or damage the equipment.

To avoid this, significant DG installment might mean a complete upgrade of the protections within an entire distribution network, which can be very costly .

DG and Power Quality

Since DG are not likely to be controlled by central operator any more, there is likely to be a lot of switching actions without acknowledging the power system opreator, which can bring about transient voltage variations
– Considering relatively large current changes during connection and disconnection of the generator are allowed.
– Some forms of prime mover (e.g. fixed speed wind turbines) may cause cyclic variations in the generator output current
– Despite the challenge it may bring about during a transient, rotating machine based DG is good for maintaining voltage at steady state due to the prescience of their physical inertia.

Some DG may bring about harmonic distortion of the network voltage; this may affect the power equipment or appliance near by and cause more power losses.
– Improperlydesigned/controlledPEsystemmaycauseoscillationor resonance with shunt capacitors and network impedances
– Thyristor based PE system may cause harmonics of low orders

DG and Power System Stability

Stability is the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact.

Very small-scale DG is not likely cause instability to power system itself.

When the total rating of DG accumulates and play significant roles in a power system collectively, the stability can potentially becomes a big issue, in a weak grid in particular.

(Weak grid means: a power grid connection point with low inertia or far from physical inertia)

DG and Network Operation

Power system operations on maintenance are based on the full control of all the known sources.

Due to the presence of DGs, power network can be energized from a number of unknown points.

For the purpose of planned maintenance, it becomes more difficult to ensure a de-energized network; hence more risk in safety hazards.

Therefore the flexibility for work on a network with DG connected to it becomes more restricted

Implications for policies of isolation and earthing for safety consequently becomes more complicated.