Activity 1

If the actual maximum demand at any one time is typically 60 GW, what diversity factor should you apply?

Reveal Solution

Activity 2

Imagine you live in a rural area and are one of six houses supplied from an 11 kV radial feeder. You can work out the diversity factor which the supply authority has applied to the collective installation.

Look around your house and note the rating of all the electrical appliances then imagine a high load situation such as a dismal winter Sunday morning when, perhaps, the electric cooker is heavily used, immersion heater on, numerous lights, entertainment equipment, extra heating, washing machine and tumble dryer, kettle, hair dryer, vacuum cleaner and so on. (This is your domestic maximum demand.)

Add up these loadings and multiply by 6 for all the houses.

If the power factor is to be near enough unity, then the total kW for the collective installation will be the total kVA.

H the houses are fed from an 11 kV/240 V, 45 kVA single phase transformer, find the diversity factor which has been applied.

Reveal Solution

Diversity Factor

ln fact, this state of affairs is highly unlikely. It is always the case that when a piece of equipment is manually or automatically switched on, another piece of equipment is switched off somewhere else. So, in practice, a diversity factor can be applied.

Diversityfactor = \frac{sum~of~all~possible~demands}{actual~demand}

Activity 3:

Referring to the variation in Load Demand diagram and looking at A, the area under the curve;

Diagram 1: Variation in Load Demand

(a) What does it represent

(b) Estimate the load factor for this customer

Reveal Solution

Activity 4:

Using the domestic maximum demand found in activity 2, work out your load factor (LF) by looking at your last electricity bill to determine the number of units used in the last quarter and dividing by the maximum demand (MD) x number of hours in the quarter.

i.e. ~ LF = \frac{Number~of~units~used}{MD \times 24 \times days~in~quarter}

Reveal Solution

Load factor is defined as the energy consumed in a given period compared with the energy which would have been consumed if the maximum demand had been sustained during that period.

So that for any given period

Load Factor ...

\frac{Energy~consumed}{Energy~which~would~have~been~consumed~had~maximum~demand~been~sustained}

The following diagram illustrates the importance of load factor.

Variation in Load Demand

Diagram 1: Variation in Load Demand

The above diagram shows a consumer’s loading with high peaks and also times of light load during one day (the 24 hour cycle).

Power Factor

A low or poor Power Factor will require a high current for a given power. It is assumed here that you are familiar with the term Power Factor and aware of the implications of a poor power factor on current demands. If you have any difficulty with this consult your tutor.

Supply authorities encourage consumers to improve both load factor and power factor. This enables plant rating to be kept within reason and also makes spare capacity available for other consumers and is achieved through well-constructed tariffs.

🔗 https://en.wikipedia.org/wiki/Power_factor

Load and Demand

In order to determine realistically the size of plant required for generation and transmission it is necessary to take into account a number of important factors. All these factors involve - either directly or indirectly - the problem of maximum demand.

Maximum demand is defined as the actual maximum load on a system during a given period of time (day/month/year). However, consideration must also be given to what might be the maximum demand if all consumers switched on their loads at the same time.

The domestic evening peak begins when industry and commerce are reaching the end of their working day and some overlap occurs resulting in an overall daily peak indemand between 1800 - 2000 hours.

As we go about our daily lives our personal demand for energy (directly and indirectly) remains but changes location. For example, as we leave our place of work and switch off lights or equipment or machinery we may take the train home so using electrical energy in another form. At home we light and heat rooms, cook meals, switch on equipment for entertainment, constantly using electrical energy throughout our waking hours.

The domestic evening peak occurs when industry and commerce are reaching the end of the working day and there is some overlap with increased early evening domestic demand.

The total possible load for all UK consumers would be in the order of 3000 W. But this would only occur if every consumer made their maximum demand simultaneously.

This section provides an insight into how the demand for power varies depending on the time of day or season and the facilities available to the Control Engineer attempting to match the output from the generators to the load demand.

All conventional power stations are listed in order of merit. This is a system of comparing the cost per unit generated between stations so that the low cost plant is used in preference. By taking a number of factors into consideration. such as weather predictions and television 'pick-up', the engineer is able to use the high capital cost/low running cost stations (such as nuclear) as base load plant.

Electricity, unlike other forms of energy, cannot be easily stored and the supplier has little control over the load at any time. So the control engineers try to keep the output from the generators equal to the connected load at the specified voltage and frequency. A measure of how well they do this is how close the national frequency is to 50 Hz throughout any 24 hours.

The demand for electricity on a system varies considerably during any 24 hour period and the demand during the winter months is much greater than that in the summer months.

Load Curve

A load curve is a graphica1 representation of the amount of power supplied by the system against time and the diagram below is a typical example.

Diagram 3: Load Curve

You can see that the daily load consists of a 'base' component plus peaks and troughs depending on the time of day.

Spinning Reserve and Static Reserve

Sudden peaks are met using spinning reserve and static reserve which can amount to 3000 MW during the day and 1250 MW at night.

Spinning Reserve

Spinning reserve involves machine sets being run up and synchronised to the system but supplying little or no power.

Steam boilers are held at full working pressure with sufficient steam passing through the turbines to maintain working temperatures. This enables the set to be fully loaded in a matter of minutes instead of hours as would be the case from cold.

Static Reserve

Static reserve is provided in two ways.

i) Some steam sets are kept turning over slowly (a few rpm) by what is known as baring gear, with again sufficient steam passing through to keep the turbine at a reasonable temperature.

The boiler is kept at somewhat less than full pressure but with enough steam in reserve to enable the set to be run-up and load taken in about an hour. This system is known as Hot Standby.

ii) Water turbines (hydro power) and large gas turbines are able to run-up from standstill and be fully loaded in a very short time (typically 5 minutes).

Such sets are invaluable for meeting sudden, unpredicted peak demands and in keeping the system frequency stable. The drawback with hydro generation is the large civil works and water catchment required (typical single set 10 - 20 MW). The disadvantage of gas turbine driven sets on the other hand is the cost of fuel and maintenance together with their short life (typical single set 20 - 30 MW).

A recent addition to the generation system in the last 20-25 years has been the development of hydraulic pumped storage. This involves a much larger than usual hydro station (typically 400 MW) being supplied from a relatively minute head pond (reservoir).

At times of peak demand the station generates in the normal manner supplying power to the grid. The tail water runs into a lower pond and not to 'waste' or into a further downstream station on the same scheme as is usually the case with a conventional hydro station.

At times of low demand the electrical machines are synchronised to the grid but run in reverse so that they drive the water turbines as large pumps. In this way they pump water from the tail pond up into the head pond.

Activity 5:

Although more costly and complex than a conventional hydro station, hydraulic pumped storage has advantages.

Can you list 2 or 3 ?

Reveal Solution

We now need to examine the way in which large consumers are charged for their electricity supply and how the structure of the tariffs encourages consumers to reduce their maximum demands and improve power factors.

The maximum demand made on a large supply system determines the size and cost of the installation. You will remember that Load Factor is defined as the energy consumed in a given period compared with the energy which would have been consumed if the maximum demand had been sustained.

So, for a fixed total annual energy consumption, a low load factor results in a high maximum demand. In tum a high maximum demand results in a high financial cost per electrical unit consumed in terms of supply plant and equipment. Since annual energy consumption is (hopefully) proportional to goods produced and services provided, a high maximum demand means an unnecessarily high cost in a competitive market.

Well-constructed tariffs are designed to provide the necessary financial incentives to influence consumers.

They are encouraged to consume a higher proportion of energy at reduced cost. It is quite possible to double the energy consumption (and so double output more or less) and only put 15% to 20% on the electricity bill.

Financial incentives can reduce overall maximum demands, and consequently the heavy costs associated with just getting a supply to a large consumer.

Customers who use more energy during the night get cheaper electricity because this keeps the large steam generators operating at or near their most efficient.

There are incentives to keep demands down during the winter months when an overall upsurge takes place.

Tariffs are also designed to improve power factors towards unity, so that power stations are only producing and transmission lines are only carrying active power.

Two-Part Tariffs

The tariffs are constructed so that consumers pay charges to meet the various costs of providing a heavy supply (irrespective of how much energy is used). Such charges are termed fixed charges although in practice they can vary considerably month by month. You'll see how this operates when you do the worked examples overleaf. ·

Customers also pay a running charge which depends on the number of units of energy used.

These two charges together form what is known as a Two-Part Tariff.

Chargeable Maximum Demand Monitoring

Chargeable Maximum Demand is the monitored maximum demand divided by the average power factor for the month and is referred to as the maximum kVA demand.

This is often a preferable system since it takes into account - and charges for - a low power factor. The average power factor is determined by measuring the kW hrs and kVAr hrs over the month.

i.e.~ average~ \Theta = tan^{-1}kVarhr

kWhr

and~average~power~factor = cos~\Theta

The Running Charge levied (cost per kWhr consumed) is usually dependent on the time of day. There is one charge between 0730 and 2330 and another much reduced charge between 2330 and 0730. Itis also dependent on the time of year, with cheaper summer tariffs.

Most industrial tariffs are billed on a monthly basis.

Demand Monitoring

It is important to note the following points.

For the units reading a running total is maintained. In contrast, once the demand reading has been reset to zero by the meter reader, the recording has been lost for all time.

As a result, most companies insist that their representative is present when readings are taken.

Some supply authorities make separate charges for maximum demand and poor power factor, while others combine the two into one charge. When done in this way it will result in an average kVA demand.

You will recall that

kVA = \underline{kW}

p.f.

To ensure a fair spread of capital cost it is necessary to measure or monitor a heavy consumer's maximum demand. However, many industrial loads demand large amounts of power for short periods - accelerating a motor and its load for instance - so that monitoring instantaneous demand is considered unfair.

In any event, the national system can cope with short duration heavy demands due to the inertia of all the rotating machinery connected to the grid.

A more satisfactory solution is to average out the demand over a relatively short time period and refer to this as monitored demand as opposed to instantaneous demand. The time period used by most supply authorities is 30 minutes.

MonUored Maximum Demand is defined as twice the number of kWhr supplied during any thirty consecutive minutes in any accounting period. Consider the following example.

A consumer uses 600 kWh between 0900 and 0930.

Since energy (kWhrs) ... No. of kW x No.of hours monitored then 600 ... No. of kW x t hrs

So that the average demand between 0900 and 0930 is given by

No.~of~kW == 600 + f

= 12pQkW

In practice between 0900 and 0930 the instantaneous demand may well have exceeded 1200 kW and at other times been much less.

This maximum demand is divided into blocks of kWhrs and a decreasing scale of charges is applied to successive blocks.

To monitor the power flowing into a consumer's installation, a special meter is installed at the intake. As energy is used, a pointer moves round a scale driven at twice the speed at which energy (kWh) is used.

In order to record the average maximum demand, the meter carries a second pointer which is 'pushed' upscale by the first (driven) pointer. The second pointer simply sits at the highest position to which it is pushed.

At the end of the 30 minute period the first pointer is automatically set to zero. During the next 30 minute period it may or may not reach and cant forward the second pointer, depending on the demand. At the end of the month a meter reader takes the maximum demand reading and resets the second pointer to zero.

Monitored Maximum Demand

Monitored Maximum Demand is defined as twice the number of kWhr supplied during any thirty consecutive minutes in any accounting period.

Consider the following example.

A consumer uses 600 kWh between 0900 and 0930.

Since energy (kWhrs) = No. of kW x No.of hours monitored

then 600 = No. of kW x 0.5 hrs

So that the average demand between 0900 and 0930 is given by

No. of kW = 600 ÷ 0.5

= 1200 kW

In practice between 0900 and 0930 the instantaneous demand may well have exceeded 1200 kW and at other times been much less.

This maximum demand is divided into blocks of kWhrs and a decreasing scale of charges is applied to successive blocks.

To monitor the power flowing into a consumer's installation, a special meter is installed at the intake. As energy is used, a pointer moves round a scale driven at twice the speed at which energy (kWh) is used.

In order to record the average maximum demand, the meter carries a second pointer which is 'pushed' upscale by the first (driven) pointer. The second pointer simply sits at the highest position to which it is pushed.

At the end of the 30 minute period the first pointer is automatically set to zero. During the next 30 minute period it may or may not reach and carry forward the second pointer, depending on the demand. At the end of the month a meter reader takes the maximum demand reading and resets the second pointer to zero.

It is important to note the following points.

For the units reading a running total is maintained. In contrast, once the demand reading has been reset to zero by the meter reader, the recording has been lost for all time. As a result. most companies insist that their representative is present when readings are taken.

Some supply authorities make separate charges for maximum demand and poor power factor, while others combine the two into one charge. When done in this way it will result in an average kVA demand.

You will recall that   kVA = kW

Chargeable Maximum Demand is the monitored maximum demand divided by the average power factor for the month and is referred to as the maximum kVA demand. This is often a preferable system since it takes into account - and charges for - a low power factor. The average power factor is determined by measuring the kW hrs and kVAr hrs over the month.

i.e.~~average~t/J \cdots tan \cdot \frac{kVar~hr}{kW~hr}

and~average~power~factor == cos~t/J

The Running Charge levied (cost per kWhr consumed) is usually dependent on the time of day. There is one charge between 0730 and 2330 and another much reduced charge between 2330 and 0730. Itis also dependent on the time of year, with cheaper summer tariffs.

Most industrial tariffs are billed on a monthly basis.

Demand must be monitored in order to ensure that the two-part tariff is as fair as possible and to encourage large consumers to improve their load factor (i.e. reduce maximum demand). The examples below illustrates the importance of monitoring.

Two factories are fed from the high voltage grid via transformers.

The first one belongs to MacIntosh's Highland Clearances, a scrap metal process works. It operates on full load day and night (including weekends) constantly drawing 1000 kW.

Therefore, at the end of a month. MacIntosh's has consumed

1000 kW \times 730 hours = 730 000 kWh = 0.73 \times 106 kWh~of~energy

The second factory belongs to Ferguson's Jumpers, a firm specialising in knitwear for tall people. It consumes approximately the same amount of energy over the month (say 7 x 105 kWh) but its load is constantly changing.

Demand fluctuates between 50 kW and 3000 kW, occasionally reaching peaks of 6000 kW. Due to this, the capacity of the lines, cables and transformers must be greater than those supplying MacIntosh's Highland Clearances. The supply authority must therefore invest more capital to serve Ferguson's Jumpers.

Consequently it is reasonable that Ferguson's Jumpers should pay more for its energy.

To ensure a fair spread of capital cost it is necessary to measure or monitor a heavy consumer's maximum demand. However, many industrial loads demand large amounts of power for short periods - accelerating a motor and its load for instance - so that monitoring instantaneous demand is considered unfair.

In any event, the national system can cope with short duration heavy demands due to the inertia of all the rotating machinery connected to the grid.

A more satisfactory solution is to average out the demand over a relatively short time period and refer to this as monitored demand as opposed to instantaneous demand. The time period used by most supply authorities is 30 minutes.

Monitored Maximum Demand is defined as twice the number of kWhr supplied during any thirty consecutive minutes in any accounting period. Consider the following example.

A consumer uses 600 kWh between 0900 and 0930.

Since energy (kWhrs) = No. of kW x No.of hours monitored

then 600 = No. of kW x 0.5 hrs

So that the average demand between 0900 and 0930 is given by

No. of kW == 600 + 0.5

= 1200 kW

In practice between 0900 and 0930 the instantaneous demand may well have exceeded 1200 kW and at other times been much less.

This maximum demand is divided into blocks of kWhrs and a decreasing scale of charges is applied to successive blocks.

To monitor the power flowing into a consumer's installation, a special meter is installed at the intake. As energy is used, a pointer moves round a scale driven at twice the speed at which energy (kWh) is used.

In order to record the average maximum demand, the meter carries a second pointer which is 'pushed' upscale by the first (driven) pointer. The second pointer simply sits at the highest position to which it is pushed.

At the end of the 30 minute period the first pointer is automatically set to zero. During the next 30 minute period it may or may not reach and carry forward the second pointer, depending on the demand. At the end of the month a meter reader takes the maximum demand reading and resets the second pointer to zero.

It is important to note the following points.

For the units reading a running total is maintained. In contrast, once the demand reading has been reset to zero by the meter reader, the recording has been lost for all time.

As a result, most companies insist that their representative is present when readings are taken.

Some supply authorities make separate charges for maximum demand and poor power factor, while others combine the two into one charge. When done in this way it will result in an average kVA demand.

You will recall that

kVA = \underline{kW}

p.f.

The following examples are based on the tariff system used by Scottish Hydro Electric pie and are worked using 2016 – 2017charges.

A copy of the 2016 tariff sheet is enclosed for reference (page 19). You should also have obtained a copy of the current tariffs from your local supply authority shop or office.

Look at the heading Maximum demand tariff on the 2016 Hydro Electric sheet.

You will notice four sub-headings (block letters) with charges opposite them depending whether the supply is taken at high or low voltage.

Maximum Demand Tariff Sheet

Maximum Demand Tariff Sheet

The company may require the customer to enter into a special agreement for a supply of electricity under this tariff.

The Maximum Authorised Capacity under this tariff is 20kVA	For High Voltage Supply (Code 54)	For Low Voltage Supply (Code 55)
Availability Charge
Per month for each of the first 20 kVA of charge capacity	£2.55	£2.55
Per month for each kVA of chargeable capacity in excess of 20 kVA	£0.75	£1.04
Demand Charge
Per month for each of the first 750 kilowatts of maximum demand made in the month	£1.36	£1.43
Per month for each of the next 3250 kilowatts of maximum demand made in the month	£0.60	£1.36
Per month for each kilowatt of maximum demand in excess of 4000 kilowatts made in the month plus per kilowatt of maximum demand made between 0730 and 2330 hours in each of the months mainly in	£0.15	£1.36
November and March	£3.82	£3.92
December, January and February	£7.33	£7.46
The average power factor shall be measured at all times. Between 2330 and 0730 hours one kilowatt of maximum demand shall be deemed to be half of one kilowatt.
Unit Charge
For each unit supplied between 0730 and 2330 hours in the months of November to March inclusive	4.77p	5.11p
For each unit supplied between 0730 and 2330 hours in the months of April to October inclusive	4.08p	4.27p
For each unit supplied between 2330 and 0730 hours	2.50p	2.55p
Reactive Kilovoltampere Hours Charge
For each kVArh supplied at any time in excess of half the number of units supplied	0.38p	0.59p
Catering Charge	0.0667 per kWh

Maximum Demand Tariff Sheet

Change of Tariff

With the consent of the company a customer may change from one tariff to another where alternative Tariffs are available for the particular type of supply, provided that notice in writing is given by the customer to the Company not less than one month before the meter reading from which the alternative Tariff is required to apply, but the date of application of the alternative Tariff will depend on the availability of suitable metering equipment. A customer shall not be entitled to change from one Tariff to another more often than once a year. Full details of these Tariffs are available from our offices and shops.

Value Added Tax

All charges are inclusive of Value Added Tax which will be charged at the appropriate rate presently at 20%.

Definitions

Expressions used in this leaflet have the following meanings:

All times quoted are Greenwich Mean Time.

The unit of electricity is the kilowatt hour.

“Authorised Capacity” – means the capacity of supply expressed in kilovoltamperes (kVA) requested by the customer and authorised by the company.

“Basic Charge” – means for the purpose of the General Tariffs, a sum, payable whether or not electricity is used, in respect of fixed costs associated with the supply of electricity including meter reading and billing costs.

“Chargeable Capacity” – means the Authorised Capacity of the supply expressed in kilovoltamperes or such higher capacity as may be determined from the recorded maximum demand in kilowatts in the month of the account or in any of the preceding eleven months, whichever is the greater, and the average power factor in the same month.

“High Voltage Supply” – means a supply metered at more than 1000 volts.

“Low Voltage Supply” – means a supply metered at 1000 volts or less

“Maximum Demand” – means twice the largest number of units supplied in any half-hour during a specified charging period. The maximum demand may be addressed by the company in cases where maximum demand indicating equipment is not installed.

“Month” – means a period of four or five consecutive weeks between one meter reading and the next such reading.

“Quarter” – means a period of approximately three consecutive months between one meter reading and the next such reading.

“Reactive kilovoltampere hours” – means the wattles current in the circuit measured over time and is recorded as kVArh.

“Weekday” – means Mondays to Friday inclusive.

{The above extract is reproduced by kind permission of Scottish Hydro-Electric plc}

Availability charge is one of the fixed charges and is based on the capacity of the supply which has been made available to the consumer.

The following is one way of looking at this. Supposing your area is supplied from a 10 MVA transformer with its associated switchgear, lines and cables. If you have agreed authorised capacity of say 800 kVA then this portion of the network has been reserved for you.

It is nearly always the case that a consumer asks for, and is granted (on the grounds of future growth), a larger capacity than is really needed. So it may well be that the aforementioned 10 MVA transformer is theoretically fully utilised when in practice its average loading may be as little as 35-40% of its rating.

In order to feed new consumers without installing new equipment (which usually isn't necessary anyway) the supply authority makes a substantial charge every month for every kVA of authorised capacity. This encourages consumers to be more realistic in their request.

Since the introduction of the availability charge in the past few years, many consumers have re-submitted requests for unrealistically low capacities and Scottish Hydro Electric have had to be clever in the extreme in dealing with the matter!