Embedded generation

Voltage rise the big issue when c nnecting embedded generat on to long 11 kV overhead lines There has recently been much interest in embedding small generators deep within distribution systems. The steady-state voltage rise resultingfrom the connection of these generators can be a major obstacle to their connection at the lower voltage levels. This article summarises the results of some generic studies, explaining this voltage rise issue and how it may be overcome.

by C. L. Masters

here has recently been much interest in connecting small generators, between 200kW and lOMW, deep within distri- T bution systems. These networks are,

by tradition, passive networks. They were designed to pass power from the national grid system, down the voltage levels, to LV customers. They were generally not designed for the connection of generators. There are many technical issues that must be considered when connecting a generating scheme to the distribution system, such as:

thermal rating of equipment system fault levels stability reverse power flow capability of tap-changers line-drop compensation steady-state voltage rise losses power quality (such as flicker, harmonics) protection.

This article concentrates on the steady-state voltage rise that occurs when connecting small generators to l l k V networks and often seriously impacts on the technical feasibility of such schemes.

Allowable voltage variations The Electricity Supply Regulations’ stipulate that, unless otherwise agreed, the steady-state voltage of systems between lOOOV and 132kV

should be maintained within +6% of the nominal voltage. For systems above 50V and below lOOOV, variations of between +lo% and -6% of nominal voltage are permitted. Prior to the 1994 amendments, variations of +6% were permitted. This change was a result of proposals to harmonise the UK electricity system with those in Europe.

The Electricity Supply Regulations are soon to be replaced with the Electricity Safety, Quality and Continuity Regulations.’ They were due to come into force in October 2001, but have been delayed due to the numerous comments made during the consultation process. The Electricity Safety, Quality and Continuity Regulations do not propose to make any immediate changes to the permitted voltage variations. However, it is proposed that, with effect from January 2003, the permitted voltage variations for systems between 50V and lOOOV will change to +lo%.

It is the Distribution Network Operator’s (DNO’s) responsibility to ensure that its systems are operated within the limits permitted by the Electricity Supply Regula- tions. However, at the planning stage, the 1lkV system is often designed to maintain voltages within *3% of nominal, so that the voltage variations seen by the LV connected customers remain within the permitted +lo% and -6% limits.

When a generator is to be connected to the distribution system, the DNO will consider

POWER ENGINEERING JOURNAL FEBRUARY 2002 5

Embedded generation

1 Voltage profile along the heavily loaded 11 kV overhead line used in the example

primary substation

IO8 r - nominal voltage at primary substation - 103% of nominal voltage at primary substation - 106% of nominal voltage at primary substation _ _ _ +6% voltage limit

106

104

102

100

98

96

94

92 I I I I I 0 4 8 12 16 20

distance from the primary substation, km

the worst case operating scenarios and ensure that their network and customers will not be adversely affected. Typically, these scenarios are:

no generation and maximum system demand maximum generation and maximum system

maximum generation and minimum system demand

demand.

Some DNOs take into account the diversity of the local load and consider the system with the minimum expected demand. Others do not, and assume no load as the worst case scenario.

Distribution systems with no embedded generation To transmit power from an l l k V primary substation to a typical LV connected customer some distance away will require the voltage at the primary substation to be higher than the voltage at the point of connection of the customer to the 11 kV system. This is explained using Panel 1.

Generally the X/R ratio of an l l k V overhead

line tends to be low, so neither of the terms RP or X Q can be neglected. This, coupled with the fact that the reactive power pushed down the line is usually much lower in magnitude than the power (assuming the customer imports reactive power), leads to there being a voltage drop along the line from the primary substation to the point of connection of the customer.

To demonstrate this, consider the following example (Fig. 1): connected to a primary substation is a 2Okm long, 1 l k V overhead line, comprising 16mm2 copper conductors. Every 4km along the line is a three-phase load of lOOkW and 20kvar. As the distance from the primary substation increases the voltage falls. With the primary substation at nominal voltage (IlkV), the far end of the line falls to 10.3kV (6% below the nominal voltage). This is right on the permitted limit. If the line had been longer or the load greater, the voltage would have fallen even further.

To maintain system voltages within permit- ted limits, DNOs often maintain primary substations above nominal voltage using automatic voltage control (AVC), on-load tap-changers and line-drop compensation.

6 POWER ENGINEERING JOURNAL FEBRUARY 2002

Embedded generation

where

VPS is the primary substation voltage VC is the voltage at the customer connection point R, X are the resistance and reactance of the overhead line P, Q are the power and reactive power transmitted from

the primary substation into the overhead line

Controlling the primary substation, in this example, to 103% and 106% of nominal voltage (11.3 kV and 11.7 kV) maintains the end of the l lkV line well within the permitted voltage limits.

Although the Electricity Supply Regulations allow voltage variations on the 11kV system of c6%, DNOs often impose limits of *3% at the planning stage. This is in order to maintain the LV connected customers within the permitted +lo% and -6% of nominal voltage. In this generic study the +3% planning limit is ignored. The l l k V system voltages are allowed to vary by +6% of nominal voltage, to more clearly demonstrate the effect of connecting a generator.

Effect of connecting generation to distribution systems Connecting a generator to the distribution system will affect the flow of power and the voltage profiles. To export its power, a genera- tor is likely to have to operate at a higher

able to absorb a significant amount of reactive power. This is explained using Panel 2.

As the XIR ratio of the l l kV line is small, neither RP nor X Q is negligible. The XQ term may be positive or negative, depending on whether the generator is exporting or importing reactive power. However, as the magnitude of the reactive power will be small compared to that of the power (unless some form of compensation is used), the RP+XQ term will tend to be positive. Thus, the voltage at the point of connection of the generator to the l l k V system will rise above that of the primary substation.

To demonstrate this, a 300kW generator (operating at unity power factor) is connected l2km from the primary substation (controlled at 103% of nominal voltage). The output of the generator is equal to the downstream demand, so the direction of the power flow from the primary substation is not altered. The voltage falls as the distance from the primary sub- station increases, as before. But the magnitude

voltage than the primary substation, unless it is of the voltage drop is less profound (Fig. 2). -- I_ _ _ _ _I I - . -I I I ___

where VGEN t VPS

1 VPS is the primary substation voltage I I VGEN IS the voltage at the generator connection point R, X are the resistance and reactance of the overhead line P, Q are the power and reactive power transmitted from

the generator into the overhead line I i

.I . . .. " ....

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Embedded generation

- no generation - 300 kW generator - 1 MW generator /--

g 109 m c < 107

._ 2 105 E e 103

8 101

-

c

97 99 0 1 4 8 12 16 20 0 4 8 12 16 20 no demand on the line full demand on the line

distance from the primary substation, km

2 Effect of connecting a Increasing the generation to 1MW reverses - - generator On thevoltage profile along the 11 kV line used in the

the flow of power along the line, from the generator towards the primary substation. The voltage at the generator rises above that elsewhere, thus allowing the power to be exported in both directions. In this example, the voltage in some parts of the system rises above the permitted +6% voltage limit.

The voltage rise is more onerous when there is no demand on the system, as all the generation is exported back to the primary substation. With 1MW of generation connected, the voltage rises to 112% of nominal. This suggests that it is the voltage rise during periods of no/minimum demand that limits how much generation can be connected.

When connecting a generator to the distri- bution system, a DNO must consider whether

3 the power may be exported back through the primary substation primary substation and must ensure that

rural 3311 1 kV

k

the transformer's tap-changers are capable of operating with a reverse power flow.

How can this voltage rise be counteracted? If the connection of a generator to an 11 kV overhead line causes an excessive voltage rise, there are several techniques that can be employed to alleviate the situation, for example:

reduce the primary substation voltage allow the generator to import reactive power (reducing the RP+XQ term) install auto transformers, or voltage regu- lators as they are often called, along the line (resetting the voltage along the line) increase the conductor size (reducing the resistance) constrain the generator at times of low demand (reducing the transmitted power) a combination of the above.

Reduce the primary substation voltage It is common practice for DNOs to maintain l l kV primary substations above nominal voltage to ensure that system voltages remain within the permitted -6% voltage limit. In the previous example, the voltage at the 1MW generator is 109% of nominal (under full- load conditions). Lowering the voltage at the primary substation from 103% to 100% of nominal reduces the voltage rise to just below the permitted +6% voltage limit (Fig. 4). It also reduces the voltage during periods of no system demand to around 110% of nominal, which is not sufficient.

Before lowering the voltage at a primary substation, a DNO must ensure that it will not adversely impact on any of its customers. If there are other feeders connected to the primary substation or teed off the l l k V line, the voltage profile along these circuits may be depressed. This may reduce the voltage of the LV customers connected to these feeders below the permitted -6% limit.

Also, if the generator is not exporting power, the system voltages will be depressed. In this example, the primary substation is maintained at 103% of nominal to ensure that the voltage 2Okm away is satisfactory. If the primary substation voltage is reduced to l l kV in order to connect the generator, the voltage at the end of the line will drop to 94% of nominal whenever the generator is not exporting power. The DNO must consider how it will correct this voltage depression. One solution may involve

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Embedded generation

- example system - effect of reducing primary substation to nominal voltage I effect of the generator operating at 0 9 power factor leading - effect of installing an auto transformer, 8 km from the

- effect of upgrading the line with 70 mm* copper conductor - effect of constraining the generator

primary substation

110 ... --- +6% volta

I I I I I I I I I I

0 4 8 12 16 20 0 4 8 12 16 20

full demand on the line no demand on the line

I distance from the primary substation, km

customer minutes lost while the off-circuit tap-changers are reset on the 11/0-415kV distribution transformers. However, this may not be practical if there are long lines or many distribution transformers involved.

Import reactive power DNOs may stipulate that generators operate at leading, lagging or even unity power factor, depending on the X/R ratio of the system, voltage regulation, local load etc. Generators are typically operated at a power factor such that if they trip, when at rated generation, the disturbance to the system is minimised.

The amount of reactive power that can be imported is generally governed by the parameters of the generator. Typically a synchronous generator can import reactive power at a 0.95 power factor. Wind turbines, with uncompensated induction generators, can import reactive power at around a 0.9 power factor.

In the initial example the 1MW generator operates at unity power factor. The voltage rises to almost 109% of nominal (under full load conditions) and 113% of nominal (under no load conditions). Allowing the generator to operate at a leading power factor of 0.9 limits the voltage rise to 104% and 108% of nominal, respectively (Fig. 4). With maximum demand on the system, this brings the voltages within the permitted +6% voltage limit. During periods of no system demand, the voltage is not

4 Effect of using various methods to reduce the voltage rise on the 11 kV line used in the example

lowered sufficiently. If a generator is to import significant levels

of reactive power, it may be necessary to agree a charging mechanism with a supplier to cover the costs involved with purchasing and transporting these extra kvars. The DNO must also consider the effect that this additional reactive power flow will have on system losses and the loading on circuits. The effect of the generator tripping must also be considered, as this will cause a transient voltage rise. It may take the transformer tap-changers at the primary substation several seconds to respond and restore the voltages. Under such circum- stances a DNO may be able to use a switched

'capacitor bank or some other form of reactive 5 Typical rural 1 lkv overhead line

' POWER ENGINEERING JOURNAL FEBRUARY 2002 9

Embedded generation

6 Some guidance as to the level of generation that can be accepted onto an 11 kV overhead line - 16 mm2 conductor, 11 kV at primary substation - 16 mm2 conductor, 11.3 kV at primary substation - 16 mm2 conductor, 11.6 kV at primary substation

- - - 70 mm2 conductor, 11 kV at primary substation - - - 70 mm* conductor, 11.3 kV at primary substation - - - 70 mm2 conductor, 11.6 kV at primary substation

0 2 4 6 8 IO 12 14 16 18 20 22 24 26 28 30

distance, km

power compensation to restore the system voltages.

Install auto transformers along the line Auto transformers (voltage regulators or voltage boosters) are simply transformers with a voltage ratio of 1:l and on-load tap-changers for voltage regulation. Essentially, inserting an auto transformer into a long circuit splits it into two sections. The voltage along one section will be regulated by the AVC, tap-changers and line-drop compensation at the primary substation. The auto transformer will regulate the voltage along the other section.

Inserting an auto transformer 8km from the primary substation, in the initial example, has little effect on the voltage profile between itself and the primary substation. Under full and no load conditions the primary of the auto transformer rises to 106% and 109% of nominal voltage, respectively (Fig. 4). The on- load tap-changer, in this example, is set to control the voltage at the secondary of the auto transformer to nominal voltage (using a tap range of ~ 5 % in five steps). Under both full and no load conditions it operates to reduce the voltage to 101% of nominal, thus maintaining the voltage rise along the remainder of the l l kV line below the permitted +6% voltage limit. In this example, the auto transformer does not prevent this limit being exceeded when there is no demand. However, by careful positioning of either one or two auto

transformers, the voltages may be maintained within limits.

Auto transformers have not traditionally been used by DNOs in this manner because there has been little generation connected to the distribution system. However, as the levels of embedded generation are set to increase their use may become more common.

When installing an auto transformer into the distribution system the DNO must consider its effect' on the system voltages under all the worst case operating scenarios to ensure that no customers will be adversely affected. The effect of the auto transformer on the line loading must also be taken into account, as it may increase the flow of reactive power along the line. The DNO must also consider how the presence of the auto transformer will affect system security, as it will introduce another factor of unreliability into the system.

Upgrade the conductor Small overhead line conductors have higher impedance than large conductors. A 70mm2 copper conductor has approximately one-third of the resistance and 90% of the reactance of a 16mm2 conductor. Thus, upgrading the con- ductor on an l lkV overhead line will signifi- cantly reduce its resistance and will smooth the voltage profile along the line.

In the initial example, the voltage at the 1MW generator was 109% of nominal (under full load conditions) and 113% of nominal

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Embedded generation

(under no load conditions). The voltage profile along the line is improved by replacing the 16mm2 conductor with 70mm2 copper (Fig. 4). It reduces the voltage at the generator to less than 105% of nominal (under full load conditions). With no demand on the line, it is marginally above the permitted +6% voltage limit.

This suggests that upgrading the conductors is a very effective method of counteracting the voltage rise problem. However, replacing the conductors can be expensive and may make a scheme uneconomic.

Constrain the generation The sophisticated control systems available these days will allow a generator to control its output in line with the system voltage. Thus if the voltage is approaching the permitted +6% voltage limit, a generator can reduce its output in order to maintain the voltage below the threshold. This will allow the generator to continue operating, rather than being con- strained off during periods of low system demand. Conversely, should the system voltage fall below nominal, a generator may be able to respond by increasing its output.

The initial example suggests that the 1MW generator cannot be accepted onto the llkV line, even when it is fully loaded. Its output has to be constrained to 750kW to maintain the system voltages within the permitted +6% limit (Fig. 4). It will have to be constrained further as the system loading is reduced. Under no load conditions the generator has to be constrained to 300kW to maintain the voltages below the permitted +6% threshold.

Constraining an embedded generator will obviously affect the economic benefit of the scheme. It is usually only-a viable option when the constraints are expected to be infrequent and where significant system reinforcement costs are avoided.

How much generation can be connected to an 11 kV overhead line? The level of generation that can be absorbed onto the distribution system is determined by many factors, such as:

voltage level voltage at the primary substation distance from the primary substation size of conductor demand on the system other generation on the system

operating regime of the generation.

Fig. 6 gives some indication as to the amount of generation that can be connected to an 11kV overhead line. It is clear that, as the distance from the primary substation increases, the amount of generation that can be accepted reduces.

Case studies Three brief case studies are presented here to show how Innogy plc has approached this voltage rise issue when developing small generating schemes.

ChiRex.CHP scheme The ChiRex combined heat and power (CHP) scheme in Northumberland (Fig. 7) comprises a 4.5MW gas turbine. It has been operational since June 1994, providing electricity and steam to the ChiRex pharmaceutical plant. Both are normally connected to the l lkV primary substation by a single l l k V cable. During some periods, such as Christmas, the demand at the pharmaceutical plant falls dramatically, and the CHP scheme exports the majority of its power into the distribution system. This causes the voltage to rise and the generator was once tripped off by the overvoltage protection.

This problem was overcome by altering the operating procedure of the CHP scheme. The output and power factor of the generator are 7 ChiRexCHPscheme

a

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Embedded generation

8 Typical small hydro generating scheme (Blantyre, Scotland)

now manually adjusted by the operators who monitor the local demand and the system voltage.

9 Jenbacher gas engine, produced by Clarke, for small embedded generation schemes (photo: courtesy of Clarke Energy, www.clarke- energy.co.uk) I

500k W hydro-generating scheme Innogy Hydro is in the early stages of developing a 500kW hydro-generating scheme in the north of Scotland (Fig. 8). The generator is to be connected to an l l k V overhead line, comprising 16"' copper conductors, approximately 15km from the primary substation. Also connected to this 1lkV line are numerous domestic customers fed by individual 1 U0.415 kV distribution trans- formers with off-circuit tap-changers.

The DNO has stipulated that the voltage along this l l kV line must not exceed 11.13 kV (1.2% above nominal voltage) as this will raise customers' voltages above 253\3 the +lo% tolerance specified in the Electricity Supply Regulations.

Provisional studies have shown that the existing l l kV system cannot accept 500kW of generation. Reducing the voltage at the primary substation is not feasible, as there are other l l k V circuits connected to the primary substation. Upgrading the line with 70"' copper conductors increases the amount of generation that can be connected, but not sufficiently.

I

The cost and feasibility of two methods of overcoming the voltage rise problem are currently being considered-installing reactive power compensation at the generator, or an auto transformer part way along the 11 kV line.

1 OMW mines gas generating scheme Cogen, an Innogy subsidiary, is currently developing a lOMW generating scheme to burn methane gas from a disused coal mine. The scheme will comprise two 5MW plants (Fig. 9) connected separately to two existing l l kV cables that run along the edge of the proposed site. Studies have shown that the generation cannot be accepted onto the existing system due to the excessive voltage rise.

The local DNO currently operates the primary substation at 11.6kV. Following tests on the system, the DNO has agreed to reduce the primary substation voltage to 11.3kV so the generation can be connected. However, should the generating scheme be out of service, the system voltages will be depressed and the voltage of a few customers will fall below the permitted limit. As the generating scheme is expected to operate at base load, this scenario will not occur frequently. It has been agreed that, when this does occur, the DNO will dispatch an engineer to manually alter the distribution transformers' tap-changers and 'the DNO will be compensated appropriately by Innogy.

Conclusions In conclusion, there are many factors that determine the level of generation that can be connected to the distribution system at 1lkV Thus every scheme will face different technical and commercial issues and must be studied on a site-by-site basis. One of the major technical difficulties is the voltage rise resulting from the reversed power flow. There are methods of counteracting this voltage rise; however, a developer must consider whether the addi- tional costs are justified.

References 1 The Electricity Supply Regulationsl988: Regulation

30, paragraph 2, amended in 1994 2 The Electricity Safety, Quality and Continuity

Regulations. 2001: Draft copy-available for con- sultation purposes on the DTI wehsite

0 IEE: 2002

Dr C. L. Masters is a Power Systems Engineer in Operations and Engineering, lnnogy plc. She is a Member of the IEE.

12 POWER ENGINEERING JOURNAL FEBRUARY 2002