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Load shedding practices, restoration practices, and critical & essential loads

Feedback and current initiativesNRS 048-9 Code of Practice

14 May 2009

© Eskom Holdings Limited

Copyright, Eskom 2007 2

Framing the context

• The country’s infrastructure, of which the power system forms an essential part, is exposed to a wide variety of threats:

– For example: Severe weather events / impacts of climate change, wilful damage to infrastructure (incl. vandalism), key network/plant failures, load shedding required in the event of system capacity shortages.

• Unlike countries more regularly exposed to the impact of such threats, South African society is relatively unprepared for the associated disruptions..

– For example: South East USA and countries such as Canada and France that undertake national

simulations of severe power system events.

• The increasing dependency of society on the power system, and the potential for an increase in the frequency of such threats materialising, implies the need for a country approach to enhancing societal resilience.

• From a power system perspective, a comprehensive “country” approach to addressing the requirements of identified critical and essential loads needs to be developed so as to contain the impact from the perspectives of safety, environmental impact, society impact, economic impact etc.

Critical & essential loads

Emergency Response Protocols


Approach

• Coordinated and transparent industry approach via a National Code of Practice

– Metro/Munic Task Team Meeting – Agreement on developing a common approach in response to DPE proposed list of exclusions from schedules . NRS process agreed as a suitable vehicle (15 January 2009).

– Industry Task Team Meeting - Similar agreement on this process (incl. active involvement)

• NRS 048 Code of Practice (Fast-track target date for completion: July 2009)

– Stakeholder involvement: Eskom/AMEU/Government/NERSA/Customer project.

– Project approval: ToR developed and approved by ESLC

– Progress: Draft 2 will be finalised on the 14th May for circulation

• Critical & essential loads information and status:

– DPE discussions – possible regulations (OHSACT) for providing essential load data

– Roll out of programme – National Electricity Response Team(NERT)-project office in DME has been established for electricity issues.

– Requirements – face-to-face meetings with customers / regulators linked to related Eskom initiatives: (i) Blackstart preparedness, (ii) FIFA Cup, (iii) Societal resilience initiatives.

– Funding mechanisms – based on requirements in Code of Practice

Load shedding, restoration, and critical & essential loads



Roadmap

Enhancement of national emergency load shedding and restoration practices

Protocols

Prior Jan 08

Protocols

Prior Jan 08

Protocols

Revision 1

April 2008

Protocols

Revision 1

April 2008

Protocols

Revision 2

Mar 2009

Protocols

Revision 2

Mar 2009

Protocols

Revision 3

July 2009

Protocols

Revision 3

July 2009

Safety & environment

Industry Task Team

engagement on

KIC load reduction

& essential load data

Predictability

Equitable participation

Social impact

Economic impact

“Brutal audit”

Summer 2008

load shedding

Development of

national code of practice

critical / essential

loads & societal

resilience

Confederations

& World Cup

Temporary

Schedules

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■

■

■

■


Predictability


Social impact

Economic impact

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■

■

■

■


Predictability


Social impact

Economic impact

■

■

■

■

■


Predictability


Social impact

Economic impact

■

■

■

■

■

■ Not optimised ■ Optimised

Country

implementation

of code of practice

■ Partially optimised


Technical issues Technical issues Technical issues Technical issues ■■■■


NRS Code of Practice

• Provides electricity suppliers (licensees) with best practice for developing load shedding practices;

• Provides electricity suppliers (licensees) with best practice for prioritising load restoration after a major system event;

• Defines and categorises critical and essential loads;

• Provides electricity suppliers (licensees), customers, and local government with best practice on how to address these loads in the context of load shedding or system restoration practices;

• Identifies the responsibilities that customers have in relation to information sharing on critical and essential loads,

• Identifies the responsibilities that the licensee utility has in terms of information sharing related to load shedding schedules.

• Provides standard definitions related to load reduction principles in order to facilitate common understanding between stakeholders.

• Defines the roles, responsibilities, and limitations of licensees and customers in addressing various aspects of load shedding

SCOPE

“Load Shedding Practices, System Restoration Practices, and Critical & Essential loads”



• Essential load requirement – customer load requirement (e.g. MW, notification time, and duration) to avoid a direct and significant impact on the safety of people, the environment, and physical plant/equipment of a nationally critical product, and which has been specifically notified as such by the customer to the licensee.

– NOTE: The load required is generally part of the total load that a customer installation normally requires, and a customer will be required to demonstrate the potential impact on safety, the environment, and physical equipment when on notification to the licensee.

– NOTE: The load may be part of a larger installation that will still be required to participate in load shedding or curtailment schemes.

– NOTE: Not contracted (as no guarantee can be given).

– NOTE: These loads should receive priority during restoration of the system (e.g. in the event of a major local interruption, or of a regional or national blackout) and should as far as possible be excluded from at least the initial stages of load shedding/curtailment.

– NOTE: Those loads that in the past were categorised as SATEPSA loads under the Republic of South Africa Telecommunications and Electrical Power Supply Authority

• Critical loads – loads that should as far as possible be protected from the impact of load shedding to either maintain the operational integrity of the power system, or to avoid a cascading impact on public infrastructure.

– NOTE: Protection from the impact of load shedding includes measures such as excluding from load shedding schedules, installing back-up facilities, or implementing specific response protocols

Definitions


• Principle 1: The integrity of the National automatic under-frequency load shedding system shall not be compromised by manual load-shedding or curtailment

• Principle 2: All customer installations shall be considered for emergency (manual) load reduction, based on broadly equitable participation by customers.

• Principle 3: Time-based manual load shedding shall be applied.

• Principle 4: Load shedding schedules shall be developed, maintained, and be available to customers.

• Principle 5: Customer reduction due to power conservation programmes shall not be considered as emergency load reduction.

• Principle 6: Critical and essential load requirements shall be addressed in accordance with this code of practice

• Principle 7:Predefined load schedules will be defined up to a predefined maximum load. Where more load shedding is required, this is regarded is an extreme system condition explicitly excluded from principles 1 to 6 and which will be handled in accordance with the situation prevalent at the time

Loadshedding Principles




• Two primary categories of load reduction are specified – i.e. (i) Load reduction required from Eskom, Metro, and Municipal customers, and (ii) Load reduction required from customers who meet the curtailment requirements. These requirements are specified in Table 1:

Stage Type Reduction Required by load shedding

of customers

Reduction Required by end-use customers

eligible for curtailment

Stage 1 Scheduled 5% of national non-curtailment load MW 10% of normal demand after energy

conservation savings within 2h of

notificationStage 2 Scheduled 2000 MW

Stage 3 Scheduled 4000 MW 20% of normal demand after energy

conservation savings

Stage 4 Unscheduled > 4000 MW Minimum load as required

Requirements- loadshedding



• Stage 1 (Scheduled): The required national distribution load shall be scheduled for shedding, with customers impacted for predefined intervals. Schedules shall be prepared from 06:00 – 22:00 daily. Customer loads are assigned to specific time slots.

– Example : Published schedules may indicate that customers are impacted for 2h every 2nd day (excl. Sunday). In practice, each time slot may overlap its adjacent time slot by an additional 0,5h to facilitate smooth change over.

• Stage 2 (Scheduled): The load reduction requirement for national distribution load is scheduled to be shed at 2h intervals every day, for example by doubling the frequency of the Stage 1 block.

• Stage 3 (Scheduled): Emergency schedules are prepared to meet the additional requirement on a 24 hour basis, but may not necessarily be published.

• Stage 4 (Unscheduled): The amount of Distribution load required to be reduced in Manual Load Shedding Stage 4 is greater than 4000MW. The management of manual load shedding for Stage 4 is handled in accordance with the situation prevalent at the time.

Requirements:schedules



• An essential loads register is required by a licensee primarily for the prioritisation of restoration of supply in the case of a blackout. The essential load requirement is also the minimum load requirement should load shedding reach stage 4.

• The relevant supply authority shall collate such information and compile a registry of essential loads within their area of supply. The essential load requirement may be subject to verification by the licensee in terms of the following criteria:

– Critical safety requirement

– Critical environment impact requirement

– National critical product requirement

Requirements: essential loads



• Loads that should as far as possible be protected from the impact of load shedding or loss of supply in order to either maintain the operational integrity of the power system, or to avoid a cascading impact on public infrastructure. (e.g. refineries, fuel pipelines)

– NOTE: Protection from the impact of load shedding includes measures such as excluding from load shedding schedules, installing back-up facilities, or implementing specific response protocols.

• The following loads are considered critical loads in terms of this code of practice:

– Public transport (e.g. rail, airports)

– Water pumping (power station requirements)

– Refineries

– Fuel pipelines

– Coal mines supplying power stations

– Essential services (e.g. police, fire fighting)

– Telecommunications infrastructure (exchanges)

– Traffic lights

– Airports

– National key points reliant on electricity for their core operations

Requirements: critical loads


Calculation of load shedding quotas

In determining load shedding quotas for the various licensees, the following methodology shall be applied:

– Each Control Centre will determine the load under its control. Possible exclusions include KSACS loads from Distribution regions.

– From this reduced load, critical loads served by the Control Centre are subtracted to obtain the base load on which the required reduction for that Control Centre is determined.

– All the base loads in the country will be added together to determine the national base load.

– Each Control Centre’s base load is divided by this overall national base load to calculate it’s required contribution (as a percentage) per load shedding event.

– The allocation will be determined at the upstream Control Centre based initially on the values given to Eskom Distribution Control Centres by the National Control Centre.

Calculation of load shedding quotas



• Example of the calculation of schedules taking automatic under-frequency load shedding into consideration.

• Given a UFLS 1st stage set of loads totalling 100MWs, 105% of the requirement, and a schedule of 10 time slots:

Time slot Manual load shedding UFLS on load shedding UFLS available UFLS % available

6-8 200 11 89 93%

8-10 200 8 92 97%

10-12 200 12 88 92%

12-2 200 9 89 93%

2-4 200 10 90 95%

4-6 200 10 90 95%

6-8 200 11 89 93%

8-10 200 9 91 95%

10-12 200 10 90 95%

• What can be seen is that the UFLS loads are split among all the various time slots. This means that all the loads contribute to load shedding, but the UFLS system is not materially affected. The gap is only the amount of UFLS used at any one time, in this case the largest is 12MWs, which reduces the scheme in the worst case to 92% of requirement. Therefore an additional installation of 12MWs of UFLS will cover the deficit.

• The downside is a slight over-shedding during a UFLS event, when not load shedding, which is likely.

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Example of integrating UFLS with load shedding

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